Abstract:Atmospheric Motion Vectors (AMVs) can supply plenty of useful information for numerical weather prediction. With the improvement in the image navigation, data calibration and derivation algorithm, the quality of FY2E is expected to be improved. Therefore, it is necessary to evaluate the improvement of FY2E AMVs for the analysis field and precipitation forecast in GRAPES (Global/Regional Assimilation and Prediction System) of China. In this study, the old and the reprocessed FY2E AMVs are used to analyze the characteristics of their horizontal and vertical structures and applied to GRAPES3Dvar Global Assimilation Prediction System to compare their differences on the assimilation and prediction. The experiments using the data collected in August 2013 show some encouraging results, which show neutral to positive impact on wind analysis field, especially in high levels. Furthermore, due to the improvement of the initial fields for the model prediction, the performance of the anomaly correlation coefficient (ACC) and root mean square error (RMSE) slightly improved. Especially the observation error of the reprocessed FY2E is lower than the old from 600 hPa to 200 hPa, which needs a further investigation. Conclusively, the reprocessed FY2E AMVs have more positive impact on wind assimilation and forecast improvement in GRAPES.

Abstract:The occurrence and development of mesoscale vortices directly influence heavy rainfalls during Meiyu period. Identifying mesoscale vortices accurately helps to improve the prediction of heavy precipitation. An objective method is proposed to automatically detect the centers of mesoscale vortices from gridded wind field in this study. Using global analysis from National Centers for Environmental Prediction (NCEP), USA, two cases during the 2013-2014 Meiyu periods were selected to compare the existing two algorithms which are based on vorticity or height distribution and the new one. The results indicate that since smallscale systems do not obey geostrophic wind relationship, the centers of wind, vortex and low pressure scarcely overlap Meiyu fronts, thus decreasing the accuracy of detection on the basis of vorticity or pressure. However, the new method is based on winds and can precisely identify most vortex centers with a higher precision than the other two algorithms. Then the new method is used to analyze the vertical structure and time evolution of different vortices in the two cases. The analysis shows that the new method does not need any manual intervention, specific level or time restriction. It can identify the position, 3D structure and time evolution of any mesoscale vortex falling in the domain in a very short time. Also, it can be used to detect mesoscale vortices on stationary fronts during Meiyu periods, very helpful for forecasters to do realtime analysis and forecast severe precipitation.

Abstract:By using the data of conventional observation and surface automatic station observation, FY2D and FY2E satellite cloud images, NCEP 1°×1° FNL analysis data and EC 0.25°×0.25° finegrid model data, the evolution features of mesoscale convective system (MCS), low tropospheric wind impact on MCS development and vertical circulation structure of Meiyu front in a Meiyu heavy rainfall event that happened during 15-18 June 2015 were studied in this paper. It was found that the local heavy rainfall in south of Jiangsu Province was caused by emergence and downstream movement of a series of MCSs along synopticscale Meiyu front and their strengthening through the positive vorticity belt in central Anhui and southern Jiangsu. There was good agreement on locations between the heavy rainfall and lowlevel jet (LLJ) core. In vertical direction, the right side of highlevel jet (HLJ) was overlaid on the leftfront side of LLJ core resulting in obvious coupling between the two systems. During the heavy rainfall, there was both strong vertical shear in low tropospheric horizontal wind and distinct inhomogeneous distribution of upward motion in southern Jiangsu which were favorable for the enhancement of vertical vorticity and development of MCS.High vertical wind shear in lowtroposphere was also helpful for merging water vapor from different moisture sources along Meiyu front.The drycold air in the north of Meiyu front moved close to the front zone through a northeast (northwest) track in lowlevel (middlelevel) troposphere. And warmmoisture air in the south of Meiyu front approached the front zone and went upward by a southwest way. As the warmmoisture air particles approached Meiyu front zone, they turned to move eastward gradually and flowed away quickly through the pumping effect of HLJ.There was crossfront motion for near surface air. The vertical secondary circulation of Meiyu front was induced by strong highlevel divergence and supplement and circulation of mass during the moving process of atmosphere.

Abstract:The raindrop spectrum shape parameter, which has significant spatiotemporal distribution features, is set as a constant in the Doublemoment Bulk Microphysics Scheme (DBMS) using Gamma distribution function. Based on Milbrandt 2mon (MY) DBMS, four Meiyu precipitation cases are simulated coupled with four empirical relationships between shape parameter (μr) and slope parameter of raindrops which are concluded from the observations of raindrop distribution. The analysis results suggest that μr has some influences on precipitation. Adopting the diagnostic formulas of μr is positively helpful for improving systematic biases of Meiyu rainfall and shows certain correction ability to the localization of rainfall distribution. The good response of heavy precipitation to μr reflects the clear tendency of improvement. Calculated by the empirical formula, μr increases generally. In the mid and lowertroposphere, μr decreases with the increase of rainfall intensity. The decline in raindrop water content and the increased raindrop massweighted average terminal velocity, which is directly related to μr, are the direct causes for the changes of precipitation. The difference in improving degree between the light and heavy rainfalls mainly results from the different response features of their variable fields. The varying trend of warmcloud particle features of severe rainfalls is consistent with that of light rainfalls, but there are considerable differences in their varying degrees. While it rains lightly, the response characteristics of physical fields to μr present similar varying trends and some linear features. However, in the case of heavy precipitation, the cloud microphysical process and ambient condition experience complex interactions with each other and no significant laws can be followed.

Abstract:Based on the conventional observation data, NCEP geopotential height and wind reanalysis data from National Centers for Environmental Prediction, the process of fog is analyzed by taking use of the method of synoptic meteorology. It turns out that there are three types of synoptic situation at 500 hPa when the regional fog around the Bohai Sea coastal areas occurs, which are zonal flow, low trough, high pressure ridge, and four types of surface synoptic situation, which are frontal cyclone, high pressure forepart, uniform pressure field and weak high pressure. According to the characteristics of the ridges and troughs, high and low pressure centers, an objective and automatic identification system is designed. Furthermore, multiple moisture condition, stratification stability condition and wind speed condition are calculated on the basis of T639 model output. The 1000 hPa depression of dew point, 1000 hPa wind speed, temperature difference between 925 hPa and 850 hPa, and M index are selected as physical diagnosis factors after diagnostic and correlation analyses. Thus, a forecasting system combining automatic identification of synoptic situation and T639 is established, which can forecast the space and time of the fog around the Bohai Sea coastal areas within the next 1-3 days. Experimental results show that good results have been achieved.

Abstract:To enhance the understanding of the squall line maintaining mechanism under influence of Hangzhou Bay, the diagnosic analyses are made on the squall line in northern Zhejiang on 27 July 2014, using Doppler radar data, convectional and denselyobserved surface data and NECP GFS data. The focus is on the evolution of convection that moved into Hangzhou Bay from the north and the south coasts and its effect on the overall development of squall lines. The result indicates that the northsouth convergence lines and moderate strength of deep vertical wind shear are the key environmental factors of the generation and development of the squall line event. Convections are strengthened after moving into Hangzhou Bay from the north and the south coasts, leading to squall line maintaining. The better moisture conditions and unstable energy conditions in Hangzhou Bay result in the further strengthening of convection from the north shore of Hangzhou Bay, and the connection of two linear convections in southern Jiangsu, northern Zhejiang as well as the sustainable development of the squall line. The temperature and humidity on the surface of Hangzhou Bay is equal to the surface of south coast， and the strong wind shear in Hangzhou Bay leads to the maintaining strength of the convection into Hangzhou Bay from the south coast. Under the interaction of cold pool and rear inflow, bow echo develops on the sea, becoming an important factor for the maintenance of the squall line.

Abstract:The objective assessment of forecast is an important part in the whole weather forecasting cycle. Based on the assessment, qualities of different operational forecasts can be noted and considered, so enhancing the level of forecast. The verification of national severe convective weather categorical forecasts from April to September during 2010-2015 is displayed by adopting the objective indices of the “pointtoarea” thret score (TS), the false alarm rate (FAR) and the missing alarm rate (MAR) in this paper. At the same time, analysis is carried out for different time interval forecasts of the annual mean verification index (mainly is from April to September of the year). Furthermore, the problems existing in the verification of severe convective weather categorical forecasts and its development in the future are also discussed. Over the past six years, in addition to the decrease of thunderstorm forecast TS during 2012-2013, there was a rising trend for TS of severe convective weather forecasting. For the 6-24 h forecasts, the TS of thunderstorm was 0.22-0.34, the TS of the shorttime heavy rain was 0.18-0.24, the TS of thunderstorm gale and hailstone was 0.01-0.07; for the 48-72 h forecasts, the TS of thunderstorm was 0.30-0.40, the TS of severe convective weather was 0.16-0.23. The TS of thunderstorm gale and hailstone was lower than that of the other two kinds of severe convective weather. The FAR of thunderstorm was twice as large as MAR of thunderstorm; the FAR of shorttime heavy rain was close to its MAR, while the FAR and MAR of thunderstorm gale and hailstone were all larger than 0.8. Compared with the TSs of similar products in Storm Prediction Center (SPC) of USA, the TSs of thunderstorm and shorttime heavy rain are higher than that of SPC and the TS of thunderstorm gale and hailstone is lower than that of SPC. Verification of typical case forecasting shows that the TS of systemic and widespread thunderstorm gale and hailstone is higher than that in other situations. The reason is that the predictability of systemic and widespread thunderstorm gale and hailstone is higher.

Abstract:By using the daily precipitation data of 68 stations in Hubei and NCEP/NCAR circulation reanalysis data, the characteristics of the heavy rain events and lowfrequency atmospheric features between the Meiyu and midsummer periods are studied. The results indicate that the quasibiweekly oscillation of precipitation exists obviously in summer over Hubei. Compared with the rains in the midsummer period, lowfrequency heavy rain events in the Meiyu period are seen more often with stronger intensities. There are significant differences of lowfrequency heavy rain events between the Meiyu and midsummer period. During Meiyu period, circulations show the distribution of northsouth wave trains above the East Asia Littoral in the midlevel troposphere. The combined impact of strong Somali crossequatorial flow and southwest air current around subtropical high contribute to the abudant water vapor. The saddletype field of circulations in East Asia and transformed flow field are beneficial to the formation of mesoscale cyclonic systems. In the midsummer period, however, circulations distribute in the pattern of Eurasian wave trains in middle troposphere. In lower troposphere, water vapor comes from subtropical peripheral with the weak Somali crossequatorial flow. The north air streams from the west of the cyclone to the west of Japan Sea and the south warmwet air streams from the periphery of subtropical high get intersected and maintained over 30°N. Before and after the heavy rain events, there are obvious differences of the lowfrequency positive vorticity propagation in low troposphere. The lowfrequency positive vorticity during the Meiyu period shows the characteristics of standing wave while in the midsummer, it propagates westward, southward and northward obviously.

Abstract:Employing the observation surface wind data from Miyun Station and Shangdianzi Station from 1994 to 2013, the wind observation difference between the first stage and the second stage at Miyun Station is analyzed. The results indicate that the average wind speed of the second stage is reduced by 0.5 m·s-1 compared to the winds in the first stage. The environment around the station affected the annual average wind speed significantly in 2013. The seasonal and monthly wind speeds are affected by the obstacles around the station at different degrees. The maximum attenuation is in spring and March, while the minimum attenuation is in summer and August. The surrounding environment has the greatest impact on the WSW wind velocity and also on the wind speed frequency between 1 m·s-1 and 2 m·s-1. Comparing the first stage, the static wind frequency in the second stage increases by 5.5%, and the frequency of static wind has increased at each season. The most obvious increase is in winter and minimum increase is in spring. The increase in fall is greater than in summer.

Abstract:A selfdeveloped precipitation microphysical characteristics sensor was applied in field experiment and a large quantity of raindrop images were obtained. The causing reasons and handling methods for outlet raindrop images were discussed, and a vectorized method based on the Akima interpolation under the polar coordinate was proposed in this paper. Then, the characteristics of average raindrop shape and axis ratio distribution were analyzed. The results show that the small drops (<1 mm in diameter) have spheroids shapes, the moderate drops (<1.0 mm and <3.0 mm in diameter) have oblate spheroids shapes and the larger drops (>3.0 mm in diameter) have more bulged tops and flattened bases. The mean axis ratio distribution is in good agreement with the empirical relationships, and the deviation between them is smaller than 0.04. The raindrops ranging from 3.4 to 3.8 mm are more spherical than those from empirical model.

Abstract:This paper employs the data from atmospheric sounding comprehensive test site in CMA (China Meteorological Administration) from August to December in 2015, and carries on the contrast analysis about cloud base height, cloud top height and cloud vertical structure to the Kaband millimeter wave cloud radar and Lband sounding data. The analysis result indicates that the electromagnetic wave transmitted by the millimeter wave cloud radar can penetrate the thick clouds to detect cloud vertical structure, keeping a good consistency with the results of sounding. Further analysis of some large differences in individual cloud base and top height shows that the difference is correlated with the sounding balloon drifting caused regional deviation and the Lband sounding relative humidity detection error. The results finally show that the detection capability of Kaband millimeter wave cloud radar is strong, and the detecting precision is higher, thus the radar is an effective means of the groundbased cloud detection.

Abstract:Using data of power transmission line galloping (which happened 3 times from 2009 to 2010) in Henan Province and observations from representative stations, characteristics of line galloping are analyzed and meteorological indices for transmission line galloping are obtained. It shows that transmission line galloping will happen when the following conditions are satisfied: (1) A cold layer exists near 850 hPa where the temperature is less than -4℃ and a layer warmer than 0℃ exists below the cold layer. Alternatively, a layer warmer than 0℃ near 700-850 hPa and a cold layer exists below the warm layer; (2) the difference between temperature and dew point temperature below 700 hPa is less than 2℃; (3) the surface temperature ranges between -4℃ and 1℃, the wind speed is greater than 3 m·s-1 and the relative humidity is greater than 70%. Finally, the meteorological indices are used to draw the galloping distribution map for the galloping process monitored in 22-24 November 2015 in Henan Province. It is found the galloping grids predicted with the indices can cover the galloping monitored towerpoles. Thus, the indices are reasonable and applicable.

Abstract:Two rainfall belts were observed over eastern China during the summer of 2016, with the southern one in the Yangtze River Valley (YRV) and the northern one over North China. In June and July, precipitation occurred mainly in the YRV. In August, however, precipitation was below normal in most part of eastern China, except for more precipitation over South China. The more intensified and more westwardextending west Pacific subtropical high (WPSH) was one of the most important circulation factors for the southern rainfall belt over the YRV in June and July. The mean position of the high ridge was near normal. However, the high ridge retreated southward sometimes, causing more precipitation over the YRV during that time. The associated anomalous lowlevel Philippine Sea anticyclone resulted in anomalous convergence of moisture flux over the middle and lower reaches of the YRV. Moreover, the persistent warming in the entire basin of the tropical Indian Ocean during the decaying year of extremely strong El Ni〖AKn~D〗o event acted as the important externalforcing factor for the above tropical and subtropical circulation anomalies. In August, however, the WPSH fractured. Anomalous lowlevel cyclonic circulation controlled the northwestern Pacific, which caused anomalous divergence of moisture flux over most part of eastern China. Therefore, high temperature and little precipitation occurred in the middle and lower reaches of the YRV. Further analysis indicated that the unusually active MaddenJulian Oscillation (MJO) in August was the important cause for the significant turning of the tropical and subtropical circulations. MJO transmitted eastward to the western Pacific and stayed there for 25 days, which is seldom seen in history.

Abstract:The main characteristics of the general atmospheric circulation in October 2016 are as follows. Over the Northern Hemisphere, the polar vortex conformed to the dipolar nature distribution, which is more intensive than the usual. In the midhigh latitudes, the circulation presented a fourwave pattern. The West Pacific subtropical high system was stronger than the usual, with its ridge extending to west and north obviously. The monthly mean precipitation over China is 55.4 mm, 55% higher than the climatological mean (35.8 mm), and is the highest for the same period since 1951. The national average temperature is 10.9℃, 0.6℃ higher than its climatological mean 10.3℃. Six severe rainfall processes occurred during this month, two of which were led by Typhoons Sarika and Haima, respectively, while the others were caused by vortex sheers. There were five cold air processes, more than the usual, and four fog and haze events in the region of BeijingTianjinHebei. Typhoon Sarika, the No.21 this year, landed the city of Wanning, Hainan Province, on 18 October and Typhoon Haima, the No.22, landed the city of Shanwei, Guangdong Province on 21 October, both of which have severely adverse effects.