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Impact of Cyclone Nilam on Tropical Lower Atmospheric Dynamics

doi: 10.1007/s00376-016-5285-x

  • A deep depression formed over the Bay of Bengal on 28 October 2012, and developed into a cyclonic storm. After landfall near the south coast of Chennai, cyclone Nilam moved north-northwestwards. Coordinated experiments were conducted from the Indian stations of Gadanki (13.5°N, 79.2°E) and Hyderabad (17.4°N, 78.5°E) to study the modification of gravity-wave activity and turbulence by cyclone Nilam, using GPS radiosonde and mesosphere-stratosphere-troposphere radar data. The horizontal velocities underwent large changes during the closest approach of the storm to the experimental sites. Hodograph analysis revealed that inertia gravity waves (IGWs) associated with the cyclone changed their directions from northeast (control time) to northwest following the path of the cyclone. The momentum flux of IGWs and short-period gravity waves (1-8 h) enhanced prior to, and during, the passage of the storm (0.05 m2 s-2 and 0.3 m2 s-2, respectively), compared to the flux after its passage. The corresponding body forces underwent similar changes, with values ranging between 2-4 m s-1 d-1 and 12-15 m s-1 d-1. The turbulence refractivity structure constant (Cn2) showed large values below 10 km before the passage of the cyclone when humidity in the region was very high. Turbulence and humidity reduced during the passage of the storm when a turbulent layer at ∼17 km became more intense. Turbulence in the lower troposphere and near the tropopause became weak after the passage of the cyclone.
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  • Abdullah A. J., 1966: The spiral bands of a Hurricane: A possible dynamic explanation. J. Atmos. Sci., 23, 367- 375.10.1175/1520-0469(1966)0232.0.CO; this paper an attempt is made to explain the shape and behavior of the spiral bands of a hurricane. The main hypothesis is that the bands are associated with gravitational waves of finite amplitude propagating at. the interface of a high-level inversion. An external source of disturbance is postulated in the form of a fresh surge of air at the exterior region of the hurricane. It is shown that this mechanism leads to the formation of bands of the required shape. The spiral bands are therefore related to the squall lines of temperate latitudes. Two numerical examples are worked out to illustrate the proposed mechanism.
    Alexand er, M. J., R. A. Vincent, 2000: Gravity waves in the tropical lower stratosphere: A model study of seasonal and interannual variability. J. Geophys. Res.,105, 17 983-17 993, doi: 10.1029/2000JD900197.10.1029/ model study is presented to clarify the relationship between gravity-wave properties observed in the stratosphere and the sources for the waves, presumed to be in the troposphere. The observations are balloon-borne radiosondes launched from Cocos Island in the tropical Indian Ocean (12°S, 97°E), and the analysis of these data is described in a companion paper [Vincent and Alexander, this issue]. The dominant time variations in the observed gravity wave activity are annual and quasi-biennial patterns in the zonal momentum flux and kinetic energy density. The background zonal winds at this site vary with the same periods, and these are known to be capable of causing dramatic variations in the observable properties of the waves even if the sources for the waves are constant in time. The results presented here clarify (1) the nature of the sources for the gravity waves observed in the stratosphere, (2) the limitations of the observations for observing the full range of gravity wave perturbations potentially present in the atmosphere, and (3) the role the observed waves can play in forcing the quasibiennial oscillation (QBO) in the zonal winds at this latitude. The stratospheric waves appear to originate near the height of the tropopause, so the source is apparently related to deep convection. No seasonal or interannual variations in the convection need be assumed to understand the observations. The waves at the tropopause appear to have a phase speed distribution that is narrowly confined near zero phase speed relative to the ground. The source is likely related to slowly propagating tropospheric convection and the wind near the tropopause. Variations observed in the stratospheric data are caused by both the wind shear in the stratosphere and the ability of waves with these characteristics to propagate vertically without severe dissipation. Higher phase speed waves may be present and could carry significant momentum flux vertically into the stratosphere and mesosphere but would be extremely difficult to see in these radiosonde data. The observed waves can contribute substantially to the descent of the eastward shear zones characteristic of the "westerly" phase of the QBO in the lower stratosphere zonal winds.
    Alexand er, M. J., J. R. Holton, D. R. Durran, 1995: The gravity wave response above deep convection in a squall line simulation. J. Atmos. Sci., 52, 2212-
    Allen S. J., R. A. Vincent, 1995: Gravity wave activity in the lower atmosphere: Seasonal and latitudinal variations. J. Geophys. Res., 100, 1327- 1350.10.1029/ climatology of gravity wave activity in the lower atmosphere based on high-resolution radiosonde measurements provided by the Australian Bureau of Meteorology is presented. These data are ideal for investigating gravity wave activity and its variation with position and time. Observations from 18 meteorological stations within Australia and Antarctica, covering a latitude range of 12 deg S - 68 deg S and a longitude range of 78 deg E - 159 E, are discussed. Vertical wavenumber power spectra of normalized temperature fluctuations are calculated within both the troposphere and the lower stratosphere and are compared with the predictions of current gravity wave saturation theories. Estimates of important model parameters such as the total gravity wave energy per unit mass are also presented. The vertical wavenumber power spectra are found to remain approximately invariant with time and geographic location with only one significant exception. Spectral amplitudes observed within the lower stratosphere are found to be consistent with theoretical expectations but the amplitudes observed within the troposphere are consistently larger than expected, often by as much as a factor of about 3. Seasonal variations of stratospheric wave energy per unit mass are identified with maxima occurring during the low-latitude wet season and during the midlatitude winter. These variations do not exceed a factor of about 2. Similar variations are not found in the troposphere where temperature fluctuations are likely to be contaminated by convection and inversions. The largest values of wave energy density are typically found near the tropopause.
    Arunachalam Srinivasan, M., S. V. B. Rao, R. Suresh, 2014: Investigation of convectively generated gravity wave characteristics and generation mechanisms during the passage of thunderstorm and squall line over Gadanki (13.5\circN,79.2\circE). Annales Geophysicae, 32, 57-68, doi: 10.5194/angeo-32-57-2014.10.5194/ study illustrates the convectively generated gravity wave generation mechanisms during the passage of thunderstorms and squall line using Indian MST radar. For the first time, it has been shown that all three generation mechanisms have been involved in the generation of gravity waves during the passage of squall line event. It is observed that the periodicities in the range of 8-80 min in the tropospheric and 8-32 min in the lower stratospheric regions and vertical wavelengths in the range of 3.2-4.8 km in the tropospheric and 1.2-1.92 km in the lower stratospheric regions are found to be dominant in the present study and are distinctly different during initial, mature and dissipative phases of convection. Amplitude of vertical wind has been weakened (from ~ 4-6 m sto ~ 1 m s) considerably after 10-30 min of a convection event. It appears that the wind shear associated with the convective clouds acted like an obstacle to the mean background flow during the squall line passage generated gravity waves. The phase profiles corresponding to the dominant period show both downward and upward propagation of waves. The vertical extent of heating is found to be deeper during squall line event compared with thunderstorm event. From the phase profiles, during 27 September 2004, two peaks of constant phase region are observed. One is due to convective elements and the other is due to strong background wind shear; however, only one peak is observed on 29 September 2004, which is only due to convective processes.
    Chane-Ming F., Z. Chen, and F. Roux, 2010: Analysis of gravity-waves produced by intense tropical cyclones. Annales Geophysicae,28, 531-547, doi: 10.5194/angeo-28-531-2010.10.5194/ and wavelet methods are combined to characterize gravity-waves (GWs) produced by two intense tropical cyclones (TCs) in the upper troposphere and lower stratosphere (UT/LS) from GPS winsonde data. Analyses reveal large contribution of GWs induced by TCs to wave energy densities in the UT/LS. An increase in total energy density of about 30% of the climatological energy density in austral summer was estimated in the LS above Tromelin during TC Dina. Four distinct periods in GW activity in relation with TC Faxai stages is observed in the UT. Globally, GWs have periods of 6 h–2.5 days, vertical wavelenghts of 1–3 km and horizontal wavelengths <1000 km in the UT during the evolution of TCs. Horizontal wavelengths are longer in the LS and about 2200 km during TCs. Convective activity over the basin and GW energy density were modulated by mixed equatorial waves of 3–4 days, 6–8 days and 10–13 days confirmed by H vm ller diagram, Fourier and wavelet analyses of OLR data. Moreover, location of GW sources is below the tropopause height when TCs are intense otherwise varies at lower tropospheric heights depending on the strength of convection. Finally, the maximum surface wind speeds of TCs Dina and Faxai can be linearly estimated with total energy densities.
    Chen S. M., Y. Y. Lu, W. B. Li, and Z. P. Wen, 2015: Identification and analysis of high-frequency oscillations in the eyewalls of tropical cyclones. Adv. Atmos. Sci.,32(6), 624-634, doi: 10.1007/s00376-014-4063-x.10.1007/ oscillations, with periods of about 2 hours, are first identified by applying wavelet analysis to observed minutely wind speeds around the eye and eyewall of tropical cyclones (TCs). Analysis of a model simulation of Typhoon Hagupit (2008) shows that the oscillations also occur in the TC intensity, vertical motion, convergence activity and air density around the eyewall. Sequences of oscillations in these variables follow a certain order.
    Cho J. Y. N., 1995: Inertio-gravity wave parameter estimation from cross-spectral analysis. J. Geophys. Res.,100(D9), 18 727-18 737, doi: 10.1029/95JD01752.10.1029/ outline a method for extracting inertio-gravity wave parameters using the autospectra and cross spectra of the horizontal perturbation winds. In essence, we define a statistical hodograph for each spectral bin, thus combining the advantages of the rotary spectrum and hodograph methods. Furthermore, we include the effects of the background vertical shear in the parameter estimation equations, a step that had often been omitted in the past. Applying this technique to a long-period data set taken with the Arecibo 430-MHz radar, we explore its usefulness as well as its limitations. Our analysis of this data set also supports the interpretation of horizontal wind-perturbation rotation in the lower stratosphere over Arecibo as inertio-gravity waves rather than mountain waves imbedded within a background vertical shear.
    Clark T. L., T. Hauf, and J. P. Kuettner, 1986: Convectively forced internal gravity waves: Results from two-dimensional numerical experiments. Quart. J. Roy. Meteor. Soc., 112, 899- 925.10.1002/ Two-dimensional numerical simulations were performed to investigate the nature of tropospheric internal gravity waves of the type which are observed to occur above active thermal convection over an unstable boundary layer. These gravity waves are believed to be excited by a combination of pure thermal forcing and by the boundary layer eddies and cumulus clouds acting as obstacles to the flow in the presence of mean environmental wind shear. Large amplitude internal gravity waves were obtained in the simulations with amplitudes and horizontal scales similar to the 12 June 1984 aircraft observations over western Nebraska. This was a day with strong wind shear in the lowest 3 km above the ground and with scattered cumulus clouds topping the boundary layer. The simulations show that there is significant difference between the early time solutions (as might be predicted by linear theory) and late time solutions for the boundary layer eddy structure. A layer interaction occurs in which gravity waves of the stable layer are excited by the boundary layer convection. There is evidence to suggest that this layer interaction occurs both with and without shear but that it is stronger in the presence of low-level shear. Results indicate that shear (or the obstacle) effect is a more efficient generator of gravity waves than is the pure thermal forcing. The simulations show that the gravity waves initially forced by the boundary layer eddies lead to a feedback mechanism that acts to organize the boundary layer eddies and the cumulus convection. The solutions suggest that the character of fair weather convection (moist or dry) is a non-local problem involving at times the full depth of the troposphere. The clouds produced in the simulations have very little influence on the wave field or boundary layer eddy structure as they are relatively small cumuli. On the other hand the clouds are strongly influenced by the interactions between the wave and eddy fields. Upshear growth of cumulus clouds similar to that which is frequently observed in nature is reproduced in the simulations. The development of feeder (-榝eeder- is used here in a dynamical sense only) clouds on (typically) the upshear side of the cloud is found to be a result of the interaction between the gravity wave field and the dry and moist convection. The relative phase velocity between the gravity waves and the cloud plays a crucial role in determining the character of the cumulus cloud growth in the present simulations. These simulations suggest that the dynamics both internal and external to the boundary of a cumulus cloud is a complicated mix between wave dynamics and the usually considered convection dynamics. A brief discussion of the implications of the present results to cloud boundary baroclinic instability dynamics is also presented.
    Das S. S., K. K. Kumar, and K. N. Uma, 2010: MST radar investigation on inertia-gravity waves associated with tropical depression in the upper troposphere and lower stratosphere over Gadanki (13.5\circN,79.2\circE). Journal of Atmospheric and Solar-Terrestrial Physics, 72, 1184-1194, doi: 10.1016/j.jastp. 2010. measurements of 3-dimensional winds, spectral parameters, and tropopause height for 藴114 h during the passage of a tropical depression using mesosphere-stratosphere-troposphere (MST) radar at Gadanki (13.5°N, 79.2°E) are discussed. The spectral analysis of zonal and meridional winds shows the presence of inertia-gravity wave (IGW) with the dominant periodicity of 56 h and intrinsic period of 27 h in the upper troposphere and lower stratosphere (UTLS). The strengthening of easterly jet and associated wind shears during the passage of the depression is one of the causative mechanisms for exciting the IGW. A well-established radar method is used to identify the tropopause and to study its response to the propagating atmospheric disturbances. The significance of the present study lies in showing the response of tropopause height to the IGW during tropical depression for the first time, which will have implications in stratosphere-troposphere exchange processes. Continuous measurements of 3-dimensional winds, spectral parameters, and tropopause height for 藴114 h during the passage of a tropical depression using mesosphere-stratosphere-troposphere (MST) radar at Gadanki (13.5°N, 79.2°E) are discussed. The spectral analysis of zonal and meridional winds shows the presence of inertia-gravity wave (IGW) with the dominant periodicity of 56 h and intrinsic period of 27 h in the upper troposphere and lower stratosphere (UTLS). The strengthening of easterly jet and associated wind shears during the passage of the depression is one of the causative mechanisms for exciting the IGW. A well-established radar method is used to identify the tropopause and to study its response to the propagating atmospheric disturbances. The significance of the present study lies in showing the response of tropopause height to the IGW during tropical depression for the first time, which will have implications in stratosphere-troposphere exchange processes.
    Das S. S., K. N. Uma, and S. K. Das, 2012: MST radar observations of short-period gravity wave during overhead tropical cyclone. Radio Sci., 47,RS2019, doi: 10.1029/2011RS 004840.
    Dewan E. M., 1979: Stratospheric wave spectra resembling turbulence. Science,204, 832-835, doi: 10.1126/science.204. 4395.832.10.1126/ effects on ozone raise the question of the significance of turbulence in vertical transport in the stratosphere. The aircraft in situ measurements of velocity fluctuations previously employed to estimate turbulence transport were, it is hypothesized, due to atmospheric waves, despite their classical turbulence spectrum. This new hypothesis implies that previous turbulence estimates are invalid. Experimental tests are suggested.
    Dhaka S. K., P. K. Devrajan, Y. Shibagaki, R. K. Choudhary, and S. Fukao, 2001: Indian MST radar observations of gravity wave activities associated with tropical convection. Journal of Atmospheric and Solar-Terrestrial Physics, 63, 1631- 1642.10.1016/S1364-6826(01) radar observations at Gadanki ja:math of high frequency (few tens of minutes) gravity waves generated most likely by convection are presented. The experiments were conducted during the months of May–June (19 May 1995, 05 June 1995 and 06 June 1996) of summer season, which is likely to be highly convective after the onset of south–west monsoon over southern part of India. The excitation and vertical propagation of gravity waves are found to display specific characteristics pointing convection as a main source. The intriguing characteristics of rising reflectivity pattern, coupled with rise in vertical wind component and turbulence from lower troposphere to upper troposphere within hour of time (on 19 May 1995 and 06 June 1996) are noticed. On the other day of experiment, horizontal large spread in reflectivity pattern confined mostly below ja:math of altitude has been observed. During this period wind disturbances are found to posses comparatively large magnitude of momentum flux at mid-tropospheric levels. Usually enhanced reflectivity regions are found to be accompanied by strong updraft and downdraft in vertical wind component ( w ). An interesting feature in the case of 05 June 1995 is the appearance of vertical wind disturbances well up to lower stratosphere. The effect of these enhanced vertical wind is in conformity with the observed increase in the momentum flux values even up to lower stratospheric altitudes. Typical wave amplitude in vertical wind disturbances during convection vary from 1– ja:math with some sudden enhancement in amplitude of the order of 8– ja:math for some short interval of time. An effort has been made to discuss these results in the light of existing theoretical concepts viz. mechanical oscillator effect, obstacle effect and direct thermal forcing for generating the convection waves (gravity waves generated due to convection).
    Dhaka S. K., M. Takahashi, Y. Kawatani, S. Malik, Y. Shibagaki, and S. Fukao, 2003: Observations of deep convective updrafts in tropical convection and their role in the generation of gravity waves. J. Meteor. Soc.Japan, 81, 1185- 1199.10.2151/ A few sequential strong updrafts of magnitude about 8&ndash;10 m/s in the upper troposphere were ob-served by the MST radar at Gadanki (13.5 N, 79.2 E), India on 21&ndash;22 and 22&ndash;23 June 2000. On both days, convective storms with rainfall appeared over the radar site. The updrafts region shifted upward by a distance of about 3&ndash;4 km within a time-range of 8&ndash;10 minutes, and terminated around 15&ndash;16 km (the level of neutral buoyancy). The signature of the gravity wave was seen in both the upper tropo-sphere and lower stratosphere. The main mechanism involved in the generation of the gravity waves most likely came from a vertically oriented oscillator, which triggered by convective updrafts near the neutral buoyancy level analogous to the mechanical oscillator. The resultant gravity waves had a verti-cal wavelength of about 2&ndash;5 km, and dominant wave periods of 10&ndash;20 minutes (above tropopause) and @10 minute (below tropopause). However, variations whose period is below 10 minutes in the tropo-sphere are thought to be not due to the gravity waves, but due to oscillatory behavior of the updrafts. The horizontal wavelengths, and intrinsic group velocity corresponding to these gravity waves, in the lower stratosphere, are estimated in the range of 10 to 20 km, and 10&ndash;12 m/s, respectively. The direction of average group velocity is estimated at about 15&ndash;20 degrees from the horizontal.
    Dutta G., B. Bapiraju, T. S. P. L. N. Prasad, P. Balasubrahmanyam, and H. A. Basha, 2005a: Vertical wave number spectra of wind fluctuations in the troposphere and lower stratosphere over Gadanki,a tropical station. Journal of Atmospheric and Solar-Terrestrial Physics, 67, 251-258, doi: 10.1016/J.JASTP.2004. analyses of horizontal and vertical winds measured by Gadanki (13.5°N, 79.2°E) MST radar in the troposphere and lower stratosphere have been presented. Average slopes of the wave number spectra obtained in the power-law region are 612.5 for horizontal winds and 612.2 for vertical winds. Spectral indices are found to vary in different altitude zones with steepest slopes in the tropopause region for both horizontal and vertical winds. A seasonal variation of the slope has also been observed with higher value in summer. The ratio of the spectral magnitudes between the stratosphere and troposphere is found to be quite variable with an average value of 4.2 and 5.5 for horizontal and vertical winds, respectively. The power-law region seems to be dominated by gravity wave fluctuations and high correlations are observed between horizontal and vertical winds and between stratospheric and tropospheric energy densities. It is suggested that the saturated gravity wave model may be modified to match the observed spectral features. Spectral growth is found to be inhibited on some days which cannot be explained by gravity wave theory. It is felt that mesoscale spectra can be influenced by mechanism like 2D-turbulence.
    Dutta G., B. Bapiraju, P. V. Rao, A. I. Sheeba, M. C. Ajay Kumar, P. Balasubrahmanyam, and H. A. Basha, 2005b: Comparison of gravity wave momentum fluxes estimated by different methods using mesosphere-stratosphere-troposphere radar. Radio Sci.,40, RS4009, doi. 10.1029/2004RS003031.10.1029/ radar measurements have been used to estimate momentum flux of gravity waves in a 2-6 hour period in the lower atmosphere over Gadanki (13.5°N, 79.2°E), a tropical station. Different methods have been adopted to compute momentum flux profiles, and a comparative study shows that momentum flux estimates produced by a hybrid method (Worthington and Thomas, 1996) which measures the vertical wind with a vertical beam and the horizontal wind with a pair of radial beams and those obtained by direct computation of spatial covariances (? and ?) show satisfactory agreement. The symmetric beam radar method of Vincent and Reid (1983) has the unique advantage since it does not require vertical beam measurement, and it is found to produce momentum flux estimates compatible with other methods except in high wind shear zones. It is observed that the result of momentum flux obtained by the symmetric beam method shows excellent matching with other methods when the average of vertical winds derived from E-W and N-S beams is used in the formula for both zonal and meridional fluxes.
    Dutta G., M. C. Ajay Kumar, P. Vinay Kumar, M. Venkat Ratnam, M. Chand rashekar, Y. Shibagaki, M. Salauddin, and H. A. Basha, 2009a: Characteristics of high-frequency gravity waves generated by tropical deep convection: Case studies. J. Geophys. Res., 114,D18109, doi: 10.1029/2008JD011332.10.1029/ gravity waves generated by tropical deep convection play a major role in shaping the general circulation of the middle atmosphere. Special experiments were conducted to capture two convective events on 16 May and 5 June 2006 using VHF radar located at Gadanki (13.5°N, 79.2°E), a tropical Indian station. Control day observations were also made for necessary comparisons. Background wind and temperature information was obtained by GPS radiosonde flights launched from the same site. This work has utilized these valuable data sets to delineate characteristics of convectively generated gravity waves. A superposition of gravity waves is observed with different scales after the deep convective events. Vertical wave number spectra of radial velocities show steeper slopes and higher power spectral densities during convection which slowly reduce to their normal values. The present case studies suggest the mechanical oscillator mechanism to be a major source of convective gravity wave generation in the tropics. Estimates of vertical wind variances and momentum fluxes of short-period (<2 h) wind fluctuations show large enhancements on convective days in comparison to control days. The momentum flux frequency spectra revealed a higher contribution of 30-65 min wave periods to the mean profile in the lower stratosphere. The wavelet transform momentum flux spectra displayed the temporal variability and discretization of the gravity wave momentum fluxes in frequency and time.
    Dutta G., M. C. Ajay Kumar, P. Vinay Kumar, P. V. Rao, B. Bapiraju, and H. A. Basha, 2009b: High resolution observations of turbulence in the troposphere and lower stratosphere over Gadanki. Annales Geophysicae, 27, 2407- 2415.10.5194/ resolution (150 m) wind measurements from 13-17 July 2004 by Mesosphere-Stratosphere-Troposphere (MST) radar and 15-16 July 2004 by Lower Atmospheric Wind Profiler (LAWP) have been used to study the time variation of turbulence intensity. Layers of higher turbulence are observed in the lower stratosphere on 15-16 July which give rise to mixing in the region. Enhancement in short-period gravity wave activity and turbulent layers are observed after 22:00 LT which could be due to a dry convection event that occurred at that time. The breakdown of the convectively generated high frequency waves seems to have given rise to the turbulence layers. Wind shear is found to be high above the easterly jet, but very poor correlation is observed between square of wind shear and turbulence parameters in the region. The heights of the turbulent layers in the lower stratosphere do not correlate with levels of minimum Richardson number. A monochromatic inertia gravity wave could be identified during 13-17 July 2004. A non-linear interaction between the waves of different scales as proposed by Hines (1992) might also be responsible for the breakdown and generation of turbulence layers.
    Fovell R., D. Durran, and J. R. Holton, 1992: Numerical simulations of convectively generated stratospheric gravity waves. J. Atmos. Sci.,49, 1427-1442, doi: 10.1175/1520-0469(1992) 049<1427:NSOCGS>2.0.CO;2.10.1175/1520-0469(1992)049<1427:NSOCGS>2.0.CO; cloud model results are compared with results from a dry model in which waves are excited by a specified compact momentum source designed to mimic the mechanical forcing caused by the regular development and rearward propagation of updraft cells. Results from this analog strongly support the notion that squall-line-generated gravity waves arise from mechanical forcing rather than thermal effects.
    Fritts D. C., T. Tsuda, T. E. VanZand t, S. A. Smith, T. Sato, S. Fukao, and S. Kato, 1990: Studies of velocity fluctuations in the lower atmosphere using the MU radar. Part II: Momentum fluxes and energy densities. J. Atmos. Sci., 47, 51- 66.10.1175/1520-0469(1990)047<0051:SOVFIT>2.0.CO; Available
    Fukao S., T. Sato, T. Tsuda, S. Kato, M. Inaba, and I. Kimura, 1988: VHF Doppler radar determination of the momentum flux in the upper troposphere and lower stratosphere: Comparison between the three- and four-beam methods. J. Atmos. Oceanic Technol., 5, 57- 69.10.1175/1520-0426(1988)0052.0.CO; paper discusses methods for measuring the vertical flux of horizontal momentum in the upper troposphere and lower stratosphere with VHF Doppler radars. Two versions of the mesosphere-stratosphere-troposphere radar technique are compared: one using three beams, of which one is vertical and two oblique; the other using four beams, which were two pairs of oblique beams symmetrically offset from the vertical. Using an MU (middle and upper atmosphere) radar which made it possible to make the three- and four-beam measurements simultaneously, it was found that the three-beam flux agreed with the four-beam flux only for long-period (longer than 6 h) wind fluctuations. For shorter periods, a systematic error in the three-beam method, caused by wind fluctuations, was observed.
    Gage K. S., 1979: Evidence for a k-5/3 law inertial range in mesoscale two-dimensional turbulence. J. Atmos. Sci., 36, 1950- 1954.
    Ghosh A. K., V. Siva Kumar, K. Kishore Kumar, and A. R. Jain, 2001: VHF radar observation of Atmospheric winds, associated shears and Cn2 at a tropical location: Interdependence and seasonal pattern. Annales Geophysicae, 19, 965- 973.10.5194/ turbulence refractivity structure constant (C) is an important parameter of the atmosphere. VHF radars have been used extensively for the measurements of C. Presently, most of such observations are from mid and high latitudes and only very limited observations are available for equatorial and tropical latitudes. Indian MST radar is an excellent tool for making high-resolution measurements of atmospheric winds, associated shears and turbulence refractivity structure constant (C). This radar is located at Gadanki (13.45° N, 79.18° E), a tropical station in India. The objective of this paper is to bring out the height structure of Cfor different seasons using the long series of data (September 1995 August 1999) from Indian MST radar. An attempt is also made to understand such changes in the height structure of Cin relation to background atmospheric parameters such as horizontal winds and associated shears. The height structure of C, during the summer monsoon and post-monsoon season, shows specific height features that are found to be related to Tropical Easterly Jet (TEJ) winds. It is important to examine the nature of the radar back-scatterers and also to understand the causative mechanism of such scatterers. Aspect sensitivity of the received radar echo is examined for this purpose. It is observed that radar back-scatterers at the upper tropospheric and lower stratospheric heights are more anisotropic, with horizontal correlation length of 10 20 m, as compared to those observed at lower and middle tropospheric heights.
    Hines C. O., 1995: Comments on "Observations of low-frequency inertia-gravity waves in the lower stratosphere over Arecibo". J. Atmos. Sci., 52, 607- 610.10.1175/1520-0469(1995)052<0607:COOLFI>2.0.CO; on the article `Observations of low-frequency inertia-gravity waves in the lower stratosphere over Arecibo,' by C. R. Cornish and M. F. Larsen from the Journal of Atmospheric Sciences. Horizontal wavelengths; Theory of freely propagating inertia-gravity waves.
    Hocking W. K., 1985: Measurement of turbulent energy dissipation rates in the middle atmosphere by radar techniques: A review. Radio Sci., 20, 1403- 1422.10.1029/ operating in the frequency band between 2 MHz and several hundred megahertz are capable of supplying a large data base of measurements of turbulent energy dissipation rates in the middle atmosphere. So far this has not been achieved; only occasionally have such radars been used to produce estimates of turbulence intensities. In order to encourage a greater emphasis on this aspect of radar studies of the middle atmosphere, this review summarizes the various techniques which can be used to measure turbulent energy dissipation rates. It is shown how absolute measurements of backscatter cross section can be used to measure turbulence intensities. A new theory is presented which shows that the power backscattered from the mesosphere depends on the turbulent energy dissipation rate, the electron density gradient, the neutral density scale height, the total electron density and the temperature gradient. The effects of turbulence on the width of signal spectra received by these radars are discussed, and it is shown how turbulence intensities may be extracted from spectral width measurements. The importance of removing nonturbulent processes which also broaden the width of the power spectra, such as wind shear broadening and beam width broadening, are stressed.
    Hooper D. A., L. Thomas, 1998: Complementary criteria for identifying regions of intense Atmospheric turbulence using lower VHF radar. Journal of Atmospheric and Solar-Terrestrial Physics, 60, 49- 61.10.1016/S1364-6826(97) is made of data from three campaigns of combined multi-beam lower VHF radar and radiosonde observations of the troposphere and lower stratosphere at Aberystwyth (52.4°N, 4.1°W). These provide altitude profiles of the usual radar signal parameters in addition to those of the Brunt-V01is01l01 frequency, ω, and the gradient Richardson number, Ri. It is shown that the signal strength observed by a radar beam directed 12° off-vertical, which is expected to represent backscatter alone, is not necessarily enhanced within regions identified by Ri as containing intense turbulence. Furthermore, perturbations of the signal strength altitude profiles are associated with those of the ωprofile. The possibility that Fresnel scatter contributes to radar returns at such a large zenith angle cannot be discounted. It is shown that regions of intense turbulence can, however, be identified by a combination of an absence of aspect sensitivity of radar returns, an enhancement of vertical beam spectral width, which has been corrected for the effect of beam broadening, and values of Ri close to or less than 1/(4). This method has highlighted the presence of layers of intense turbulence which follow descending phase fronts of long-period gravity waves in the lower stratosphere.
    Ito H., 1963: Aspects of typhoon development. Proc. Inter-Regional Seminar on Tropical Cyclones, Tokyo, Japan Meteor Agency, 103- 119.
    Kim S. Y., H. Y. Chun, and J. J. Baik, 2005: A numerical study of gravity waves induced by convection associated with Typhoon Rusa. Geophys. Res. Lett., 32, L24816, doi. 10.1029/2005GL024662.10.1029/[1] Typhoon Rusa (2002) is simulated using a three-dimensional mesoscale model (MM5), and the characteristics of gravity waves generated by convection associated with the typhoon are investigated. The gravity waves in the stratosphere propagate in two directions, northwestward and southeastward according to the convective bands propagating in the same directions, although the typhoon itself moves north-northeastward. Spectral analyses show that the inertio-gravity waves (IGWs) in the stratosphere generated by Rusa have a dominant horizontal wavelength of 300–600 km, a vertical wavelength of 3–11 km, and a period of 6–11 hrs. A large fraction of the IGWs is filtered out in the upper troposphere and stratosphere mainly due to the critical-level filtering process. The decreased magnitude of the momentum flux with height in a non-filtered region is likely due to the damping process, with a minor contribution by the wave breaking process that can occur exclusively near the critical-level phase speeds.
    Kim S. Y., H. Y. Chun, and J. J. Baik, 2007: Sensitivity of Typhoon-induced gravity waves to cumulus parameterizations. Geophys. Res. Lett., 34,L15814, doi: 10.1029/2007GL 030592.10.1029/ of typhoon-induced gravity waves to cumulus parameterizations is examined using a mesoscale model (MM5). For this, Typhoon Rusa (2002) is simulated with four cumulus parameterizations (Kain-Fritsch, Grell, Anthes-Kuo, and Betts-Miller schemes) and the characteristics of typhoon-induced gravity waves are compared. The experiments show differences in rainband structure and vertical motion, resulting in different forcing spectra for zonal wavelength and period. As a result, induced stratospheric gravity waves are different in amplitude and spectral shape. However, the difference is not as large as that in the forcing spectrum, since a large portion of the waves generated by major forcing components is filtered out by the background wind, which is nearly the same in all the experiments. Instead, variation in zonal wavelength and period of forcing modifies the characteristics of stratospheric gravity waves by changing damping time scale in the nonfiltered region.
    Kudeki E., S. J. Franke, 1998: Statistics of momentum flux estimation. Journal of Atmospheric and Solar-Terrestrial Physics, 60, 1549- 1553.10.1016/S1364-6826(98) effect of geophysical noise on the precision of momentum flux measurements is examined. The dominant contribution to the uncertainty of momentum flux estimates scales with the geometric mean of the energy in horizontal and vertical wind fluctuations. To obtain statistically significant measurements of the momentum flux, long integration times are necessary since the flux is typically a small fraction of the geometric mean energy. For example, when the fraction is 1%, at least 16 days of stratospheric measurements will be needed for reliable flux estimation.
    Larsen M. F., 1995: Reply with comments on "Modulated mountain waves". J. Atmos. Sci., 52, 611- 612.10.1175/1520-0469(1995)0522.0.CO; No abstract available
    Lilly D. K., 1983: Stratified turbulence and the mesoscale variability of the atmosphere. J. Atmos. Sci., 40, 749-
    McLandress C., M. J. Alexand er, D. L. Wu, 2000: Microwave Limb Sounder observations of gravity waves in the stratosphere: A climatology and interpretation. J. Geophys. Res., 105, 11 947- 11 967.10.1029/ temperature data from the Microwave Limb Sounder (MLS) are analyzed to obtain information about high intrinsic frequency gravity waves in the stratosphere. Global climatologies of temperature variance at solstice are computed using six years of data. A linear gravity wave model is used to interpret the satellite measurements and to infer information about tropospheric wave sources. Globally uniform sources having several different spectral shapes are examined and the computed variances are filtered in three-dimensional space in a manner that simulates the MLS weighting functions. The model is able to reproduce the observed zonal mean structure, thus indicating that the observations reflect changes in background wind speeds and provide little information about the latitudinal variation of wave sources. Longitudinal variations in the summer hemisphere do reflect source variations since the modeled variances exhibit much less variation in this direction as a consequence of the zonal symmetry of the background winds. A close correspondence between the MLS variances and satellite observations of outgoing-longwave radiation suggests that deep convection is the probable source for these waves. The large variances observed over the tip of South America in winter are most certainly linked to orographic forcing but inferences about wave sources in Northern Hemisphere winter are difficult to make as a result of the high degree of longitudinal and temporal variability in the stratospheric winds. Comparisons of model results using different source spectra suggest that the tropospheric sources in the subtropics in summer have a broader phase speed spectrum than do sources at middle latitudes in winter.
    Nastrom G. D., F. D. Eaton, 2001: Persistent layers of enhanced Cn2 in the lower stratosphere from VHF radar observations. Radio Sci., 36, 137- 149.10.1029/ climatologies of persistent layers of enhanced refractive index structure parameter C 2 N were developed for the lower stratosphere from VHF radar observations at White Sands Missile Range, New Mexico, for the period January 1991 to September 1996. Knowledge of the nature of enhanced refractivity layers is of high interest to the atmospheric sciences, propagation, and remote sensing communities. The layers reported have C 2 N enhanced at least 7 dB above the background continuously for at least 11 hours and migrate vertically no more than one radar range gate (150 m) over 1 hour. The cumulative frequency of the lengths (11-37 hours) of the 259 persistent layers identified shows that 25% of the layers last over 17 hours. Comparisons of profiles of wind speeds, variances of the wind components, vertical shear of the horizontal wind, Doppler spectral width, temperature, Brunt-Vaisala frequency, and Richardon's number for times with and without persistent layers at 17 km show that wind speed at 5.6 km in addition to spectral width, wind shear, and vertical velocity variances at 17 km are stronger during enhanced layer episodes than during nonlayer periods. Possible sources for the persistent layers are suggested, and the shortcomings of each hypothesis are discussed. Several case studies of radiosonde ascents during persistent layers give no obvious indication of the source of these layers.
    Nastrom G. D., T. E. VanZand t, and J. M. Warnock, 1997a: Vertical wavenumber spectra of wind and temperature from high-resolution balloon soundings over Illinois. J. Geophys. Res., 102, 6685- 6701.10.1029/ spectrally analyzed high-resolution balloon measurements of vertical profiles of temperature and horizontal wind in the troposphere and lower stratosphere over very flat terrain in Illinois. This paper is principally concerned with spectra in the power law range at vertical wavenumbers m ≥10cycle/m. The logarithmic spectral slopes and amplitudes are found to have only insignificant dependencies on meteorological parameters, including time of day, season, wind speed and direction, vertical shear, etc., except that between the troposphere and stratosphere the spectral amplitude scales as (N)with q 000.3, where N is the buoyancy frequency. The mean slopes are ≈-3 in the stratosphere and ≈ -2.6 in the troposphere. On the average the individual spectra with larger amplitudes have less negative spectral slopes. The wide variation of spectral slopes (σ≈0.5) and amplitudes and the weak dependence on Nare quite inconsistent with the predictions of theories of saturated spectra. Further, the wind spectra in the troposphere and stratosphere are correlated, which suggests some unsaturated propagation between the regions. The ratio of kinetic to potential energy spectra is constant versus m, consistent with the linear gravity wave polarization relations. The magnitude of the model ratio can be brought into agreement with the observed ratio by assuming a model intrinsic frequency spectrum varying as ωwith p 005/3 to 2 plus an enhancement of energy near the inertial frequency.
    Nastrom G. D., T. E. VanZand t, and F. D. Eaton, 1997b: Long-lived layers of enhanced reflectivity in the lower stratosphere. Paper presented at 8th Workshop on Technical and Scientific Aspects of MST Radars, Bangalore, India, Scientific Communication on Solar Terr Phys Bangalore, 15- 20.
    Nath D., W. Chen, 2013: Investigating the dominant source for the generation of gravity waves during Indian Summer Monsoon using ground-based measurements. Adv. Atmos. Sci.,30, 153-166, doi: 10.1007/s00376-012-1273-y.10.1007/ the tropics, convection, wind shear (i.e., vertical and horizontal shear of wind and/or geostrophic adjustment comprising spontaneous imbalance in jet streams) and topography are the major sources for the generation of gravity waves. During the summer monsoon season (June-August) over the Indian subcontinent, convection and wind shear coexist. To determine the dominant source of gravity waves during monsoon season, an experiment was conducted using mesosphere-stratosphere-troposphere (MST) radar situated at Gadanki (13.5°N, 79.2°E), a tropical observatory in the southern part of the Indian subcontinent. MST radar was operated continuously for 72 h to capture high-frequency gravity waves. During this time, a radiosonde was released every 6 h in addition to the regular launch (once daily to study low-frequency gravity waves) throughout the season. These two data sets were utilized effectively to characterize the jet stream and the associated gravity waves. Data available from collocated instruments along with satellite-based brightness temperature (TBB) data were utilized to characterize the convection in and around Gadanki. Despite the presence of two major sources of gravity wave generation (i.e., convection and wind shear) during the monsoon season, wind shear (both vertical shear and geostrophic adjustment) contributed the most to the generation of gravity waves on various scales.
    Nayar S. R. P., S. Sreeletha, 2003: Momentum flux associated with gravity waves in the low-latitude troposphere. Annales Geophysicae, 21, 1183- 1195.10.5194/ vertical fluxes of horizontal momentum at tropospheric heights are calculated for four days, 25-28 August 1999. The mean zonal wind during these days show the presence of strong westward wind at the upper troposphere. Both the symmetric beam radar method and the power spectral method of evaluation of vertical flux of zonal and meridional momentum shows nearly the same result for quiet conditions. The temporal evolution of the momentum flux is estimated for a day with strong zonal shear and convection. These results indicate that on 28 August 1999, the strong downward vertical wind in the lower altitude range is associated with upward vertical flux of zonal momentum, and strong upward vertical wind is associated with downward vertical flux. Similarly, the strong shear in zonal wind is associated with the increase in negative values in vertical flux in the upper troposphere. Analysis of the role of wave periods in the transport of momentum flux indicates that the vertical momentum flux magnitude is not evenly distributed in all wave periods, but instead it peaks at certain wave periods in the range 10 to 100 min. Key words. Meteorology and atmospheric dynamics (convective process; tropical meteorology; precipitation)
    Niranjan Kumar K., T. K. Ramkumar, 2008: Characteristics of inertia-gravity waves over Gadanki during the passage of a deep depression over the Bay of Bengal. Geophys. Res. Lett., 35,L13804, doi: 10.1029/2008GL033937.10.1029/ MST Radar located at the Indian tropical station of Gadanki (13.5°N, 79.2°E near the eastern coast of India), studies have been made on the characteristics of inertia-gravity waves generated in the lower troposphere during deep depression developed over the Bay of Bengal on 20-24 June 2007. Filtering and the hodograph analyses of horizontal winds indicate that the low-pressure system has generated inertia gravity waves, propagating outward from the core of the depression. Strong enhancement of radar reflectivity (SNR) in the heights of ~4-7 km for a few days around 22 June 2007 and the upward propagation of gravity wave energy above this height range indicate that the source of the waves is located at ~4-7 km. This is in agreement with earlier theoretical expectations. The vertical and horizontal wavelengths of gravity waves are found to be ~2.2 km and ~240 km respectively in the troposphere.
    Niranjan Kumar, K., T. K. Ramkumar, M. Krishnaiah, 2011: MST radar observation of inertia-gravity waves generated from tropical cyclones. Journal of Atmospheric and Solar-Terrestrial Physics, 73, 1890- 1906.10.1016/ a Mesosphere Stratosphere Troposphere (MST) radar, operating at 5302MHz at Gadanki (13.5°N, 79.2°E), India, the present study reports on the temporal and spatial characteristics of inertia-gravity waves (IGWs) generated from four tropical cyclones that formed over the Bay of Bengal. IGWs are observed with intrinsic frequencies, vertical and horizontal wavelengths in the ranges of 1.2 f –3.0 f , 2.5–5, and 300–160002km, respectively, where f is the Coriolis frequency at the Gadanki site. It is found that both the convective and geostrophic adjustment processes in the troposphere play a major role in the generation of IGWs. Also it is found that the horizontal propagation direction of IGWs is aligned along the motion of convective rain bands.
    Pfister, L., Coauthors, 1993: Gravity waves generated by a tropical cyclone during the STEP tropical field program: A case study. J. Geophys. Res., 98( D5), 8611- 8638.10.1029/ of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere-Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone's entire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110-km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100-to 150-km-diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil show that these cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. A three-dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110-km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400-600 m, a lifetime of 4.5-6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi-biennial oscillation. There is strong evidence that some of the mesoscale gravity waves break below 20 km as well. The effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above-cloud diabatic cooling.
    Rao D. N., P. Kishore, T. N. Rao, S. V. B. Rao, K. K. Reddy, M. Yarraiah, and M. Hareesh, 1997: Studies on refractivity structure constant, eddy dissipation rate, and momentum flux at a tropical latitude. Radio Sci., 32( 5), 1375- 1389.10.1029/ and UHF Doppler radars provide a unique database to estimate the refractivity structure constant C, eddy dissipation rate 鈭, and vertical flux of horizontal momentum. Using the data collected from the Indian MST radar, these parameters are studied at a tropical latitude. The refractivity turbulence structure constant is estimated from the backscattered power of the received echoes. C(radar) and C(model), derived from radiosonde observations, are compared, and a fairly good agreement is seen. Diurnal and seasonal variations of Care also presented. The eddy dissipation rate is estimated from the radar echoes employing the power and spectral width methods. A fairly good agreement is seen between the two methods. Values of 蓻 are found to vary from 10to 10msin a height range of 4-19 km. Cand 蓻 are observed to be minimum during a moderate jet stream wind of 50-60 m s. Vertical flux of horizonal momentum is computed using the symmetrical two-beam method. Significant fluxes of westward and northward momentum are observed, and the values lie in the range of -1 to +1 ms. The implied accelarations are also estimated. The results presented are largely consistent with the results available in the literature.
    Rao J. Y., 1998: A study of Atmospheric stable layers in the tropical atmosphere using Indian MST radar. Ph.D. dissertation, Sri Venkateswara University, 172
    Rao P. B., A. R. Jain, P. Kishore, P. Balamuralidhar, S. H. Damle, and G. Viswanathan, 1995: Indian MST radar 1. System description and sample vector wind measurements in ST mode. Radio Sci., 30, 1125- 1138.
    Ratnam M. V., T. Tsuda, Y. Shibagaki, T. Kozu, and S. Mori, 2006: Gravity wave characteristics over the equator observed during the CPEA campaign using simultaneous data from multiple stations. J. Meteor. Soc.Japan, 84A, 239- 257.10.2151/ vertical and temporal variations of inertia-gravity waves are studied by means of an intensive radiosonde campaign conducted from 10 Apri1 to 9 May, 2004 at five sites, including the Equatorial Atmosphere Radar (EAR) site at Koto Tabang (0.2°S, 100.32°E) in west Sumatra, Indonesia. The four other balloon sounding sites are located about 75-400 km away from EAR. Dominant gravity waves with periods of 2-3 days and vertical wavelengths ofapproximately 3-5 km showing clear downward phase propagation were detected, particularly in the upper troposphere and lower stratosphere (UTLS) region. The gravity wave energy is found to become the largest at an altitude of approximately 20 km, although the enhancement was not continuous, but intermittent. The wave activity was similar at all five sites, having only a slight phase shift, which suggests that the horizontal scale of the wave is larger than the distance between the sites. We have applied a correlative analysis to delineate the horizontal propagation characteristics of gravity waves, and estimated the horizontal wavelength (位h) to be approximately 1,700 km propagating toward 30° south from the east from 26-30 April, 2004, which is further verified by hodograph analysis for individual profiles. From 10-14 April, 2004 and 5-9 May, 2004, 位h and the direction of the propagation were found to be 2,700 km and 3,250 km, and 26° and 3° north from the east, respectively. The spatial and temporal variations in the convection, which is thought to be a major source in the generation of gravity waves, is also studied using satellite data of outgoing long-wave radiation (OLR). We noticed clear eastward advection of large super cloud clusters (SCCs) from the Indian Ocean to the maritime continent, with occasional movement towards the observational sites. The source of the gravity waves is strongly related to this slowly eastward-advecting tropospheric convection, implying that the wave activity observed in the UTLS region was generated by far distant sources located west of the EAR. In addition, we present a case study in which large wave activity did not correspond to the particular cloud convection.
    Sato K., 1989: An inertial gravity wave associated with a synoptic-scale pressure trough observed by the MU radar. J. Meteor. Soc.Japan, 67, 325- 333.
    Sato K., 1993: Small-scale wind disturbances observed by the MU radar during the passage of typhoon Kelly. J. Atmos. Sci.,50, 518-537, doi: 10.1175/1520-0469(1993)050<0518: SSWDOB>2.0.CO;2.10.1175/1520-0469(1993)050<0518:SSWDOB>2.0.CO; paper describes small-scale wind disturbances associated with Typhoon Kelly (October 1987) that were observed by the MU radar, one of the MST (mesosphere, stratosphere, and troposphere) radars, continuously for about 60 hours with fine time and height resolution. First, in order to elucidate the background of small-scale disturbances, synoptic-scale variation in atmospheric stability related to the typhoon structure during the observation is examined. When the typhoon passed near the MU radar site, the structure was no longer axisymmetric. There is deep convection only in the front (north-northeast) side of the typhoon while convection behind it is suppressed by a synoptic-scale cold air mass moving eastward to the west of the typhoon. A drastic change in atmospheric stability over the radar site as indicated by echo power profiles is likely due to the passage of the sharp transition zone of convection.Strong small-scale wind disturbances were observed around the typhoon passage. It is shown that the statistical characteristics are significantly different before (BT) and after (AT) the typhoon passage, especially in frequency spectra of vertical wind fluctuations. The spectra for BT are unique compared with earlier studies of vertical winds observed by VHF radars. Another difference is dominance of a horizontal wind component with a vertical wavelength of about 3 km, which is observed only in AT.Further analyses are made of detailed characteristics and vertical momentum fluxes for dominant disturbances. It is found that some of the disturbances are generated so as to remove the momentum of cyclonic wind rotation of the typhoon. Deep convection, topographic effects in strong winds, and strong vertical shear of horizontal winds around an inversion layer are possible sources of the dominant disturbances. Moreover, two monochromatic disturbances lasting for more than 10 h in the lower stratosphere observed in BT and AT, respectively, are identified as inertio-gravity waves, by obtaining wave parameters consistent with all observed quantities. Both of the inertio-gravity waves propagate energy away from the typhoon.
    Thomas L., I. T. Prichard, and I. Astin, 1992: Radar observations of an inertia-gravity wave in the troposphere and lower stratosphere. Annales Geophysicae, 10, 690-;cpsidt=5544176
    Tsuda T., P. T. May, T. Sato, S. Kato, and S. Fukao, 1988: Simultaneous observations of reflection echoes and refractive index gradient in the troposphere and lower stratosphere. Radio Sci., 23, 655- 665.10.1029/ have studied some characteristics of clear air echoes in the lower stratosphere and troposphere from simultaneous observations of vertical echo power and temperature profiles. The vertical echo power has been oversampled every 75 m with a height resolution of 150 m by the middle and upper atmosphere (MU) radar (35°N, 136°E). During the radar observations a radiosonde was launched at the MU radar site in order to measure temperature, humidity, and pressure with a height resolution of a few tens of meters, from which the mean gradient of generalized potential refractive index, M, was determined. In the lower troposphere (below 10 km altitude), M is enhanced owing to humidity by about 10-20 dB, and its fine structure is mainly determined by the vertical gradient of humidity. The relatively large time-height variation of tropospheric echo power seems to be attributed to rapid changes in the humidity profile. On the other hand, in the upper troposphere (above 10 km altitude) and stratosphere the vertical structure of M is mainly determined by the Brunt-V01is01l01 frequency and air density, where the former determines fine vertical structure of M and the latter the gradual decrease in M with a scale height of about 7 km. The measured Mprofile agrees well with the vertical echo power profile down to the radar height resolution of 150 m. That is, the vertical structure of the reflection coefficient is mainly determined by M, and therefore the energy density of 3-m scale fluctuations E(2k) seems to be distributed uniformly with height. The vertical spacing of intense reflection layers usually ranges from 500 m to a few kilometers, which corresponds to the dominant vertical scale of fluctuations in the Brunt-V01is01l01 frequency profile. The vertical distribution of intense reflection layers seems to be explained by a predominance of a saturated vertical wave number spectrum of gravity waves with a slope of -3 and a dominant vertical scale of a few kilometers.
    Tsuda T., Y. Murayama, H. Wiryosumarto, S. W. B. Harijono, and S. Kato, 1994: Radiosonde observations of equatorial atmosphere dynamics over Indonesia: 2. Characteristics of gravity waves. J. Geophys. Res., 99, 10 507- 10 516.10.1029/ paper discusses the characteristics of gravity waves in the equatorial region revealed by analyzing radiosonde measurements of wind velocity and temperature fluctuations at 0–35 km, with a height resolution of 150 m, made every 5–7 hours between February 27 and March 22, 1990, in East Java, Indonesia. We conducted hodograph analysis to delineate vertical and horizontal propagation characteristics and found that most gravity waves were generated in the middle of the troposphere and that they propagated upward into the stratosphere. The amplitudes of wind velocity and temperature fluctuations due to gravity waves were larger in the stratosphere than in the troposphere. The direction of horizontal propagation of gravity waves was rather uniformly distributed in the troposphere, but it became eastward in the lower stratosphere, being opposite to that of the mean winds because of quasi-biennial oscillation. The vertical wavenumber spectra of wind velocity were described fairly well by a saturated model spectrum, although their amplitudes were smaller in the stratosphere. A typical vertical wavelength was 2–2.5 km, while the horizontal phase velocities were 5–7 and 12 m/s in the troposphere and stratosphere, respectively. The amplitudes of small-scale gravity waves were significantly larger in the equatorial stratosphere than at middle latitudes. Time-height variation of the wind velocity variance due to gravity waves showed a clear correlation with that for high relative humidity, which implies that cloud convection is an important mechanism of gravity wave generation in the equatorial region.
    VanZand t, T. E., 1982: A universal spectrum of buoyancy waves in the atmosphere. Geophys. Res. Lett., 9, 575- 578.10.1029/ - Scientific documents that cite the following paper: A universal spectrum of buoyancy waves in the atmosphere
    Vincent R. A., I. M. Reid, 1983: HF Doppler measurements of mesospheric gravity wave momentum fluxes. J. Atmos. Sci., 40, 1321- 1333.10.1175/1520-0469(1983)040<1321:HDMOMG>2.0.CO; probable importance of internal gravity waves in balancing the momentum budget of the mesosphere has been emphasized in recent theoretical studies. The present investigation is concerned with a method by which the vertical flux of horizontal momentum can be measured by ground-based radars. The method requires the comparison of the Doppler shifts in backscattered radar echoes from the upper atmosphere using two or more narrow beams offset from the vertical. It is assumed that fluctuations of the Doppler shifts are caused by gravity wave wind oscillations. Provided the atmospheric motions are horizontally homogeneous, the momentum flux is proportional to the difference of the variances of the Doppler velocities. The technique has been applied to observations of the upper mesosphere made with a HF radar located near Adelaide, Australia.
    Vincent R. A., M. J. Alexander, 2000: Gravity waves in the tropical lower stratosphere: An observational study of seasonal and interannual variability. J. Geophys. Res., 105, 17 971- 17 982.10.1029/ observations made at Cocos Islands (12°S, 97°E) in the Indian Ocean between September 1992 and June 1998 are used to study seasonal and interannual variations in gravity wave activity in the lower stratosphere (18-25 km). The islands are located in a region of generally strong convection that occurs at all times of the year, with the period of strongest convective activity between December and July (wet season). The prevailing zonal winds during the observational period and height range are westward with a quasi-biennial oscillation (QBO) superimposed. Time series of wave energy show that largest wave amplitudes occur during the wet season when convection is strongest, but a QBO-like variation is also apparent. Maximum energy densities of about 25 J kgoccur early 1993, 1995, and 1997 at the times when the westward shears are largest. Wave energy is found to be propagating upward, and in the horizontal there is considerable azimuthal anisotropy, with predominate eastward propagation against the prevailing wind. Upward fluxes of zonal momentum flux (u'w'炉) are estimated by combining the temperature and wind information. Fluxes show a similar temporal behavior to the energy. The motion and temperature fields are dominated by waves with vertical wavelengths 藴2 km. Using a Stokes parameter analysis, it is found that the intrinsic frequencies are, on average, 2-3 times the inertial frequency, corresponding to intrinsic periods of 20-25 hours. Horizontal wavelengths between 200 and 2000 km are inferred, with a mean value of about 1000 km. The mean intrinsic phase speeds are about 10 ms, but ground-based phase speeds are centered on 0 ms.
    Worthington R. M., L. Thomas, 1997: Long-period unstable gravity-waves and associated VHF radar echoes. Annales Geophysicae, 15, 813- 822.10.1007/ atmospheric radar is used to measure the wind velocity and radar echo power related to long-period wind perturbations, including gravity waves, which are observed commonly in the lower stratosphere and tropopause region, and sometimes in the troposphere. These wind structures have been identified previously as either inertia-gravity waves, often associated with jet streams, or mountain waves. At heights of peak wind shear, imbalances are found between the echo powers of a symmetric pair of radar beams, which are expected to be equal. The largest of these power differences are found for conditions of simultaneous high wind shear and high aspect sensitivity. It is suggested that the effect might arise from tilted specular reflectors or anisotropic turbulent scatterers, a result of, for example, Kelvin -Helmholtz instabilities generated by the strong wind shears. This radar power-difference effect could offer information about the onset of saturation in long-period waves, and the formation of thin layers of turbulence.
    Zhang F. Q., S. G. Wang, and R. Plougonven, 2004: Uncertainties in using the hodograph method to retrieve gravity wave characteristics from individual soundings. Geophys. Res. Lett., 31,L11110, doi: 10.1029/2004GL019841.10.1029/ hodograph method is commonly used to retrieve inertio-gravity wave characteristics from individual vertical profiles of the winds. In order to estimate the uncertainties of this method, we have analyzed mesoscale numerical simulations of a gravity wave event in which a coherent quasi-monochromatic inertio-gravity wave packet is present. Single profiles are extracted from the simulations, analyzed using the hodograph method, and the derived wave characteristics are compared to the reference values determined from the four-dimensional simulated fields. Although the conditions favor the use of the hodograph method, the derived wave parameters possess significant uncertainties.
    Zong H. J., L. G. Wu, 2015: Re-examination of tropical cyclone formation in monsoon troughs over the western North Pacific. Adv. Atmos. Sci.,32(8), 924-934, doi: 10.1007/ s00376-014-4115-2.10.1007/ monsoon trough (MT) is one of the large-scale patterns favorable for tropical cyclone (TC) formation over the western North Pacific (WNP). This study re-examines TC formation by treating the MT as a large-scale background for TC activity during May-揙ctober. Over an 11-year (2000-10) period, 8.3 TC formation events on average per year are identified to occur within MTs, accounting for 43.1% of the total TC formation events in the WNP basin. This percentage is much lower than those reported in previous studies. Further analysis indicates that TC formation events in monsoon gyres were included at least in some previous studies. The MT includes a monsoon confluence zone where westerlies meet easterlies and a monsoon shear line where the trade easterlies lie north of the monsoon westerlies. In this study, the large-scale flow pattern associated with TC formation in the MT is composited based on the reference point in the confluence zone where both the zonal and meridional wind components are zero with positive vorticity. While previous studies have found that many TCs form in the confluence zone, the composite analysis indicates that nearly all of the TCs formed in the shear region, since the shear region is associated with stronger low-level relative vorticity than the confluence zone. The prevailing easterly vertical shear of zonal wind and barotropic instability may also be conducive to TC formation in the shear region, through the development of synoptic-scale tropical disturbances in the MT that are necessary for TC formation.
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Manuscript History

Manuscript received: 29 December 2015
Manuscript revised: 19 March 2016
Manuscript accepted: 06 April 2016
通讯作者: 陈斌,
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Impact of Cyclone Nilam on Tropical Lower Atmospheric Dynamics

  • 1. Vignana Bharathi Institute of Technology, Hyderabad, TS 501301, India
  • 2. National Atmospheric Research Laboratory, Gadanki, AP 517112, India
  • 3. Jawaharlal Nehru Technological University, Hyderabad, TS 500085, India

Abstract: A deep depression formed over the Bay of Bengal on 28 October 2012, and developed into a cyclonic storm. After landfall near the south coast of Chennai, cyclone Nilam moved north-northwestwards. Coordinated experiments were conducted from the Indian stations of Gadanki (13.5°N, 79.2°E) and Hyderabad (17.4°N, 78.5°E) to study the modification of gravity-wave activity and turbulence by cyclone Nilam, using GPS radiosonde and mesosphere-stratosphere-troposphere radar data. The horizontal velocities underwent large changes during the closest approach of the storm to the experimental sites. Hodograph analysis revealed that inertia gravity waves (IGWs) associated with the cyclone changed their directions from northeast (control time) to northwest following the path of the cyclone. The momentum flux of IGWs and short-period gravity waves (1-8 h) enhanced prior to, and during, the passage of the storm (0.05 m2 s-2 and 0.3 m2 s-2, respectively), compared to the flux after its passage. The corresponding body forces underwent similar changes, with values ranging between 2-4 m s-1 d-1 and 12-15 m s-1 d-1. The turbulence refractivity structure constant (Cn2) showed large values below 10 km before the passage of the cyclone when humidity in the region was very high. Turbulence and humidity reduced during the passage of the storm when a turbulent layer at ∼17 km became more intense. Turbulence in the lower troposphere and near the tropopause became weak after the passage of the cyclone.

1. Introduction
  • Tropical cyclones are large-scale and violent weather systems that originate over ocean areas where SSTs exceed 26°C-27°C and other favorable conditions, such as wind shear and wind direction, prevail. (Zong and Wu, 2015) examined the formation of tropical cyclones and reported that the vertical shear in zonal wind and barotropic instability could be conducive to their formation in the shear region through the development of synoptic-scale disturbance in the monsoon trough region during summer months. Cyclones are accompanied by typical thermal and wind structures. The associated drops in surface pressure are not systematic and abrupt changes are observed (Ito, 1963). The convection associated with a cyclone can generate gravity waves throughout the full range of wave frequencies (Sato, 1993; Dhaka et al., 2003; Kim et al., 2007; Niranjan Kumar and Ramkumar, 2008; Dutta et al., 2009a; Chane-Ming et al., 2010; Das et al., 2010; Niranjan Kumar et al., 2011). Convectively generated nonstationary gravity waves contribute significantly to the momentum budget of the middle atmosphere. The generation mechanism of the gravity waves due to deep convection is highly complex. Three simple mechanisms that describe the wave generation are thermal forcing (Alexander et al., 1995), the obstacle effect (Clark et al., 1986), and the mechanical oscillator effect (Fovell et al., 1992). Evidence of thermal forcing as a mechanism to generate gravity waves was reported by (McLandress et al., 2000), but the contribution of this mechanism compared to others in the spectrum of gravity waves could not be quantified. The obstacle effect is also referred to as the "transient mountain effect" and is equivalent to topographic generation of gravity waves. The evidence for this mechanism's responsibility for wave generation was reported from stratospheric aircraft observations over convection (Pfister et al., 1993). This mechanism gives rise to an anisotropic wave spectrum and was also observed in radiosonde analyses of low-frequency waves (Alexander and Vincent, 2000; Vincent and Alexander, 2000). The third mechanism——the mechanical oscillator effect——is described as a method where the source oscillates with a regular period and generates mostly high-frequency gravity waves. In general, the spectrum of gravity waves generated during tropical cyclones cannot be linked to a distinct source mechanism, since they are coupled with one another (Dhaka et al., 2001; Arunachalam Srinivasan et al., 2014).

    Large convective systems like typhoons/cyclones are reported to be major sources of inertia gravity waves (IGWs) (Abdullah, 1966; Kim et al., 2005; Niranjan Kumar and Ramkumar, 2008; Das et al., 2010; Niranjan Kumar et al., 2011). (Sato, 1993) observed different wave characteristics before and after the passage of typhoon Kelly (1981). Small-scale disturbances were observed during the passage of the typhoon. Kim et al. (2005, 2007) simulated typhoon Rusa, which occurred in 2002, using different mesoscale models and found that the convective bands associated with the typhoon gave rise to IGWs. They reported the wave spectrum in the stratosphere to be asymmetric due to the critical level filtering by the background wind in the troposphere. The horizontal and vertical wavelengths of IGWs reported in the study were 300-600 km and 3-11 km, respectively, with periods of 6-11 h. (Chane-Ming et al., 2010) analyzed the signature of gravity waves during the evolution of two tropical cyclones and observed periods between 6 h to 2.5 days. The vertical wavelengths were between 1 and 3 km, and the horizontal wavelengths were found to be less than 2000 km in the upper troposphere and greater than 2000 km in the lower stratosphere. (Dhaka et al., 2003) reported the wavelengths of gravity waves generated in the vicinity of cyclones to vary between 6 and 11 km. The generation of IGWs during cyclones has also been observed with Gadanki mesosphere-stratosphere-troposphere (MST) radar (Niranjan Kumar and Ramkumar, 2008; Niranjan Kumar et al., 2011; Das et al., 2012). The vertical and horizontal wavelengths reported in these studies lie in the ranges 2-5 km and 200-1000 km, respectively. (Das et al., 2012) observed stratospheric gravity waves with a period of approximately 42 min during the passage of a cyclone. Large wind shear associated with a cyclone may generate IGWs with short wavelength. Using MST radar (Gadanki) and satellite data, (Nath and Chen, 2013) analyzed in detail the source of gravity waves and concluded that wind shear associated with the tropical easterly jet contributes significantly to producing IGWs with wavelengths ranging between 3 and 5 km.

    A depression developed in the Bay of Bengal on 28 October 2012, which culminated in a severe cyclonic storm [Nilam (2012)] with intensity 3 and a northwestward path of movement. It eventually dissipated on 2 November 2012 in the Rayalaseema region at around 0530 LST. The cyclone was closest to Gadanki (64 km) in the evening of 31 October 2012 (2030 LST), and to Hyderabad (468 km) in the morning of 1 November 2012 (0830 LST). The track of the cyclone and the observational sites are shown in Fig. 1. The contours of outgoing longwave radiation (OLR), along with background winds at 850 hPa for the period 30 October 2012 to 2 November 2012 (active days), and 7-8 November 2012 (control days), are shown in Fig. 2. OLR and wind data were obtained from the NCEP-NCAR and ECMWF (ERA-Interim) reanalysis datasets, respectively. Coordinated experiments were conducted from two stations-Gadanki (13.5°N, 79.2°E) and Vignana Bharathi Institute of Technology (VBIT), Hyderabad (17.4°N, 78.5°E)——to study the different characteristics of the gravity waves. This paper presents the impact of cyclone Nilam (2012) on different gravity wave parameters, with a focus on the gradual change in momentum flux and turbulence of the upper troposphere, using continuous radar data.

    Figure 1.  Track of cyclone Nilam (2012) between 28 October and 1 November 2012.

    Figure 2.  Contours of OLR, along with background winds, at 850 hPa, for the periods 30 October to 2 November 2012 and 7-8 November 2012.

    Figure 3.  Histogram of maximum height reached by the balloons flown from both stations.

    Figure 4.  Shifted profiles (30-min average) of zonal, meridional and vertical winds measured by the radar during 29 October to 2 November 2012. Successive profiles are displayed by 5 m s$^-1$ for the zonal and meridional components, and 0.5 m s$^-1$ for the vertical component.

2. Experimental methods and data
  • High-resolution GPS radiosonde flights were carried out with Meisei (Japan) radiosondes from Gadanki at 1630 LST everyday between 25 October and 10 November 2012, and with iMet (USA) radiosondes from Hyderabad at 1130 LST between 30 October and 10 November 2012. The accuracies of wind and temperature, as provided by the manufacturers, are (0.1 m s-1, 0.5 K) for the Meisei radiosonde and (1 m s-1, 0.2 K) for the iMet radiosonde. There was no flight from Gadanki on 27 October. A histogram illustrating the maximum heights reached by the balloons flown from both stations is shown in Fig. 3. The GPS radiosonde data of winds and temperature were interpolated every 100 m to smooth the profiles. Outliers, if any, were removed by visual inspection.

    The MST radar at Gadanki operates at a frequency of 53 MHz, with an average power-aperture product of 7× 108 W m-2 and a one-way beam width of 3°. The radar system is described in detail in (Rao et al., 1995). Continuous measurements were made by the radar in eight-beam mode (East, Zenith X, West, Zenith Y, North, Zenith X, South, Zenith Y) from 29 October (1130 LST) to 2 November (0810 LST). The winds measured by the MST radar——with a height resolution of 150 m, from 1130 LST 29 October to 0930 LST 30 October——were continuous and with a data gap of 2.5 min. The data were averaged over 5 min (two profiles). After 0930 LST it was not possible to run the radar continuously with the same experimental specification file. Thereafter, the experiment was conducted with a 300 m height resolution and the data gaps were, on average, between 15 and 30 min, except at two places where the gaps were 45 min and 1 h. The data after 0930 LST 30 October were averaged over 30 min to obtain continuous data up to 0800 LST 2 November. The two gaps in between, as mentioned, were linearly interpolated to obtain continuous datasets with a 30 min time resolution. The quality of the radar data was checked by visual inspection and outliers were removed. The data were acceptable only up to 18 km, above which they showed large variability. The data between 0350 to 0600 LST 1 November were also discarded due to their poor quality. The shifted plots of winds (30-min averaged) of all the components are shown in Fig. 4. The available continuous data, averaged for 5 min and 30 min, were used for computation of different parameters.

    Figure 5.  (a-e) Profiles of zonal wind velocities over Gadanki during 29 October to 2 November 2012, along with the average profile of the control days. (f-j) As in (a-e) but for meridional winds. (k-o) As in (a-e) but for temperature. The tropopause height and temperature are shown in the insets.

    Figure 6.  As in Fig. 5 but for Hyderabad between 30 October and 3 November 2012.

    Figure 7.  Comparison of mean profiles of radar winds during 1630-1800 LST on each day and simultaneous radiosonde winds over Gadanki during 29 October to 1 November 2012. Horizontal error bars are plotted at a few heights for radar mean winds.

    Figure 8.  Hodographs of horizontal wind fluctuations during (a, e) active and (c, g) control days for radar and radiosonde data, respectively, over Gadanki. An open circle and a solid circle in each hodograph indicate the lowest and highest altitudes, respectively. (b, d, f, h) As in (a, c, e, g) but for zonal wind and temperature fluctuations. The curves with thin lines represent the elliptical fits.

    Figure 9.  Comparison of mean wave-number spectra during active and control days in standard form for (a) zonal and (b) meridional winds in the troposphere over Gadanki. (c, d) As in (a, b) but over Hyderabad. (e, f) As in (c, d) but for stratospheric winds. Corresponding slopes are shown for active and control cases with dashed and solid lines respectively.

    Figure 10.  (a, b) Zonal and meridional momentum flux profiles of IGWs prior to, during, and after the passage of the cyclone. (c, d) As in (a, b) but for 1-8 h gravity waves. Corresponding error bars are shown with horizontal lines. (e, f) As in (a, b) but for wind shear profiles.

    Figure 11.  (a, b) As in Figs. 10a and b but for body forces. (c, d) As in (a, b) but for 1-8 h gravity waves.

    Figure 12.  Contours of log $C_n^2$ (a) prior to, (b) during and (c) after the passage of the cyclone over Gadanki. (d-f) As in (a-c) but for wind shear. (g) Contours of relative humidity using ERA-Interim data from 31 October to 1 November 2012.

3. Analysis and discussion
  • The GPS radiosonde flight data of winds and temperature from Gadanki and Hyderabad were used for hodograph analysis to find the direction of IGW propagation during Nilam. The maximum effect of the cyclone could be observed from 1800 LST onwards on 31 October 2012 at Gadanki, and on 1 November 2012 during the whole day at Hyderabad. Heavy rains and high winds were found at ground level. The GPS radiosonde could reach near the tropopause (17 km) at Gadanki, but the balloon got snapped from the thread at around 7 km at Hyderabad, after which there was no contact between the balloon and the surface station. A second GPS radiosonde was launched from Hyderabad at 1730 LST on the same day (1 November 2012), but this one could reach only reached 5 km before flying away due to the turbulent atmosphere. The wind and temperature profiles obtained from the radiosonde flights are depicted in Figs. 5a-o and Figs. 6a-o for the periods 29 October to 2 November and 30 October to 3 November for Gadanki and Hyderabad, respectively, which were considered as active days. The profiles of the active days were also compared with the average of control day profiles——the mean profiles for 7-9 November. The radar data of Gadanki from 1630-1800 LST on each day were averaged and compared with the corresponding radiosonde profiles (Figs. 7a-h). Reasonable agreement was found, which increased our confidence in the reliability of the data.

  • Cyclone Nilam (2012) crossed Gadanki on 31 October 2012, and Hyderabad on 1 November 2012. Wind and temperature profiles obtained from the Gadanki radiosonde flights, along with the simultaneous radar data of horizontal winds, were used to plot hodographs for Gadanki; whereas, the radiosonde data alone provided the hodograph for Hyderabad. Gravity-wave fluctuations were extracted from the wind and temperature profiles by removing the quadratic fits (Allen and Vincent, 1995; Nastrom et al., 1997a). The fluctuation profiles were quite noisy due to the superposition of different waves of various vertical wavelengths. Hence, a narrow band-pass filter of 2-4 km wavelength was applied to each profile, which improved the hodographs of quasi-monochromatic gravity waves (Ratnam et al., 2006). Note that this method is approximate, sometimes carrying large uncertainties. The uncertainty, however, is lower for shorter wavelength gravity waves like IGWs (Zhang et al., 2004). The dominant propagation directions, based on velocity perturbations from radiosonde and radar measurements, were in agreement in the upper tropospheric region (10-15 km). Though the horizontal propagation direction of the wave can be obtained from the u'-v' hodograph, the exact direction of propagation can be determined by analyzing the corresponding u'-t' hodograph (Ratnam et al., 2006).

    The 30-min averaged profiles of horizontal winds measured by radar and the temperature profile obtained from the radiosonde (1630 LST) were used to identify the propagation direction, assuming that there was no major change in temperature during this period. Figures 8a, b, e and f display one pair of these hodographs for both radar and radiosonde data, on 31 October 2012 during the passage of the cyclone, revealing the direction of propagation as northwest. Figures 8c, d, g and h show similar hodographs for a control day (8 November), and the direction of propagation is northeast. The majority (65%) of the hodographs constructed for active and control days show the same trends. In fact, the continuous radar data were utilized to identify the variation in wave propagation direction with the passage of the cyclone. The results reveal that the direction between 0730 and 1530 LST on 31 October was mostly northeast, which then changed to northwest during 1830 LST of the same day to 0730 LST of the following day (1 November). The northwest trend continued for a couple of hours after the passage of the cyclone.

    Similar observations were reported for the same site by (Niranjan Kumar and Ramkumar, 2008) and (Niranjan Kumar et al., 2011), for cyclones during southwest and northeast monsoons and due to similar cyclonic tracks. The hodographs below approximately 8-9 km showed different directions and were inconclusive. It appears that the direction of gravity waves generated during the cyclone followed its path. The direction of propagation using the Hyderabad data could not be confirmed unambiguously since there was only one flight per day, and the flight on 1 November was aborted at an altitude of around 7 km. The number of hodographs derived from this profile was quite low, making it difficult to reach a firm conclusion.

    Some wave parameters can be estimated from hodographs using linear dispersion relations (Tsuda et al., 1994; Cho, 1995). The intrinsic wave frequency (ω) can be computed from the ratio of minor to major axes of the ellipse, \begin{equation} \label{eq1} \frac{{v}'}{{u}'}=-i\left(\dfrac{f}{\omega}\right), (1)\end{equation} where f is the inertial frequency and (v',u') are the meridional and zonal wind fluctuations, respectively. f is calculated as \begin{equation} \label{eq2} f=\sin\varphi/\tau , (2)\end{equation} where φ is the latitude of the location, and τ is equal to half a day, i.e., τ =12 h. The horizontal wave number k is determined using the relation \begin{equation} \label{eq3} k={m\omega}/{N} , (3)\end{equation} where N is the Brünt-Väisälä frequency and m is the vertical wave number. The wave parameters obtained in the present study are shown in Table 1.

  • The wind fluctuations in the middle atmosphere are supposed to represent a superposition of gravity waves. The frequency and wave number spectra are believed to have constant slopes of -5/3 and -3, respectively, throughout the lower and middle atmosphere (Dewan, 1979; VanZandt, 1982). However, (Gage, 1979) and (Lilly, 1983) interpreted mesoscale fluctuations in terms of 2D turbulence. Observational reports have seriously contradicted the universality of the spectra (Allen and Vincent, 1995; Nastrom et al., 1997a). (Fritts et al., 1990) observed asymmetry between the low- and high-frequency portion of the gravity-wave energy spectrum, which supports the dominance of wave theory. A report by (Dutta et al., 2005a) suggested that the spectrum deviates from the universal spectrum in different atmospheric conditions, and that mesoscale spectra can coexist with mechanisms like 2D turbulence.

    The altitude profiles of the wind fluctuations at Gadanki and Hyderabad were subjected to PSD (power spectral density) analyses. The spectra of the active days (29-31 October and 1-2 November) for Gadanki and those (30-31 October and 2 November) for Hyderabad were averaged and compared with the control spectra——the averages of 7-9 November at the respective stations——and are illustrated in Figs. 9a-f in standard form. The balloon flights from Gadanki reached a height of around 17 km on all the days, whereas the flights from Hyderabad went beyond 30 km, except on 1 November, as described in section 2. The spectra over Gadanki were computed only for the troposphere (1-15 km), while those over Hyderabad were split into the troposphere (1-15 km) and stratosphere (18-30 km). The slopes for the control days were found to be approximately -3, while those for the active days were approximately -3.5 to -4. The stratospheric slopes were slightly higher than the tropospheric slopes. The enhancements in spectral indices during active days would have mostly been due to the extra energy input during the cyclonic storm (Alexander et al., 1995; Dutta et al., 2009a). The vertical wavelengths deciphered from the wave-number spectra of both the stations were in the range of 2.5 to 4.5 km, which agreed with those computed from the hodograph analyses.

  • The spatial and temporal variations of gravity-wave momentum flux play a crucial role in atmospheric dynamics. Cyclones/typhoons are accompanied by convective bands, which are believed to be strong sources of IGWs. (Kim et al., 2007) observed higher powers for high-frequency gravity waves, which dominate the wave field during such events. A number of studies on the momentum flux of gravity waves suggest local momentum flux can be much larger than the mean values during thunderstorms (Dhaka et al., 2001; Nayar and Sreeletha, 2003; Dutta et al., 2009a) and events like typhoons/cyclones (Sato, 1993; Das et al., 2012). (Chen et al., 2015) identified high-frequency waves with periods of about 2 h in winds around the eye and eyewall of tropical cyclones. Studies on the momentum flux and turbulence generated during cyclones over Gadanki are lacking. In the present study, we estimated momentum flux of 1-8 h gravity waves and IGWs in three stages——prior to, during, and after the passage of cyclone Nilam (2012) over Gadanki——using continuous MST radar winds. The momentum flux of gravity waves is an extremely small parameter and needs a long period of averaging to obtain a meaningful estimate (Kudeki and Franke, 1998; Dutta et al., 2005b). The long and continuous 30-min averaged radial velocities of 31 October and 1 November 2012 were split into three sets of 8 h each (0730-1530 LST 31 October; 1830 LST 31 October to 0230 LST 1 November; 0630-1430 LST 1 November), which were considered as prior to, during, and after the passage of the storm. The continuous 16 profiles in each slot were detrended (mean removed) to obtain the fluctuations of radial velocities, which were then used to compute the momentum flux profiles of 1-8 h gravity waves. Time series of the radial velocity data were also used, to calculate the fluctuations by removing the quadratic fits from each profile and then filtering them between 2 and 4 km to obtain the IGW perturbations. These fluctuations were utilized to compute the momentum fluxes of IGWs. The perturbation profiles of IGWs and short-period gravity waves were used to derive the momentum flux profiles following the conjugate beam method developed by (Vincent and Reid, 1983): \begin{eqnarray} \label{eq4} \overline {{u}'{w}'}&=&\dfrac{\overline {(v_{E}^2-v_{W}^2)}}{2\sin 2\theta} ,(4)\\[1.5mm] \label{eq5} \overline {{v}'{w}'}&=&\dfrac{\overline {(v_{N}^2-v_{S}^2)}}{2\sin 2\theta} ,(5)\\\nonumber \end{eqnarray} where w' is vertical wind fluctuations, v E,v W,v N and v S represent the radial perturbation velocities in the east, west, north and south beams, respectively, and θ is the zenith angle of the beam. The momentum flux profiles were then smoothed by taking a 3-point running mean with a weighted average. The momentum flux profiles with large variabilities were removed and the mean profiles of the three slots computed. Comparisons of these mean momentum flux profiles of the three slots obtained for IGWs and 1-8 h gravity waves are depicted in Figs. 10a-d with standard deviations at a few heights which, on average, were found to be less compared to the flux estimates. The momentum flux of IGWs varied between -0.05 to +0.05 m2 s-2 and -0.05 to +0.07 m2 s-2 for the zonal and meridional components, respectively. Larger values were found between 10 and 15 km during the event for both wind cases. The short period (1-8 h) gravity waves carried higher momentum fluxes, which ranged between -0.1 to +0.15 m2 s-2 and -0.3 to +0.15 m2 s-2 for the two wind components, respectively. The meridional flux estimate in the lower troposphere (6 km) showed a large value. In general, the estimates showed larger values during the passage of Nilam (2012), followed by the prior-to and after-the-storm periods, when the atmosphere returned almost to normal.

    The atmosphere became disturbed as the cyclone approached the site, with an enhancement in corresponding momentum flux estimates, as evident from the larger values prior to the event compared with those after the passage of the storm. (Das et al., 2012) reported maximum flux to be -0.6 and -0.3 m2 s-2 for zonal and meridional components above the tropopause during a tropical cyclone over Gadanki. The flux estimates were found to be larger below the tropopause (10-15 km) prior to the event, and slowly returned to normal values after the passage of the storm. The associated wind shear profiles are shown in Figs. 10e and f. The positive/negative wind shear regions filter out most of the westerly/easterly and slower easterly/westerly propagating waves, particularly in the case of IGWs (Kim et al., 2005, 2007). It can also be explained in terms of critical level filtering in the region where the gravity-wave phase speed matches with the background wind. The positive momentum flux of IGWs (zonal component) in the lower troposphere does not follow this trend. Otherwise, the structure of momentum flux profiles can be explained with the help of corresponding wind shear profiles.

    Mean profiles of induced zonal (\(\overline F_u\)) and meridional (\(\overline F_v\)) drags can be derived from the corresponding momentum flux profiles using the relations \begin{equation} \label{eq6} \overline {F_u}=-\dfrac{1}{\rho_0}\dfrac{\partial}{\partial z}(\rho_0\overline {{u}'{w}'}) (6)\end{equation} and \begin{equation} \label{eq7} \overline {F_v}=-\dfrac{1}{\rho_0}\dfrac{\partial}{\partial z}(\rho_0\overline {v'w'}) , (7)\end{equation} where ρ0 is the atmospheric density and \(\rho_0\overline u'w'\) and \(\rho_0\overline v'w'\) are the mean density weighted zonal and meridional momentum fluxes, respectively. The profiles of induced drag for IGWs and short-period (1-8 h) gravity waves are illustrated in Figs. 11a-d. The body forces show large enhancement due to the cyclonic effect, and vary between 2 m s-1 d-1 (zonal) and 4 m s-1 d-1 (meridional) for IGWs, and 12-15 m s-1 d-1 for both wind components of 1-8 h gravity waves. The normal range of the drag after the cyclone is 10 m s-1 d-1, which is in agreement with earlier reports (Fukao et al., 1988; Rao et al., 1997).

  • Turbulent mixing in the atmosphere is very important in general, and even more so during dynamic events like cyclones and typhoons. Atmospheric turbulence is known to depend on background wind, temperature and humidity, which undergo large changes during such events and are worthy of investigation. MST radar provides an excellent opportunity to determine the refractivity structure constant (Cn2), since it is believed that the reflected echoes are mainly due to the refractivity structures associated with atmospheric turbulence (Hooper and Thomas, 1998). The high-resolution (5-min averages) continuous data of the MST radar were used to calculate the turbulent parameters of volume reflectivity (η) and the refractivity structure constant (Cn2) in three available data slots (0000-0530 LST and 1820-2330 LST 31 October, and 0600-1130 LST 1 November) to reveal the behavior of Cn2 prior to, during, and after the closest (64 km) approach of the cyclone to Gadanki, following the relations given by (Rao, 1998) and (Ghosh et al., 2001): \begin{equation} \eta=\dfrac{32(\ln2)KB(\alpha_{r}T_{c}+T_{r})r^2}{P_{t}\alpha_{r}\alpha_{t}A_{e}n_{B}n_{C}\Delta r} \left(\dfrac{S}{N}\right) , (8)\end{equation} where K is Boltzmann's constant, B is the effective receiver band width, α r is the receiver path loss, T c is the cosmic noise temperature, T r is the receiver noise temperature, r is the range of the back scatter echo, P t is the peak transmitter power, α t is the transmitter path loss, A e is the effective antenna area, n B is the number of bauds for coded pulse, and n C is the number of coherent integrations. The average signal to noise ratio (SNR) of four oblique beams was taken as (S/N) to avoid specular echo reflections (Hocking, 1985; Dutta et al., 2009b). The refractivity structure constant Cn2 was estimated using the volume reflectivity as follows: \begin{equation} \label{eq8} \eta=0.38C_n^2\lambda^{-\frac{1}{3}} , (9)\end{equation} where Λ is the radar wavelength. Contours of Cn2 and wind shear for the three slots are depicted in Figs. 12a-f.

    Tropospheric turbulence showed high values below 10 km before the passage of the cyclone (Fig. 12a), which then

    reduced when the cyclone was closer to the site (Fig. 12b). However, a turbulent layer close to approximately 17 km became stronger during this time, having been weaker before. This layer weakened again as the cyclone passed away (Fig. 12c). Figures 12d-f show the corresponding wind shear contours, revealing a layer of intense wind shear close to approximately 17 km during the passage of the cyclone, which was found to be somewhat weaker before and after the passage of the storm. The strong layer of Cn2, which persisted for a few hours even beyond the passage of the cyclone, was possibly due to the effect of wind shear. Enhancement of Cn2 has often been linked to enhanced shear (Tsuda et al., 1988; Sato, 1989; Nastrom et al., 1997b; Worthington and Thomas, 1997). (Nastrom and Eaton, 2001) conducted an extensive study of persistent layers of Cn2 using White Sands radar data and observed these layers even when wind shear was low, making them inconclusive about the source. Wave breaking might also cause the turbulent layer near the tropopause. Layers of enhanced reflectivity have also been attributed to IGWs and mountain waves (Thomas et al., 1992; Hines, 1995; Larsen, 1995). Gadanki is surrounded by hills of 700 m to 1 km in altitude. Hence, the longer-period waves generated during the passage of the storm might have contributed to the layer of Cn2. Large values of Cn2 below 10 km in the troposphere are reported to be due to the structure of humidity and its gradient (Tsuda et al., 1988). Humidity values (every 6 h) were obtained from the ERA-Interim data over (13.5°N, 79.5°E), which is close to Gadanki. The contours of relative humidity below 10 km are illustrated in Fig. 12g, which reveals larger values of humidity before the passage of cyclone, which then reduced during and after its passage. These results confirm the earlier reports mentioned above.

4. Summary
  • Convection associated with tropical cyclones is known to generate gravity waves in all frequency ranges. High-resolution (300 m in height and 2.5 min in time) wind measurements were taken with the Gadanki MST radar during the passage of cyclone Nilam (2012). In addition, GPS radiosonde flights were conducted from Gadanki and Hyderabad, once a day. These data gave a unique opportunity to study the effects of tropical cyclones on the characteristics of gravity waves, and also on dynamical parameters like momentum flux of gravity waves and turbulence in the lower atmosphere.

    The propagation direction of IGWs was determined by hodograph analyses using u'-v' and u'-t', revealing a change from northeast to northwest during the passage of the cyclone. The vertical wavelength of the wave remained in the same range, whereas the horizontal wavelength shortened compared to the control day.

    The momentum flux of IGWs and short-period (1-8 h) gravity waves, as well as the induced drag and turbulence, were computed in three slots: prior to, during, and after the passage of the cyclone. The vertical fluxes of horizontal momentum and the associated body forces enhanced prior to and during the passage of the storm, as compared to the values after its passage. Flux estimates ranged between 0.05 m2 s-2 (IGWs) and 0.8 m2 s-2 (1-8 h gravity waves), and the corresponding body forces varied between 2 and 4 m s-1 d-1, and 12-15 m s-1 d-1. The turbulent parameter Cn2 showed larger values before the passage of the cyclone below 10 km, along with high humidity. Turbulence and humidity reduced in this region during the passage of the cyclone, and a turbulent layer at around 17 km at this time grew in strength. The layers of large Cn2 near the tropopause and below 10 km weakened after the storm moved far away from the site.




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