Distinct Raindrop Size Distributions of Convective Inner‐ and Outer‐Rainband Rain in Typhoon Maria (2018)

Abstract

Unlike previous studies from a single disdrometer, this study investigates and compares the raindrop size distribution (RSD) characteristics of convective inner-rainband rain (CIR) and convective outer-rainband rain (COR) in Typhoon Maria (2018), simultaneously captured for the first time by a newly established observational network including nine disdrometers in the northeastern Fujian Province of China. It is shown that the radar reflectivity for the CIR increases sharply with decreasing height below the melting layer, whereas it remains nearly unchanged for the COR. This suggests the dominance of collision-coalescence process in the CIR, that is, the collection of small raindrops by larger ones as they fall. Thus, the surface-level CIR generally has far lower concentration of small raindrops and larger mean raindrop diameter than those previously found in the COR. Close to the tropical cyclone (TC) eyewall, it is found for the first time that although the raindrops are relatively large, the raindrop concentration is too low to yield a high rain rate. In contrast, high rain rates are concentrated at a distance of about 1.5-2.5 times the radius of maximum wind from the TC center, where there are appropriate normalized concentrations (log 10 N w) and corresponding raindrop diameters. In addition, this study demonstrates differences between the CIR and COR in terms of the RSD evolution with increasing rain rate and the radar reflectivity-rain rate (Z-R) and shape-slope (μ-Λ) relationships, confirming the existence of different rain microphysics in various rain regions of a TC.

A Comparison of Convective Raindrop Size Distributions in the Eyewall and Spiral Rainbands of Typhoon Lekima (2019)

“…The different RSD characteristics in various spiral rainbands of a TC have been discussed in previous studies (Bao et al, 2019(Bao et al, , 2020Chen et al, 2012;Ulbrich & Lee, 2002;Wang et al, 2016;Wen et al, 2018). In recent years, more and more Doppler weather radars are being (or have been) upgraded with dual-polarization capability, and then used to observe TCs (Zhang et al, 2019).…”

“…As in Bao et al (2020, hereafter Bao20), the China New Generation Doppler Weather Radar (CINRAD/SA) data from Wenzhou station (black triangle in Figure 1a) are also utilized by this study. The radar reflectivity shows that Lekima maintained a structure of concentric eyewalls for at least 15 hr prior to landfall (Figure 1).…”

“…In order to measure the RSD characteristics simultaneously in different rain regions of a TC, a number of disdrometers have been deployed in eastern China by the China Meteorological Administration (CMA) since 2017 (Bao et al, 2020). Super Typhoon Lekima made landfall in this area in 2019, with a wind speed of 52 m s −1 , before moving over this observational network (Figure 1).…”

“…Different shape-slope ( μ-Λ) relationships, which can reflect physical characteristics of actual rain, have been used widely in the retrieval of polarimetric radar observations in different rain regions (Brawn & Upton, 2008;Chang et al, 2009;Chen et al, 2012;Zhang, 2015;Zhang et al, 2003). Thus, different μ-Λ relationships derived from RSDs have been suggested for the polarimetric retrieval in various spiral rainbands of a TC by Bao et al (2019Bao et al ( , 2020. In contrast, the RSD within a mature TC eyewall has not been well observed, although Chen et al (2012) argued that a few rain samples were collected by a single disdrometer close to the center of Typhoon Morakot (2009).…”

“…The extreme precipitation of tropical cyclones (TCs) is associated mainly with the convective rain in their eyewall, spiral rainbands, and other rainbands caused by the interaction between the TC circulation and other weather systems (Chen et al, 2013; Czajkowski et al, 2013; Houze, 2010; Rappaport, 2014; Zhang et al, 2009). Previous studies have identified various formation mechanisms for convection in different rain regions of a TC, resulting in different cloud structures and microphysical processes (Bao et al, 2019, 2020; Didlake et al, 2017; Hence & Houze, 2008; Houze, 2010; Li & Wang, 2012; Montgomery & Kallenbach, 1997; Moon & Nolan, 2015; Willoughby, 1988). Conversely, microphysical processes (e.g., release of latent heat) play a critical role in the convection development and eventual precipitation in TCs (Chan & Chan, 2016; Khain et al, 2016; Li et al, 2013; Wang, 2009; Williams, 2019).…”