Spatial Inhomogeneity of Synoptic-Induced Precipitation in a Region of Steep Topographic Relief: A Case Study

The topography plays an essential role in the initiation and development of precipitating clouds, therefore has a profound effect on the ultimate spatial distribution of precipitation. This study investigates the fine-scale characteristics of a synoptic-induced precipitation event in Southwest China, a region characterized by a sequence of steep mountains aligned roughly north-south. The convection-permitting simulation successfully reproduces the observed rainband induced by a synoptic-scale shear line. The spatial distribution of accumulated precipitation over three small-scale mountains (named M1, M2, and M3 from east to west) exhibits distinct inhomogeneity. The accumulated precipitation is significantly enhanced on the western slope of M1, the high-altitude area of M2, and the eastern slope of M3. The low-level vortex generated on the western slope of M1, as well as the convergence established over M2 and the eastern slope of M3, dynamically contributes to the enhanced precipitation over the various mountain locations. As the highest of the three mountains, M2 exhibits pronounced blocking effects on the mesoscale circulations. Additional sensitivity experiment demonstrates that the mesoscale circulations and corresponding precipitation are sensitive to the highest elevation in the continuous mountains. The rainfall accumulation over M2 (M3) could decrease (increase) by 54.3% (63.4%), when the terrain of M2 is reduced to the comparable height of surrounding mountains. With more upslope wind flowing over M2, the convergence over M3 is strengthened. The more intense upward motion and stronger potential instability both contribute to the notable increase of precipitation over M3. This study implies the possible mechanisms of the inhomogeneous precipitation over Southwest China and could deepen the current understanding of the topographic effects on precipitation over complex terrains. Southwest China is characterized by high average elevation with a succession of quasi-north-south-oriented mountains. Such topography with local variance tends to generate inhomogeneous distribution of precipitation. In this study, we explore the spatial characteristics of a typical synoptic-induced rainfall case that occurs in Southwest China and examine the role of steep topography relief in this rainfall process by using a convection-permitting model. The simulation reasonably reproduces the distribution and evolution features of the observed rainband. We find significant enhancement of precipitation in the different locations of the small-scale mountains. The steep topographic relief modulates the distribution of precipitation by exerting dominant influences on local circulations. The results from the sensitivity experiment, in which the terrain of Cang Mountain reduced to the comparable height as the surrounding mountains, further reveal that the highest mountain peaks in this complex terrain region plays a key role in the near-surface flow patterns and thermal conditions. Consequently, this leads to variations in the enhancement of precipitation over the mountains. The precipitation under strong synoptic forcing is significantly enhanced by the steep topography relief of Southwest China The mesoscale circulations modulated by the steep mountains play an essential role in shaping the distribution of precipitation The mesoscale circulations and corresponding precipitation are sensitive to the highest elevation in the continuous mountain ranges