A Multiscale Analysis of a Nocturnal Extreme Rainfall Event of 14 July 2017 in Northeast China

A multiscale observational analysis of a nocturnal extreme rainfall event that occurred at Changtu in Northeast China on 14 July 2017 is performed using global analysis, automated surface observations, Doppler radar, rawinsonde, and disdrometer data. Results show that the large-scale environment was characterized by high convective available potential energy and precipitable water, moderate convective inhibition, and a southwesterly low-level jet (LLJ) capped by an inversion layer. The first and subsequent convective cells developed along a quasi-stationary surface convergence zone in a convection-void region of a previously dissipated meso-a-scale convective line. Continuous convective initiation through backbuilding at the western end and the subsequent merging of eastward-moving convective cells led to the formation of a near-zonally oriented meso-b-scale rainband, with reflectivity exceeding 45 dBZ (i.e., convective core intensity). This quasistationary rainband was maintained along the convergence zone by the LLJ of warm moist air, aided by local topographical lifting and convectively generated outflows. A maximum hourly rainfall amount of 96mm occurred during 0200-0300 Beijing standard time as individual convective cores with a melting layer of.55 dBZ reflectivity moved across Changtu with little intermittency. The extreme-rain-producing stage was characterized with near-saturated vertical columns, and rapid number concentration increases of all raindrop sizes. It is concluded that the formation of the meso-b-scale rainband with continuous convective backbuilding, and the subsequent echo-training of convective cores with growing intensity and width as well as significant fallouts of frozen particles accounted for the generation of this extreme rainfall event. This extreme event was enhanced by local topography and the formation of a mesovortex of 20-30 km in diameter.