Organization of Tropical Convection in Low Vertical Wind Shears: The Role of Cold Pools

Abstract

An investigation is conducted to document the role convectively generated cold pools play in determining the spatial organization of tropical deep convection. Using a high-resolution cloud-resolving model, the evolution of cold pools produced by deep convection is examined, in the situation of limited large-scale wind shear, and a homogeneous underlying sea surface temperature. Ignoring the cold pools resulting from multiple deep convective events, the mean model cold pool attained a minimum temperature and water vapor mixing ratio depression of 1 K and 1.5 g kg?1, respectively; a horizontal velocity increase of 4.8 m s?1; and the latent and sensible heat fluxes are increased by a factor of 1.9 and 2.6, respectively. The cold pools had a mean lifetime of approximately 2.5 h and attained maximum radii ranging from 3 to 18 km, with a mean of 8.6 km. Taking the organization of convection into account, these figures are consistent with observational studies of convective wakes. The composite cold pool showed that development occurred in three distinct stages. As seen in observations, the air in the vicinity of deep convection has a higher equivalent potential energy than average. In the first stage, before the downdraft develops and reaches the subcloud layer, the area below the convection is cooled and moistened by the evaporation of rainfall. The downdraft then injects cold and dry air into the boundary layer, and the spreading cold pool is consequentially moister than average just inside the gust front but drier in the central regions. Finally, mass conservation requires that air from above the boundary layer be entrained into the wake of the expiring downdraft?thus causing the central regions of the cold pool to recover very quickly in temperature?but increases further the moisture perturbation. These features are confirmed by a number of observational studies. The key to the triggering of new deep convective cells lies with the band of high equivalent potential temperature, but negatively buoyant air, situated inside the boundary of the spreading cold pools. It is this air that forms the new convective cells. The radius at which this occurs is determined by the time taken for surface fluxes to remove the negative temperature perturbation, thereby reducing convective inhibition energy. In summary, the primary mechanism by which cold pools organize tropical deep convection in low wind shear conditions is principally thermodynamical, and not dynamical as previously assumed.