Clouds and Climate
The Romps Group at the University of California, Berkeley
Recent work with global climate models (GCMs) has shown that convective momentum transport (CMT) has a large influence on the simulated climate. Although CMT is simple to define (it is just the vertical transport by clouds of horizontal momentum), it has proven difficult to model. Over the past 60 years, several proposals have been made as to how this effect should be parameterized in GCMs. The differences between these different schemes has to do with how the pressure-gradient force is represented.
In this paper, it is shown that the GKI scheme used in the Community Atmosphere Model (CAM) version 5 is exactly equivalent to a scheme in which no pressure-gradient force is used, but in which the convective mass flux is reduced. This leads to an underestimation of the compensating subsidence of wind profiles, which leads to some poor behavior by the GKI scheme. A simple remedy is proposed for CAM.
Figure 3. The steady-state wind profile generated in a convecting atmosphere when the air is accelerated at 6 km and damped to zero everywhere on a 12-hour timescale in the LES (solid), the simple theory (dashed), and the GKI scheme (dotted).