15 JUNE 2020 Y A N G E T A L . 5013 Subseasonal and Diurnal Variability in Lightning and Storm Activity over the Yangtze River Delta, China, during Mei-yu Season a,b,c a,b d,e a,b c JI YANG, KUN ZHAO, XINGCHAO CHEN, ANNING HUANG, YUANYUAN ZHENG, AND a,b,c KANGYUAN SUN a Key Laboratory for Mesoscale Severe Weather/Ministry of Education and School of Atmospheric Science, Nanjing University, Nanjing, China b State Key Laboratory of Severe Weather and Joint Center for Atmospheric Radar Research of the China Meteorological Administration and Nanjing University, Beijing, China c Jiangsu Research Institute of Meteorological Sciences, Nanjing, China d Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, Pennsylvania e Center for Advanced Data Assimilation and Predictability Techniques, The Pennsylvania State University, University Park, Pennsylvania (Manuscript received 20 June 2019, in final form 16 March 2020) ABSTRACT Using 5 years of operational Doppler radar, cloud-to-ground (CG) lightning observations, and National Centers for Environmental Prediction reanalysis data, this study examined the spatial and temporal char- acteristics of and correlations between summer storm and lightning activity over the Yangtze River Delta (YRD), with a focus on subseasonal variability and diurnal cycles. The spatiotemporal features of storm top, duration, maximum reflectivity, size, and cell-based vertical integrated liquid water were investigated using the Storm Cell Identification and Tracking algorithm. Our results revealed that there was high storm activity over the YRD, with weak diurnal variations during the mei-yu period. Specifically, storms were strongly associated with mei-yu fronts and exhibited a moderate size, duration, and intensity. On average, afternoon storms exhibited stronger reflectivity and higher storm tops than morning storms, as evidenced by the af- ternoon peak in CG lightning. The storm intensity strengthened in the post-mei-yu period, due to an increase in atmospheric instability; this was accompanied by a higher frequency of CG lighting. The diurnal cycles of storm frequency and CG lightning in the post-mei-yu period followed a unimodal pattern with an afternoon peak. This is attributable to increased thermodynamic instability in the afternoon, as little diurnal variation was observed for wind shear and moisture. An inverse correlation between lightning occurrence and mean peak current (MPC) for negative CG lightning was evident during the pre-mei-yu and mei-yu periods. The diurnal variation in MPC for negative CG lightning agreed well with that for storm intensity. 1. Introduction weather and climate predictions. For example, the temperature and humidity in the upper troposphere The spatial distribution and diurnal cycles of storms can be affected by the height and intensity of storms and precipitation over East Asia during the monsoon (Folkins 2002). In addition, storm properties can be have been investigated extensively in recent years (Lin used as evaluation benchmarks for numerical models et al. 2011; M. Chen et al. 2014; Chen et al. 2015, 2017, and parameterization schemes (Davis et al. 2003; 2018). However, few studies have focused on the prop- Arakawa 2004; Liang et al. 2004; Starzec et al. 2018). erties of warm-season storms or their diurnal cycles over The mei-yu (baiu in Japan) season over the YRD is the Yangtze River Delta (YRD) in China, which is lo- associated with a northward shift of the East Asian cated in one of three maximum rainfall centers collec- summer monsoon (Sampe and Xie 2010; Luo et al. tively referred to as the Yangtze–Huaihe River basin 2013). During the mei-yu period (mid-June to mid- (YHRB) (Ding and Chan 2005). An understanding July), a zonal rainband with heavy rainfall is frequently of these storms and their properties is important for observed. This rainband, referred to as the mei-yu front, is controlled by frontal structures in the lower tropo- Corresponding author: Kun Zhao, [email protected] sphere and characterized by a sea level pressure trough, DOI: 10.1175/JCLI-D-19-0453.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/01/21 11:32 AM UTC 5014 JOURNAL OF CLIMATE VOLUME 33 horizontal shear, sharp moisture gradients, and rela- from the Chinese operational radar network provide a new tively weak temperature gradients (Ding and Chan opportunity to gain insight into the three-dimensional (3D) 2005; Sampe and Xie 2010). Previous studies have shown structure and evolution of storms during different phases of that the associated convection that becomes organized the monsoon period for characterizing diurnal variations in in the midnight-to-morning hours during the mei-yu storm properties. period is closely related to ascending moisture flow over Lightning activity is an effective indicator of deep convectively generated cold pools (Luo et al. 2014; He convection and climate change, and has been widely et al. 2018). Another possible mechanism driving the investigated over different regions (Hondl and Eilts mei-yu front is the synoptic-scale frontal convergence 1994; Zipser and Lutz 1994; Watson et al. 1995; created by accelerated southerly moisture flows result- Gremillion and Orville 1999; Zipser et al. 2006; Yuan ing from clockwise rotation of low-level winds (Chen and Qie 2008; Mosier et al. 2011; Seroka et al. 2012; et al. 2010; Chen et al. 2017). Xue et al. (2018) used Metzger and Nuss 2013; Qie et al. 2014; Zheng et al. convection-permitting simulations to investigate the key 2016). Laboratory studies show that riming electrifi- mechanism controlling the mei-yu precipitation diurnal cation is the main charge separation mechanism asso- cycle. They found that the convergence forced by low- ciated with thunderstorms, occurring mainly during level ageostrophic winds results in an early morning graupel–ice crystal collisions with supercooled water peak and an evening minimum in precipitation, which (Takahashi 1978; Saunders 1993). Many previous stud- can be explained by the boundary layer inertial oscilla- ies concentrated on the lightning rate, spatial distribu- tion theory (Blackadar 1957). Diurnal variations in the tion, and diurnal cycle (Zajac and Rutledge 2001; Qie precipitation amount, frequency, and intensity have re- et al. 2003; Yuan and Qie 2008; Xia et al. 2015, 2018; ceived more attention in recent years (Chen et al. 2010; Yang et al. 2016). Observational studies have indicated Bao et al. 2011; Xu and Zipser 2011; M. Chen et al. 2012; that the peak current of negative lightning is closely G. Chen et al. 2012; G. Chen et al. 2014; Chen et al. 2017; related to thunderstorm intensity. This putative rela- Xu 2013). During the mei-yu period, frequent nocturnal tionship is associated with the small eddies created by long-duration rainfall events with shallow echo tops strong turbulence within afternoon storms; under these contribute to the early-morning rainfall maximum over conditions, the distances between electrostatic thun- the YRD, whereas deep convection produces a sec- dercloud charges become smaller, resulting in frequent ondary afternoon peak in precipitation (Chen et al. lightning occurrences but with less charge available to 2010; Xu and Zipser 2011). During the post-mei-yu flow into the return stroke, resulting in a smaller peak period, the diurnal cycle of deep convection, precipi- current and flash size (Bruning and MacGorman 2013; tation, and lightning exhibits a single peak (Xu and Chronis et al. 2015a,b). However, there have been rel- Zipser 2011). Luo et al. (2013) compared the summer atively few studies investigating the relationship be- storm properties over southern China and the YHRB tween various storm properties and the peak amount of based on satellite observations. They found that the lightning (Shafer et al. 2000; Carey and Rutledge 2003; contribution of heavy rainfall to total rainfall amount is Zheng et al. 2016). greater in the YHRB than in southern China. They also In this study, the summer storm properties and light- showed that convective intensity strengthens progres- ning activity over the YRD during the pre-mei-yu, mei- sively from the premonsoon period to the monsoon yu, and post-mei-yu periods from 2010 to 2014 were period and further to the postmonsoon period over the investigated. In contrast to previous studies based on YHRB and southern China. Furthermore, in most of satellite imagery, high-resolution ground-based radar the Asian monsoon regions, convection is stronger during and cloud-to-ground (CG) lightning observations were the monsoon break and postmonsoon periods than dur- used here for the first time to reveal the spatiotemporal ing monsoon activity (Kodama et al. 2005; Yuan and Qie structure of storms and lightning activity with fine resolu- 2008; Xu et al. 2009; Xu and Zipser 2011; Luo et al. tion. Relevant atmospheric conditions were also examined 2013). The abovementioned studies focused mainly on using reanalysis data. Based on statistical characterization, the spatial distribution of storms and cloud tops across the environmental conditions influencing storm activity different regions and/or periods based on satellite data and CG lightning over the YRD are discussed with respect such as data from the Tropical Rainfall Measuring to the different monsoon phases. Mission that recorded rainfall from a particular location The rest of the paper is organized as follows. The data twice a day. Temporal resolution limitations and atten- and methodology are introduced in section 2. Section 3 uation (Xu 2013) have limited the ability to investigate describes the large-scale flow and environmental con- the properties of storms, including storm lifetime, ditions during the pre-mei-yu, mei-yu, and post-mei-yu movement, vertical structure, and diurnal cycle. Observations periods. Section 4 presents the spatial distribution of Unauthenticated | Downloaded 10/01/21 11:32 AM UTC 15 JUNE 2020 Y A N G E T A L .