Convective quasi-equilibrium revisited, part II: cloud resolving model simulations

Impulsive cooling perturbations to either the upper or lower troposphere in a cloud-resolving model initially in radiative-convective equilibrium (RCE) are made and the return of the system to RCE is then followed in time. The relaxation process is significantly different for upper and lower tropospheric pulses. Two thermodynamic variables are studied in particular, the specific moist entropy, which is approximated as the dry entropy plus a scaled mixing ratio, and a variable we call the adiabatic warming, which is the dry entropy minus the scaled mixing ratio. The latter variable captures combined changes in the temperature and mixing ratio which leave the moist entropy unmodified. The perturbation in the rainfall rate is also examined.

As in the study of Tompkins and Craig (1998), the adjustment time has distinctive short term (1 hour) and long-term (days) behavior. When the cooling is confined to the lower troposphere, the short term adjustment is very strong in the vertically integrated adiabatic warming and in the precipitation, which exhibits a strong, short term pulse. However, for cooling of the upper atmosphere, the short term response in adiabatic warming is much weaker, with a much diminished and delayed pulse of precipitation. The long term response is comparable for the two cases. These results are largely consistent with the theoretical conclusions of part I of this presentation.