Deep Convective Adjustment of Temperature and Moisture

Simple process models and complex climate models are remarkably sensitive to the time scale of convective adjustment tau, but this parameter remains poorly constrained and understood. This study uses the linear-range slope of a semiempirical relationship between precipitation and a lower-free-tropospheric buoyancy measure B-L. The B-L measure is a function of layer-averaged moist enthalpy in the boundary layer (150-hPa-thick layer above surface), and temperature and moisture in the lower free troposphere (boundary layer top to 500 hPa). Sensitivity parameters with units of time quantify the B-L response to its component perturbations. In moist enthalpy units, B-L is more sensitive to temperature than equivalent moisture perturbations. However, column-integrated moist static energy conservation ensures that temperature and moisture are equally altered during the adjustment process. Multiple adjusted states with different temperature-moisture combinations exist; the B-L sensitivity parameters govern the relationship between adjusted states, and also combine to yield a time scale of convective adjustment similar to 2 h. This value is comparable to tau values used in cumulus parameterization closures. Disparities in previously reported values of tau are attributed to the neglect of the temperature contribution to precipitation, and to averaging operations that include data from both precipitating and nonprecipitating regimes. A stochastic model of tropical convection demonstrates how either averaging operations or neglected environmental influences on precipitation can yield tau estimates longer than the true tau value built into the model. The analysis here culminates in construction of a precipitation closure with both moisture and temperature adjustment (q-T closure), suitable for use in both linearized and nonlinear, intermediate-complexity models.