Monday, 9 July 2018: 1:30 PM
Regency E/F (Hyatt Regency Vancouver)
High resolution large-eddy simulations (LES) often serve as references in the development of parameterizations to be used in larger scales atmospheric models (e.g. convection, turbulence, microphysics). These reference simulations are commonly forced at the surface by latent- and sensible-heat fluxes in order to constrain the system. This generally implies that radiative fluxes and heating rates are not explicitly computed. They are practically independent of the current state of the atmosphere, if not completely ignored. Doing so neglects the strong and complex coupling between the surface and the atmosphere, in particular through the radiative budget, which is a critical driver of convection and cloud formation. It also overlooks the strong interactions between clouds and radiation, which shape the meso-scale organization of clouds. In this study, we perform LES simulations of continental cumuli with the model Meso-NH, and compare the cloud fields obtained with surface forcings on the one hand, and with an interactive surface and explicit radiation on the other hand. Various setups are used, where surface radiative fluxes can be spatially or temporally averaged, or artificially shifted. The life cycle of clouds and their statistics are investigated to stress the impact of surface fluxes on cloud development. This puts light on the large spatial and temporal variability of radiative fluxes, in particular through the shading effect of clouds. This study thus gives an insight into the limits of forced simulations, and paves the way for the parameterization of radiative transfer in unresolved broken clouds fields.
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