Satellite Observations of Organizational Morphology Associated with the Extreme-rain-producing Mesoscale Convective Systems over Northwestern China

The mesoscale convection system (MCS) is known as a substantial contributor to flash flooding, hailstorms, wind gusts, and tornadoes. The organizational morphology of mesoscale convection systems (MCSs) can reflect the initiation, development, and propagation processes of convective cells in the system, which are the keys to determining whether heavy precipitation can be produced. So far, the organizational modes for the MCSs over mountainous areas or areas with sparse radar stations often fail to be wholly understood due to geographical conditions that restrict radar observation. This paper proposes a method for making an objective MCS classification and segmentation based on geostationary satellite observations. In this study, the algorithm is developed using Himawari/AHI multiple infrared and water vapor channel observations, which have a 2 km and 10-min spatial-temporal resolution. Validation of the product relative to the classification in radar composite reflectivity imagery indicates that the algorithm offers skill for discriminating between convective and stratiform areas and matches 65% of convective area identifications in radar imagery with a false alarm rate of 39% and an accuracy of 94%. A quantitative evaluation of the similarity between 50 given MCSs' structures derived from satellite and radar observations shows that the similarity between the two is as high as 60%. For further testing, organizational modes of the MCS that induced the heavy precipitation in northwest China on 21 August 2016 (hereafter known as the "0821" rainstorm) are identified. It is found the MCS accompanied by the "0821" rainstorm successively exhibits modes of the isolated cell (IC), squall line with parallel stratiform rain (PS), and nonlinear system (NL) during its lifecycle. In the developing stage of the MCS, the system is characterized by PS and NL structures. As compared to precipitation intensity, the PS mode might play a key role in producing this flooding. These findings suggest that the PS-organized MCS may be an essential structural feature of storms that triggers extreme heavy rainfall in the semi-arid region of northwest China.