Initiation and Evolution of Long-Lived Eastward-Propagating Mesoscale Convective Systems over the Second-Step Terrain alongYangtze-Huaihe River Valley

Based on the previous statistical analysis of mesoscale convective systems (MCSs) over the second-step terrain alongYangtze-Huaihe River Valley, eight representative long-lived eastward-propagating MCSs are selected for model-basedsensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstreamareas. We subject each MCS to a semi-idealized (CNTL) simulation and a sensitivity (NOLH) simulation that neglectscondensational heating in the formation region. The CNTL experiment reveals convection forms in the region downstreamof a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere. Upon merging with otherconvective systems, moist convection develops into an MCS, which propagates eastward under the influence of midtroposphericwesterlies, and moves out of the second-step terrain. The MCS then merges with pre-existing local convectionover the plains; the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa. Whilethis vortex moves eastward to regions with local vortex at 850 hPa, another vortex at 925 hPa is also intensified. Finally,the vortices at 850 and 925 hPa merge together and develop into a mesoscale convective vortex (MCV). In contrast, MCSsfail to form and move eastward in the NOLH experiment. In the absence of eastward-propagating MCSs, moist convectionand mesoscale vortices still appear in the plains, but the vortex strength and precipitation intensity are significantlyweakened. It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact thedevelopment and enhancement of moist convection and vortices in the downstream areas.