In order to meet the demand of nowcasting convective storms in Beijing, the climatological characteristics of convective storms in Beijing and its vicinity were analyzed based on the infrared (IR) temperature of black body (TBB) data during May―August of 1997―2004. The climatological probabilities, the diurnal cycle and the spatial distribution of convective storms are given respectively in this paper. The results show that the climatological characteristics of convective storms denoted by TBB≤-52℃ are consistent with those statistic studies based on the surface and lightning observations. Furthermore, the climatological characteristics of May and June are very different from those of July and August, showing that there are two types of convective storms in this region. One occurs in the transient polar air mass on the midlatitude continent during the late spring and early summer. This type of convection arises with thunder, strong wind gust and hail over the mountainous area in the northern part of this region from afternoon to nightfall, the other occurs with heavy rainfall in the warm and moist air mass over the North China Plain and vicinity of Bohai Sea. This study also shows that the long-term data of IR TBB observed by geostationary satellite can complement the temporal and spatial limitation of the weather radar and surface observations.
Spiral rainband is a prominent structure of tropical cyclone. Though its forming mechanism, vortex Rossby wave theory, has been widely accepted in recent years, its internal structural features are still not well known. The spiral rainband in the severe tropical storm Kammuri (2002), which caused heavy rainfall in southeast China, is simulated using the mesoscale model MM5 (V3). Results show that the simulated spiral rainband propagates azimuthally at a speed close to that of vortex Rossby wave in theory, and is accom- panied with energy dispersion in the radial direction. The structural features of simulated spiral rainband are analyzed with the high-resolution model output including the full physical process. Positive vorticity, ascending motion, hori- zontal momentum and so on are highly concentrated in the spiral rainband. The convergent moisture of spiral rainband comes mostly from the planetary boundary layer under 1 km. Airflow from the outside of spiral rainband is convective instability, which can provide instability energy for convec- tion development. However, the atmospheric stratification in the inside of spiral rainband is neutral, implying that the instability energy has been released. There is a mesoscale strong wind band just near the spiral rainband in the outer side with a maximum wind speed exceeding 30 m/s, which results from the pressure force acceleration when the air flows into the spiral rainband along the gradient of pressure.
The complicated evolutive process of how a tropical cyclone transforms into an extratropical cyclone is still an unresolved issue to date, especially one which arises in a weakly baroclinic environment. Typhoon Winnie （1997） is studied during its extratropical transformation stage of extratropical transition （ET） with observational data and numerical simulations. Results show that Winnie experienced its extratropical transformation to the south of the subtropical high without intrusion of the mid-latitude baroclinic zone. This is significantly different from previous studies. Analyses reveal that the cold air, which appeared in the north edge of Winnie circulation, resulted from the precipitation drag and cooling effect of latent heat absorption associated with the intense precipitation there. The cooling only happened below 3 km and the greatest cooling was below 1 km. With the cold air and its advection by the circulation of Winnie, a front was formed in the lower troposphere. The front above is related not only to the cooling in the lower level but also to the warming effect of latent heat release in the middle-upper levels. The different temperature variation in the vertical caused the temperature gradient over Winnie and resulted in the baroclinicity.
This study analyzed the climatological characteristics of severe convective storms in the Beijing and Tianjin region and its vicinity based on the Doppler radar data of Tanggu during May―August of 2003― 2007. The climatological characteristics, e.g. storm area, volume, top height, max reflectivity, life time and motion, are analyzed. The results include: 75% of all storms in the Beijing-Tianjin region last no more than 30 minutes, and most storms have a volume less than 400 km3; most storms move from southwest to northeast while the speed is between 10―30 km/h; the mean storm top height is about 6 km, but some strong convective storms can have a top height larger than 15 km; finally, storm area and volume have a similar geographical distribution character showing increasing trends from west to east. Compared with the statistic results based on the conventional surface meteorological observations, the results based on the radar data can present not only 3D spatial statistic results of convective storms (e.g., volume and top height), but also the quantitative climatological characteristics, such as the life-time and speed distributions. These statistical results are useful for studying the climatic characteristics of convective storms in the Beijing-Tianjin region and its vicinity.
HAN LeiYU XiaoDingZHENG YongGuangCHEN MingXuanWANG HongQingLIN YinJing
