Variable Raindrop Size Distributions in Different Rainbands Associated With Typhoon Fitow (2013)

Abstract

The microphysical characteristics of rain may vary in different rain regions of a tropical cyclone (TC), but few studies have demonstrated the differences in raindrop size distributions (RSDs) of convective rain in different rainbands of a specific TC. This study examines the RSD characteristics and evolution of convective rain within outer rainbands and a coastal-front-like rainband associated with Typhoon Fitow, based on observational data from a disdrometer at Shibo station in Shanghai, China, during 6-7 October 2013. Considering the fast passage of convective TC rainbands over the disdrometer and the low rain rate of stratiform rain in the outer area, this study proposes a modified rain-type classification method based on the disdrometer data. This study indicates that convective outer-rainband rain (ORR) and coastal-front rain (CFR) have different rain parameters, three parameters of the gamma model, radar reflectivity-rain rate (Z-R), and shape-slope (μ-Λ) relationships. The convective ORR has higher concentrations at all drop sizes than the convective CFR as well as larger spectral width, leading to the greater rainfall rate. The different Z-R relationships suggest the necessity of a variable relationship for quantitative precipitation estimation (QPE) in different rain regions of the TC. This study also demonstrates for the first time that the RSD evolution with increasing rain rate is different in various convective rainbands associated with Fitow, suggesting that different microphysical parameterization schemes may be required for different rainbands in TC models.Note. The terms R, Z, W, N T , D m , D max , and log 10 N w represent rain rate, radar reflectivity, rain water content, total drop concentration, mass-weighted mean diameter, maximum diameter, and normalized intercept parameter, respectively. The rainfall duration and accumulated amount are also given for the different rain types in each stage. "C," "S," and "T" denote the convective, stratiform, and total rainfall, respectively.

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

“…The coefficient A in the power law relationship ( Z = AR b ) generally increases as the radius decreases, due to increasing average D m (Figure 2a), so the eyewall (outer‐rainband) CR has the largest (smallest) A . The Z‐R relationship of outer‐rainband CR in this study is in close agreement with the result in Bao et al (2019), whereas the A of inner‐rainband CR in this study is smaller than that in Bao20. This may be because CR samples (with large D m ) between RMW and 1.5 RMW are not included in the inner‐rainband CR in this study (Figure 2a).…”

“…It is interesting that in the eyewall there are no samples categorized as stratiform rain (SR) by the classification method modified by Bao et al (2019). Note moreover that the percentage of SR is much higher than that of CR within an annular region between 3 and 5 RMW from the TC center, which supports the suggestion that the SR samples in this region result mainly from the upper‐level cloud initiated from the eyewall updraft, characterized by a high concentration of small raindrops and low rain rate (Bao et al, 2019). These results may explain why TC rain mainly consists of a high concentration of small raindrops (Tokay et al, 2008; Wang et al, 2016; Wen et al, 2018), but sometimes has a large concentration of medium‐to‐large‐sized raindrops within some convective rainbands (Brauer et al, 2020; Wolff et al, 2019).…”

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

“…Figure 11. Scatterplots and corresponding best fit lines of radar reflectivity Z (mm 6 m −3 ) versus rain rate R (mm hr −1 ) for convective inner-rainband rain (CIR; red) and convective outer-rainband rain (COR; blue), as well as the best fit lines of previous results reported by Wen et al (2018) and Bao et al (2019).…”

“…The COR has far fewer samples with D m > 1.5 mm than has the CIR, and thus, the COR has fewer samples with rain rates >30 mm hr −1 than has the CIR (Figure 9c). Although these COR samples have rather high raindrop concentrations, the absence of a middle-to low-level updraft over this area is not conducive to ) versus the mass-weighted diameter D m (mm) for the convective inner-rainband rain (CIR; red points) and convective outer-rainband rain (COR; blue points) using the rain-type classification method modified by Bao et al (2019). The cyan circle and plus indicate mean values for the CIR and COR, respectively, and contours denote the rain rate (mm hr −1 ; thick contour is 30 mm h −1 ).…”

“…However, the composite results of these studies may obscure different RSD characteristics in the various TC rainbands. In fact, several investigations have noticed different rain microphysics in different TC rainbands (Bao et al, 2019;Chen et al, 2012;Ulbrich & Lee, 2002;Wang et al, 2016; Wu et al, 2018 hereafter Bao19). Unfortunately, these investigations were constrained by measurements from a single disdrometer and could not simultaneously capture and compare the RSD characteristics of inner and outer TC rainbands.…”