2012 Vol. 18, No. 3
column
2012, 18(3): 275-283.
Abstract:
In order to provide an operational reference for tropical cyclone precipitation forecast, this study investigates the spatial distributions of precipitation associated with landfalling tropical cyclones (TCs) affecting China using Geostationary Meteorological Satellite 5 (GMS5)-TBB dataset. All named TCs formed over the western North Pacific that made direct landfall over China during the period 2001-2009 are included in this study. Based on the GMS5-TBB data, this paper reveals that in general there are four types of distribution of precipitation related to landfalling TCs affecting China. (a) the South-West Type in which there is a precipitation maximum to the southwestern quadrant of TC; (b) the Symmetrical South Type in which the rainfall is more pronounced to the south side of TC in the inner core while there is a symmetrical rainfall distribution in the outer band region; (c) the South Type, in which the rainfall maxima is more pronounced to the south of TC; and (d) the North Type, in which the rainfall maxima is more pronounced to the north of TC. Analyses of the relationship between precipitation distributions and intensity of landfalling TCs show that for intensifying TCs, both the maximum and the coverage area of the precipitation in TCs increase with the increase of TC intensity over northern Jiangsu province and southern Taiwan Strait, while decreasing over Beibu Gulf and the sea area of Changjiang River estuary. For all TCs, the center of the torrential rain in TC shifts toward the TC center as the intensity of TC increases. This finding is consistent with many previous studies. The possible influences of storm motion and vertical wind shear on the observed precipitation asymmetries are also examined. Results show that the environmental vertical wind shear is an important factor contributing to the large downshear rainfall asymmetry, especially when a TC makes landfall on the south and east China coasts. These results are also consistent with previous observational and numerical studies.
In order to provide an operational reference for tropical cyclone precipitation forecast, this study investigates the spatial distributions of precipitation associated with landfalling tropical cyclones (TCs) affecting China using Geostationary Meteorological Satellite 5 (GMS5)-TBB dataset. All named TCs formed over the western North Pacific that made direct landfall over China during the period 2001-2009 are included in this study. Based on the GMS5-TBB data, this paper reveals that in general there are four types of distribution of precipitation related to landfalling TCs affecting China. (a) the South-West Type in which there is a precipitation maximum to the southwestern quadrant of TC; (b) the Symmetrical South Type in which the rainfall is more pronounced to the south side of TC in the inner core while there is a symmetrical rainfall distribution in the outer band region; (c) the South Type, in which the rainfall maxima is more pronounced to the south of TC; and (d) the North Type, in which the rainfall maxima is more pronounced to the north of TC. Analyses of the relationship between precipitation distributions and intensity of landfalling TCs show that for intensifying TCs, both the maximum and the coverage area of the precipitation in TCs increase with the increase of TC intensity over northern Jiangsu province and southern Taiwan Strait, while decreasing over Beibu Gulf and the sea area of Changjiang River estuary. For all TCs, the center of the torrential rain in TC shifts toward the TC center as the intensity of TC increases. This finding is consistent with many previous studies. The possible influences of storm motion and vertical wind shear on the observed precipitation asymmetries are also examined. Results show that the environmental vertical wind shear is an important factor contributing to the large downshear rainfall asymmetry, especially when a TC makes landfall on the south and east China coasts. These results are also consistent with previous observational and numerical studies.
2012, 18(3): 284-296.
Abstract:
The results from a hybrid approach that combines a mesoscale meteorological model with a diagnostic model to produce high-resolution wind fields in complex coastal topography are evaluated. The diagnostic wind model (California Meteorological Model, CALMET) with 100-m horizontal spacing was driven with outputs from the Weather Research and Forecasting (WRF) model to obtain near-surface winds for the 1-year period from 12 September 2003 to 11 September 2004. Results were compared with wind observations at four sites. Traditional statistical scores, including correlation coefficients, standard deviations (SDs) and mean absolute errors (MAEs), indicate that the wind estimates from the WRF/CALMET modeling system are produced reasonably well. The correlation coefficients are relatively large, ranging from 0.5 to 0.7 for the zonal wind component and from 0.75 to 0.85 for the meridional wind component. MAEs for wind speed range from 1.5 to 2.0 m s-1 at 10 meters above ground level (AGL) and from 2.0 to 2.5 m s-1 at 60 m AGL. MAEs for wind direction range from 30 to 40 degrees at both levels. A spectral decomposition of the time series of wind speed shows positive impacts of CALMET in improving the mesoscale winds. Moreover, combining the CALMET model with WRF significantly improves the spatial variability of the simulated wind fields. It can be concluded that the WRF/CALMET modeling system is capable of providing a detailed near-surface wind field, but the physics in the diagnostic CALMET model needs to be further improved.
The results from a hybrid approach that combines a mesoscale meteorological model with a diagnostic model to produce high-resolution wind fields in complex coastal topography are evaluated. The diagnostic wind model (California Meteorological Model, CALMET) with 100-m horizontal spacing was driven with outputs from the Weather Research and Forecasting (WRF) model to obtain near-surface winds for the 1-year period from 12 September 2003 to 11 September 2004. Results were compared with wind observations at four sites. Traditional statistical scores, including correlation coefficients, standard deviations (SDs) and mean absolute errors (MAEs), indicate that the wind estimates from the WRF/CALMET modeling system are produced reasonably well. The correlation coefficients are relatively large, ranging from 0.5 to 0.7 for the zonal wind component and from 0.75 to 0.85 for the meridional wind component. MAEs for wind speed range from 1.5 to 2.0 m s-1 at 10 meters above ground level (AGL) and from 2.0 to 2.5 m s-1 at 60 m AGL. MAEs for wind direction range from 30 to 40 degrees at both levels. A spectral decomposition of the time series of wind speed shows positive impacts of CALMET in improving the mesoscale winds. Moreover, combining the CALMET model with WRF significantly improves the spatial variability of the simulated wind fields. It can be concluded that the WRF/CALMET modeling system is capable of providing a detailed near-surface wind field, but the physics in the diagnostic CALMET model needs to be further improved.
2012, 18(3): 297-304.
Abstract:
The International Centre for Theoretical Physics (ICTP, Italy) Regional Climate Model version 3.0 (RegCM3) is used to simulate spatio-temporal distribution characteristics and radiative forcing (RF) of organic carbon (OC) aerosols in and around China. The preliminary simulation results show that OC aerosols are mostly concentrated in the area to the south of Yellow River and east of Tibetan Plateau. There is a decreasing trend of column burden of OC aerosols from south to north in China. The maximum value of column burden of OC aerosols is above 3 mg/m2 and located in the central and southern China, southeastern Tibet, and southwestern China’s Yunnan, Guizhou, Sichuan provinces. The simulation on the seasonal variation shows that the maximum value of column burden of OC aerosols appears in winter and the secondary value is in spring and the minimum in summer. The RF of OC aerosols which varies seasonally is negative at the top of the atmosphere (TOA) and surface. The spatio-temporal characteristics of the RF of OC aerosols are basically consistent with that of IPCC, implying the high accuracy of the parameterization scheme for OC aerosols in RegCM3.
The International Centre for Theoretical Physics (ICTP, Italy) Regional Climate Model version 3.0 (RegCM3) is used to simulate spatio-temporal distribution characteristics and radiative forcing (RF) of organic carbon (OC) aerosols in and around China. The preliminary simulation results show that OC aerosols are mostly concentrated in the area to the south of Yellow River and east of Tibetan Plateau. There is a decreasing trend of column burden of OC aerosols from south to north in China. The maximum value of column burden of OC aerosols is above 3 mg/m2 and located in the central and southern China, southeastern Tibet, and southwestern China’s Yunnan, Guizhou, Sichuan provinces. The simulation on the seasonal variation shows that the maximum value of column burden of OC aerosols appears in winter and the secondary value is in spring and the minimum in summer. The RF of OC aerosols which varies seasonally is negative at the top of the atmosphere (TOA) and surface. The spatio-temporal characteristics of the RF of OC aerosols are basically consistent with that of IPCC, implying the high accuracy of the parameterization scheme for OC aerosols in RegCM3.
2012, 18(3): 314-321.
Abstract:
Based on the Global Regional Assimilation and Prediction System-Tropical Cyclone Model (GRAPES-TCM), an ensemble forecast experiment was performed, in which Typhoon Wipha during the period immediately prior to landfall was selected for the study and the breeding of growing mode (BGM) method was used to perturb the initial conditions of the vortex field and the environment field. The results of the experiment indicate that each member had a different initial status in BGM processing and they show a reasonable spread among members along with the forecast phase. Changes in the large-scale field, thermodynamic structure, and spread among members took place when Wipha made landfall. The steering effect of the large-scale field and the interaction between the thermodynamics and the dynamics resulted in different tracks of the members. Meanwhile, the forecast uncertainty increased. In summary, the ensemble mean did not perform as well as the control forecast, but the cluster mean provided some useful information, and performed better than the control in some instances. The position error was 34 km for 24 h forecast, 153 km for 48 h forecast, and 191 km for 66 h forecast. The strike probability chart qualitatively described the forecast uncertainty.
Based on the Global Regional Assimilation and Prediction System-Tropical Cyclone Model (GRAPES-TCM), an ensemble forecast experiment was performed, in which Typhoon Wipha during the period immediately prior to landfall was selected for the study and the breeding of growing mode (BGM) method was used to perturb the initial conditions of the vortex field and the environment field. The results of the experiment indicate that each member had a different initial status in BGM processing and they show a reasonable spread among members along with the forecast phase. Changes in the large-scale field, thermodynamic structure, and spread among members took place when Wipha made landfall. The steering effect of the large-scale field and the interaction between the thermodynamics and the dynamics resulted in different tracks of the members. Meanwhile, the forecast uncertainty increased. In summary, the ensemble mean did not perform as well as the control forecast, but the cluster mean provided some useful information, and performed better than the control in some instances. The position error was 34 km for 24 h forecast, 153 km for 48 h forecast, and 191 km for 66 h forecast. The strike probability chart qualitatively described the forecast uncertainty.
2012, 18(3): 322-331.
Abstract:
A quantitative scheme is put forward in our work of forecasting the storm rainfall of typhoons for specific sites. Using the initial parameters, weather situations and physical quantities as well as numerical weather prediction products, the scheme constructs multivariate, objective and similarity criteria for environmental factors for the time between the current and forthcoming moment within the domain of forecast. Through defining a non-linear similarity index, this work presents a comprehensive assessment of the similarity between historical samples of typhoons and those being forecast in terms of continuous dynamic states under the multivariate criteria in order to identify similar samples. The historical rainfall records of the similar samples are used to run weighted summarization of the similarity index to determine site-specific and quantitative forecasts of future typhoon rainfall. Samples resembling the typhoon being forecast are selected by defining a non-linear similarity index composed of multiple criteria. Trial tests have demonstrated that this scheme has positive prediction skill.
A quantitative scheme is put forward in our work of forecasting the storm rainfall of typhoons for specific sites. Using the initial parameters, weather situations and physical quantities as well as numerical weather prediction products, the scheme constructs multivariate, objective and similarity criteria for environmental factors for the time between the current and forthcoming moment within the domain of forecast. Through defining a non-linear similarity index, this work presents a comprehensive assessment of the similarity between historical samples of typhoons and those being forecast in terms of continuous dynamic states under the multivariate criteria in order to identify similar samples. The historical rainfall records of the similar samples are used to run weighted summarization of the similarity index to determine site-specific and quantitative forecasts of future typhoon rainfall. Samples resembling the typhoon being forecast are selected by defining a non-linear similarity index composed of multiple criteria. Trial tests have demonstrated that this scheme has positive prediction skill.
2012, 18(3): 332-340.
Abstract:
Based on the NCEP/NCAR reanalysis data and the observed precipitation data in the south of China from 1958 to 2000, the impact of 30 to 60 day oscillation of atmospheric heat sources on the drought and flood events in June in the south of China is discussed. During the flood (drought) events, there exists an anomalous low-frequency anticyclone (cyclone) at the low level of the troposphere over the South China Sea and the northwestern Pacific, accompanied with anomalous low-frequency heat sinks (heat sources), while there exists an anomalous low-frequency cyclone (anticyclone) with anomalous heat sources (sinks) over the area from the south of China to the south of Japan. On average, the phase evolution of the low-frequency in drought events is 7 to 11 days ahead of that in flood events in May to June in the south of China. In flood events, low-frequency heat sources and cyclones are propagated northward from the southern South China Sea, northwestward from the warm pool of the western Pacific and westward from the northwestern Pacific around 140°E, which have very important impact on the abundant rainfall in June in the south of China. However, in drought events, the northward propagations of the low-frequency heat sources and cyclones from the South China Sea and its vicinity are rather late compared with those in flood events, and there is no obvious westward propagation of the heat sources from the northwestern Pacific. The timing of the low-frequency heat source propagation has remarkable impact on the June rainfall in the south of China.
Based on the NCEP/NCAR reanalysis data and the observed precipitation data in the south of China from 1958 to 2000, the impact of 30 to 60 day oscillation of atmospheric heat sources on the drought and flood events in June in the south of China is discussed. During the flood (drought) events, there exists an anomalous low-frequency anticyclone (cyclone) at the low level of the troposphere over the South China Sea and the northwestern Pacific, accompanied with anomalous low-frequency heat sinks (heat sources), while there exists an anomalous low-frequency cyclone (anticyclone) with anomalous heat sources (sinks) over the area from the south of China to the south of Japan. On average, the phase evolution of the low-frequency in drought events is 7 to 11 days ahead of that in flood events in May to June in the south of China. In flood events, low-frequency heat sources and cyclones are propagated northward from the southern South China Sea, northwestward from the warm pool of the western Pacific and westward from the northwestern Pacific around 140°E, which have very important impact on the abundant rainfall in June in the south of China. However, in drought events, the northward propagations of the low-frequency heat sources and cyclones from the South China Sea and its vicinity are rather late compared with those in flood events, and there is no obvious westward propagation of the heat sources from the northwestern Pacific. The timing of the low-frequency heat source propagation has remarkable impact on the June rainfall in the south of China.
2012, 18(3): 341-348.
Abstract:
An analysis of high-resolution precipitation data for 1978-2006 indicates that the precipitation over southern China in June experienced a low-value period in 1980-1989 and a high-value period in 1992-2001. It also reveals that exceptional heavy (light) precipitation occurred in June 2005 (2004) since 1951. For these variations on both interdecadal and interannual timescales, fairly uniform anomalies of precipitation appeared over Vietnam, southern China, and southeastern China. Corresponding to positive (negative) precipitation anomalies, anomalous southeasterly (northwesterly) flow at 850 hPa reached Vietnam and anomalous southwesterly (northeasterly) flow expanded to the coastal regions of southern and southeastern China. Precedent to the positive (negative) precipitation anomalies during 1992-2001 (1980-1989), positive (negative) anomalies of sea surface temperature appeared over the extratropical northwestern Pacific in the winter and spring seasons, associated with a strong (weak) extension of the warm Kuroshio Current that affects the coastal region of eastern China. The above-normal precipitation in June 2005 was associated with the pseudo-ENSO event in the previous winter, and the below-normal precipitation in June 2004 was associated with negative anomalies of sea surface temperature over the equatorial central Pacific and positive anomalies over the equatorial western and eastern Pacific.
An analysis of high-resolution precipitation data for 1978-2006 indicates that the precipitation over southern China in June experienced a low-value period in 1980-1989 and a high-value period in 1992-2001. It also reveals that exceptional heavy (light) precipitation occurred in June 2005 (2004) since 1951. For these variations on both interdecadal and interannual timescales, fairly uniform anomalies of precipitation appeared over Vietnam, southern China, and southeastern China. Corresponding to positive (negative) precipitation anomalies, anomalous southeasterly (northwesterly) flow at 850 hPa reached Vietnam and anomalous southwesterly (northeasterly) flow expanded to the coastal regions of southern and southeastern China. Precedent to the positive (negative) precipitation anomalies during 1992-2001 (1980-1989), positive (negative) anomalies of sea surface temperature appeared over the extratropical northwestern Pacific in the winter and spring seasons, associated with a strong (weak) extension of the warm Kuroshio Current that affects the coastal region of eastern China. The above-normal precipitation in June 2005 was associated with the pseudo-ENSO event in the previous winter, and the below-normal precipitation in June 2004 was associated with negative anomalies of sea surface temperature over the equatorial central Pacific and positive anomalies over the equatorial western and eastern Pacific.
2012, 18(3): 349-359.
Abstract:
Guangdong suffered from the most serious precipitation of its corresponding time during the dragon-boat race of 2008 since 1951. The relationship between the strong dragon-boat precipitation in 2008 and atmospheric low-frequency oscillation was analyzed with the methods of wavelet analysis, correlation and Lanczos filter. Results showed that the daily rainfall exhibits a significant 7 to 12-day quasi-periodic oscillation (namely quasi-10-day oscillation) during the precipitation, the daily 500 hPa height over Guangdong exhibits a significant 8 to 13-day quasi-periodic oscillation, and the daily 850 hPa zonal wind averaged over the north of the South China Sea presents a significant quasi-12-day periodic oscillation. The Guangdong rainfall during the annually first rainy season is most closely correlated with monsoon over the north of South China Sea, and less closely with an upper-level trough at 500 hPa affecting Guangdong. Strong monsoon surges induced two heavy rainfall processes in 2008. The monsoon surges joined with a westward-propagating quasi-10-day oscillation that originated from the central Pacific and was enhanced in a strong convective region east of the Philippines and a northward-propagating monsoon that originated from the southern South China Sea was enhanced. With composite analysis of typical phases, the common evolution characteristics of atmospheric circulation of the two heavy rainfall processes were analyzed for different phases. These features can be used as reference for medium prediction of heavy rainfall processes in Guangdong.
Guangdong suffered from the most serious precipitation of its corresponding time during the dragon-boat race of 2008 since 1951. The relationship between the strong dragon-boat precipitation in 2008 and atmospheric low-frequency oscillation was analyzed with the methods of wavelet analysis, correlation and Lanczos filter. Results showed that the daily rainfall exhibits a significant 7 to 12-day quasi-periodic oscillation (namely quasi-10-day oscillation) during the precipitation, the daily 500 hPa height over Guangdong exhibits a significant 8 to 13-day quasi-periodic oscillation, and the daily 850 hPa zonal wind averaged over the north of the South China Sea presents a significant quasi-12-day periodic oscillation. The Guangdong rainfall during the annually first rainy season is most closely correlated with monsoon over the north of South China Sea, and less closely with an upper-level trough at 500 hPa affecting Guangdong. Strong monsoon surges induced two heavy rainfall processes in 2008. The monsoon surges joined with a westward-propagating quasi-10-day oscillation that originated from the central Pacific and was enhanced in a strong convective region east of the Philippines and a northward-propagating monsoon that originated from the southern South China Sea was enhanced. With composite analysis of typical phases, the common evolution characteristics of atmospheric circulation of the two heavy rainfall processes were analyzed for different phases. These features can be used as reference for medium prediction of heavy rainfall processes in Guangdong.
2012, 18(3): 360-368.
Abstract:
This paper diagnoses and analyses the developmental mechanism of a process of extratropical transition of a tropical cyclone which occurred over West Pacific Ocean based on a diagnosis method of potential vorticity inversion of frontogenesis. The study diagnoses quantitatively the role and effect of dynamic influence of westerly cold troughs, middle-latitude baroclinic frontal zones, cyclone cycles and unbalanced wind fields during the different stages of the extratropical transition of a tropical cyclone, and also discusses the interaction between them and the developmental mechanism. The results show that there are different developmental mechanisms during each stage of the extratropical transition and the processes are also unbalanced.
This paper diagnoses and analyses the developmental mechanism of a process of extratropical transition of a tropical cyclone which occurred over West Pacific Ocean based on a diagnosis method of potential vorticity inversion of frontogenesis. The study diagnoses quantitatively the role and effect of dynamic influence of westerly cold troughs, middle-latitude baroclinic frontal zones, cyclone cycles and unbalanced wind fields during the different stages of the extratropical transition of a tropical cyclone, and also discusses the interaction between them and the developmental mechanism. The results show that there are different developmental mechanisms during each stage of the extratropical transition and the processes are also unbalanced.
2012, 18(3): 369-376.
Abstract:
The sensitivity of precipitation to sea surface temperature (SST) and its diurnal variation is investigated through a rainfall partitioning analysis of two-dimensional cloud-resolving model experiments based on surface rainfall budget. For all experiments, the model is set up using zero vertical velocity and a constant zonal wind and is integrated over 40 days to reach quasi-equilibrium states. The 10-day equilibrium grid-scale simulation data and a time-invariant SST of 29℃ are used in the control experiment. In the sensitivity experiments, time-invariant SSTs are 27℃ and 31℃ with an average value of 29℃ when the minimum and maximum values of diurnal SST differences are 1℃ and 2℃, respectively. The results show that the largest contribution to total rainfall is from the rainfall with water vapor convergence and local atmospheric drying and hydrometeor gain/divergence (~30%) in all experiments. When SST increases from 27℃ to 29℃, the contribution from water vapor convergence decreases. The increase of SST reduces the contribution of the rainfall with water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric drying and hydrometeor gain/divergence and the rainfall with local atmospheric moistening and hydrometeor loss/convergence. The inclusion of diurnal variation of SST with the diurnal difference of 1℃ decreases the rainfall contribution from water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric moistening and hydrometeor loss/convergence. The contribution of the rainfall from water vapor convergence is barely changed as the diurnal difference of SST increases from 1℃ to 2℃.
The sensitivity of precipitation to sea surface temperature (SST) and its diurnal variation is investigated through a rainfall partitioning analysis of two-dimensional cloud-resolving model experiments based on surface rainfall budget. For all experiments, the model is set up using zero vertical velocity and a constant zonal wind and is integrated over 40 days to reach quasi-equilibrium states. The 10-day equilibrium grid-scale simulation data and a time-invariant SST of 29℃ are used in the control experiment. In the sensitivity experiments, time-invariant SSTs are 27℃ and 31℃ with an average value of 29℃ when the minimum and maximum values of diurnal SST differences are 1℃ and 2℃, respectively. The results show that the largest contribution to total rainfall is from the rainfall with water vapor convergence and local atmospheric drying and hydrometeor gain/divergence (~30%) in all experiments. When SST increases from 27℃ to 29℃, the contribution from water vapor convergence decreases. The increase of SST reduces the contribution of the rainfall with water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric drying and hydrometeor gain/divergence and the rainfall with local atmospheric moistening and hydrometeor loss/convergence. The inclusion of diurnal variation of SST with the diurnal difference of 1℃ decreases the rainfall contribution from water vapor convergence primarily through the decreased contribution of the rainfall with local atmospheric moistening and hydrometeor loss/convergence. The contribution of the rainfall from water vapor convergence is barely changed as the diurnal difference of SST increases from 1℃ to 2℃.
2012, 18(3): 377-386.
Abstract:
The spatial variation of sea surface temperature anomalies (SSTA) in the North Pacific Ocean during winter is investigated using the EOF decomposition method. The first two main modes of SSTA are associated with Pacific Decadal Oscillation (PDO) mode and North Pacific Gyre Oscillation (NPGO) mode, respectively. Moreover, the first mode (PDO) is switched to the second mode (NPGO), a dominant mode after mid-1980. The mechanism of the modes’ transition is analyzed. As the two oceanic modes are forced by the Aleutian Low (AL) and North Pacific Oscillation (NPO) modes, the AR-1 model is further used to examine the possible effect and mechanism of AL and NPO in generating the PDO and NPGO. The results show that compared to the NPO, the AL plays a more important role in generating the NPGO mode since the 1970s. Likewise, both the AL and NPO affect the PDO mode since the 1980s.
The spatial variation of sea surface temperature anomalies (SSTA) in the North Pacific Ocean during winter is investigated using the EOF decomposition method. The first two main modes of SSTA are associated with Pacific Decadal Oscillation (PDO) mode and North Pacific Gyre Oscillation (NPGO) mode, respectively. Moreover, the first mode (PDO) is switched to the second mode (NPGO), a dominant mode after mid-1980. The mechanism of the modes’ transition is analyzed. As the two oceanic modes are forced by the Aleutian Low (AL) and North Pacific Oscillation (NPO) modes, the AR-1 model is further used to examine the possible effect and mechanism of AL and NPO in generating the PDO and NPGO. The results show that compared to the NPO, the AL plays a more important role in generating the NPGO mode since the 1970s. Likewise, both the AL and NPO affect the PDO mode since the 1980s.
2012, 18(3): 387-392.
Abstract:
Based on data from satellite and surface observations, the horizontal and vertical distributions of clouds over eastern China and the East China Sea are examined. Three maximum centers of cloud cover are clearly visible in the horizontal distribution of total cloud cover. Two of these maxima occur over land. As the clouds mainly originate from the climbing airflows in the southern and eastern slopes of the Tibetan Plateau, they can be classified as dynamic clouds. The third center of cloud cover is over the sea. As the clouds mainly form from the evaporation of the warm Kuroshio Current, they can be categorized as thermodynamic clouds. Although the movement of the cloud centers reflect the seasonal variation of the Asian summer monsoon, cloud fractions of six cloud types that are distinct from the total cloud cover show individual horizontal patterns and seasonal variations. In their vertical distribution, cloud cover over the land and sea exhibits different patterns in winter but similar patterns in summer. In cold seasons, limited by divergent westerlies in the middle troposphere, mid-level clouds prevail over the leeside of the Tibetan Plateau. At the same time, suppressed by strong downdraft of the western Pacific subtropical high, low clouds dominate over the ocean. In warm seasons both continental and marine clouds can penetrate upward into the upper troposphere because they are subject to similar unstable stratification conditions.
Based on data from satellite and surface observations, the horizontal and vertical distributions of clouds over eastern China and the East China Sea are examined. Three maximum centers of cloud cover are clearly visible in the horizontal distribution of total cloud cover. Two of these maxima occur over land. As the clouds mainly originate from the climbing airflows in the southern and eastern slopes of the Tibetan Plateau, they can be classified as dynamic clouds. The third center of cloud cover is over the sea. As the clouds mainly form from the evaporation of the warm Kuroshio Current, they can be categorized as thermodynamic clouds. Although the movement of the cloud centers reflect the seasonal variation of the Asian summer monsoon, cloud fractions of six cloud types that are distinct from the total cloud cover show individual horizontal patterns and seasonal variations. In their vertical distribution, cloud cover over the land and sea exhibits different patterns in winter but similar patterns in summer. In cold seasons, limited by divergent westerlies in the middle troposphere, mid-level clouds prevail over the leeside of the Tibetan Plateau. At the same time, suppressed by strong downdraft of the western Pacific subtropical high, low clouds dominate over the ocean. In warm seasons both continental and marine clouds can penetrate upward into the upper troposphere because they are subject to similar unstable stratification conditions.
2012, 18(3): 393-402.
Abstract:
The radiative and microphysical effects of ice clouds on a torrential rainfall event over Hunan, China in June 2004 are investigated by analyzing the sensitivity of cloud-resolving model simulations. The model is initialized by zonally-uniform vertical velocity, zonal wind, horizontal temperature and vapor advection from National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data. The exclusion of radiative effects of ice clouds increases model domain mean surface rain rates through the increase in the mean net condensation associated with the increase in the mean radiative cooling during the onset phase and the increases in the mean net condensation and the mean hydrometeor loss during the mature phase. The decrease in the mean rain rate corresponds to the decreased mean net condensation and associated mean latent heat release as the enhanced mean radiative cooling by the removal of radiative effects of ice clouds cools the mean local atmosphere during the decay phase. The removal of microphysical effects of ice clouds decreases the mean rain rates through the decrease in the mean net condensation during the onset phase, while the evolution of mean net condensation and the mean hydrometeor changes from decrease to increase during the mature phase. The reduction in the mean rain rate is primarily associated with the mean hydrometeor change in the absence of microphysical effects of ice clouds during the decay phase.
The radiative and microphysical effects of ice clouds on a torrential rainfall event over Hunan, China in June 2004 are investigated by analyzing the sensitivity of cloud-resolving model simulations. The model is initialized by zonally-uniform vertical velocity, zonal wind, horizontal temperature and vapor advection from National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data. The exclusion of radiative effects of ice clouds increases model domain mean surface rain rates through the increase in the mean net condensation associated with the increase in the mean radiative cooling during the onset phase and the increases in the mean net condensation and the mean hydrometeor loss during the mature phase. The decrease in the mean rain rate corresponds to the decreased mean net condensation and associated mean latent heat release as the enhanced mean radiative cooling by the removal of radiative effects of ice clouds cools the mean local atmosphere during the decay phase. The removal of microphysical effects of ice clouds decreases the mean rain rates through the decrease in the mean net condensation during the onset phase, while the evolution of mean net condensation and the mean hydrometeor changes from decrease to increase during the mature phase. The reduction in the mean rain rate is primarily associated with the mean hydrometeor change in the absence of microphysical effects of ice clouds during the decay phase.
2012, 18(3): 305-313.
Abstract:
The NCEP/NCAR reanalysis datasets and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rain data are used to investigate the large scale seasonal transition of East Asian subtropical monsoon (EASM) and its possible mechanism. The key region of EASM is defined according to the seasonal transition feature of meridional wind. By combining the 'thermal wind' formula and the 'thermal adaptation' equation, a new 'thermal-wind-precipitation' relation is deduced. The area mean wind directions and thermal advections in different seasons are analyzed and it is shown that in summer (winter) monsoon period, the averaged wind direction in the EASM region varies clockwise (anticlockwise) with altitude, and the EASM region is dominated by warm (cold) advection. The seasonal transition of the wind direction at different levels and the corresponding meridional circulation consistently indicates that the subtropical summer monsoon is established between the end of March and the beginning of April. Finally, a conceptual schematic explanation for the mechanism of seasonal transition of EASM is proposed.
The NCEP/NCAR reanalysis datasets and Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rain data are used to investigate the large scale seasonal transition of East Asian subtropical monsoon (EASM) and its possible mechanism. The key region of EASM is defined according to the seasonal transition feature of meridional wind. By combining the 'thermal wind' formula and the 'thermal adaptation' equation, a new 'thermal-wind-precipitation' relation is deduced. The area mean wind directions and thermal advections in different seasons are analyzed and it is shown that in summer (winter) monsoon period, the averaged wind direction in the EASM region varies clockwise (anticlockwise) with altitude, and the EASM region is dominated by warm (cold) advection. The seasonal transition of the wind direction at different levels and the corresponding meridional circulation consistently indicates that the subtropical summer monsoon is established between the end of March and the beginning of April. Finally, a conceptual schematic explanation for the mechanism of seasonal transition of EASM is proposed.
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