The Relationship Between Isolated Deep Convection Initiation and Topography in the North China Area

The characteristics of isolated deep convection initiation (DCI) and its relation to topography in the North China area are studies statistically and numerically. The infrared brightness temperature data from satellite Himawari-8 are utilized to identify DCI events in three summers. A total of 2534 DCI events are obtained and their locations show clustering over mountains and hills, suggesting the significance of local topography. Topography is described with elevation and relief amplitude. DCI events and grid boxes are counted. DCI events per grid box increases with elevation and relief amplitude. Among different types of topography, DCI is favored in mountains and hilly areas. Moreover, the morning cloud cover condition also shows notable impact on the relation of DCI and topography. For the regime characterized with less morning clouds (regime one), DCI strongly depends on elevation and relief amplitude, while for the regime with more morning clouds (regime two), topography shows a moderate impact on DCI. The time of DCI events are also recorded, and regime one shows a stronger diurnal variation and a peak occurring 2 hours earlier than that of regime two. The synoptic patterns show the difference of large-scale environment between the two regimes, which can explain their differences in DCI to some extent. To clarify the mechanism of topographic effect in DCI process, quasi-idealized numerical simulation in North China is conducted with WRF. The averaged 6-hourly ERA-Interim reanalysis data, which can maintain the major patterns of large-scale circulations, are inputted into the model as initial and boundary conditions. The elevation and relief amplitude of the study domain is varied in the model. The preliminary result shows that the speed of upscale convection growth changes with elevation and relief amplitude, which indicates that mechanisms involving topography-induced variation of solar heating may exist and need further numerical study. We suggest that special attention should be paid to elevation and relief amplitude (or topography type), as well as morning cloud cover condition when forecasting DCI in the North China area and mountainous areas around the world.