2015 Vol. 32, No. 9
Display Method:
2015, 32(9): 1173-1185.
doi: 10.1007/s00376-015-4257-x
Abstract:
Daily precipitation series at 15 stations in the Beijing metropolitan region (BMR) during 1960-2012 were homogenized using the multiple analysis of series for homogenization method, with additional adjustments based on analysis of empirical cumulative density function (ECDF) regarding climate extremes. The cumulative density functions of daily precipitation series, the trends of annual and seasonal precipitation, and summer extreme events during 1960-2012 in the original and final adjusted series at Beijing station were comparatively analyzed to show the necessity and efficiency of the new method. Results indicate that the ECDF adjustments can improve the homogeneity of high-order moments of daily series and the estimation of climate trends in extremes. The linear trends of the regional-mean annual and seasonal (spring, summer, autumn, and winter) precipitation series are -10.16, 4.97, -20.04, 5.02, and -0.11 mm (10 yr)-1, respectively. The trends over the BMR increase consistently for spring/autumn and decrease for the whole year/summer; however, the trends for winter decrease in southern parts and increase in northern parts. Urbanization affects local trends of precipitation amount, frequency, and intensity and their geographical patterns. For the urban-influenced sites, urbanization tends to slow down the magnitude of decrease in the precipitation and extreme amount series by approximately -10.4% and -6.0%, respectively; enhance the magnitude of decrease in precipitation frequency series by approximately 5.7%; reduce that of extremes by approximately -8.9%; and promote the decreasing trends in the summer intensity series of both precipitation and extremes by approximately 6.8% and 51.5%, respectively.
Daily precipitation series at 15 stations in the Beijing metropolitan region (BMR) during 1960-2012 were homogenized using the multiple analysis of series for homogenization method, with additional adjustments based on analysis of empirical cumulative density function (ECDF) regarding climate extremes. The cumulative density functions of daily precipitation series, the trends of annual and seasonal precipitation, and summer extreme events during 1960-2012 in the original and final adjusted series at Beijing station were comparatively analyzed to show the necessity and efficiency of the new method. Results indicate that the ECDF adjustments can improve the homogeneity of high-order moments of daily series and the estimation of climate trends in extremes. The linear trends of the regional-mean annual and seasonal (spring, summer, autumn, and winter) precipitation series are -10.16, 4.97, -20.04, 5.02, and -0.11 mm (10 yr)-1, respectively. The trends over the BMR increase consistently for spring/autumn and decrease for the whole year/summer; however, the trends for winter decrease in southern parts and increase in northern parts. Urbanization affects local trends of precipitation amount, frequency, and intensity and their geographical patterns. For the urban-influenced sites, urbanization tends to slow down the magnitude of decrease in the precipitation and extreme amount series by approximately -10.4% and -6.0%, respectively; enhance the magnitude of decrease in precipitation frequency series by approximately 5.7%; reduce that of extremes by approximately -8.9%; and promote the decreasing trends in the summer intensity series of both precipitation and extremes by approximately 6.8% and 51.5%, respectively.
2015, 32(9): 1186-1196.
doi: 10.1007/s00376-015-4222-8
Abstract:
Based on a vector radiative transfer model of the atmosphere-ocean system, the influence of oceanic components on radiation processes, including polarization effects, was investigated in the wavelength region ranging from 0.380 to 0.865 μm. The components considered were phytoplankton, inorganic suspended material (sediment), and colored, dissolved organic matter. Due to their important roles in oceanic radiation processes, the sensitivity of the bidirectional reflectance to the rough ocean surface, represented by the wind velocity 10 m above the ocean surface, and aerosol, were taken into account. The results demonstrated that both radiance and polarized radiance just below the ocean surface were sensitive to the change of the concentrations of the considered components, while the dependence of polarized radiance on the observation geometry was more sensitive than radiance. Significant differences in the specular plane existed between the impacts of the phytoplankton and sediment on the degree of polarization just above the ocean surface at 670 nm. At the top of the atmosphere (TOA), polarization was relatively insensitive to changing concentrations of ocean particles at longer wavelengths. Furthermore, the radiance at the TOA in the solar plane was more sensitive to the aerosol optical thickness than wind velocity. In contrast, wind velocity strongly influenced the radiance at the TOA in the sun glint region, while the polarization degree showed less dependence in that region. Finally, a nonlinear optimal inversion method was proposed to simultaneously retrieve the aerosol and wind velocity using radiance measurement.
Based on a vector radiative transfer model of the atmosphere-ocean system, the influence of oceanic components on radiation processes, including polarization effects, was investigated in the wavelength region ranging from 0.380 to 0.865 μm. The components considered were phytoplankton, inorganic suspended material (sediment), and colored, dissolved organic matter. Due to their important roles in oceanic radiation processes, the sensitivity of the bidirectional reflectance to the rough ocean surface, represented by the wind velocity 10 m above the ocean surface, and aerosol, were taken into account. The results demonstrated that both radiance and polarized radiance just below the ocean surface were sensitive to the change of the concentrations of the considered components, while the dependence of polarized radiance on the observation geometry was more sensitive than radiance. Significant differences in the specular plane existed between the impacts of the phytoplankton and sediment on the degree of polarization just above the ocean surface at 670 nm. At the top of the atmosphere (TOA), polarization was relatively insensitive to changing concentrations of ocean particles at longer wavelengths. Furthermore, the radiance at the TOA in the solar plane was more sensitive to the aerosol optical thickness than wind velocity. In contrast, wind velocity strongly influenced the radiance at the TOA in the sun glint region, while the polarization degree showed less dependence in that region. Finally, a nonlinear optimal inversion method was proposed to simultaneously retrieve the aerosol and wind velocity using radiance measurement.
2015, 32(9): 1197-1207.
doi: 10.1007/s00376-014-4175-3
Abstract:
Heat waves have attracted increasing attention in recent years due to their frequent occurrence. The present study investigates the heat wave intensity and duration in China using daily maximum temperature from 753 weather stations from 1960 to 2010. In addition, its relationships with soil moisture local forcing on the ten-day period and monthly scales in spring and summer are analyzed using soil moisture data from weather stations and ERA40 reanalysis data. And finally, a mechanistic analysis is carried out using CAM5.1 (Community Atmosphere Model, version 5.1) coupled with CLM2 (Community Land Model, version 2). It is found that the heat wave frequency and duration show a sandwich distribution across China, with high occurrence rates in Southeast China and Northwest China, where the maximum frequency and duration exceeded 2.1 times and 9 days per year, respectively. The increasing trends in both duration and intensity occurred to the north of 35°N. The relationships between heat wave frequency in northern China in July (having peak distribution) and soil moisture in the earlier stage (from March to June) and corresponding period (July) are further analyzed, revealing a strong negative correlation in March, June and July, and thus showing that soil moisture in spring and early summer could be an important contributor to heat waves in July via positive subtropical high anomalies. However, the time scales of influence were relatively short in the semi-humid and humid regions, and longer in the arid region. The contribution in the corresponding period took place via positive subtropical high anomalies and positive surface skin temperature and sensible heat flux anomalies.
Heat waves have attracted increasing attention in recent years due to their frequent occurrence. The present study investigates the heat wave intensity and duration in China using daily maximum temperature from 753 weather stations from 1960 to 2010. In addition, its relationships with soil moisture local forcing on the ten-day period and monthly scales in spring and summer are analyzed using soil moisture data from weather stations and ERA40 reanalysis data. And finally, a mechanistic analysis is carried out using CAM5.1 (Community Atmosphere Model, version 5.1) coupled with CLM2 (Community Land Model, version 2). It is found that the heat wave frequency and duration show a sandwich distribution across China, with high occurrence rates in Southeast China and Northwest China, where the maximum frequency and duration exceeded 2.1 times and 9 days per year, respectively. The increasing trends in both duration and intensity occurred to the north of 35°N. The relationships between heat wave frequency in northern China in July (having peak distribution) and soil moisture in the earlier stage (from March to June) and corresponding period (July) are further analyzed, revealing a strong negative correlation in March, June and July, and thus showing that soil moisture in spring and early summer could be an important contributor to heat waves in July via positive subtropical high anomalies. However, the time scales of influence were relatively short in the semi-humid and humid regions, and longer in the arid region. The contribution in the corresponding period took place via positive subtropical high anomalies and positive surface skin temperature and sensible heat flux anomalies.
2015, 32(9): 1208-1216.
doi: 10.1007/s00376-015-4201-0
Abstract:
Thirty strong Madden-Julian Oscillation (MJO) events in boreal winter 1982-2001 are selected to investigate the triggering processes of MJO convection over the western equatorial Indian Ocean (IO). These MJO events are classified into three types, according to their dynamic and thermodynamic precursor signals in situ. In Type I, a remarkable increase in low-level moisture occurs, on average, 7 days prior to the convection initiation. This low-level moistening is mainly due to the advection of the background mean moisture by easterly wind anomalies over the equatorial IO. In Type II, lower-tropospheric ascending motion anomalies develop, on average, 4 days prior to the initiation. The cause of this ascending motion anomaly is attributed to the anomalous warm advection, set up by a suppressed MJO phase in the equatorial IO. In Type III, there are no clear dynamic and thermodynamic precursor signals in situ. The convection might be triggered by energy accumulation in the upper layer associated with Rossby wave activity fluxes originated from the midlatitudes.
Thirty strong Madden-Julian Oscillation (MJO) events in boreal winter 1982-2001 are selected to investigate the triggering processes of MJO convection over the western equatorial Indian Ocean (IO). These MJO events are classified into three types, according to their dynamic and thermodynamic precursor signals in situ. In Type I, a remarkable increase in low-level moisture occurs, on average, 7 days prior to the convection initiation. This low-level moistening is mainly due to the advection of the background mean moisture by easterly wind anomalies over the equatorial IO. In Type II, lower-tropospheric ascending motion anomalies develop, on average, 4 days prior to the initiation. The cause of this ascending motion anomaly is attributed to the anomalous warm advection, set up by a suppressed MJO phase in the equatorial IO. In Type III, there are no clear dynamic and thermodynamic precursor signals in situ. The convection might be triggered by energy accumulation in the upper layer associated with Rossby wave activity fluxes originated from the midlatitudes.
2015, 32(9): 1217-1230.
doi: 10.1007/s00376-015-4092-0
Abstract:
A major issue in radar quantitative precipitation estimation is the contamination of radar echoes by non-meteorological targets such as ground clutter, chaff, clear air echoes etc. In this study, a fuzzy logic algorithm for the identification of non-meteorological echoes is developed using optimized membership functions and weights for the dual-polarization radar located at Mount Sobaek. For selected precipitation and non-meteorological events, the characteristics of the precipitation and non-meteorological echo are derived by the probability density functions of five fuzzy parameters as functions of reflectivity values. The membership functions and weights are then determined by these density functions. Finally, the non-meteorological echoes are identified by combining the membership functions and weights. The performance is qualitatively evaluated by long-term rain accumulation. The detection accuracy of the fuzzy logic algorithm is calculated using the probability of detection (POD), false alarm rate (FAR), and clutter-signal ratio (CSR). In addition, the issues in using filtered dual-polarization data are alleviated.
A major issue in radar quantitative precipitation estimation is the contamination of radar echoes by non-meteorological targets such as ground clutter, chaff, clear air echoes etc. In this study, a fuzzy logic algorithm for the identification of non-meteorological echoes is developed using optimized membership functions and weights for the dual-polarization radar located at Mount Sobaek. For selected precipitation and non-meteorological events, the characteristics of the precipitation and non-meteorological echo are derived by the probability density functions of five fuzzy parameters as functions of reflectivity values. The membership functions and weights are then determined by these density functions. Finally, the non-meteorological echoes are identified by combining the membership functions and weights. The performance is qualitatively evaluated by long-term rain accumulation. The detection accuracy of the fuzzy logic algorithm is calculated using the probability of detection (POD), false alarm rate (FAR), and clutter-signal ratio (CSR). In addition, the issues in using filtered dual-polarization data are alleviated.
2015, 32(9): 1231-1243.
doi: 10.1007/s00376-014-4239-4
Abstract:
The impact of assimilating radiances from the Advanced Microwave Sounding Unit-A (AMSU-A) on the track prediction of Typhoon Megi (2010) was studied using the Weather Research and Forecasting (WRF) model and a hybrid ensemble three-dimensional variational (En3DVAR) data assimilation (DA) system. The influences of tuning the length scale and variance scale factors related to the static background error covariance (BEC) on the track forecast of the typhoon were studied. The results show that, in typhoon radiance data assimilation, a moderate length scale factor improves the prediction of the typhoon track. The assimilation of AMSU-A radiances using 3DVAR had a slight positive impact on track forecasts, even when the static BEC was carefully tuned to optimize its performance. When the hybrid DA was employed, the track forecast was significantly improved, especially for the sharp northward turn after crossing the Philippines, with the flow-dependent ensemble covariance. The flow-dependent BEC can be estimated by the hybrid DA and was capable of adjusting the position of the typhoon systematically. The impacts of the typhoon-specific BEC derived from ensemble forecasts were revealed by comparing the analysis increments and forecasts generated by the hybrid DA and 3DVAR. Additionally, for 24 h forecasts, the hybrid DA experiment with use of the full flow-dependent background error substantially outperformed 3DVAR in terms of the horizontal winds and temperature in the lower and mid-troposphere and for moisture at all levels.
The impact of assimilating radiances from the Advanced Microwave Sounding Unit-A (AMSU-A) on the track prediction of Typhoon Megi (2010) was studied using the Weather Research and Forecasting (WRF) model and a hybrid ensemble three-dimensional variational (En3DVAR) data assimilation (DA) system. The influences of tuning the length scale and variance scale factors related to the static background error covariance (BEC) on the track forecast of the typhoon were studied. The results show that, in typhoon radiance data assimilation, a moderate length scale factor improves the prediction of the typhoon track. The assimilation of AMSU-A radiances using 3DVAR had a slight positive impact on track forecasts, even when the static BEC was carefully tuned to optimize its performance. When the hybrid DA was employed, the track forecast was significantly improved, especially for the sharp northward turn after crossing the Philippines, with the flow-dependent ensemble covariance. The flow-dependent BEC can be estimated by the hybrid DA and was capable of adjusting the position of the typhoon systematically. The impacts of the typhoon-specific BEC derived from ensemble forecasts were revealed by comparing the analysis increments and forecasts generated by the hybrid DA and 3DVAR. Additionally, for 24 h forecasts, the hybrid DA experiment with use of the full flow-dependent background error substantially outperformed 3DVAR in terms of the horizontal winds and temperature in the lower and mid-troposphere and for moisture at all levels.
2015, 32(9): 1244-1254.
doi: 10.1007/s00376-015-4251-3
Abstract:
In this study, a newly developed method, termed moving empirical orthogonal function analysis (MEOF), is applied to the study of midlatitude baroclinic waves over the wintertime North Pacific from 1979 to 2009. It is shown that when the daily, high-pass filtered (2-10 days) meridional wind at 250 hPa is chosen as the variable of the MEOF analysis, typical features of baroclinic waves/storm tracks over the wintertime North Pacific can be well described by this method. It is found that the first two leading modes of the MEOF analysis, MEOF1 and MEOF2, assume quite different patterns. MEOF1 takes the form of a single wave train running in the east-west direction along 40°N, while MEOF2 is a double wave train pattern running in the east-west direction along 50°N and 30°N, respectively. The shift composites of various anomalous fields based on MEOF1 and MEOF2 assume typical baroclinic wave features. MEOF1 represents a primary storm track pulsing with an intrinsic time scale of two days. It shows significant "midwinter suppression" and apparent interannual variability. It is stronger after the mid-1990s than before the mid-1990s. MEOF2 represents a double-branch storm track, also with an intrinsic time scale of approximately two days, running along 50°N and 30°N, respectively. It shows no apparent seasonal variation, but its interannual and decadal variation is quite clear. It oscillates with larger amplitude and longer periods after the mid-1990s than before the mid-1990s, and is heavily modulated by El Niño-Southern Oscillation (ENSO).
In this study, a newly developed method, termed moving empirical orthogonal function analysis (MEOF), is applied to the study of midlatitude baroclinic waves over the wintertime North Pacific from 1979 to 2009. It is shown that when the daily, high-pass filtered (2-10 days) meridional wind at 250 hPa is chosen as the variable of the MEOF analysis, typical features of baroclinic waves/storm tracks over the wintertime North Pacific can be well described by this method. It is found that the first two leading modes of the MEOF analysis, MEOF1 and MEOF2, assume quite different patterns. MEOF1 takes the form of a single wave train running in the east-west direction along 40°N, while MEOF2 is a double wave train pattern running in the east-west direction along 50°N and 30°N, respectively. The shift composites of various anomalous fields based on MEOF1 and MEOF2 assume typical baroclinic wave features. MEOF1 represents a primary storm track pulsing with an intrinsic time scale of two days. It shows significant "midwinter suppression" and apparent interannual variability. It is stronger after the mid-1990s than before the mid-1990s. MEOF2 represents a double-branch storm track, also with an intrinsic time scale of approximately two days, running along 50°N and 30°N, respectively. It shows no apparent seasonal variation, but its interannual and decadal variation is quite clear. It oscillates with larger amplitude and longer periods after the mid-1990s than before the mid-1990s, and is heavily modulated by El Niño-Southern Oscillation (ENSO).
2015, 32(9): 1255-1265.
doi: 10.1007/s00376-015-4280-y
Abstract:
This study explores the characteristics of high temperature anomalies over eastern China and associated influencing factors using observations and model outputs. Results show that more long-duration (over 8 days) high temperature events occur over the middle and lower reaches of the Yangtze River Valley (YRV) than over the surrounding regions, and control most of the interannual variation of summer mean temperature in situ. The synergistic effect of summer precipitation over the South China Sea (SCS) region (18°-27°N, 115°-124°E) and the northwestern India and Arabian Sea (IAS) region (18°-27°N, 60°-80°E) contributes more significantly to the variation of summer YRV temperature, relative to the respective SCS or IAS precipitation anomaly. More precipitation (enhanced condensational heating) over the SCS region strengthens the western Pacific subtropical high (WPSH) and simultaneously weakens the westerly trough over the east coast of Asia, and accordingly results in associated high temperature anomalies over the YRV region through stimulating an East Asia-Pacific (EAP) pattern. More precipitation over the IAS region further adjusts the variations of the WPSH and westerly trough, and eventually reinforces high temperature anomalies over the YRV region. Furthermore, the condensational heating related to more IAS precipitation can adjust upper-tropospheric easterly anomalies over the YRV region by exciting a circumglobal teleconnection, inducing cold horizontal temperature advection and related anomalous descent, which is also conducive to the YRV high temperature anomalies. The reproduction of the above association in the model results indicates that the above results can be explained both statistically and dynamically.
This study explores the characteristics of high temperature anomalies over eastern China and associated influencing factors using observations and model outputs. Results show that more long-duration (over 8 days) high temperature events occur over the middle and lower reaches of the Yangtze River Valley (YRV) than over the surrounding regions, and control most of the interannual variation of summer mean temperature in situ. The synergistic effect of summer precipitation over the South China Sea (SCS) region (18°-27°N, 115°-124°E) and the northwestern India and Arabian Sea (IAS) region (18°-27°N, 60°-80°E) contributes more significantly to the variation of summer YRV temperature, relative to the respective SCS or IAS precipitation anomaly. More precipitation (enhanced condensational heating) over the SCS region strengthens the western Pacific subtropical high (WPSH) and simultaneously weakens the westerly trough over the east coast of Asia, and accordingly results in associated high temperature anomalies over the YRV region through stimulating an East Asia-Pacific (EAP) pattern. More precipitation over the IAS region further adjusts the variations of the WPSH and westerly trough, and eventually reinforces high temperature anomalies over the YRV region. Furthermore, the condensational heating related to more IAS precipitation can adjust upper-tropospheric easterly anomalies over the YRV region by exciting a circumglobal teleconnection, inducing cold horizontal temperature advection and related anomalous descent, which is also conducive to the YRV high temperature anomalies. The reproduction of the above association in the model results indicates that the above results can be explained both statistically and dynamically.
2015, 32(9): 1266-1276.
doi: 10.1007/s00376-015-4235-3
Abstract:
Assuming spheroidal and spherical particle shapes for mineral dust aerosols, the effect of particle shape on dust aerosol optical depth retrievals, and subsequently on instantaneous shortwave direct radiative forcing (SWDRF) at the top of the atmosphere (TOA), is assessed based on Moderate Resolution Imaging Spectroradiometer (MODIS) data for a case study. Specifically, a simplified aerosol retrieval algorithm based on the principle of the Deep Blue aerosol retrieval method is employed to retrieve dust aerosol optical depths, and the Fu-Liou radiative transfer model is used to derive the instantaneous SWDRF of dust at the TOA for cloud-free conditions. Without considering the effect of particle shape on dust aerosol optical depth retrievals, the effect of particle shape on the scattering properties of dust aerosols (e.g., extinction efficiency, single scattering albedo and asymmetry factor) is negligible, which can lead to a relative difference of at most 5% for the SWDRF at the TOA. However, the effect of particle shape on the SWDRF cannot be neglected provided that the effect of particle shape on dust aerosol optical depth retrievals is also taken into account for SWDRF calculations. The corresponding results in an instantaneous case study show that the relative differences of the SWDRF at the TOA between spheroids and spheres depend critically on the scattering angles at which dust aerosol optical depths are retrieved, and can be up to 40% for low dust-loading conditions.
Assuming spheroidal and spherical particle shapes for mineral dust aerosols, the effect of particle shape on dust aerosol optical depth retrievals, and subsequently on instantaneous shortwave direct radiative forcing (SWDRF) at the top of the atmosphere (TOA), is assessed based on Moderate Resolution Imaging Spectroradiometer (MODIS) data for a case study. Specifically, a simplified aerosol retrieval algorithm based on the principle of the Deep Blue aerosol retrieval method is employed to retrieve dust aerosol optical depths, and the Fu-Liou radiative transfer model is used to derive the instantaneous SWDRF of dust at the TOA for cloud-free conditions. Without considering the effect of particle shape on dust aerosol optical depth retrievals, the effect of particle shape on the scattering properties of dust aerosols (e.g., extinction efficiency, single scattering albedo and asymmetry factor) is negligible, which can lead to a relative difference of at most 5% for the SWDRF at the TOA. However, the effect of particle shape on the SWDRF cannot be neglected provided that the effect of particle shape on dust aerosol optical depth retrievals is also taken into account for SWDRF calculations. The corresponding results in an instantaneous case study show that the relative differences of the SWDRF at the TOA between spheroids and spheres depend critically on the scattering angles at which dust aerosol optical depths are retrieved, and can be up to 40% for low dust-loading conditions.
2015, 32(9): 1277-1290.
doi: 10.1007/s00376-015-4234-4
Abstract:
Fall velocity-diameter relationships for four different snowflake types (dendrite, plate, needle, and graupel) were investigated in northeastern South Korea, and a new algorithm for classifying hydrometeors is proposed for distrometric measurements based on the new relationships. Falling ice crystals (approximately 40 000 particles) were measured with a two-dimensional video disdrometer (2DVD) during a winter experiment from 15 January to 9 April 2010. The fall velocity-diameter relationships were derived for the four types of snowflakes based on manual classification by experts using snow photos and 2DVD measurements: the coefficients (exponents) for different snowflake types were 0.82 (0.24) for dendrite, 0.74 (0.35) for plate, 1.03 (0.71) for needle, and 1.30 (0.94) for graupel, respectively. These new relationships established in the present study (PS) were compared with those from two previous studies. Hydrometeor types were classified with the derived fall velocity-diameter relationships, and the classification algorithm was evaluated using 3× 3 contingency tables for one rain-snow transition event and three snowfall events. The algorithm showed good performance for the transition event: the critical success indices (CSIs) were 0.89, 0.61 and 0.71 for snow, wet-snow and rain, respectively. For snow events, the algorithm performance for dendrite and plate (CSIs = 1.0 and 1.0, respectively) was better than for needle and graupel (CSIs = 0.67 and 0.50, respectively).
Fall velocity-diameter relationships for four different snowflake types (dendrite, plate, needle, and graupel) were investigated in northeastern South Korea, and a new algorithm for classifying hydrometeors is proposed for distrometric measurements based on the new relationships. Falling ice crystals (approximately 40 000 particles) were measured with a two-dimensional video disdrometer (2DVD) during a winter experiment from 15 January to 9 April 2010. The fall velocity-diameter relationships were derived for the four types of snowflakes based on manual classification by experts using snow photos and 2DVD measurements: the coefficients (exponents) for different snowflake types were 0.82 (0.24) for dendrite, 0.74 (0.35) for plate, 1.03 (0.71) for needle, and 1.30 (0.94) for graupel, respectively. These new relationships established in the present study (PS) were compared with those from two previous studies. Hydrometeor types were classified with the derived fall velocity-diameter relationships, and the classification algorithm was evaluated using 3× 3 contingency tables for one rain-snow transition event and three snowfall events. The algorithm showed good performance for the transition event: the critical success indices (CSIs) were 0.89, 0.61 and 0.71 for snow, wet-snow and rain, respectively. For snow events, the algorithm performance for dendrite and plate (CSIs = 1.0 and 1.0, respectively) was better than for needle and graupel (CSIs = 0.67 and 0.50, respectively).
2015, 32(9): 1291-1302.
doi: 10.1007/s00376-015-4258-9
Abstract:
A deep understanding of turbulence structure is important for investigating the characteristics of the atmospheric boundary layer, especially over heterogeneous terrain. In the present study, turbulence intensity and turbulent kinetic energy (TKE) parameters are analyzed for different conditions with respect to stability, wind direction and wind speed over a valley region of the Loess Plateau of China during December 2003 and January 2004. The purpose of the study is to examine whether the observed turbulence intensity and TKE parameters satisfy Monin-Obukhov similarity theory (MOST), and analyze the wind shear effect on, and thermal buoyancy function of, the TKE, despite the terrain heterogeneity. The results demonstrate that the normalized intensity of turbulence follows MOST for all stability in the horizontal and vertical directions, as well as the normalized TKE in the horizontal direction. The shear effect of the wind speed in the Loess Plateau region is strong in winter and could enhance turbulence for all stability conditions. During daytime, the buoyancy and shear effect together constitute the generation of TKE under unstable conditions. At night, the contribution of buoyancy to TKE is relatively small, and mechanical shearing is the main production form of turbulence.
A deep understanding of turbulence structure is important for investigating the characteristics of the atmospheric boundary layer, especially over heterogeneous terrain. In the present study, turbulence intensity and turbulent kinetic energy (TKE) parameters are analyzed for different conditions with respect to stability, wind direction and wind speed over a valley region of the Loess Plateau of China during December 2003 and January 2004. The purpose of the study is to examine whether the observed turbulence intensity and TKE parameters satisfy Monin-Obukhov similarity theory (MOST), and analyze the wind shear effect on, and thermal buoyancy function of, the TKE, despite the terrain heterogeneity. The results demonstrate that the normalized intensity of turbulence follows MOST for all stability in the horizontal and vertical directions, as well as the normalized TKE in the horizontal direction. The shear effect of the wind speed in the Loess Plateau region is strong in winter and could enhance turbulence for all stability conditions. During daytime, the buoyancy and shear effect together constitute the generation of TKE under unstable conditions. At night, the contribution of buoyancy to TKE is relatively small, and mechanical shearing is the main production form of turbulence.
2015, 32(9): 1308-1318.
doi: 10.1007/s00376-015-4250-4
Abstract:
The present study evaluates the precipitation variability over the South China Sea (SCS) and its relationship to tropical Indo-Pacific SST anomalies during spring-to-summer transition (April-May-June, AMJ) simulated by 23 Intergovernmental Panel on Climate Change Coupled Model Intercomparison Project Phase 5 coupled models. Most of the models have the capacity to capture the AMJ precipitation variability in the SCS. The precipitation and SST anomaly (SSTA) distribution in the SCS, tropical Pacific Ocean (TPO), and tropical Indian Ocean (TIO) domains is evaluated based on the pattern correlation coefficients between model simulations and observations. The analysis leads to several points of note. First, the performance of the SCS precipitation anomaly pattern in AMJ is model dependent. Second, the SSTA pattern in the TPO and TIO is important for capturing the AMJ SCS precipitation variability. Third, a realistic simulation of the western equatorial Pacific (WEP) and local SST impacts is necessary for reproducing the AMJ SCS precipitation variability in some models. Fourth, the overly strong WEP SST impacts may disrupt the relationship between the SCS precipitation and the TPO-TIO SST. Further work remains to be conducted to unravel the specific reasons for the discrepancies between models and observations in various aspects.
The present study evaluates the precipitation variability over the South China Sea (SCS) and its relationship to tropical Indo-Pacific SST anomalies during spring-to-summer transition (April-May-June, AMJ) simulated by 23 Intergovernmental Panel on Climate Change Coupled Model Intercomparison Project Phase 5 coupled models. Most of the models have the capacity to capture the AMJ precipitation variability in the SCS. The precipitation and SST anomaly (SSTA) distribution in the SCS, tropical Pacific Ocean (TPO), and tropical Indian Ocean (TIO) domains is evaluated based on the pattern correlation coefficients between model simulations and observations. The analysis leads to several points of note. First, the performance of the SCS precipitation anomaly pattern in AMJ is model dependent. Second, the SSTA pattern in the TPO and TIO is important for capturing the AMJ SCS precipitation variability. Third, a realistic simulation of the western equatorial Pacific (WEP) and local SST impacts is necessary for reproducing the AMJ SCS precipitation variability in some models. Fourth, the overly strong WEP SST impacts may disrupt the relationship between the SCS precipitation and the TPO-TIO SST. Further work remains to be conducted to unravel the specific reasons for the discrepancies between models and observations in various aspects.
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