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2005 Vol. 22, No. 5

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Intraseasonal Oscillation in the Tropical Indian Ocean
LI Chongyin, HU Ruijin, YANG Hui
2005, 22(5): 617-624. doi: 10.1007/BF02918705
The features of the intraseasonal oscillation (ISO) of the tropical Indian Ocean are studied using several sources of observational data. It is shown that there are intraseasonal oscillations in the tropical Indian Ocean, but their periods vary with latitude: the major period is about 20-30 days in the equatorial region, about 30-50 days at 10°N/10°S latitude and 60-90 days at 20°N/20°S latitude. The intensity of the ISO increases with latitude but the speed of the westward propagation of the ISO decreases with latitude. The intensity and propagation speed of the ISO have clear interannual variation features. The atmospheric intraseasonal oscillation over the tropical Indian Ocean is also analyzed and compared with the oceanic intraseasonal oscillation. It is shown that the major period is in the range 30-60 days and the intensity and period of the atmospheric ISO decrease with latitude slightly. The zonal propagation of the atmospheric ISO also has some differences with the oceanic ISO. It is necessary to study the relationship between the atmospheric ISO and oceanic ISO in the tropical Indian Ocean deeply.
A Numerical Investigation of a Slow-Moving Convective Line in a Weakly Sheared Environment
Changhai LIU
2005, 22(5): 625-639. doi: 10.1007/BF02918706
A series of three-dimensional, cloud-resolving numerical simulations are performed to examine a slowpropagating, quasi-two-dimensional convective system in a weakly sheared environment during the Tropical Rainfall Measuring Mission Large-Scale Biosphere-Atmosphere (TRMM-LBA) field campaign. The focus is on the kinematics and thermodynamics, organization mechanisms, and dynamical effects of low-level shear, ice microphysics and tropospheric humidity. The control simulation, which is initialized with the observed sounding and includes full microphysics, successfully replicates many observed features of the convective system, such as the linear structure, spatial orientation, life cycle, and sluggish translation.The system at the mature stage displays a line-normal structure similar to that associated with squalltype convective systems, but the corresponding mesoscale circulation and thermodynamic modification are much weaker. Ice-phase microphysical processes are not necessary to the formation of the convective system, but they play a non-trivial role in the late evolution stage. In contrast, the low-level shear, albeit shallow and weak, is critical to the realistic realization of the convective line. The tropospheric moisture above the planetary boundary layer has an important impact on the behavior of convective organization.In particular, a dry layer in the lower troposphere significantly suppresses convective development and inhibits the generation of organized convection even though the convective available potential energy is substantial. The free-atmosphere humidity has received little attention in previous studies of organized convection and warrants further investigation.
Black Carbon Particles in the Urban Atmosphere in Beijing
WANG Gengchen, BAI Jianhui, KONG Qinxin, Alexander EMILENKO
2005, 22(5): 640-646. doi: 10.1007/BF02918707
A study of the concentration of black carbon particles and its variation in the urban atmosphere has been carried out since 1996 in the Beijing area. The measurements were done in the late autumn and early winter each year, the period before and after domestic heating activities begin. The results show the presence of black carbon particles at the high level that vary over a large range in the urban atmosphere in Beijing. The mean value of daily average concentration for the whole observation period of 1996-2004is 20.0μg m-3. An evident decrease of black carbon particle concentration in the Beijing area is observed after 2000, and the daily average concentration of black carbon particles is estimated to be 16.0μg m-3with a variation range of 2.10-50.50μg m-3 for the period of 2000-2004. The observation method and main variation behavior characteristics of black carbon particles in the urban atmosphere in the Beijing area are given and discussed.
Atmospheric Profile Retrieval with AIRS Data and Validation at the ARM CART Site
WU Xuebao, LI Jun, ZHANG Wenjian, WANG Fang
2005, 22(5): 647-654. doi: 10.1007/BF02918708
The physical retrieval algorithm of atmospheric temperature and moisture distribution from the Atmospheric InfraRed Sounder (AIRS) radiances is presented. The retrieval algorithm is applied to AIRS clear-sky radiance measurements. The algorithm employs a statistical retrieval followed by a subsequent nonlinear physical retrieval. The regression coefficients for the statistical retrieval are derived from a dataset of global radiosonde observations (RAOBs) comprising atmospheric temperature, moisture, and ozone profiles. Evaluation of the retrieved profiles is performed by a comparison with RAOBs from the Atmospheric Radiation Measurement (ARM) Program Cloud And Radiation Testbed (CART) in Oklahoma,U. S. A.. Comparisons show that the physically-based AIRS retrievals agree with the RAOBs from the ARM CART site with a Root Mean Square Error (RMSE) of 1 K on average for temperature profiles above 850 hPa, and approximately 10% on average for relative humidity profiles. With its improved spectral resolution, AIRS depicts more detailed structure than the current Geostationary Operational Environmental Satellite (GOES) sounder when comparing AIRS sounding retrievals with the operational GOES sounding products.
Climate-Induced Variability of Sea Level in Stockholm: Influence of Air Temperature and Atmospheric Circulation
Deliang CHEN, Anders OMSTEDT
2005, 22(5): 655-664. doi: 10.1007/BF02918709
This study is focused on climate-induced variation of sea level in Stockholm during 1873-1995. After the effect of the land uplift is removed, the residual is characterized and related to large-scale temperature and atmospheric circulation. The residual shows an overall upward trend, although this result depends on the uplift rate used. However, the seasonal distribution of the trend is uneven. There are even two months (June and August) that show a negative trend. The significant trend in August may be linked to fresh water input that is controlled by precipitation. The influence of the atmospheric conditions on the sea level is mainly manifested through zonal winds, vorticity and temperature. While the wind is important in the period January-May, the vorticity plays a main role during June and December. A successful linear multiple-regression model linking the climatic variables (zonal winds, vorticity and mean air temperature during the previous two months) and the sea level is established for each month. An independent verification of the model shows that it has considerable skill in simulating the variability.
South China Sea Summer Monsoon Onset in Relation to the Off-Equatorial ITCZ
ZHOU Wen, Johnny Chung-Leung CHAN, LI Chongyin
2005, 22(5): 665-676. doi: 10.1007/BF02918710
Observations of the South China Sea summer monsoon (SCSSM) demonstrate the different features between the early and late onsets of the monsoon. The determining factor related to the onset and the resultant monsoon rainfall might be the off-equatorial ITCZ besides the land-sea thermal contrast. The northward-propagating cumulus convection over the northern Indian Ocean could enhance the monsoon trough so that the effect of the horizontal advection of moisture and heat is substantially increased, thus westerlies can eventually penetrate and prevail over the South China Sea (SCS) region.
Evaluation of Mid-Depth Currents of NCEP Reanalysis Data in the Tropical Pacific Using ARGO Float Position Information
XIE Jiping, ZHU Jiang, XU Li, GUO Pinwen
2005, 22(5): 677-684. doi: 10.1007/BF02918711
The global project of the Array for Real-time Geostrophic Oceanography (ARGO) provides a unique opportunity to observe the absolute velocity in mid-depths of the world oceans. A total of 1597 velocity vectors at 1000 (2000) db in the tropical Pacific derived from the ARGO float position information during the period November 2001 to October 2004 are used to evaluate the intermediate currents of the National Centers for Environmental Prediction reanalysis. To derive reliable velocity information from ARGO float trajectory points, a rigorous quality control scheme is applied, and by virtue of a correction method for reducing the drift error on the surface in obtaining the velocity vectors, their relative errors are less than 25%. Based on the comparisons from the quantitative velocity vectors and from the space-time average currents, some substantial discrepancies are revealed. The first is that the velocities of the reanalysis at mid-depths except near the equator are underestimated relative to the observed velocities by the floats.The average speed difference between NCEP and ARGO values ranges from about -2.3 cms-1 to -1.8cms-1. The second is that the velocity difference between the ocean model and the observations at 2000dB seems smaller than that at 1000 dB. The third is that the zonal flow in the reanalysis is too dominant so that some eddies could not be simulated, such as the cyclonic eddy to the east of 160°E between 20°N and 30°N at 2000 dB. In addition, it is noticeable that many floats parking at 1000 dB cannot acquire credible mid-depth velocities due to the time information of their end of ascent (start of descent) on the surface in the trajectory files. Thus, relying on default times of parking, descent and ascent in the metadata files gravely confines their application to measuring mid-depth currents.
Diurnal Variation of Tropical Convection during TOGA COARE IOP
Jae-Young BYON, Gyu-Ho LIM
2005, 22(5): 685-702. doi: 10.1007/BF02918712
Diurnal variation of tropical convection and kinematic and thermodynamic conditions was investigated for different large-scale environments of the convectively active and inactive periods by using satellite observations and surface measurements during the Intensive Observation Period (IOP) of the Tropical Ocean Global Atmosphere/Coupled Ocean-Atmosphere Response Experiment (TOGA/COARE). During the convectively active period, the features of nocturnal convection appear in vertical profiles of convergence, vertical velocity, heat source, and moisture sink. The specific humidity increases remarkably in the middle troposphere at dawn. On the other hand, the altitude of maximum convergence and that of the upward motion is lower during the convectively inactive period. The specific humidity peaks in the lower troposphere in the daytime and decreases in the middle troposphere. Spectral analyses of the time series of the infrared (IR) brightness temperature (TBB) and amounts of rainfall suggest multiscale temporal variation with a prominent diurnal cycle over land and oceanic regions such as the Intensive Flux Array (IFA) and the South Pacific Convergence Zone (SPCZ). Over land, the daily maximum of deep convection associated with cloud top temperature less than 208 K appears at midnight due to the daytime radiative heating and the sea-land breeze. Over the ocean, convection usually tends to occur at dawn for the convectively active period while in the afternoon during the inactive period. Comparing the diurnal variation of convection with large-scale variables, the authors inferred that moisture in the middle troposphere contributes mostly to the development of nocturnal convection over the ocean during the convectively active period.
Statistical Prediction of Heavy Rain in South Korea
Keon Tae SOHN, Jeong Hyeong LEE, Soon Hwan LEE, Chan Su RYU
2005, 22(5): 703-710. doi: 10.1007/BF02918713
This study is aimed at the development of a statistical model for forecasting heavy rain in South Korea. For the 3-hour weather forecast system, the 10 km× 10 km area-mean amount of rainfall at 6 stations (Seoul, Daejeon, Gangreung, Gwangju, Busan, and Jeju) in South Korea are used. And the corresponding 45 synoptic factors generated by the numerical model are used as potential predictors. Four statistical forecast models (linear regression model, logistic regression model, neural network model and decision tree model) for the occurrence of heavy rain are based on the model output statistics (MOS) method. They are separately estimated by the same training data. The thresholds are considered to forecast the occurrence of heavy rain because the distribution of estimated values that are generated by each model is too skewed.The results of four models are compared via Heidke skill scores. As a result, the logistic regression model is recommended.
A Model for Retrieval of Dual Linear Polarization Radar Fields from Model Simulation Outputs
LIU Liping, ZHANG Pengfei, Qin XU, KONG Fanyou, LIU Shun
2005, 22(5): 711-719. doi: 10.1007/BF02918714
An algorithm for retrieving polarimetric variables from numerical model fields is developed. By using this technique, radar reflectivity at horizontal polarization, differential reflectivity, specific differential phase shift and correlation coefficients between the horizontal and vertical polarization signals at zero lag can be derived from rain, snow and hail contents of numerical model outputs. Effects of environmental temperature and the melting process on polarimetric variables are considered in the algorithm. The algorithm is applied to the Advanced Regional Prediction System (ARPS) model simulation results for a hail storm. The spatial distributions of the derived parameters are reasonable when compared with observational knowledge. This work provides a forward model for assimilation of dual linear polarization radar data into a mesoscale model.
The Soil Moisture of China in a High Resolution Climate-Vegetation Model
DAN Li, JI Jinjun, ZHANG Peiqun
2005, 22(5): 720-729. doi: 10.1007/BF02918715
The spatial distribution of soil moisture, especially the temporal variation at seasonal and interannual scales, is difficult for many land surface models (LSMs) to capture partly due to the deficiencies of the LSMs and the highly spatial variability of soil moisture, which makes it problematic to simulate the moisture for climate studies. However the soil moisture plays an important role in influencing the energy and hydrological cycles between the land and air, so it should be considered in land surface models. In this paper, a soil moisture simulation in China with a T213 resolution (about 0.5625°× 0.5625°) is compared to the observational data, and its relationship to precipitation is explored. The soil moisture distribution agrees roughly with the observations, and the soil moisture pattern reflects the variation and intensity of the precipitation. In particular, for the 1998 summer catastrophic floods along the Yangtze River, the soil moisture remains high in this region from July to August and represents the flood well. The seasonal cycle of soil moisture is roughly consistent with the observed data, which is a good calibration forthe ground simulation capacity of the Atmosphere-Vegetation Interaction Model (AVIM) with respect to this tough problem for land surface models.
A Study of the Extratropical Transformation of Typhoon Winnie (1997)
ZHU Peijun, ZHENG Yongguang, ZHANG Chunxi, TAO Zuyu
2005, 22(5): 730-740. doi: 10.1007/BF02918716
The complicated evolutive process of how a tropical cyclone transforms into an extratropical cyclone is still an unresolved issue to date, especially one which arises in a weakly baroclinic environment. Typhoon Winnie (1997) is studied during its extratropical transformation stage of extratropical transition (ET)with observational data and numerical simulations. Results show that Winnie experienced its extratropical transformation to the south of the subtropical high without intrusion of the mid-latitude baroclinic zone.This is significantly different from previous studies. Analyses reveal that the cold air, which appeared in the north edge of Winnie circulation, resulted from the precipitation drag and cooling effect of latent heat absorption associated with the intense precipitation there. The cooling only happened below 3 km and the greatest cooling was below 1 km. With the cold air and its advection by the circulation of Winnie, a front was formed in the lower troposphere. The front above is related not only to the cooling in the lower level but also to the warming effect of latent heat release in the middle-upper levels. The different temperature variation in the vertical caused the temperature gradient over Winnie and resulted in the baroclinicity.
On the Mechanism of the Locking of the El Ni o Event Onset Phase to Boreal Spring
YAN Bangliang
2005, 22(5): 741-750. doi: 10.1007/BF02918717
The mechanism of the locking of the El Nino event onset phase to boreal spring (from April to June)in an intermediate coupled ocean-atmosphere model is investigated. The results show that the seasonal variation of the zonal wind anomaly over the equatorial Pacific associated with the seasonal variation of the ITCZ is the mechanism of the locking in the model. From January to March of the El Nino year, the western wind anomaly over the western equatorial Pacific can excite the downwelling Kelvin wave that propagates eastward to the eastern and middle Pacific by April to June. From April to December of the year before the El Nino year, the eastern wind anomaly over the equatorial Pacific forces the downwelling Rossby waves that modulate the ENSO cycle. The modulation and the reflection at the western boundary modulate the time of the transition from the cool to the warm phase to September of the year before the El Nino year and cause the strongest downwelling Kelvin wave from the reflected Rossby waves at the western boundary to arrive in the middle and eastern equatorial Pacific by April to June of the El Nino year. The superposition of these two kinds of downwelling Kelvin waves causes the El Nino event to tend to occur from April to June.
HCl Quasi-Biennial Oscillation in the Stratosphere and a Comparison with Ozone QBO
CHEN Yuejuan, SHI Chunhua, ZHENG Bin
2005, 22(5): 751-758. doi: 10.1007/BF02918718
HALOE data from 1992 to 2003 are used to analyze the interannual variation of the HCl volume mixing ratio and its quasi-biennial oscillation (QBO) in the stratosphere, and the results are compared with the ozone QBO. Then, the NCAR two-dimensional interactive chemical, dynamical and radiative model is used to study the effects of the wind QBO on the distribution and variation of HCl in the stratosphere.The results show that the QBO signals in the HCl mixing ratio are mainly at altitudes from 50 hPa to 5 hPa; the larger amplitudes are located between 30 hPa and 10 hPa; a higher HCl mixing ratio usually corresponds to the westerly phase of the wind QBO and a lower HCl mixing ratio usually corresponds to the easterly phase of the wind QBO in a level near 20 hPa and below. In the layer near 10 hPa-5 hPa, the phase of the HCl QBO reverses earlier than the phase of the wind QBO; the QBO signals for HCl in the extratropics are also clear, but with reversed phase compared with those over the Tropics. The HCl QBO signals at 30°N are clearer than those at 30°S; the QBOs for HCl and ozone have a similar phase at the 50hPa-20 hPa level while they are out of phase near 10 hPa; the simulated structures of the HCl QBO agree well with observations. The mechanism for the formation of the HCl QBO and the reason for differences in the vertical structure of the HCl and ozone QBO are attributed to the transport of HCl and ozone by the wind QBO-induced meridional circulation.
Retrieval of Water Vapor Profiles with Radio Occultation Measurements Using an Artificial Neural Network
WANG Xin, Lü Daren
2005, 22(5): 759-764. doi: 10.1007/BF02918719
A new method applying an artificial neural network (ANN) to retrieve water vapor profiles in the troposphere is presented. In this paper, a fully-connected, three-layer network based on the backpropagation algorithm is constructed. Month, latitude, altitude and bending angle are chosen as the input vectors and water vapor pressure as the output vector. There are 130 groups of occultation measurements from June to November 2002 in the dataset. Seventy pairs of bending angles and water vapor pressure profiles are used to train the ANN, and the sixty remaining pairs of profiles are applied to the validation of the retrieval. By comparing the retrieved profiles with the corresponding ones from the Information System and Data Center of the Challenging Mini-Satellite Payload for Geoscientific Research and Application (CHAMP-ISDC), it can be concluded that the ANN is relatively convenient and accurate. Its results can be provided as the first guess for the iterative methods or the non-linear optimal estimation inverse method.
Simple General Atmospheric Circulation and Climate Models with Memory
2005, 22(5): 765-769. doi: 10.1007/BF02918720
This article examines some general atmospheric circulation and climate models in the context of the notion of "memory". Two kinds of memories are defined: statistical memory and deterministic memory.The former is defined through the autocorrelation characteristic of the process if it is random (chaotic),while for the latter, a special memory function is introduced. Three of the numerous existing models are selected as examples. For each of the models, asymptotic (at t →∞) expressions are derived. In this way,the transients are filtered out and that which remains concerns the final behaviour of the models.
A Laboratory Study of the Turbulent Velocity Characteristics in the Convective Boundary Layer
SUN Jianning, JIANG Weimei, CHEN Ziyun, YUAN Renmin
2005, 22(5): 770-780. doi: 10.1007/BF02918721
Based on the measurement of the velocity field in the convective boundary layer (CBL) in a convection water tank with the particle image velocimetry (PIV) technique, this paper studies the characteristics of the CBL turbulent velocity in a modified convection tank. The experiment results show that the velocity distribution in the mixed layer clearly possesses the characteristics of the CBL thermals, and the turbulent eddies can be seen obviously. The comparison of the vertical distribution of the turbulent velocity variables indicates that the modeling in the new tank is better than in the old one. The experiment data show that the thermal's motion in the entrainment zone sometimes fluctuates obviously due to the intermittence of turbulence. Analyses show that this fluctuation can influence the agreement of the measurement data with the parameterization scheme, in which the convective Richardson number is used to characterize the entrainment zone depth. The normalized square velocity w2i/w2* at the top of the mixed layer seems to be time-dependent, and has a decreasing trend during the experiments. This implies that the vertical turbulent velocity at the top of the mixed layer may not be proportional to the convective velocity (w*).