What Is the Relationship between Precipitation Symmetry, Vortex Tilt, and Intensification in an Ensemble of Idealized Moderately Sheared Tropical Storms?

Moderately sheared tropical cyclones tend to have higher model and forecast intensity errors (Bhatia and Nolan, 2013). Yet there remains a dearth of modeling studies such as Rios-Berrios et al. (2015) that have examined moderately sheared storms, and even fewer using a shear-relative quadrant based analysis necessary to analyze asymmetric structure. Alvey et al. (2015), Zawislak et al. (2016), and Munsell et al. (2017) showed the importance of precipitation symmetrization and vortex alignment prior to rapid intensification (RI). Building upon previous studies, we hypothesize that an increase in convection downshear-left (Chen and Gopalakrishnan, 2015) is linked to vortex alignment and precipitation symmetrization, a precursor to rapid intensification. The degree of necessary vertical alignment and symmetry for intensification, however, is sensitive to changes in environmental conditions including vertical wind shear, mid-tropospheric relative humidity, and sea surface temperature (SST).

We use the Weather Research and Forecasting Model (WRF) model – Advanced Research WRF (ARW; Skamarock et al., 2008) and apply time-varying point-downscaling (TVPDS; Onderlinde and Nolan, 2017) to a balanced, TC-like vortex, creating an idealized, moderately-shear tropical storm. After allotting the vortex a sufficient amount of time to spin-up and tilt in response to the sheared wind profile, we smoothly transition the vertical wind shear. In a series of sensitivity tests varying the shear transition, we focus on the shear-relative thermodynamic and precipitation evolution. We test the dynamical mechanisms responsible for precipitation symmetry to determine if it’s a cause or a result of vortex alignment. In addition, we examine other environmental influences including the hypothesis that a sheared storm can overcome an unfavorable tilt with higher SSTs. Future work will include additional analyses that break down the components of temperature, water vapor mixing ratio, and vorticity budgets within quadrant based frameworks to demonstrate the processes governing the transition from an asymmetric to more symmetric tropical cyclone.