Examining Storm Asymmetries in Hurricane Irma (2017) Using Polarimetric Radar Observations

Abstract

Dual‐polarization radar observations of Hurricane Irma (2017) provide new insight into the microphysical structure of a mature tropical cyclone that can be tied to the cyclone dynamics. The primary eyewall exhibited a radar signature of hydrometeor size sorting, which implied that large drops fell out near persistent upward motion in the front‐right quadrant of the storm, while smaller drops were advected downstream. In the outer rainbands, convective initiation was also preferred in the front‐right quadrant, whereas stratiform precipitation was predominant downwind. For both the primary eyewall and outer rainbands, the preferred quadrant for convective initiation was consistent with the expected kinematic asymmetry of a tropical cyclone in weak environmental wind shear but with moderate translation speed. The developing secondary eyewall exhibited a different asymmetry that indicated a stratiform‐to‐convective transition associated with heavy precipitation in the rear quadrants. This transition is consistent with hypothesized dynamical theories for secondary eyewall formation.

A Comparison of the Polarimetric Radar Characteristics of Heavy Rainfall From Hurricanes Harvey (2017) and Florence (2018)

“…Meanwhile, the inner rainband shows more modest increases in Z H and K DP near the surface, and the Z DR profile varies little below 4 km. These localized profiles combined with the examples from Harvey underscore the variety of microphysical processes that can exist in a TC, consistent with prior studies (Didlake & Kumjian, 2018; Wang et al, 2016; Wu et al, 2018). A more systematic examination of the vertical polarimetric profiles with respect to both TC features and surface rainfall is beyond the scope of this study but is recommended for future work.…”

Quantifying Precipitation Efficiency and Drivers of Excessive Precipitation in Post-Landfall Hurricane Harvey

“…Polarimetric radar observations can also provide useful information regarding enhanced rainfall rates in convective storms through identification of Z DR and K DP columns (Homeyer and Kumjian 2015). These columns of enhanced Z DR and K DP can help identify large supercooled drops being lofted by convective updrafts (e.g., Herzegh and Jameson 1992;Loney et al 2002;Kumjian et al 2014), and can help distinguish embedded convective features from stratiform elements within the outer rainbands (e.g., Griffin et al 2014;Didlake Jr. and Kumjian 2018) The supercells in Harvey were identified using 3-6 MRMS rotation tracks and were collocated with swaths of enhanced polarimetric radar variables. Within these supercells from 0300-0400 UTC on 27 August, the 1-hour mean values of Z H , Z DR , and K DP were as high as 50 dBZ, 1.25 dB, and 2 degrees/km.…”

“…Radar observations from the WSR-88D network are useful in cases of landfalling tropical cyclones as they provide observations at a high temporal resolution (e.g., Medlin et al 2007;Didlake Jr. and Kumjian 2018) allowing for the identification of mesoscale features contained within the spiral rainbands and eyewall. The processes occurring in these features can ultimately enhance rainfall rates at the surface.…”

Evaluation of Simulated Drop Size Distributions and Microphysical Processes Using Polarimetric Radar Observations for Landfalling Typhoon Matmo (2014)

“…Microphysical parameterizations (MPs) have been widely used in numerical weather prediction models to represent complex cloud microphysical and precipitation processes. Nonlinear interactions between microphysical and dynamical processes through latent heat release and absorption in microphysical processes are particularly critical for TC forecasts (Didlake & Kumjian, ; Feng & Bell, ; Igel et al, ). To describe the complex microphysical processes, more complicated MP schemes have been developed that include more hydrometeor species, more physical processes, and higher degrees of freedom in describing the particle size distribution (PSD) functions.…”