The Fundamental Global Cooling Influence of Enhanced Cumulonimbus (Cb) Convection

Monday, 18 April 2016: 12:00 AM
Ponce de Leon C (The Condado Hilton Plaza)
William M. Gray, Colorado State Univ., Fort Collins, CO

Cumulonimbus (Cb) clouds occur in clusters and must typically have substantial initiating low-level mechanical updraft mass forcing of at least 2 m/s over widths of at least 2 km. Downdraft mass forcing from dying surrounding Cb's acts as the primary mass source for the generation of new pre-Cb updraft elements. The strong low-level wind speeds and low humidity of downdrafts greatly enhances surface evaporation. Recent extensive buoyancy and entrainment calculations have been made which show the low-level updraft velocity and width requirements needed for Cb penetration into the upper troposphere. This is where atmospheric temperatures are too cold to hold anything but miniscule amounts of water-vapor and ice.

Cb mass penetration into the upper troposphere forces compensating surrounding return flow mass subsidence of this very dry air to lower levels. This acts to reduce the upper tropospheric relative humidity (RH) near the 300 mb (10 km) level. This subsidence allows more infrared (IR) energy to be fluxed to space through a small lowering of the infrared Emission Level (EL) to a warmer level where σT4 is higher.

Enhanced global Cb convection acts to cool the troposphere by causing more IR energy to be fluxed to space. By contrast, reduced surface evaporation and reduced deep Cb convection act as an influence to raise global temperature. This scenario is opposite to the positive temperature and water-vapor feedback assumptions of the numerical climate models which are not able to explicitly threat the dynamics of individual Cb elements.

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