Adapting RKW Theory to the Study of Tropical Cyclone Rainband Convection

First applied to mid-latitude squall lines with leading convective lines and trailing stratiform precipitation, the RKW theory (Rotunno et al., 1988) is a widely known framework to examine the impact of variations in low-level environmental shear on squall line dynamical evolution. Tropical cyclone (TC) rainbands often present as quasi-linear mesoscale phenomena, with some characteristics like squall lines. Still, significant differences between squall lines and TC rainbands do exist in their kinematic and thermodynamic structures. Analyses based on RKW theory have not been broadly and thoroughly applied to realistic TC rainbands. This study reinterprets RKW's tenets to provide unique insight into convection control mechanisms within TC rainbands.

Our analysis uses a high-resolution, convection-permitting simulation of Hurricane Matthew (2016) run with the Penn State WRF-EnKF system, wherein we focus on individual convective elements within the TC's rainband complex. We compute a low-level (0-2.5 km altitude) shear measurement analogous to the classic RKW approach, solving for the total local shear vector by calculating the magnitude and direction of maximum local shear. This approach allows one to measure the local shear balance on either side of the updraft, independent of factors such as the movement or orientation of the rainband updrafts. Our results substantially affirm original RKW principles by identifying distinct low-level shear patterns concurrent with specific characteristics and phases of rainband convective updraft evolution.

Convective initiation is prominent in the upwind region of the rainband complex, located on the right side of the environmental deep-layer (850-200 hPa) shear vector. Here, storm-scale radial inflow, experiencing friction at the air-sea interface, develops inward-pointed low-level shear. With no other significant shear sources, this local unbalanced shear causes updrafts to tilt slightly radially inward to vertically upright, defining the tilt of new convective updrafts in the rainband. Moving downwind in the rainband complex, we observe precipitation-induced outward-pointing shear offsetting the inward shear. This results in a more balanced low-level shear with strong, deep updrafts tilted slightly radially outward. Further downwind in the rainband complex, mature updrafts are tilted substantially outward due to strong cold-pool-induced outward shear, disrupting the earlier balance. Convection in this stage exhibits signs of transitioning completely to stratiform precipitation.

You can cite the paper here [DC1]