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CAPE in Tropical Cyclones

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  • 1 Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York
  • | 2 Department of Earth and Planetary Science, University of California, Berkeley, and the Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
  • | 3 Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York
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Abstract

Convective available potential energy (CAPE) and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the NOAA Gulfstream-IV aircraft. Calculations were done with and without the effects of condensate loading, entrainment, and the latent heat of fusion. CAPE showed larger values downshear than upshear within 400 km of the center, consistent with the observed variation of convective intensity. The larger downshear CAPE arose from (i) higher surface specific humidity, (ii) lower midtropospheric temperature, and, for entraining CAPE, (iii) larger free-tropospheric relative humidity.

Reversible CAPE had only one-half the magnitude of pseudoadiabatic CAPE. As shown previously, reversible CAPE with fusion closely resembled pseudoadiabatic CAPE without fusion. Entrainment had the most dramatic impact. Entraining CAPE was consistent with the observed radial distribution of convective intensity, displaying the largest values downshear at inner radii. Without entrainment, downshear CAPE was smallest in the core and increased outward to the 600-km radius.

The large number of sondes allowed the examination of soundings at the 90th percentile of conditional instability, which reflect the conditions leading to the most vigorous updrafts. Observations of convection in tropical cyclones prescribe the correct method for calculating this conditional instability. In particular, the abundance and distribution of vigorous deep convection is most accurately reflected by calculating CAPE with condensate retention and a fractional entrainment rate in the range of 5%–10% km−1.

Corresponding author address: John Molinari, Department of Atmospheric and Environmental Sciences, ES-225, University at Albany/SUNY, 1400 Washington Avenue, Albany, NY 12222. E-mail: jmolinari@albany.edu

Abstract

Convective available potential energy (CAPE) and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the NOAA Gulfstream-IV aircraft. Calculations were done with and without the effects of condensate loading, entrainment, and the latent heat of fusion. CAPE showed larger values downshear than upshear within 400 km of the center, consistent with the observed variation of convective intensity. The larger downshear CAPE arose from (i) higher surface specific humidity, (ii) lower midtropospheric temperature, and, for entraining CAPE, (iii) larger free-tropospheric relative humidity.

Reversible CAPE had only one-half the magnitude of pseudoadiabatic CAPE. As shown previously, reversible CAPE with fusion closely resembled pseudoadiabatic CAPE without fusion. Entrainment had the most dramatic impact. Entraining CAPE was consistent with the observed radial distribution of convective intensity, displaying the largest values downshear at inner radii. Without entrainment, downshear CAPE was smallest in the core and increased outward to the 600-km radius.

The large number of sondes allowed the examination of soundings at the 90th percentile of conditional instability, which reflect the conditions leading to the most vigorous updrafts. Observations of convection in tropical cyclones prescribe the correct method for calculating this conditional instability. In particular, the abundance and distribution of vigorous deep convection is most accurately reflected by calculating CAPE with condensate retention and a fractional entrainment rate in the range of 5%–10% km−1.

Corresponding author address: John Molinari, Department of Atmospheric and Environmental Sciences, ES-225, University at Albany/SUNY, 1400 Washington Avenue, Albany, NY 12222. E-mail: jmolinari@albany.edu
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