The Contrastive Analysis And Mechanism Research With Observation On Different And Typical Precipitation

The forecasting of severe convective weather is still difficult now. A comparative analysis of two typical convective rainstorms on 31 July 2006 ( 731 case) and 1 August 2006 ( 801 case) is shown in this paper by using the local conventional and unconventional observation data. The characteristics of three-dimensional wind and temperature on boundary layer of the two cases, and the influence of easterly flow on the initiation of boundary layer convergence line are discussed by using the CINRAD radar data and the VDRAS (Variational Doppler Radar Analysis System). The purpose is to reveal the trigger mechanism of convective storms in Beijing area. It is aimed to provide some scientific basis in forecasting of this kind of convective weather and laid a foundation for the further research in this area.The preliminary conclusion as followed:1,It has been revealed that the 731 convective rainstorm is a very typical event of regional rainfall (most area of Beijing had a 25-55mm precipitation and the main rainfall lasted for about 6h from 08:00-14:00 LST). The westerly trough and surface cold front are the main influence system. We have experience to forecast this kind of weather. While the 801 event was a very local severe convective storm (the squall line system lasted 4h and the severe rainfall continued about 20 minutes but the distribution was inhomogenous) . Beijing was in the rear of northeast low-vortex. It is difficult to analysis this event through analyzing synoptic scale systems.2,The low-level convergence and high-level divergence had already established before the 731 case. The value of CAPE was about 2551.8J/kg at 2:00 LST in the morning. The lifting condensation level was at only 983.7hPa level at the time of rainfall beginning and the unstable energy accumulated a lot. Southeast low level jet provided water vapor for the rainfall. Before 801 case, Beijing was in the cold advection behind the trough and the air was tending to stable. It was moist in the ground and dry in the mid-high altitude because of the rainfall in the previous day. High-level jet occurred at 200hPa level. CAPE decreased from 788.6 J/kg to 369.3 J/kg and the lifting condensation level became to 790.7hPa at 14:00 LST but the CIN decreased to 0 J/kg since the morning. A strong vertical wind shear appeared on the level of 1000-1300m at 16:30 LST by wind profiler observations. In other words, it is possible that the severe convection appeared though the unstable energy was weak. 3,From the observations of radar:(1)The 731 event was caused by the large-scale precipitation echo that came from Hebei province. Meso-scale convergence line provided the trigger condition of the rainfall initiation. The 801 event was a strong squall line with obvious bow echo characteristic. But it could be seen that the intensity of 801 is much stronger than 731 by lifting the radar elevation. There was still severe radar echo of about 60dBZ at 3.4°elevation level.(2)The 0 isanemone of 731 event appeared S -type distribution, but 801 did not. There was a strong wind shear in vertical and a cattle eye -type characteristic appeared that combined with the outflow of the gust front.(3)The primary analysis of VAD wind profiler data showed that for the 731 case, the wind rotated clockwise as the height rising. But for 801 case, a strong vertical wind shear appeared on the level about 1.2-1.8km at 16:28 LST. It indicated the severe convection will develop later.(4)By calculating the VIL, it is found that the value of VIL in 801 was about 10 kg/m2 more than 731 during the storm development. It may be the cause of the hail and strong wind in 801 case but not in 731 event.4,Analysis from the retrieval of three-dimensional wind and perturbation temperature by VDRAS showed that:(1)In the 731 event, the wind structure is typical of regional rainstorm. The convergence line at low level did not play a leading role. In the 801 event, the intensification, weakening and north-south movement of the easterly flow at low level (about 900-1300m) had a distinct effect on initiation, enhancement and dissipation of the squall line, as well as persistence of the convergence line.(2)The temperature was cold at high-level and warm at low-level of 731 event, and the temperature of urban zone is obvious higher than suburbs. The development of 801 case was closely combined with the temperature gradient between the urban zone and suburbs. As the squall line developed, a strong cold pool appeared in the rear of storm.5,The preliminary discussed of the trigger mechanism of 801 severe convection as follows:(1) A strong easterly flow initiated at low level in Beijing urban zone and southeast of great Beijing area before the squall line reached. It brought the warm and dry air (comparing to the surface) to the top of Beijing. The air accumulated and formed a dry and warm lid; On the other hand, it collided with across mountain northwesterly wind and be forced by the topographic uplift to trigger a boundary layer convergence line which maybe one of the trigger mechanism of the severe convection.(2) The distribution of the vertical velocity displayed symmetrically, and the temperature at mid-high level was higher than low-level. It is even more wet at low-level than the upper air. The vertical velocity center located at about 1-2km level.