General Features of Radar‐Observed Boundary Layer Convergence Lines and Their Associated Convection over a Sharp Vegetation‐Contrast Area

Boundary layer convergence lines (referred to as boundaries hereafter), by acting as a lifting mechanism, have long been recognized as an important trigger to convective initiation and development. Boundaries and their associated convective activities often develop as a result of the differential heating due to land surface contrasts. Some of such land surface contrasts have been well documented in relevant statistical studies, such as mountain‐valley contrast, land‐sea contrast, and land‐lake contrast. However, how boundaries and their associated convection respond to vegetation contrast has little been studied. Answering this question is particularly important for arid/semiarid areas where the vegetation cover keeps changing due to both natural processes and anthropogenic modifications (e.g., desertification, afforestation, and irrigation).

The irrigation area in the bend of the Yellow River and its vicinity in Inner Mongolia, North China (hereafter referred to as Hetao Irrigation District), provides an ideal vegetation contrast for studying boundaries and their associated convection. Hetao Irrigation District is one of the most extensive irrigations in Asia, and the cultivated land is mainly irrigated by the Yellow River. It is surrounded by wide arid areas, such as deserts. Many long‐lasting mesoscale convective systems that had a high impact in East China originated in this area. Using satellite images, previous studies proposed that the cloud formation frequency over the irrigation district was lower than that over the surrounding arid area. However, the vegetation contrast has not been related with boundaries or associated convective processes.

The present study intends to explore the climatology of boundaries and their associated convection in response to vegetation contrast around Hetao Irrigation District based on five‐year summertime radar observations. A total of 323 boundaries were identified with 44% being convection‐associated. Results showed that the boundaries especially the convective boundaries were more frequent over the arid area than those over the vegetated area and tended to have an orientation parallel to the vegetation contrast line. The boundary activities collocated well with the diurnal variation in surface temperature difference across the vegetation contrast. Compared with the nonconvective boundaries, the convective boundaries formed earlier and moved faster into the inner arid area, obtained maximum length around midday, and then initiated convection. It was also found that vegetation contrast might also affect the high‐frequency location and magnitude of boundary‐associated convective precipitation.