The generation of available potential energy by sensible heating: a case study

Abstract

The importance of the sensible heat transfer from the ocean to the atmosphere within an east coast cyclone is studied using the concept of available potential energy. Both the generation of the storm's available potential energy by this diabatic component and the boundary layer frictional dissipation are estimated. The sensible heat transfer through the interface is calculated by employing the bulk aerodynamic method. The ten meter wind speed is estimated from the surface geostrophic wind models of Rossby & Montgomery (1935) and Lettau (1961). For each model, both a variable wind dependent and a fixed roughness parameter are utilized. The sensible heat flux from the four different computations varied by 50% showing the sensitivity of the flux estimates to the modeling of the boundary layer winds. From the flux calculations both upper and lower bound estimates of the storm generation by the sensible heating are determined. The upper bound estimate is made by adding all the thermal energy to the superadiabatic surface layer, a few millibars in vertical extent, while the lower bound is determined by distributing the energy uniformly within the dry adiabatic mixing layer extending from a few to several hundred millibars vertically. These bounds reflect the uncertainty in the nature of the energy transfer between the turbulent scale of the boundary layer and the quasi-horizontal scale of the storm. It is postulated that the true generation is bounded by these upper and lower estimates. For the small area of 5.4 x 102 m2, the lower bound generation estimates range from 0.7 to 1.9 watts m-2 at OOz and 1.3 to 3.1 watts m-2 at 12z while the upper bound ranges from 1.5 to 3.9 at OOz and 2.6 to 6.1 at 12z for the four different estimates of sensible heat transfer. The boundary layer frictional dissipation increases from 2.7 at OOz to 7.1 watts m-2 at 12z. The results indicate that sensible heat generation is significant in the creation of the cyclone's available potential energy, particularly during the early cyclogenetic stage.