On the interaction between marine boundary layer cellular cloudiness and surface heat fluxes
Abstract. The interaction between marine boundary layer cellular cloudiness and surface fluxes of sensible and latent heat is investigated. The investigation focuses on the non-precipitating closed-cell state and the precipitating open-cell state at low geostrophic wind speed. The Advanced Research WRF (Weather Research and Forecasting) model is used to conduct cloud system-resolving simulations with interactive surface fluxes of sensible heat, latent heat, and of sea salt aerosol, and with a detailed representation of the interaction between aerosol particles and clouds. The mechanisms responsible for the temporal evolution and spatial distribution of the surface heat fluxes in the closed- and open-cell state are investigated and explained. It is found that the closed-cell state imposes its horizontal spatial structure on surface air temperature and water vapor, and, to a lesser degree, on the surface sensible and latent heat flux. The responsible mechanism is the entrainment of dry, free tropospheric air into the boundary layer. The open-cell state is associated with oscillations in surface air temperature, water vapor, and in the surface fluxes of sensible heat, latent heat, and of sea salt aerosol. Here, the responsible mechanism is the periodic formation of clouds, rain, and of cold and moist pools with elevated wind speed. Open-cell cloud formation, cloud optical depth and liquid water path, and cloud and rain water path are identified as good predictors of the horizontal spatial structure of surface air temperature and sensible heat flux, but not of surface water vapor and latent heat flux. It is shown that the open-cell state creates conditions conducive to its maintenance by enhancing the surface sensible heat flux. The open-cell state also enhances the sea salt flux relative to the closed-cell state. While the open-cell state under consideration is not depleted in aerosol and is insensitive to variations in sea salt fluxes, in aerosol-depleted conditions, the enhancement of the sea salt flux may replenish the aerosol needed for cloud formation and hence contribute to the maintenance of the open-cell state. Spatial homogenization of the surface fluxes is found to have only a small effect on cloud properties in the investigated cases.