Convective Bursts, Downdraft Cooling, and Boundary Layer Recovery in a Sheared Tropical Storm

John Molinari Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York

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Jaclyn Frank AWS Truepower, Albany, New York

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David Vollaro Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York

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Abstract

Tropical Storm Edouard (2002) experienced episodic outbreaks of convection downshear within the storm core in the presence of 11–15 m s−1 of ambient vertical wind shear. These outbreaks lasted 2–6 h and were followed by long periods with no deep convection. Flights from U.S. Air Force reconnaissance aircraft within the boundary layer were used to investigate the cause of one such oscillation. Low equivalent potential temperature θe air filled the boundary layer as convection ceased, creating a 4–6-K deficit in θe within the convective region. Soundings within 110 km of the center were supportive of convective downdrafts, with midlevel relative humidity below 15% and large downdraft CAPE. Deep convection ceased within 75 km of the center for more than 8 h. Tangential velocity reached hurricane force locally during the convective outbreak, then became nearly symmetric after convection stopped, arguably as a result of axisymmetrization, and the storm weakened. Nevertheless, the corresponding lack of convective downdrafts during this period allowed surface heat and moisture fluxes to produce substantial increases in boundary layer entropy. A new burst of convection followed. Consistent with recent papers it is argued that tropical cyclone intensification and decay can be understood as a competition between surface heat and moisture fluxes (“fuel”) and low-entropy downdrafts into the boundary layer (“antifuel”).

Corresponding author address: John Molinari, University at Albany, State University of New York, Atmospheric and Environmental Sciences, ES-225, 1400 Washington Ave., Albany, NY 12222. E-mail: jmolinari@albany.edu

Abstract

Tropical Storm Edouard (2002) experienced episodic outbreaks of convection downshear within the storm core in the presence of 11–15 m s−1 of ambient vertical wind shear. These outbreaks lasted 2–6 h and were followed by long periods with no deep convection. Flights from U.S. Air Force reconnaissance aircraft within the boundary layer were used to investigate the cause of one such oscillation. Low equivalent potential temperature θe air filled the boundary layer as convection ceased, creating a 4–6-K deficit in θe within the convective region. Soundings within 110 km of the center were supportive of convective downdrafts, with midlevel relative humidity below 15% and large downdraft CAPE. Deep convection ceased within 75 km of the center for more than 8 h. Tangential velocity reached hurricane force locally during the convective outbreak, then became nearly symmetric after convection stopped, arguably as a result of axisymmetrization, and the storm weakened. Nevertheless, the corresponding lack of convective downdrafts during this period allowed surface heat and moisture fluxes to produce substantial increases in boundary layer entropy. A new burst of convection followed. Consistent with recent papers it is argued that tropical cyclone intensification and decay can be understood as a competition between surface heat and moisture fluxes (“fuel”) and low-entropy downdrafts into the boundary layer (“antifuel”).

Corresponding author address: John Molinari, University at Albany, State University of New York, Atmospheric and Environmental Sciences, ES-225, 1400 Washington Ave., Albany, NY 12222. E-mail: jmolinari@albany.edu
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