Characteristics Of Warm-Season Precipitation Over Beijing-Tianjin-Hebei Region

Statistics analysis is applied to explore the inter-annual variation and spectral distribution of precipitation during May to September of 2007~2011 in North China(mainly include the cities of Beijing and Tianjin, Hebei Province). The result shows that robust temporal and spatial differences of precipitation during the four years, but the spatial variation of the four years are nearly the same. The spatial distribution of precipitation over the area of Beijing shows the less precipitation in northwestern but more over the eastern and southern area along the terrain orientation, which means that the terrain effect is the main reason for the precipitation distribution. Monthly precipitation also shows variation between different months and different areas. Diurnal variation of the precipitation during Bejing area features the maximum in the evening to early morning and minimum in the afternoon to early evening.Based on the statistics analysis above, a strong precipitation case and a weak precipitation case are chosen to compare the synoptic and mesoscale characteristic of the precipitation east of the second step terrain. Though two cases originates from the eastern part of the second step terrain, the occurring times and the paths of eastward propagation are different with each other. The strong case appears before the low trough and north of the Western Pacific Tropical High. Abundant water vapor is transported into the study area by the northwestern flow, which favors the strong precipitation. The synoptic circulation of the weak case is the straight westerlies. The short-wavelength trough and the low vortex the main dynamic background for the precipitation. For the both cases, the mesoscale vortex directly triggers the rainfall. The vorticity budget shows that the horizontal advection of the vorticity is vital for the vortex development in the weak case and stretching is also positive the strengthening of the vortex. However, for the strong case, both the vertical advection and stretching are favorable for the sharp vorticity increasing and stretching is essential for the maintenance of the vortex. The common feature of the cases is positive effect of the stretching during the whole life of the two vortices.