Abstract:Completely understanding the conditions and mechanisms of deep moist convection (commonly referred to as “thunderstorm” in operational forecasting) and severe convective weather (especially tornadoes) is the basis for better forecasting them. This paper first analyzes the relationships between convective available potential energy and convective inhibition energy and temperature and humidity of lifted air parcel, physical significance of convection temperature, and water vapor amounts for deep moist convection, and then further proposes the definition of significant severe convective weather in China, presenting some spatiotemporal characteristics of severe convective weather, the relationship between extreme rainfall and surface dew point temperature, the producing mechanism of convectively high winds, the physical significance of wet bulb temperature closely related to hail melting layer, the environmental conditions for supercell storms and tornadoes, and the formation mechanism of mesocyclones and tornadoes. Finally, understandings of vorticity and hodograph, and their relationship with mesocyclones and tornadoes are given. Different interpretations and threshold values of some concepts and variables listed in this paper and our understandings are provided for readers to study and compare.

Abstract:Based on the GSI assimilation system and WRFARW model, a severe rainfall event which occurred in southern China for the period in 17-18 April 2016 was simulated by the method of rapid update cycle. The experiments used radar radial wind data and other conventional data in assimilation cycle. Several kinds of forecast variables were analyzed to find how the assimilation cycles and difference data would influence the forecast result. The results showed that despite the limitation of the single type of observation, there is a certain improvement effect on the false precipitation prediction by the use of rapid update cycle. Radar radial wind data and conventional data mixing assimilation could improve the humidity and precipitation prediction skills, especially at heavy rainfall levels ETS score. Although the simulated rainfall peak value is less than real observation, the assimilation of radar radial wind data could effectively improve the vertical velocity of the squall line, so the occurrence time and intensity of heavy rainfall are much closer to the real observation.

Abstract:By using the data of conventional surface and sounding observations as well as the associated NCEP FNL and GFS reanalysis data, the features of precipitation type of heavy rainsnow process in North China during 3-4 November 2012 are studied in this paper. It is found that in the periphery of lowlevel vortex, the rain and snow separatrix is identical with the 0℃ isotherm and streamline convergence zone between northerly and southerly wind at 925 hPa. However, near the center of lowlevel vortex, the rain and snow separatrix takes on a feature of anticlockwise revolving from the northwest quadrant to the center of the vortex. In the early stage of cyclone developing, snow is located at the northnortheast quadarant to northwest quadrant of the 850 hPa vortex while the precipitation type at southeast quadrant is rain. As the extratropical cyclone intensifies, temperature at the southwest quadrant of lowlevel vortex drops quickly due to the lowlevel cold advection. Therefore, the snow area gradually expands southward along the wind flow of the west side of lowlevel vortex and takes on a cyclonicshaped distribution ultimately. The temporal evolution of precipitation type depends on the temperature advection of the whole lowlevel troposphere (850-950 hPa). When there is warm temperature advection in either 900-850 hPa or 950-900 hPa level, the precipitation type may still be rain even if cold advection exists in other levels. The cold temperature advection of northwest wind at the west side of lowlevel vortex is very important to snow. When the lowlevel wind transforms to eastly wind, the lowlevel cold advection disappears, and finally, the precipitation type involves to rain.

Abstract:A case study of an EF2 tornado that occurred in Wenchang, Hainan on 5 June 2016 was carried out by using observation data, automatic weather station data, Haikou Doppler radar data, wind profile data and 2G high resolution visible cloud images of Fengyun Meteorological Satellite. The results showed that: (1) This tornado was triggered by a supercell, corresponding to the edge of subtropical high, front of trough at 500 hPa, shear line at 850 hPa and the south side of ground thermal low. The favorable environmental conditions for the developing of the supercell were the significant increase of vertical wind shear in 0-2 km and the lower lifting condensation level, both caused by sealand breeze, and the quick increase of CAPE with solar radiation heat. (2) Both the mesoβ scale convergence line (seabreeze front) and the outflow boundary of thunderstorm triggered and enhanced the developing of the supercell, which was on the east side of eastward squall line, spreading from east to west along the mesoβ scale convergence line, and in contrast to the average wind direction of the storm bearing layer, that is, backpropagating. Some characters of the supercell including hookshaped echo, highlevel echo overhang, mesocyclone and small scale severely shear (similar with TVS) were observed. The middle mesocyclone tornado funnel cloud was generated during extension strengthening, touchdown to low level. The tornado reached its strongest stage during the time of the small scale shear developing both upwardly and downwardly from the middle level. Tornado occurred at the maximum gradient area of weak echo reflectivity of low level. (3) The merge of sea breeze front line and sinking air outflow border of supercell, generated the breeze front located in the south of supercell, and finally a horizontal vorticity tube formed from east to west orientation. A upright vorticity tube twisted when the horizontal tube met the updraft near the weak hooklike echo, forming a lowlevel mesocyclone. The breeze front located in the south of supercell met with eastward moving gust front, and the boundary layer convergence was enhanced, favoring lowlevel mesocyclone to gain the stretch rotating acceleration, and eventually tornado formed.

Abstract:In this article, the nonhydrostatic WRF V3.6.1 (Weather Research and Forecast) model is used to simulate Typhoon ChanHom at high resolution with three nests. The model can well simulate the development and evolution of Typhoon ChanHom as well as the precipitation that it brought to the northeast of Zhejiang. Observation data indicate that the topographic friction convergence and lifting under northeast air flow make the strong convective systems gather in the northwest of the typhoon eye in the vicinity of Zhejiang, with a lot of convective cloud cluster in the convective systems. This is an important reason for the recordbreaking rainfall in northeastern Zhejiang. The control simulation also captures these features. The precipitation system is consistent with orientation of topography, i.e. the rainfall increases on the windward side. The sensitivity experiments show that topography in the northeast area of Zhejiang plays an important role in the increment of the typhoon rainfall. By lowering the terrain, the smallscale convective systems get reduced apparently. Meanwhile, the rainfall in northeastern Zhejiang weakens as well. Thus, topography can impact the structure and evolution of these convective systems, causing the abnormal changes of precipitation. Of more importance is that Typhoon ChanHom maintained the highly symmetric structure in the vicinity of Zhejiang, with the typical eyewall and warm core structure.

Abstract:This paper analyzed structure characteristics of convective clouds over the area between the Yangtze River and the Huaihe River based on Doppler radar data during 2013-2014. The used 227 convective cloud cases are classified into nine structure types. Then, we found that the isolated cell (IC), clusters of cells (CC), nonlinear systems (NL) are the dominant convective clouds over this area, accounting for 29.1%, 18.1%, and 23.3% of the total respectively. Different structural convective clouds occur under different weather backgrounds. Isolated cell is the main convection in the trough I pattern; nonlinear system is the major convection in the trough II pattern. At the same time, other structural convective clouds also occur frequently due to the common synoptic conditions. The convective clouds detected by the CFMCW radar show higher resolution of IC, CC and NL cloud structure images and the maximum falling speeds of precipitation particles, reaching 13.3 m·s-1, 8.2 m·s-1 and 11.5 m·s-1, respectively.

Abstract:Based on the observation and reanalysis data, the spatial mode and corresponding circulation characteristics of precipitation anomaly under pentad scale in Southern China during the 2015 flood season (from April to September) are analyzed by using the methods of empirical orthogonal function (EOF), composite analysis and significance test. The results show that the westeast reverse pattern of precipitation anomaly is the major mode in the developing phases of strong El Ni〖AKn~D〗o years and the pentad pattern in 2015 shows similar features. But the pentad scale of precipitation anomaly patterns can be divided into two types of westeast reverse patterns by using the first and third EOF mode, and they have obvious differences in upper, middle, lower levels of the troposphere. The precipitation anomaly pattern of more in east and less in west (A1) with the first EOF mode is dominated by the anticyclone activities around Philippines at the low level, but the pattern of more in east and less in west (A2) with the third EOF mode is dominated by tropical cyclone activities. The pattern of less in east and more in west (B1) with the first EOF mode is obviously affected by the moving southward of the cold air activities, but the pattern of less in east and more in west (B2) with the third EOF mode is dominated by abnormal position of the anticyclone activities around Philippines at the low level. The precipitation anomaly features of westeast reverse pattern in Southern China during the 2015 flood season are closely related to the positions of anticyclones around Philippines, tropical cyclone activities and the moving path of the cold air activities.

Abstract:In this paper, the yearly tropical cyclone potential impact index (YTCPI) is used as the link to preliminarily study the method of typhoon preevaluation for annual disaster situation in China. The results of preevaluation test show that, from 1991 to 2008, the preevaluation of typhoon disaster for 1996, 1997 and 2004 was lower than the facts, and for other years matched with the actual situation well. The test results of independent samples in 2009-2013 show that, only in 2009 the preevaluation was not correct. In general, it’s feasible to preevaluate the annual disaster situation of typhoon with YTCPI index.

Abstract:From 26 to 28 June 2015, Central Asia vortex caused heavy rainfall in the north slope of Tianshan Mountain. Using conventional data, NCEP reanalysis data and 9 weather stations’ GPS precipitable water vapor (GPSPWV) observations from the groundbased GPS networks, this article did indepth analysis on the vapor features in this process. The results show that: (1) before precipitation, at 500 hPa, the water vapor of the Arabian Sea was transported to Central Asia vortex through the Tibetan Plateau, the vortex was obviously moistened; during precipitation, at 700 hPa, the eastward water vapor transport channel was established, which was from Bay of Bengal through the Sichuan Basin and Hexi Corridor, and the easterly airflow and the southerly (SE) airflow of vortex itself converged the vapor in the heavy rain area. So, the lower atmosphere rapidly moistened in the heavy rain area. (2) All stations GPSPWV experienced wetting process from one to three days, the water jumped once or twice before precipitation began caused by the deep low vortex, and GPS PWV jumped more than 5 mm·(4 h)-1. (3) Under the same water vapor transport and vapor convergence conditions, the stronger the GPSPWV at stations of arid areas surges, the stronger the rain intensity is. To a certain extent, the water vapor transport and convergence has certain corresponding relationship with GPSPWV surge. (4) In 4-5 h before the heavy rainfall in Urumqi caused by Central Asia vortex occurred the growth rate of GPSPWV reached more than 4 mm, and the peak value of GPSPWV was almost twice of climatic mean.

Abstract:Same as with quantitative precipitation forecast (QPF), snowfall depth forecasting is one important operation of weather forecasting in winter, and the relationship between snowfall depth and precipitation which is called snowtoliquid ratio (SLR) abroad is an important parameter required to forecast snowfall depth. In this paper, the main research progresses in variations of SLR and its impact factors at home and aboard in recent decades are reviewed, and the SLR forecast techniques and methods are summarized and compared. Studies indicate that SLR changes obviously in temporal scale, and has seasonal and spatial distribution differences. Air temperature and relative humidity are the two most important meteorological factors affecting SLR. In addition, other meteorological factors such as pressure, vertical motion, surface wind, snowfall weight, solar radiation and snow melting also affect SLR in different degrees. With the development of prediction technology, the methods of SLR forecasting can be summarized as three categories, including climatological, statistical and physical methods. The climatological method is too simple, the application of new statistical methods could improve the prediction ability of SLR, but this method still cannot get rid of the inherent deficiencies of statistics. Comparatively, the method based on the numerical model for predicting instantaneous SLR fits the developing trend toward refinement in snowfall depth forecast in future. At present, observation data of snowfall depth are less and the observation frequency is low in China, thus it is important to effectively carry out the surface snowfall and sounding observation densely, and solve the problem of lack of observation data in the research of SLR. Besides, based on the operational numerical weather prediction model exploring the influence mechanism of meteorological factors on SLR and establishing the forecast operation system suitable for winter snowfall are the developing trend in China.

Abstract:Based on the Doppler weather radar data of Liuzhou and Guilin, and the conventional upperair and surface observation data, this paper analyzes the environmental condition and radar echo structure of the severe hailstorm which occurred in Liuzhou on 9 April 2016. The results show that: (1) The background responsible for the hailstorm includes intense heatingup of the surface which led to the development of thermal depression in the lower atmosphere, the developing eastward cold temperature trough in the midlevel, and an upperlevel jet stream above the convective location. The convective clouds triggered by surface dryline and the mesoscale convergence line evolved into supercells under the influence of the unstable stratification and significant vertical wind shear. (2) The lowlevel reflectivity of the supercell showed a typical hook echo or inverted Vnotch. Along the inflow direction and through the strongest echo, the reflectivity showed typical BWER, overhanging echo and echo wall. The strongest echo was within a vertical narrow zone along the echo wall and the value got to 65 dBz. The corresponding radial velocity in the lower atmosphere had a strong mesocyclone with rotation speed reaching 24 m·s-1. The development and maintenance of mesocyclone made the supercell storm develop and maintain. (3) Maximum reflectivity reaches 60 dBz, strong echo over 50 dBz extending to the height of -30℃ layer, VIL value over 60 kg·m-2 and VIL density over 5.0 g·m-3, mesocyclone in moderate or severe intensity can be taken as the early warning indicator of severe hail.

Abstract:In 2008, the thick icecoating caused by the severe freezing rain covered power transmission line in Zhejiang, leading to enormous damages to Zhejiang power grid. However, only one third of the weather stations observed the glaze with a short duration. In this paper, two severe freezing rain processes which occurred in 2008 and 2013 were analyzed using the global reanalysis data ERAInterim and the disaster records of icecoating on Zhejiang power grid. The results show that there are three layers of coldwarmcold with the warm layer’s temperature above 0℃ during the heavy freezing rain process in Zhejiang Province. In Zhejiang, the liquid water falling from the warm layer would be frozen into ice particles because the temperature in warm layer is lower and the thickness of the bottom ice layer is thicker. Sometimes, the ground temperature is above 0℃, so rain would be observed at weather stations during freezing rain. However, the thickness of bottom ice layer become thinner at high elevation, then tiny liquid water can be frozen into ice particles after it passes through the bottom ice layer. The freezing rain can be found mostly above 400 m elevation of central Zhejiang and above 600 m elevation of southern Zhejiang. According to the freezing rain characteristics in Zhejiang, the falling zone of freezing rain can be calculated based on the global reanalysis data, and the results match the disaster records of icecoating well. In general, this is a new method for freezing rain monitoring and forecasting, especially for some mountainous provinces like Zhejiang.

Abstract:The main characteristics of the general atmospheric circulation are as follows. There was one polar vortex center in the Northern Hemisphere, stronger than usual. The circulation in Eurasian middlehigh latitudes showed a fourwave pattern. The strength of Western Pacific subtropical high was a little weaker than normal. While the south branch trough was a little stronger than normal. The monthly mean temperature was 4.5℃, 0.4℃ higher than normal and the monthly mean precipitation amount was 36.2 mm which is more than normal by 22.7%. Two waves of cold air processes happened in this month, and three regional torrential rain processes occurred during the same period. There were mainly two dust weather events seen in the northern part of China. Besides, severe convection weather appeared in Jiangsu and Hunan.