Effects of Topographic Perturbation on the Precipitation Distribution in Sichuan

Terrain characteristics can be accurately represented in spectrum space. Terrain spectra can quantitatively reflect effects of topographic dynamic forcing on the atmosphere. The one-dimensional weighted-average spatial spectral analysis method is used to explore topographic forcing on precipitation distribution in Sichuan. Results indicate that spectral distributions of terrain and winter precipitation in zonal direction present a typical resonance coupling pattern, while that of terrain and precipitation in other seasons drifts toward the smaller scale. In meridional direction, spectral distributions of terrain and precipitation in each season present the large-scale drift pattern. Different patterns are probably relevant to the change of circulations. In winter, due to strong zonal circulation and weak meridional circulation, atmospheric fluctuations caused by zonal topographic forcing show the most significant impact on precipitation. After that season, the zonal circulation weakens gradually in agreement with the decrease of zonal topographic forcing while the meridional flow enhances, leading to the increase of the damping of the zonal wind disturbance caused by terrain, and the pattern transforms from resonance to drift. Summer rainfall is produced by interaction among different scale systems, and terrain is one of the most important factors. The maximum topographic spectral energy in zonal direction is about an order of magnitude larger than that in meridional direction, implying that effects of topographic dynamic forcing are zonally stronger than that in meridional direction. Values of meridional and zonal topographic characteristic scales are 296.8 km and 475.8 km, respectively, which reflects the characteristic of the mesoscale topographic forcing coincident with the frequent mesoscale systems in Sichuan. The peak of the precipitation spectral energy in summer is about two orders of magnitude larger than that in winter and one order of magnitude larger than that in spring or autumn, and the characteristic scale in summer is about 150 km smaller than that in winter. It illustrates that the intensity of the zonal topographic dynamic forcing in summer is significantly increased when the scale of precipitation systems decreases, which explains the high frequency of mesoscale convective precipitation, and implies the significant impact of topographic dynamic forcing on atmosphere as well. The strongest summer precipitation in Sichuan is located at Ya'an, where larger-scale topographic perturbation is more significant than other region in Sichuan. The terrain spectra and summer precipitation spectra in meridional direction are phase-locked in identical wavelength (37.1 km), implying the critical role of terrain on the occurrence of heavy rainfall, and the effect of topographic dynamic forcing in meridional direction is dominant.