Diabatic effects on potential vorticity over the global tropics

Abstract

Through the use of physical initialization, which relates to precipitation and precipitable water initialization we incorporate the observed rainfall and obtained a better description of the diabatic heating from a very high-resolution global model. Interpolating assimilated data sets (basic and derived) onto isentropic surfaces we carried out detailed analysis of the potential vorticity equation for the cases, 1 December 1992 and 4 October 1995. In addition to the horizontal advection of potential vorticity, the potential vorticity Ertel's equation includes contributions from both the vertical and horizontal differentials of diabatic heating and from frictional effects. Often most of these terms are of the order of 10^-12kg^-1m²s^-2K. A significant contribution to the local budget of potential vorticity is provided by (1) the differentials of convective heating in rain areas and (2) the radiative heating related to shallow cloud top cooling in trade wind belts. Viewing this problem over the middle troposphere (near the 325K isentropic surface) and lower troposphere (both slightly above (310K) and below (305K) the tops of undisturbed tropical stratocumulus), we find that the diabatic effects arising from differential heating contribute significantly to the generation of potential vorticity. Tropical short-range forecasts using models designed to conserve potential vorticity exhibit a very large error growth, which can be reduced significantly by including diabatic effects. Three-dimensional trajectories of air parcels in the tropical latitudes show that within 12 to 24 hours, these parcels often undergo physical processes resulting in substantial changes in potential vorticity. Non-conservation of potential vorticity appears to be important over many regions of the disturbed as well as undisturbed tropics.