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Improved estimates of the role of precipitation in tropical cyclone intensification

Event Type
Department of Atmospheric Sciences
Room 114 of the Transportation Building
Apr 9, 2014   3:00 pm  
Dan Harnos, Graduate Student, Department of Atmospheric Sciences, University of Illinois
Shirley Palmisano

Tropical cyclone (TC) intensification remains a paramount issue facing the tropical meteorology community despite continued advances in observational capabilities and computing power.  The most troubling intensification cases from a predictability standpoint are episodes of rapid intensification (RI), or a wind increase of 30 knots over a 24 hour period, that can yield catastrophic effects for coastal interests should prediction of an RI event be missed.  We simulate two episodes of RI (Hurricane Ike of 2008 and Hurricane Earl of 2010) under low and high wind shear respectively with the Weather Research and Forecasting model to investigate the causes behind each RI episode.

Recent work has highlighted the importance of the positioning of the TC heating distribution relative to the radius of maximum wind (RMW), due to the increased inertial stability existing within the RMW being more efficient for warm core development.  RMW-related work has predominantly used axisymmetric estimates, neglecting asymmetry in the horizontal and vertical.  Such asymmetry may be important to consider due TCs typically undergoing RI at tropical storm strength where the vortex structure can be readily modified by localized convection.  We introduce an objective method to quantify the 3-D RMW character, thus allowing evaluation of the heating and precipitation fields relative to a region where their TC intensification impacts can be maximized.  We show that RMW asymmetry is substantial near RI initiation, such that axisymmetric RMW estimates can introduce substantial analysis errors.

Prior studies focusing on TC inner-core precipitation predominantly highlight the role of the most intense convective elements:  stratospheric-penetrating convective bursts (CBs).  CBs represent only convective variety however, while less intense convective regimes (e.g. shallow cumuli, cumulus congestus, and deep convection failing to penetrate the tropopause) may also prove important.  We develop an objective method to quantify the relative roles of the aforementioned convective regimes at the individual 3-D updraft level.  The low shear RI case is primarily linked to heating and vertical fluxes from deep convection with secondary contributions of CBs and cumulus congestus while the high shear case is overwhelmingly CB-driven.

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