Entrainment, Mixing and Latent Heating in Tropical Deep Convection

Ed Zipser has made many outstanding contributions to tropical meteorology during his 50 years in the field of atmospheric science. One of these contributions was the evaluation of the Riehl and Malkus hot tower hypothesis. Measurements of tropical deep convective updrafts made by Zipser and others during various field campaigns showed little evidence of the undiluted convective cores proposed by Riehl and Malkus. Zipser went on to demonstrate that latent heating above the freezing level could provide the additional buoyancy necessary for convective parcels to reach the upper troposphere. More recently, research by Zipser and others resulted in a modified definition of hot towers as “any deep convective tower rooted in the boundary layer and topping in the upper troposphere” irrespective of the amount of updraft dilution. The work to be presented here evaluates these recent modifications to the hot tower hypothesis through the use of Lagrangian trajectories within high-resolution cloud-resolving model simulations run to radiative convective equilibrium (RCE).

RCE simulations were performed using the Regional Atmospheric Modeling System (RAMS). The simulations were initialized with a TOGA COARE sounding and utilized a fixed sea surface temperature of 300K, horizontal grid spacing of 1km, a grid domain of 3000 km x 200km x 25km and two-moment microphysics. A line of convective cells was one of the many convective systems that developed 15 days into this RCE simulation. A nested grid with horizontal grid spacing of 250m and 128 vertical levels was used to focus on the dynamical and microphysical processes of this convective line. Forward and backward trajectory analysis was performed. 873 forward trajectories were initiated ahead of the convective line, 1km apart, and at regular vertical intervals between the surface and 10km AGL. 1233 backward trajectories were initialized above 10km AGL within the convective anvil of the convective line. The impacts of various microphysical processes on the equivalent potential temperature and latent heating were then analyzed along these trajectories. Radiative heating was found to be negligible during the time period of analysis. Changes in equivalent potential temperature were therefore broken into those contributions from latent heating due to ice processes and a residual taken to be representative of mixing. The trajectory analysis demonstrated that equivalent potential temperature was decreased in association with mixing below the freezing level, but that latent heating due to both freezing and vapor deposition increased it above the freezing level, in line with Zipser's findings. The relative contributions of various microphysical processes have also been quantified. Furthermore, both the boundary layer and midlevel regions were found to be important sources of inflow air into the convective line. The majority of air entering the convective line originated above the lowest 2km AGL, however, the strongest updrafts are comprised of air that originated closer to the surface. The contributions of Ed Zipser to our understanding of tropical convective updrafts, latent heating and entrainment and the results from this study will be presented, occasionally interspersed with personal anecdotes.