Synoptic Impacts on the Occurrence of Mesoscale Boundaries and Their Associated Convection Over an Area of Sharp Vegetation Contrast.

This study examined synoptic impacts on the occurrence of boundaries and their associated convection during summers from 2012 to 2016 over Hetao Irrigation District in North China, where there is a sharp vegetation contrast. Objective synoptic‐pattern classification showed that boundary days, especially convective boundary days, were more frequent under a mid‐troposphere high‐pressure ridge or pre‐trough pattern. Such patterns featured a warmer near‐surface environment, stronger southerly winds, larger convective available potential energy (CAPE), and low‐level moisture convergence. Relative to unfavorable patterns, boundaries under favorable patterns showed more apparent arid‐area dependence, more dominant orientation along the line of vegetation contrast, more moved northward, formed and matured earlier, and triggered convection more dispersedly. Under each pattern, compared with nonboundary days, boundary days possessed higher near‐surface temperatures and stronger ambient flows against the vegetation‐contrast‐induced circulations near the surface; plus, compared with nonconvective boundary days, convective boundary days possessed stronger updrafts, larger CAPE, and higher moisture and K index. Plain Language Summary: Boundary layer convergence lines (boundaries), which often manifest as fine lines of clear‐air radar echoes, are an important mesoscale mechanism of convective initiation. Mesoscale boundaries induced by vegetation heterogeneity are far less well understood than those induced by the contrast between sea or lakes and land, particularly in terms of synoptic impacts on the occurrence of such boundaries and their associated convection. Based on a boundary data set during the five summers from 2012 to 2016 over Hetao Irrigation District (HID)—an area in North China with a sharp contrast in vegetation, mainly between desert and irrigation areas—the aim of the present study was to link these boundaries and their associated convection with different synoptic features via objective synoptic‐pattern classification. Results showed that the boundaries were more likely to occur and trigger convection on days when HID was under a high‐pressure ridge or pre‐trough pattern. The favorable patterns had a warmer environment with stronger southerly flows for boundaries to develop, along with larger convective available potential energy and moisture convergence for boundaries to trigger convection. Their associated boundaries showed more apparent arid‐area dependence, more dominant orientation along the vegetation interface, more northward movements, earlier formation and maturity, and more dispersed convection initiation timing. Key Points: Boundaries and their convection over Hetao Irrigation District prefer mid‐level high‐pressure‐ridge or pre‐trough synoptic patternsThe favorable patterns have a warmer boundary layer with more frequent southerly flows, larger CAPE, and stronger moisture convergenceInstability and moisture effectively distinguish convective from nonconvective boundary days but not boundary from nonboundary days [ABSTRACT FROM AUTHOR]