Composite Mesoscale Environmental Conditions Influencing Tornado Frequencies in Landfalling Tropical Cyclones

Abstract

Abstract Spatial patterns of tropical cyclone tornadoes (TCTs), and their relationship to patterns of mesoscale predictors within United States landfalling tropical cyclones (LTCs) are investigated using multicase composites from 27 years of reanalysis data from 1995 through 2021. For 72 cases of LTCs with wide ranging TC intensites at landfall, daytime TCT frequency maxima are found in the northeast, right-front, and downshear-right quadrants when their composites are constructed in ground-relative, TC-heading relative, and environmental shear relative coordinates, respectively. TCT maxima are located near maxima of 10-m to 700-hPa bulk wind difference (BWD), which are enhanced by the TC circulation. This proxy for bulk vertical shear in roughly the lowest 3 km is among the best predictors of maximum TCT frequency. Relative to other times, the position of maximum TCT frequency during the afternoon shifts ∼100 km outward from the LTC center toward larger MLCAPE values. Composites containing the strongest LTCs have the strongest maximum 10-m to 700-hPa and 10-m to 500-hPa BWDs (∼20m s−1) with nearby maximum frequencies of TCTs. Corresponding composites containing weaker LTCs but still many TCTs, had bulk vertical shear values that were ∼20% smaller (∼16 m s−1). Additional composites of cases having similarly weak average LTC strength at landfall, but few or no TCTs, had both maximum bulk vertical shears that were an additional ∼20% lower (∼12 m s−1) and smaller MLCAPE. TCT environments occurring well inland are distinguished from others by having stronger westerly shear and a west-to-east oriented baroclinic zone (i.e., north-to-south temperature gradient) that enhances mesoscale ascent on the LTC’s east side.