| Based on different spatial and temporal resolutions precipitation dataset in Shaanxi, Gansu, Ningxia Province and NCEP reanalysis data, the temporal-spatial distribution of heavy rainfall in different class during flood season, synoptic conceptual models and their characteristic of physical parameters have been firstly explored. Then a typical case of heavy rainfall happened on7th August2010in south-central part of Gansu province has been chosen to study in detail. During the study of this case, many new type observational data have been used to find large-scale environment background and the evolution of Mesoscale Convective System. The results mainly indicate that:(1) In recent20years (1991-2010a), the precipitation and the cumulative frequency of heavy rainfall in flood reason appeared increased in Shan-Gan-Ning region, especially in recent6years (2005-2010a), the heavy rainfall happened more frequently. In recent6years, heavy rainfall in flood season is mainly occurred in its east area where there are4active and3inactive areas. Heavy rainfall is most active in July and August. Last dekad of June and second dekad of August are two pieces of highly occurring period from May to September. The severe precipitation is more likely to occur in August. Diurnal variation presents the bimodal distribution. The heavy rain (1hr≥30mm) which tends to happen mainly in the evening is closely associated with special terrain such as windward slop of mountain, trumpet-shaped terrain.(2) Three synoptic conceptual models (Trough and subtropical high pattern, Low vortex and typhoon far away pattern, Shear between two high pressure pattern) have been refined from32typical cases. In addition to common features, each of conceptual models has its unique characteristic represented by concering physical parameters.(3) The "100807" process is a typical case of two high pressure pattern. The interaction of the break of high pressure dam on500hPa, the mesoscale vortex over the sourth of Gansu on700hPa and the south flank divergence’s formation of westerly jet on200hPa had provided the advantageous large-scale environment background for heavy rain. The cold air came primarily from the middle layer (700hPa). Poor cold air on the ground impacted the different areas at the different period of the process, which resulted in scattered precipitation. And the tropical depression, subtropical-high and the topography of Tibetan Plateau made a contribution to establish the moisture transmission channel. Rapid increase of water vapor on surface and strong vertical moisture transmission guaranteed that enough moisture could produce a large amount of rainfall in a short time.(4) During the period of strong rainfall, there was a strong divergence (positive vorticity) on the upper level while convergence (negative vorticity) on the lower level. In the afternoon, with shape increase in surface temperature, south-central part of Gansu province gradually accumulated amount of unstable energy. The cold air piling up the warm advection enhanced stratification instability in the evening. They all provided a favorable dynamic and thermodynamic conditons for heavy rain.(5)There were four mesoscale rain cell during the whole process which resulted from Mesoscale Convective System. It is during the MCS’s mature phase that heavy rainfall occurred most frequently and widely. The heavy rainfall often appeared in the area of low TBB(-72℃or-62℃) or the sharp and dense TBB on the east of low center. The heavy rain of LinTao in17,18h developed from the combination of two cells. After the combinatin, echo of new cell became strong and echo top height ascended, which resulted in the most severe rainfall.(6)The initial trigger for this MCS is the surface mesoscale convergency, and the surface mesoscale convergence (shear) line or mesoscale vortex had a closely corresponding with strong rainfall areas. The negative lightning flashes which usually happened before the appearance of heavy rainfall played a dominated role in the CG lightning activities. The sharp increase of lightning flashes is an obvious signal that the convective system would get rapid development in the next1-2hours. The concentrated area of lightning flashes indicates the direction of convective system’s movement and also has a closely relationship with strong rainfall area.(10) Local mesoscal topography had an impact on meso-y extreme precipitation occurred in zhouqu which triggered massive mudslide. |
PDF Full Text Request