11D.2 A parameterization of convective organization

Wednesday, 30 April 2008: 1:30 PM
Palms I (Wyndham Orlando Resort)
Brian Mapes, Univ. of Miami/RSMAS, Miami, FL; and R. Neale

A variable representing precipitation-driven mesoscale structure in deep convective cloud fields is introduced in the NCAR CAM 3.5 global atmosphere model. This variable, called org, is a forced-damped flywheel with 3h time scale whose source is precipitation. It is used to precondition the air entering the convective closure, representing the fact that air different from (more favorable than) the GCM grid-scale mean state ascends preferentially. Such a "wrapper" approach could be applied to other models: indeed, CAM is an especially imperfect vehicle for this demonstration, but it suffices. The impact of the scheme is a local positive feedback and delay, so when it rains it pours, but it takes convection a few hours to fully "organize" in this sense. The mean state becomes warmer and drier, variability (in both space and time) is enhanced, afternoon rain over land is delayed, and the distribution of rainrates becomes wider.
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