Observations of supergradient winds in the tropical cyclone boundary layer

Abstract

Secondary eyewall formation (SEF) impacts tropical cyclone (TC) intensity and structure, but the inner core dynamics of this phenomenon are not well understood. Numerical models suggest that a supergradient jet at the top of the TC boundary layer (TCBL) associated with boundary layer convergence and forcing of deep convection may play a critical role in SEF. There is a lack of consensus on the importance and magnitude of supergradient jets, due in part to limited high-resolution observations near the surface. A new spline-based, 3D variational analysis technique called Spline Analysis at Mesoscale Utilizing Radar and Aircraft Instrumentation (SAMURAI) is used to combine airborne Doppler radar, GPS dropwindsonde, and in situ flight level observations to estimate the magnitude of the supergradient wind (SGW). A detailed error analysis is presented for wind, pressure gradient, and SGW retrievals using synthetic observations in the primary eyewall of a Weather Research and Forecasting Model (WRF) simulated Hurricane Rita (2005). The new methodology is then used to examine the SGW in the primary and secondary eyewalls of the real Hurricane Rita on 22 September. Hurricane Rainband and Intensity Change Experiment (RAINEX) field campaign observations from two aircraft are used to estimate the magnitude of the SGW for the northern quadrant of the TC and for the azimuthal average. Results from the simulated primary eyewall show the methodology is successful at retrieving the tangential and radial wind fields with low errors. The pressure gradient field has a higher error, especially when dropsondes were included in the analysis. The resulting SGW magnitudes are negatively affected by the pressure gradient errors, resulting in unrealistic supergradient maxima near the surface. The root mean square error in the retrieved SGW is ~5 m s-1, consistent with an analytic error analysis. The results from the real observations provide new estimates of the magnitude of SGW in mature primary and secondary eyewalls. The primary eyewall was found to have a SGW maximum of 22 m s-1 and the secondary eyewall was found to have a SGW maximum of 16 m s-1 for the axisymmetric analysis. The new methodology shows promise to estimate SGW and quantify its importance in TCBL dynamics and SEF, but additional error analysis is necessary to refine the estimates.