Object-Oriented Composite Analysis of Warm-Sector Rainfall in North China

Abstract

Abstract Warm-sector rainfall (WSR) occurs, by definition, in a warm-air region that is isolated from any forcing related to synoptic frontal boundaries at the surface. This study explores the use of an object-oriented technique to objectively and automatically identify various WSR events over North China from June to September in 2012–17. A total of 768 substantive events are identified over the 6 years. They have a mean maximum rainfall accumulation of 35 mm h−1. Most such events occur over the plains, with two-frequency maxima: one to the south of the Yanshan Mountain Ranges, and the other near the junction of Henan, Shandong, and Jiangsu provinces. WSR-related rainstorms can form in all warm-season months but are most commonly observed between mid-July and mid-August (40% of all events occurred then). Geographically, the region at greatest risk moves gradually northward from mid-June to mid-August, consistent with the progression of the East Asian summer monsoon. There are two diurnal peaks in WSR activity, one from late afternoon to early evening and the other from late evening to early morning. Three classes of upper-level synoptic pattern seem to be conducive to WSR: (i) a “Mongolia front pattern,” (ii) a “northern China front pattern,” and (iii) a “southern front pattern.” All of these patterns are accompanied by warm and moist southwesterly flow at low levels. Prior to WSR events, there is usually an upper-level trough. According to other studies, such a feature is not usually seen for WSR events in South China.