Department of Environmental Sciences

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Seminar Abstracts
Environmental Sciences Seminar Abstract            

  Entrainment and the Diurnal Cycle of Continental Precipitation
Anthony Del Genio
NASA Goddard Institute for Space Sciences (GISS)

Over most continental regions, the climatological maximum in rainfall occurs in mid-to-late afternoon, or even in the early evening. General circulation models, both weather and climate models, fail to reproduce this fundamental feature of Earth's climate, producing a rainfall peak close to the noontime maximum in insolation instead. Many factors on large and small scales account for the observed timing of the rainfall peak, but the model-data discrepancy is most likely to arise from shortcomings in model cumulus parameterizations. A good candidate for closer study is the turbulent entrainment of drier environmental air into cumulus updrafts, a process that consumes the turbulent kinetic energy of the cumulus updraft, reduces its buoyancy, and limits its vertical penetration and rainfall production. Entrainment has also been implicated as an important player in other climate problems, including climate sensitivity and the Madden-Julian Oscillation.

We use cloud-resolving model simulations of continental convection to infer the strength of entrainment into convective clouds of varying depths. Entrainment turns out to be much stronger than general circulation models tend to assume. It also weakens as convection deepens, apparently due to the onset of downdrafts and boundary layer cold pools that help sustain convection for many hours and enhance the updraft speed at cloud base. We evaluate several candidate parameterizations against the cloud resolving model results to determine their suitability for use in general circulation models.

Last updated: 11/13/2009