Statistical Analysis of Convective Updrafts in Tropical Cyclone Rainbands Observed by Airborne Doppler Radar

Abstract

Ten years of airborne Doppler radar observations are used to study convective updrafts' kinematic and reflectivity structures in tropical cyclone (TC) rainbands. An automated algorithm is developed to identify the strongest rainband updrafts across 12 hurricane‐strength TCs. The selected updrafts are then collectively analyzed by their frequency, radius, azimuthal location (relative to the 200–850 hPa environmental wind shear), structural characteristics, and secondary circulation (radial/vertical) flow pattern. Rainband updrafts become deeper and stronger with increasing radius. A wavenumber‐1 asymmetry arises, showing that in the downshear (upshear) quadrants of the TC, updrafts are more (less) frequent and deeper (shallower). In the downshear quadrants, updrafts primarily have in‐up‐out or in‐up‐in secondary circulation patterns. The in‐up‐out circulation is the most frequent pattern and has the deepest updraft and reflectivity tower. Upshear, the updrafts generally have out‐up‐in or in‐up‐in patterns. The radial flow of the updraft circulations largely follows the vortex‐scale radial flow shear‐induced asymmetry, being increased low‐level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) quadrants. It is hypothesized that the convective‐scale circulations are significantly influenced by the vortex‐scale radial flow at the updraft base and top altitudes. Other processes of the convective life cycle, such as bottom‐up decay of aging convective updrafts due to increased low‐level downdrafts, can influence the base altitude and, thus, the base radial flow of the updraft circulation. The findings presented in this study support previous literature regarding convective‐scale patterns of organized rainband convection in a mature, sheared TC.

Lightning and Radar Characteristics of Tornadic Cells in Landfalling Tropical Cyclones

“…However, there also exists nontrivial variability in the location and magnitude of the lightning maximum with the TC‐relative azimuth and distance from the TC center of the cells (not shown). This is partially due to the direction of the TC advective winds and, hence, the convective outflow changing with TC‐relative azimuth, which includes variations in the direction of the: (a) convergent, cyclonic advective winds of the TC primary circulation and (b) the divergent, anticyclonic radial outflow of the TC secondary circulation (Barnes et al., 1983; Barron et al., 2022; Hence & Houze, 2008). Moreover, TC outer radii are also characterized by a deeper mixed phase region with greater concentrations of supercooled water due to increased CAPE, reduced scavenging of supercooled water, and weaker subsidence associated with the TC secondary circulation (Black & Hallett, 1999; Molinari et al., 2012).…”

Modulation of Asymmetric Inner‐Core Convection on Midlevel Ventilation Leading up to the Rapid Intensification of Typhoon Lekima (2019)

“…A more recent observational study by Barron et al. (2022) indicated that the structure of convectively induced overturning circulation in TC rainband is more complex when the environmental shear is at least moderate. Specifically, the radial flow direction near the base and top of the convective cell may follow the shear‐induced mean flow, that is, low‐level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) flank.…”

“…It is worth noting that the overturning circulation discussed in Barron et al. (2022) includes the wavenumber‐1 background mean flow, while the purely heating‐forced overturning circulation generally exhibits in‐up‐out pattern (Moon & Nolan, 2010).…”

“…Specifically, the radial flow direction near the base and top of the convective cell may follow the shear-induced mean flow, that is, low-level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) flank. It is worth noting that the overturning circulation discussed in Barron et al (2022) includes the wavenumber-1 background mean flow, while the purely heating-forced overturning circulation generally exhibits in-up-out pattern (Moon & Nolan, 2010). 6, but for the quadrant-averaged fields averaged between 0300 UTC and 0800 UTC 7 August.…”

“…Previous observations based on airborne Doppler radar (Didlake & Houze, 2009;Hence & Houze, 2008) have revealed that the convection cells in the inner-core rainbands produce local overturning circulation, usually composed of low-level inflow, upward motion and midtroposphere outflow. A more recent observational study by Barron et al (2022) indicated that the structure of convectively induced overturning circulation in TC rainband is more complex when the environmental shear is at least moderate. Specifically, the radial flow direction near the base and top of the convective cell may follow the shear-induced mean flow, that is, low-level inflow (outflow) and midlevel outflow (inflow) in the downshear (upshear) flank.…”