A dual mass flux framework for boundary layer convection
| Neggers, Roel | European Centre for Medium-range Weather Forecasts (ECMWF) |
Category: Modeling
A new convective boundary layer scheme is presented that is currently being developed for the Integrated Forecasting System (IFS) of the European Centre for Medium-range Weather Forecasts (ECMWF). Part of the total turbulent flux is modeled through advective mass flux by multiple updrafts, initialized at the surface. Two groups of updrafts are explicitly represented; i) updrafts that never reach their lifting condensation level, and ii) updrafts that condensate and become cloudy. Key new ingredient is the flexibility of the associated updraft area fractions, as a function of model state. As a result, an extra degree of freedom is introduced in the system of updraft equations. In a discretized updraft ensemble, this theoretically allows for i) smooth transitions between the major convective regimes, and ii) explicit representation of important negative feedback mechanisms between cloud base humidity, stability and the cumulus mass flux. Other upgrades include among others i) the updraft entrainment closure and ii) the variance budget closure, as part of a new statistical cloud scheme. In accordance with one of the main goals of the ARM program of improving models for climate and weather prediction, the new updraft scheme is evaluated against ARM continuous forcing datasets as well as large-eddy simulation results on idealized boundary layer cases. In general, the presence of a transporting dry updraft significantly improves the countergradient thermodynamic structure of the mixed layer, and better maintains the slight stability of the shallow cumulus cloud base transition layer. The entrainment model reproduces the typical vertical structure of the updraft temperature and humidity profiles. The explicit representation of cloud base mass flux feedbacks is shown to be effective in maintaining the marine PBL state close to equilibrium, up to medium-range timescales and beyond. The scheme reproduces the onset and typical gradual development of the transient convective cloud layer over land. Finally, the robustness of the scheme is demonstrated by its evaluation against ARM continuous forcings, consistently developing representative cloud layers under slightly different conditions.
This poster will be displayed at the ARM Science Team Meeting.
