Journal of the Atmospheric Sciences, submitted 8/08, accepted 12/08.
Adam H. Sobel
Department of Applied Physics and Applied Mathematics and Department of Earth and Environmental Sciences, Columbia University, New York, NY.
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY.
This paper examines the influence of imposed drying, intended to represent
horizontal advection of dry air, on parameterized deep convection interacting
with large-scale dynamics in a single column model framework.
Two single column models, one based
on the NASA GEOS5 general circulation model and the other developed
by Bony and Emanuel, are run in weak temperature gradient mode.
Drying is imposed by relaxation of the specific
humidity field towards zero within a specified vertical layer. The strength of
the drying is controlled either by specifying the relaxation time scale or the
vertically-integrated drying tendency; results are insensitive to which specification
The two models reach very different solutions for the same boundary conditions and model configuration. Even when adjustments to the boundary conditions and model parameters are made to render the precipitation rates similar, large differences in the profiles of relative humidity and large-scale vertical velocity persist. In both models, however, drying in the middle troposphere is more effective, per kg m^2/s (or W/m^2) of imposed drying, in suppressing precipitation than is drying in the lower troposphere. Even when compared at equal relaxation time (corresponding to weaker net drying in the middle than lower troposphere) middle-tropospheric drying is comparably effective to lower-tropospheric drying. Upper-tropospheric drying has a relatively small effect on precipitation, though large drying in the upper troposphere cannot be imposed in steady state due to the lack of moisture there. Consistently with the other model differences, the gross moist stabilities of the two models are quite different, and vary somewhat differently as a function of imposed drying, but in both models the gross moist stability increases as the drying is increased when it is less than around 30 $W/m^2 and located in the middle troposphere. For lower-tropospheric drying the gross moist stability either decreases with increased drying or increases more slowly than for middle-tropospheric drying.