Abstract:Changes in the frequency of typhoon occurrence and landfall location in the East China Sea are investigated by using the best typhoon track data in Northwest Pacific Ocean from 1951 to 2015.This 65-yr record is provided by Japan Meteorological Agency(JMA).This paper focuses on the East China Sea region,investigating typhoon frequency,landfall position,the relationship between typhoon occurrence(landfall position) and the Pacific Decadal Oscillation(PDO) and El Niño/La Niña and Southern Oscillation(ENSO),and the possible factors affecting typhoon generation and movement.The results indicate that:(1)July-August-September is the season of the highest occurrence for typhoons in the East China Sea,with the highest number of typhoons occurring in August.Typhoons making landfall in this region have similar behavior.(2)The occurrence of typhoons entering the East China Sea has both interannual and interdecadal trends.During the PDO warm(cool) phase,fewer(more) typhoons occur and the occurrence shows an upward(downward) trend.Fewer(more) typhoons enter the East China Sea during El Niño(La Niña) periods.Niño3.4 index and typhoon frequency show a negative correlation as a whole,with the correlation coefficient of -0.32 at 90% confidence level.(3)The latitude of the typhoon landfall point varies greatly,ranging from 24 to 36°N.A certain interdecadal change is also observed.During the PDO warm(cool) phase,the typhoon landfalls are further north(south) and have a tendency to move northward(southward).However,the relationship between typhoon landfall latitude and ENSO is relatively complex and needs further investigation.(4)The primary reason for the change of the typhoon occurrence and landfall may be attributed to the location and strength of the subtropical high in Northwest Pacific.During the PDO cold(warm) phase,the Northwest Pacific subtropical high weakens(strengthens) and the subtropical high moves in an eastward and northward(westward and southward) direction.Under the condition of a strong(weak) tropical convergence zone over Northwest Pacific,this kind of atmospheric circulation contributes to the generation of typhoon,leading to the high(low) frequency of typhoons entering the East China Sea and the further north(south) typhoon landing point.

Abstract:Most previous studies have paid much attention on changes in landfall tropical cyclone(TC) motion caused by island topography and land drift induced by flat terrain roughness but rarely focused on influences of continental topography.In this study,the landfall tropical cyclone movement changes are examined in the presence of the influences of topography and environmental flows through numerical experiments,in which an initially axisymmetric vortex makes landfall on East China coastal areas.It is found that the flat land can bring about an acceleration of tropical cyclone during landfall while the increase in tropical cyclone speed is much larger,which is attributed to the increase of steering flows.The increase of speed has not been revealed in previous studies and may be an important attribution to TC track prediction error.Of more importance is that the flat land can induce asymmetric flow within the tropical cyclone regions while the asymmetric flows induced by topography are much stronger.Non-steering factors tend to counteract between one another and contribute little to the landfall tropical cyclone motion in general but different factors have individual contributions on different vertical layers.

Abstract:In this study we examine the role of typhoon spiral rain bands and the horizontal vorticity in a rainstorm case(typhoon Morakot,which occurred in Taiwan in 2009),using real-time observations and the Weather Research and Forecasting(WRF) model.The time period of our simulation is from 12:00 on August 6 to 00:00 on August 10,when there is typically heavy rainfall in Taiwan.Our results show that the maximum precipitation occurred on August 7 to 9,while at the same time the rainstorm in Taiwan was related to two spiral rain bands,the major spiral rain band of which was located in central Taiwan,while the other was located in southern Taiwan.At the low level,the rain bands consistent with the large values of the horizontal vorticity region pointed east,while at the top,the horizontal vorticity pointed west at the northern spiral rain bands.Rainstorms occurred near the vertical circulation,and moved with the large values of the horizontal vorticity region,showing that the two were closely linked.The large value of horizontal vorticity was related to the vertical vorticity,and the horizontal vorticity converted to vertical vorticity in the rainfall area.The positive curl of the horizontal vorticity corresponded with upward movement,as well as with the spiral rain bands and rainfall.When the horizontal vorticity reduced,the rain bands remained while the positive curl of the horizontal vorticity was present.

Abstract:The diagnostic analysis on a heavy precipitation supercell storm process in northern Jiangsu province on 22 July 2008 is implemented by using the conventional observation data,NCEP reanalysis data,FY2C satellite products and CINRAD-SA Doppler radar data.Weather analysis shows that the synoptic environment is characterized by the high humidity,strong convective instability(convective available potential energy,3 445 J/kg),medium to strong vertical wind shear (0-6 km,18 m/s) and low LCL (lifting condensation level),which directly result in the generation and development of the heavy precipitation supercell storm.The radar echo analysis shows that the evolution of the heavy precipitation supercell can be generalized into three stages:the isolated cell develops increasingly,evolves into a classic heavy precipitation supercell and then weakens eastward gradually,lasting more than 2 h.Doppler radar base reflectivity and storm-relative radial velocity observations on various elevation angles are used to investigate the radar echo features of the heavy precipitation supercell storm.The results show that at its mature stage,the heavy precipitation supercell storm displays a low-middle-level bounded weak echo,the larger radar reflectivity factor sloping toward the low-level inflow from low level to high level,and associated typical front inverted "V" notch echo with the inflow in middle and lower levels.Corresponding radar radial velocity images show that a well-developed mesoscale cyclone is in company with the inverted "V" notch echo in front of the supercell storm,and coincident with the heavy precipitation area at the surrounding of the inverted "V" notch echo.Through diagnosing the moist potential vorticity on isentropic surface and the moist potential vorticity during the process,results suggest that the intrusion of the durative dry cold air in upper levels touches off the release of potential convective instability energy,which is in favor of the development of convective motion.The atmospheric convective instability and conditional symmetric instability coexist in middle and low levels,so there are vertical convection and slantwise convection during the heavy rainfall,meanwhile the easterly inflow adjacent to the boundary layer transports ample water vapor to the heavy rain area,which plays an important role on the generation and development of the heavy rainstorm.

Abstract:Using the NCEP/NCAR reanalysis data of four times one day and the daily weather observation data with 200 stations in the Northeast China from 1961 to 2014,this paper analysed the climatic characteristics of heavy rain that caused by northeast cold vortex,and the effects of low-level jet and shear on cold vortex heavy rain.Diagnosed and analyzed physical parameters such as specific humidity,water vapor flux,water vapor flux divergence,divergence,vertical velocity,and K index,which can characterize the conditions of water vapor,dynamic conditions,and thermal instability in the process of heavy rain caused by northeast cold vortex.It summarized the thresholds that need to be met for the above physical parameters in the heavy rain caused by northeast cold vortex.A forecasting model for the "ingredients method" of the heavy rain that caused by northeast cold vortex was constructed by using the ingredient factors of physical parameter threshold indicators and low-level jet and shear.Using the numerical prediction products which provided by ECWMF model and T639 model,the cold vortex heavy rain ingredient model was applied to daily forecast work.For the sake of examining the applicability of the ingredients method model,the quality of the heavy rain forecast was tested from 2015 to 2016,and the performance evaluation had been done.The results show that there is 66% possibility of leading to heavy rain by the Northeast China cold vortex activities.The times of heavy rain caused by cold vortex were more in July and August than other months.The low-level jet and the shear at lower in tropospheric could trigger rainstorm,there were low-level jets or shears at 91.2% of heavy rain caused by northeast cold vortex.The TS score by using "Ingredients method" on forecasting heavy rain caused by the Northeast China cold vortex is 7.4% and 11.1% higher than that of the ECMWF model and T639 model,which was greatly reduces the rate of missing reports on forecasting of heavy rain.Therefore,it has high practical application value and has great significance for disaster prevention and mitigation.

Abstract:When one makes short-range regional ensemble forecast by using the Breeding of Growing Modes (BGM) method,a critical problem first to be faced is what the typical characteristics of evolution of initial perturbations are in the short-range ensemble prediction system (EPS).Consequently,a short-range EPS based on the BGM method has been developed with WRF3.6.The regular rescaling scheme has also been incorporated into this system.Meanwhile,the short-range EPS that has covered the uncertainties of horizontal wind,vertical velocity,potential temperature,geopotential height and water vapor mixture ratio takes the large-range rainstorm in southern China in June 2016 as an example to recognize the evolving mechanism of perturbations.The results show that:(1)the perturbation growing process of physical quantities in the upper,middle and lower levels of model atmosphere can be divided into two stages,one of which is the rapid linear growth of perturbations and the perturbations quickly complete the total increase of themselves in this phase,another of which is the nonlinear stable phase of perturbations growing and the transition from the fast linearly growing phase to the nonlinear stable phase takes about 24 h.(2)The perturbations of physical quantities take approximately the same length of time to enter the nonlinear stable phase through the temporal evolution features of perturbation growth rate,correlation coefficient and perturbation growing modes.Nonetheless,when the perturbations come into the nonlinear stable stage,the numerical values and evolving characteristics of each assessment parameter are different for the same pressure level with different physical quantities or the same physical quantity at different pressure levels.Moreover,there is a diurnal oscillation phenomenon with time for each assessment parameter at the nonlinear stable stage.(3)For the initial ensemble of different random patterns with the same size of perturbation amplitude,the impacts of different random patterns on perturbations breeding mainly yield differences in the nonlinear stable stage while the differences between each pattern are too small to distinguish in the fast linearly growing stage.(4)For the initial ensemble of the same random pattern but with different sizes of perturbation amplitudes,the influences of different amplitudes on the evolution of perturbations mainly occurs in the fast linear growth phase,while the differences between each amplitude are quite small in the nonlinear stable phase.Additionally,the different sizes of initial amplitudes have no influence on the characteristic time scale of the perturbations getting into the nonlinear stable phase.

Abstract:Based on the ERA-interim reanalysis data of the ECMRWF(European Center for Medium-Range Weather Forecasts) from 1979 to 2013,this paper systematically analyzes the intraseasonal variation of winter meridional wind in East Asian monsoon region and its possible mechanism by using the mean square analysis,power spectrum analysis,Butterworth filtering,synthesis analysis and other statistical methods.Results show that the winter meridional wind anomaly of East Asian monsoon area significantly changes in South China area,with an oscillation period of 10-20 d (Quasi-BiWeekly Oscillation,QBWO).In the horizontal direction,the 850 hPa anomalous north wind has the characteristics of propagation from the high latitudes to the low latitudes.The anomalous meridional wind near 60°N spreads southeastward,and the anomalous weak meridional wind near 30°N spreads eastward in the subtropical zone.They converge in South China,then are divided into two centers,which continue to spread southward and eastward respectively.The oscillating evolution of winter meridional wind has the characteristics of anti-phase between Lake Baikal and South China.There exists the energy conversion from the basic air flow to the quasi-biweekly fluctuation in South China,therefore the anomalous meridional wind will enhance significantly there.In the vertical direction,the anomalous meridional wind exhibits strong-weak-strong anomalous center features in the upper,middle and lower layers of the troposphere.There is a large center of meridional wind in the lower (850 hPa) and upper (200-300 hPa) layers of the troposphere.However,the quasi-biweekly oscillations of meridional wind are relatively weak in the middle troposphere.Rossby wave changes significantly at 300 hPa.Over the eastern part of China,there is also a large value center of meridional wind with quasi-biweekly oscillation period.The fluctuations in the subtropical regions at 300 hPa are more obvious than those at 850 hPa,and the fluctuations near 60°N move southeastward,similarly merging in the eastern part of China.The Rossby waves from Europe spread eastward and the Rossby waves from North Pacific spread westward,and their convergence near the eastern part of China results in an increase of fluctuation energy.

Abstract:Based on the winter(DJF) daily surface air temperature data from China National Meteorological Information Center and the NCEP/NCAR global reanalysis data,the non-filtering method is applied to extract the low-frequency signals of 10-30 d from the air temperature and atmospheric circulation.The low-frequency air temperature in China in winter from 1979 to 2011 and associated atmospheric circulation characteristics are analyzed,focusing on the influence of the Ural mountain circulation on the low-frequency surface air temperature in China in winter.Results show that:1)the first mode of surface air temperature in winter demonstrates a consistent cold(warm) pattern over most part of China.The variance of 10-30 d low-frequency surface air temperature accounts for more than 30% of the total in the typical years.2)Circulation anomalies in the North Atlantic to the polar region,Ural Mountain and Lake Baikal are significantly related to winter temperature anomalies in China.When the polar vortex is weaker or the NAO is a positive anomaly,the high pressure ridge in the Ural mountain area is stronger,which is conducive to strengthening of Siberian cold high near Lake Baikal at the surface,so that it is cold in China;and vice versa.3)The low-frequency air temperature in China in winter is closely related to the Ural mountain circulation.When the circulation anomaly over the Ural mountain area leads 15 d earlier than the surface air temperature,their relationship is best.That is,the positive height anomalies over the Ural mountain area is conducive to strengthening of the Ural mountain high ridge and Siberian high,in such a way that East Asian winter monsoon is strengthened,which leads to low temperature in China in winter.

Abstract:In this paper,a heavy rainfall occurring in the Yangtze-Huaihe River region during the plum rain season is simulated using the mesoscale numerical model(WRF-V3.4).The Milbrandt-Yau scheme,a two-moment scheme,is chosen,in which the size distribution of each hydrometeor category is described by a Gamma distribution law,and the spectral shape parameter α is fixed.In order to understand the effects of the spectral shape parameters of the raindrops and graupel on the precipitation,the precipitation,water content and concentration distribution of the hydrometeors,as well as the source and sink terms of the raindrops and graupel,are analyzed through several sensitivity experiments with different values of raindrops and graupel,and different spectral shape parameters.The results show that the distribution of raindrops and graupel is greatly affected by the spectral shape parameters thereof,as it can affect the micro-physical processes associated with rainwater and graupel.When the spectral shape parameters of the raindrops increase,the raindrop evaporation first increases then decreases,while the collision of the graupel and clouds also decreases.At the same time,when the spectral shape parameter of the graupel increases,the graupel melting decreases.Therefore,the change of the two parameters leads to differences in precipitation distribution and magnitude.

Abstract:Based on the data from plane,radar and satellite,the structure features of a rainfall in Hebei province during 08:00 May 1,2014 to 08:00 May 2,2014 was studied.The results showed that the precipitation stratiform clouds in this weather process could approximately divided into 3 layers:2 850-4 200 m was cold-warm cloud,2 162-2 174 m was pure warm cloud and 121-265 m in the surface layer was warm cloud with low particle concentration (10¹ cm^-3).There existed obvious catalytic cloud-supply could structure before the precipitation.After the precipitation began,the higher cloud imposed catalysis effect on the lower cloud.The potential area of artificial precipitation stimulation mainly located in 3 100-4 000 m,where the intensity of corresponding radar echo was 20-30dBz and the vertical gradient of its intensity is significantly reduced.The correlation analysis of the cloud microphysical parameters at different altitudes was analyzed.According to the results,the LWC and the cloud particle concentration showed strong negative correlation with the aerosol concentration at the cloud base.The coefficient between the IWC and the cloud particle concentration was 0.434.The CCN concentration had strong positive correlation with the temperature (0.717) in the cold cloud.

Abstract:In order to reveal the meteorological reasons for the formation and development of regional atmospheric pollution,according to the large-scale weather background,NCEP reanalysis data and surface atmospheric composition observation data were used to divide the persistent air pollution processes over central Liaoning Province during 2010-2012 into five types,namely,Mongolia High Type,Denatured High Type,Mongolian Cyclone Type,Weak Depression Type and Jiang-huai Cyclone Type.Further physical characteristic study shows that the pollution events caused by Mongolian Cyclone account for the highest ratio of 32.7% whereas those caused by Jiang-huai Cyclone account for the lowest ratio of 6.1%.All the eleven events caused by Mongolia High appear in winter while all the four events caused by Weak Depression appear in summer.The fifteen events caused by Denatured High appear in winter,autumn and spring,whereas the three events caused by Jiang-huai Cyclone appear in summer,autumn and winter.The sixteen events caused by Mongolian Cyclone appear in all the seasons.All the above show that autumn and winter are the high-occurrence seasons for Mongolia High Type,Denatured High Type and Mongolian Cyclone Type,and summer is the high-occurrence season for Weak Depression Type and Mongolian Cyclone Type.Boundary layer physical mechanism study shows that the horizontal convergence,vertical transport and atmospheric stratification stability of Jiang-huai Cyclone Type and Mongolia High Type are similar,whereas the inversion of the former is stronger.As for Jiang-huai Cyclone Type,the relative humidity is as high as over 95% and the downward flow is as strong as 0.2-0.4 Pa/s,along with the inversion strength at 0.7℃/hm and inversion height at 925 hPa.Compared with these two types,the horizontal convergence of the other three is similar,but the downwind,atmospheric stratification stability and inversion are significantly weaker,even rarely appear.

Abstract:Using the FVCOM-SWAVE coupled regional ocean model,a numerical simulation and comparative analysis were performed for the wind wave effect on the air-sea heat and momentum fluxes in the Yellow Sea during the typical cold wave processes.In order to learn the basic features of the air-sea heat and momentum fluxes in the Yellow Sea,a statistical analysis by air and sea reanalysis data of CFSR(Climate Forecast System Reanalysis,US) were performed for a 5-year duration from 2009 to 2013.The statistical results showed that the air-sea heat and momentum flux exchanges were impacted by the sea surface wind,difference between air-sea temperature,ocean currents,and so on.The fluxes were stronger during the cold season from November to March,while the wave impact was more significant in autumn-winter than in spring-summer,due to the weak ocean currents during the cold season.In the cold season the heat fluxes from between the sea and air were also remarkable,as the air-sea temperature difference and SST grads between latitudes were larger and resulted in an unstable surface environment.The variables of the coastal station comparison showed that the south station faced a wider open sea with a wave height of about 20% higher than the north coastal station,while the south fluxes were also stronger than in the north part.The two typical cold wave events in the cold season were numerically simulated,in which the cold mass paths of invading the Yellow Sea moved from the north and west.The results showed that the air-sea heat and momentum flux exchange became significantly stronger during the cold wave processes,with the momentum flux increasing by about 1-4 times and the heat flux increase 1-6 times over the maximum average values in the cold season.The simulation results also showed that the air-sea heat and momentum flux exchange exhibited obvious differences,due to the different paths and intensity between the two typical cold waves.The heat and momentum flux responses were more intense in the north path cold wave process,with the momentum flux being about 1/4 times stronger than that in the west path cold wave,and the heat flux being 1/2 times stronger.The cold wave path affected the large value distribution of the fluxes.When the wind waves were coupled into the model,the air-sea heat and momentum flux exchanges were enhanced:the height of wind waves increased by 1.5 times,the maximum value of the momentum flux increased by about 2 times,and the heat flux increased by 10-160 W/m².When the height of the wind waves was reduced by 0.5 times,the maximum value of the momentum flux was reduced by about 40%,and the heat flux decreased by about 10-55 W/m².The wind wave provided an important contribution to the air-sea heat and momentum flux features and distribution.

Abstract:The quality control of satellite observations has shown significant impact on data assimilation as well as retrieving meteorological products.Satellite microwave thermal emission mixed with signals derived from the active sensors,e.g.radar technologies,air traffic control system,garage door opener remote control,applications of the global positioning system to intelligent vehicle highway systems,defense transportation tracking system,vehicle speed detection system,etc.,is referred to as radio-frequency interference(RFI).RFI remarkably causes serious problem for microwave sensing.Comparing with previous advanced microwave scanning radiometer-EOS (AMSR-E),AMSR-2 which is on board the global change observation mission 1st-water(GCOM-W1) satellite,and adds the channels at 7.3 GHz,in order to mitigate the RFI contamination at lower frequencies.AMSR-2 has seven frequencies at 6.9,7.3,10.65,18.7,23.8,36.5,and 89.0 GHz,each having dual channels at vertical and horizontal polarization states,respectively.In order to analyze the observations at this new-added channel be interfered or not,RFI at 7.3 GHz over East Asia is identified using spectral difference method based on AMSR-2 observations collected from August 1st to 16th,2014.The achieved results showed that RFI at 7.3 GHz widely spreads over East Asia in some places.The RFIs located in Gyeonggi-do Province,South Korea,Bangladesh,Vietnam and,South-East Cambodia are isolated in space and persistent in time and originate from a wide variety of coherent point target sources.The observations of spaceborne microwave radiometer in ascending orbit portions usually are interfered over Japan,while no RFI approximately was detected in descending portions.The RFI location and intensity varied with time and radiometer scanning angle within observation period.After analyzing the contamination source,it can be concluded that the source of RFI in Japan is the results of the interference of the reflected geostationary communication or down linked satellite signals with the spaceborne microwave radiometer data.

Abstract:This research proposed a kind of improved weather radar TITAN algorithm for different seasons,latitude and longitude and underlying surface in China.The weather radar data format conversion system is established in order to convert various Doppler weather radar base data in China to the MDV radar data format.The parameters threshold of radar characteristics of strong storm weather in different areas are counted based on different climate types,latitude and longitude and characteristics of land and sea.We make comparative experimental analysis between two typical weather radar base data of super-cell storm in Yunnan and Hainan regions and the improved weather radar TITAN algorithm.The results indicate that the error of using the improved TITAN algorithm to identify,track and predict strong storm weather after 30 minutes to 1 hour is relatively small,which has a higher credibility and could be conducive to identify disaster weather and commanding operation of weather modification.

Abstract:Based on the data of the NCEP global reanalysis,with a resolution of 1°×1° and cloud-to-ground lightning in Shaoyang from June to August 2008 to 2010,an SVM classification model of thunderstorm forecast was set up.First,the ground flashes data for 6 hours corresponding to the NCEP data were counted.The flashes were greater than or equal to 3,which is defined as a thunderstorm.A non-thunderstorm is defined as when the flash is zero.If the number was greater than 0 or less than 3,it was not included in the statistical samples.In addition,if the flashes were detected by the lightning location system,but the thunderstorm days were not recorded at the meteorological observation station,then the data were not included on the model samples.According to these criteria,1 007 samples were counted,286 of which were thunderstorm samples,and 721 were non-thunderstorm samples.80% of the samples were used as the training samples,and the remaining 20% were used as the test samples.Next,27 parameters were extracted from the NCEP global reanalysis data on 1°×1° degree grids,and 21 convection parameters were calculated,of which a total of 48 physical quantities were used to set up a thunderstorm-forecasting model according to the method of SVM classification.The correlation between 48 physical quantities and the occurrence of thunderstorm of training samples was calculated,then 11 significant predictors greater than 0.3 were selected.Before the forecasting model was established,all parameters were normalized.By repeatedly changing the parameter values,the highest classification accuracy of the parameters c and g were finally selected as the model parameters.When the two parameters were determined,the potential thunderstorm forecast model was set up.The test results show that the prediction accuracy of the SVM model was 86.21%,false alarm rate was 15.25%,and missing rate was 13.79%.The scope and step length were manually set during the parameter optimization,yet after several repetitions some modeling errors were still present,which led to the test results differing.In order to validate the stability of the model,70% (90%) of the modeling samples were re-selected and verified with optimal parameters.It was found that there was no significant difference in accuracy among the three kinds of model samples,which illustrates the stability and reliability of the forecast model,and the model can forecast the thunderstorm in 6 hours.Logistic regression discrimination and Bayesian discrimination are the two other forecast methods used for thunderstorms.Through comparison of the three forecast methods,it was found that the forecast model based on SVM gave the best prediction performance with TSS,and the true skill statistic was 0.79.The rates of the false alarm and pseudo fault report were lower than when using the other two methods.Therefore,it can be seen that the forecast model built using the SVM method is able to provide a certain reference value for potential thunderstorm prediction in the Shaoyang area,with a higher accuracy rate of distinguishing thunderstorms and non-thunderstorm.In other words,the prediction model can provide a reference value for potential thunderstorm forecast in the Shaoyang area.