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Latent Heating and Mixing due to Entrainment in Tropical Deep Convection

Clayton J. McGeeDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Susan C. van den HeeverDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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Abstract

Recent studies have noted the role of latent heating above the freezing level in reconciling Riehl and Malkus' hot tower hypothesis (HTH) with evidence of diluted tropical deep convective cores. This study evaluates recent modifications to the HTH through Lagrangian trajectory analysis of deep convective cores in an idealized, high-resolution cloud-resolving model (CRM) simulation that uses a sophisticated two-moment microphysical scheme. A line of tropical convective cells develops within a finer nested grid whose boundary conditions are obtained from a large-domain CRM simulation approaching radiative convective equilibrium (RCE). Microphysical impacts on latent heating and equivalent potential temperature (θe) are analyzed along trajectories ascending within convective regions of the high-resolution nested grid. Changes in θe along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. The simulations demonstrate that mixing with dry environmental air decreases θe along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase θe above the freezing level. Latent heating contributions along trajectories from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation are also quantified. Finally, the source regions of trajectories reaching the upper troposphere are identified. Much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, but the strongest updrafts are composed of air from closer to the surface. The importance of both boundary layer and midlevel inflow in moist environments is underscored in this study.

Corresponding author address: Clayton J. McGee, Department of Atmospheric Science, Colorado State University, 1371 Campus Delivery, Fort Collins, CO 80523. E-mail: cjmcgee@atmos.colostate.edu

Abstract

Recent studies have noted the role of latent heating above the freezing level in reconciling Riehl and Malkus' hot tower hypothesis (HTH) with evidence of diluted tropical deep convective cores. This study evaluates recent modifications to the HTH through Lagrangian trajectory analysis of deep convective cores in an idealized, high-resolution cloud-resolving model (CRM) simulation that uses a sophisticated two-moment microphysical scheme. A line of tropical convective cells develops within a finer nested grid whose boundary conditions are obtained from a large-domain CRM simulation approaching radiative convective equilibrium (RCE). Microphysical impacts on latent heating and equivalent potential temperature (θe) are analyzed along trajectories ascending within convective regions of the high-resolution nested grid. Changes in θe along backward trajectories are partitioned into contributions from latent heating due to ice processes and a residual term that is shown to be an approximate representation of mixing. The simulations demonstrate that mixing with dry environmental air decreases θe along ascending trajectories below the freezing level, while latent heating due to freezing and vapor deposition increase θe above the freezing level. Latent heating contributions along trajectories from cloud nucleation, condensation, evaporation, freezing, deposition, and sublimation are also quantified. Finally, the source regions of trajectories reaching the upper troposphere are identified. Much of the air ascending within convective updrafts originates from above the lowest 2 km AGL, but the strongest updrafts are composed of air from closer to the surface. The importance of both boundary layer and midlevel inflow in moist environments is underscored in this study.

Corresponding author address: Clayton J. McGee, Department of Atmospheric Science, Colorado State University, 1371 Campus Delivery, Fort Collins, CO 80523. E-mail: cjmcgee@atmos.colostate.edu
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