Mass-Flux Characteristics of Reactive Scalars in the Convective Boundary Layer

The transport of nonreactive and reactive bottom-up and top-down diffusing scalars in a solid-lid convective boundary layer is studied using large-eddy simulation (LES). The chemistry considered consists of an irreversible, binary reaction involving the bottom-up and top-down diffusing scalars. The mass-flux or top-hat characteristics of the reactive flow are determined. Also, several mass-flux schemes are run in an off-line mode, that is, with prescribed profiles of the mass flux and the updraft area fraction, and are compared to the LES. Top-hat approximations are found to capture about 25% of the covariance between two arbitrary (nonreacting or reacting) scalars and about 65% of the flux. Subplume fluxes are located either in the updraft for bottom-up diffusing scalars or in the downdraft for top-down diffusing scalars. The mass-flux scheme that is nearly identical to the exact plume-budget equations gives the best performance. For the parameterization of lateral exchange this mass-flux scheme includes gross exchange across the interface between updrafts and downdrafts, that is, includes also subinterface-scale exchange processes (like the other dynamical quantities also prescribed in an off-line mode using LES data). A simpler mass-flux scheme, which does not include the more sophisticated parameterizations of subplume fluxes and subinterface-scale lateral exchange, is found to perform only slightly worse. The results of this paper are also valid for the surface layer and lower mixed layer of the entraining convective boundary layer but not for the entrainment zone

Greenland surface mass balance simulated by a regional climate model and comparison with satellite derived data in 1990-1991

The 1990 and 1991 ablation seasons over Greenland are simulated with a coupled atmosphere-snow regional climate model with a 25-km horizontal resolution. The simulated snow water content allows a direct comparison with the satellite-derived melt signal. The model is forced with 6-hourly ERA-40 reanalysis at its boundaries. An evaluation of the simulated precipitation and a comparison of the modelled melt zone and the surface albedo with remote sensing observations are presented. Both the distribution and quantity of the simulated precipitation agree with observations from coastal weather stations, estimates from other models and the ERA-40 reanalysis. There are overestimations along the steep eastern coast, which are most likely due to the “topographic barrier effect”. The simulated extent and time evolution of the wet snow zone compare generally well with satellite-derived data, except during rainfall events on the ice sheet and because of a bias in the passive microwave retrieved melt signal. Although satellite-based surface albedo retrieval is only valid in the case of clear sky, the interpolation and the correction of these data enable us to validate the simulated albedo on the scale of the whole Greenland. These two comparisons highlight a large sensitivity of the remote sensing observations to weather conditions. Our high-resolution climate model was used to improve the retrieval algorithms by taking more fully into account the atmosphere variability. Finally, the good agreement of the simulated melting surface with the improved satellite signal allows a detailed estimation of the melting volume from the simulation