Sounding-based prediction of supercell motions in tropical cyclones

The goals of this study are to (a) evaluate the feasibility of using existing mid-latitude sounding-based methods for supercell motion prediction in tropical cyclones, and if needed, (b) develop a new sounding-based method for accurate use in tropical cyclones (TCs). By cross-referencing four primary databases, a total of 52 tornadic supercells embedded in the circulation of 27 landfalling TCs from 1997-2008 were identified following the radar classification guidelines of Edwards et al. (2012). Also, supercells must have occurred within 3 h and 111 km of a rawinsonde sounding. Observed supercell motions were obtained by tracking the hook echo and/or mesovortex signature in animated radar imagery over a 30-min time window centered on the tornado report. The observed motions were compared to eight different sounding-based methods for estimating supercell motion. None of the mid-latitude methods performed well, as all exhibit statistically significant mean errors from the observed motions for TC supercells. Thus, a new TC-specific method was developed based on shear-relative and Galilean invariant concepts, assuming supercell motion is a combination of advection by the mean wind and a deviant motion resulting from interaction between the convective updraft and the environmental vertical shear. The new “optimized” forecast method out-performs all mid-latitude methods by exhibiting statistically insignificant mean errors. Use of the new method in calculating low-level cell-relative helicity (CRH) produced substantially smaller CRH magnitudes than the currently used mid-latitude methods. Such differences may have significant implications for forecast/warning decisions unique to TC supercells.