Cumulus Friction Research Articles

The wave-CISK problem is investigated in a linearized context with two significant departures from previous models: 1) the cumulus heating parameterization is made thermodynamically consistent by requiring that cumulus heating exactly equal the latent heat of moisture converged in the wave; and 2) cumulus friction is included in the model. The parameterization of vertical transport of horizontal momentum by cumulus clouds follows the formulation of Schneider and Lindzen (1976); with a constant mean zonal wind, cumulus friction is proportional to the mean cloud mass flux. Results from the model of Stevens et at. (1977) are presented for various scales of motion. For magnitudes of cloud mass flux typical of the ITCZ, no wave-CISK modes appear on the synoptic scale of several thousand kilometers. As the horizontal scales of these long-period waves are decreased, the disturbances become frictionally controlled, neutral wave-CISK modes. When the mean cloud mass flux is reduced by a factor of 2, instabilities do occur at synoptic scales. Kelvin waves are simulated on a midlatitude f-plane with a zonal wall. For very long space and time scales, they are frictionally dominated and neutral modes. At very short time scales, they act as gravity waves. Gravity waves with the scales of observed squall lines are largely unaffected by cumulus friction. They display the instability characteristics of previous inviscid studies. By contrast, very slow gravity waves are neutralized by the action of cumulus friction.

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