Velocity and Length Scales for Moist Convection: Estimation of Scale-Aware Convective Adjustment Timescale

English Abstract:

One of the most uncertain parameters in some of the cumulus convection parameterization (CCP) schemes is the convective adjustment time scale (τ). Yet there exists no generalized τ formulation that can be used in these CCP schemes for regional and global models as well as with measurements. To address this critical research need, for continental convection I have developed a generalized and scale-aware dynamic formulation to estimate convective adjustment time scale for CCP schemes as well as for observational studies. First, appropriate length and velocity scales for subcloud and cloud layers are identified. For subcloud layer, its depth is considered as the length scale and the velocity scale is based on turbulent kinetic energy. For cloud layers, the depth of a cumulus cloud is used as length scale while a simplified form of Arakawa and Schubert’s cloud work function is used to derive the velocity scale. Then, a scale-aware convective adjustment time scale (τ) formulation for cumulus convection for shallow and deep clouds is developed as a turn-over timescale that depends on the ratio of length scale and velocity scale (simplified cloud work function). Key features of the τ formulation are: (1) it can be used in any mass flux or convective adjustment type of CCP schemes; (2) it has scale-awareness; and (3) it responds to the stabilization of subcloud layer as well as cloud layer.

Various measurement data available from the TWP-ICE and MC3 experiments are used for the estimation of τ. It is found that τ can vary from about 30 minutes to about 3 hours depending on the depth of a shallow/deep convective cloud observed during the TWP-ICE and MC3 experiments’ periods. Further, results obtained from a regional climate model simulations using the proposed τ formulation also indicated similar range of τ variability. Pertinent climate parameters as well as simulated surface precipitation are evaluated using corresponding observations and these results will be presented.