Image Processing of Radar Mosaics for the Climatology of Convection Initiation in South China

Abstract

A dataset of convection initiation (CI) is of great value in studying the triggering mechanisms of deep moist convection and evaluating the performances of numerical models. In recent years, the data quality of the operationally generated radar mosaics over China has been greatly improved, which provides an opportunity to retrieve a CI dataset from that region. In this work, an attempt is made to reveal the potential of applying a simple framework of objective CI detection for the study of CI climatology in China. The framework was tested using radar mosaic maps in South China that were accessible online. The identified CI events were validated in both direct and indirect ways. On the basis of a direct manual check, nearly all of the identified CI cells had an organized motion. The precipitation echoes of the cells had a median duration of approximately 2.5 h. The CI occurrences were further compared with rainfall estimates to ensure physical consistency. The diurnal cycle of CI occurrence exhibits three major modes: a late-night-to-morning peak at the windward coasts and offshore, a noon-to-late-afternoon peak on the coastal land, and an evening-to-early-morning peak over the northwestern highland. These spatial modes agree well with those of rainfall, indirectly suggesting the reliability of the CI statistics. By processing radar mosaic maps, such a framework could be applied for studying CI climatology over China and other regions.

Convection Initiation in Monsoon Coastal Areas (South China)

“…These radar mosaics were operationally produced by the China Meteorological Administration (CMA). The feasibility of applying such radar data to objectively retrieve a CI data set was demonstrated in Bai, Chen, et al (2020). The CMA has applied many advanced approaches to improve the quality of radar mosaics in the last decade, especially since 2013.…”

“…The identified CI tends to be associated with locally developed new convection. For more details regarding CI identification in South China, please refer to Bai, Chen, et al (2020). Nearly 25,600 CI events were detected during the 42 warm‐season months from 2013 to 2019.…”

“…Such observational data at unprecedentedly high spatiotemporal resolutions allow us to keep up with rapidly evolving convective activities and thus detect CI (e.g., Chen et al, 2012; Fabry et al, 2013; Haberlie et al, 2015; Reif & Bluestein, 2017; Weckwerth et al, 2011). Recently, we managed to retrieve the high‐quality CI data sets in South China through image processing of historical radar mosaics and verified these data through other data sources (Bai, Chen, et al, 2020). The newly available CI data sets thus provide us with a great opportunity to reveal the CI climatology on monsoon coasts.…”

“…In panel (b), the mountains labeled M1, M2, M3 and M4 are described in the text. The hatched area within the contours indicates the terrain at an altitude of 250 m. In panel (c), the diurnal variation of rainfall amplitude (red; Bai, Chen, et al, 2020) is derived from the CMORPH data during April–September from 2013 to 2018, which is averaged in the domain to the north of the blue line in panel (a). The diurnal and monthly variations of CI occurrence were estimated by counting all the CI samples shown in panel (b).…”

Relative Roles of Low‐Level Wind Speed and Moisture in the Diurnal Cycle of Rainfall Over a Tropical Island Under Monsoonal Flows

“…In this study, we will focus on the rainfall diurnal cycle over a monsoonal tropical island (Hainan Island) during the East Asian summer monsoon season. Located in South China, Hainan Island is characterized by active convection and associated precipitation in the monsoon season (e.g., Bai et al, 2020; Zhu et al, 2017). In South China, the rainfall amount from April to June (also called Mei‐Yu or pre‐rainy season) typically accounts for more than 50% of the annual rainfall (Ding, 1992).…”