Thermodynamic Observations and Flux Calculations of the Tropical Cyclone Surface Layer within the Context of Potential Intensity

Abstract

Abstract Thermodynamic variables including temperature, humidity, and equivalent potential temperature are obtained and calculated from 88 buoy and C-MAN time series of 38 Atlantic hurricanes. Radial profiles of these variables are compared to the tropical cyclone (TC) boundary layer idealization in potential intensity (PI) theory. For the composite hurricane, temperature decreases by 2.4 K between the environmental far field and the radius of maximum winds (RMW), in contrast to the PI boundary layer profile, which is radially isothermal outside the RMW. Observationally derived moisture and equivalent potential temperature (moist entropy) begin to increase with decreasing radius beyond the RMW, especially for the subset of category 3–5 hurricanes. This suggests the relevance of ocean–air fluxes beyond the RMW to increasing the moist entropy of eyewall updrafts. Ocean–air enthalpy fluxes produced by 85 time series with sea surface temperature data are explored using the bulk aerodynamic flux formulation and two methods that explicitly account for sea spray. Formulations incorporating sea spray produce greater total enthalpy fluxes, especially near the RMW. Total enthalpy fluxes calculated using composite observed conditions differ substantially from fluxes calculated using the idealizations of classic PI theory, though the sign of the difference depends on the calculation method used. Observed conditions may yield higher maximum intensities if maximum intensity is governed by the energy production–frictional dissipation balance under the eyewall. However, if TC intensity is governed by the entropy gained by inflow air, no matter where entropy is acquired, observed conditions may yield lower intensities than the classic PI theory boundary layer.