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

Based on the previous statistical analysis of mesoscale convective systems (MCSs) over the second-step terrain along Yangtze-Huaihe River Valley, eight representative long-lived eastward-propagating MCSs are selected for model-based sensitivity testing to investigate the initiation and evolution of these types of MCSs as well as their impact on downstream areas. We subject each MCS to a semi-idealized (CNTL) simulation and a sensitivity (NOLH) simulation that neglects condensational heating in the formation region. The CNTL experiment reveals convection forms in the region downstream of a shortwave trough typified by persistent southwesterly winds in the low-to midtroposphere. Upon merging with other convective systems, moist convection develops into an MCS, which propagates eastward under the influence of midtropospheric westerlies, and moves out of the second-step terrain. The MCS then merges with pre-existing local convection over the plains; the merged convection reinforces the cyclonic wind perturbation into a mesoscale vortex at 850 hPa. While this 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, MCSs fail to form and move eastward in the NOLH experiment. In the absence of eastward-propagating MCSs, moist convection and mesoscale vortices still appear in the plains, but the vortex strength and precipitation intensity are significantly weakened. It is suggested the eastward-propagating MCSs over the second-step terrain significantly impact the development and enhancement of moist convection and vortices in the downstream areas.