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.

Automated summary

This study examined the synoptic impacts on the occurrence of boundaries and associated convection in the Hetao Irrigation District in North China from 2012 to 2016. The results showed that boundary days, especially convective boundary days, were more frequent under a mid-troposphere high-pressure ridge or pre-trough pattern. These patterns were characterized by warmer near-surface environment, stronger southerly winds, larger convective available potential energy (CAPE), and low-level moisture convergence. Boundaries under favorable patterns showed more apparent arid-area dependence, more dominant orientation along the vegetation contrast, more northward movement, earlier formation and maturation, and more dispersed convection. Boundary days had higher near-surface temperatures and stronger ambient flows against the vegetation-contrast-induced circulations compared to nonboundary days. Additionally, convective boundary days had stronger updrafts, larger CAPE, and higher moisture and K index compared to nonconvective boundary days.