Investigating the Characteristics and Dynamics of Convective Updrafts in Tropical Cyclone Rainbands

Open Access
- Author:
- Barron, Nicholas
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 24, 2024
- Committee Members:
- Paul Markowski, Program Head/Chair
Colin Zarzycki, Major Field Member
Anthony Didlake, Chair & Dissertation Advisor
Xingchao Chen, Major Field Member
Jonathan Dodge, Outside Unit & Field Member - Keywords:
- tropical cyclones
tropical cyclone rainbands
convective updrafts - Abstract:
- Convective updrafts in tropical cyclone (TC) rainbands have been found to cause storm- wide changes. An automated algorithm is developed to select, track, and measure the characteristics of updrafts. By analyzing a comprehensive airborne dual-Doppler observation dataset (TC-RADAR), static simulation snapshots of Hurricanes Matthew (2016) and Harvey (2017), and high-temporal resolution simulation of Hurricane Harvey (2017), the selected updrafts are collectively analyzed by their frequency, radius, azimuthal location (relative to the 200-850 hPa environmental wind shear and low-level mean flow (LMF)), topology characteristics, low-level shear, and secondary circulation (radial/vertical) flow pattern, as well as how each of these change across the life span of the updrafts. Comparisons between observations and static simulation snapshots are favorable. A wavenumber-1 asymmetry in updraft frequency is located, finding that updrafts are more frequent and deeper in the downshear and windward quadrants, while in the upshear and leeward quadrants, they are less frequent and shallower. In a downwind progression, following the rainband, the secondary circulation generally expresses the following patterns: an in- up-in followed by out-up-out flow, or an in-up-in, in-up-out, and out-up-out progression. Convective-scale circulations are hypothesized to significantly influence the vortex-scale radial flow at the updraft base and top altitudes. Other processes, such as the bottom-up decay of aging convective updrafts due to increased low-level downdrafts, can also influence the base altitude and, consequently, the base radial flow of the updraft circulation. We hypothesize these are expressions of two convective pathways: that of a transient, buoyant convective air mass and a strong single-cell air mass thunderstorm, respectively. A modified version of Rotunno, Klemp, and Weisman’s (1988) low-level shear balance theory is derived for TC rainbands, specifically the region beneath convective updrafts. The low-level shear progresses from inward-pointed to outward-pointed from the upwind to downwind portion of the rainband. We hypothesize this behavior results from cold pool circulations and microburst-induced shear that grow in magnitude and quantity, respectively, in the downwind-most portion of the rainband. Updrafts were the largest and most intense when the shear was roughly balanced, suggesting a relationship between the low-level shear and convective maintenance. Updraft temporal tracking was performed on a high-frequency simulation of Hurricane Harvey, permitting a direct evaluation of earlier theories of how convection characteristics evolve. These findings are generally consistent with previous studies, with an additional pathway not conceived in the steady-state studies. Updrafts begin at a high base altitude that lowers as convection intensifies. These strong convective elements have high vertical velocity and depth at maturity. An in-up-in→out-up-out→in-up-out→out-up-out circulation pattern accompanies these updrafts; they tend to form in the left-of-shear.