Possible Impacts Of Land-Surface Process On Two Kinds Of Heavy Rainfall Events

Heavy rainfall is the common disastrous synoptic scale weather system, and if there is a relation to land surface processes is needed to research. This article first to the WRF model simulations of the heavy rain events were evaluated, lay the foundation for later study. Typical heavy rainfall events are selected to investigate the impacts of land-surface process on different kinds of heavy rainfall events by comparing their environmental conditions and physical variables fields based on NCEP reanalysis data. Furthermore, in order to explore the possible physical mechanism of the land surface processes on the heavy rainfall events, sensitivity experiments were carried out by using the meso-scale model WRFV3.3. The main conclusions are as follows:(1) The WRF model generally simulates heavy rainfall events well, although the simulations of large-scale precipitation outperforms the simulations of thermal convective precipitation. The model shows better performance in predicting a larger amount of large-scale precipitation, whereas it performs better in predicting convective precipitation which includes more moderate rainfall. The simulation results of convective precipitation decline with the increasing rain rates. The assessment work indicated that the model performance in simulating the events of large-scale precipitation are better than that of the thermal convective precipitation. So it’s necessary to classify the rainstorms in the study.(2) Jiangxi heavy rainstorm event taking place on12May2012belongs to the type of large-scale precipitation in nature, which is featured by its large spatial scope and long lasting time. Another heavy rainstorm event occurring in Fujian province after noon on23August,2011, is mainly under the control of the Subtropical High. The local sensible heat and latent heat fluxes evidently strengthen the instability of the lower atmosphere, which triggers the thermal convective precipitation. On account of different formation mechanism of the two types, the Jiangxi heavy rainstorm is closely linked to a deep convective system, the convergence center; vertical speed center and instable energy center correspond well to the weather system in both the time evolution and the spatial distribution. However, convergence and instable energy center of the Fujian heavy rainstorm are mainly located near the ground, which may be caused by the momentum, energy and water transport from the underlying surface. Further investigations on the water-vapor budget reveal that almost all of the precipitable water come from the convergence of water-vapor horizontal flux for Jiangxi case, whereas surface evaporation contribute about50.12percent of the precipitable water for Fujian case. From the diagnostic results, it is concluded that the land-surface process has more important impacts on the heavy rainstorm event in Fujian compared to that on the heavy rainstorm in Jiangxi.(3) After closing land-surface fluxes in the numerical experiments, it is found that the effects of land-surface process on the torrential rain are significant, which can even determine the occurrence of convective rainfall in Fujian. Furthermore, several sensitivity experiments are performed by using four land-surface parameterization schemes (LSPs) and results suggest that the two heavy rainfall cases simulated are sensitive to the LSPs. The cumulus convective precipitation is more sensitive to the LSPs in Jiangxi case, while the large-scale precipitation is more sensitive in Fujian case. On the whole, the heavy rainfall case in Fujian is more sensitive to the LSPs than that in Jiangxi. The mechanism responsible for the sensitivity is precipitation-type dependent. For large-scale rainfall, the sensitivity originates from the responses of the wind field near the surface and water vapor field to various LSPs. However, the sensitivity comes from the responses of the land surface-atmosphere flux to LSPs significantly. Disturbance tests of the land-surface parameters are also carried out, and it is found that soil porosity and surface albedo play more important roles in the sensitivity of the convective precipitation to land-surface process compared with the surface roughness and the minimal stomatal conductance. The sensitivity of the model results to the land surface parameters disturbances strongly depends on the disturbance of the land surface-atmosphere fluxes in the nature. The responses of land surface-atmosphere fluxes to the land surface parameters are usually faster, larger in the spatial scale and more regular than that of the wind field. This study can provide a reference for our better understanding on the impacts of land surface process on the weather processes (such as heavy rainfall) and improvement of the performance of the numerical model in simulating the heavy rainfall events.