Abstract
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.
Highlights
Stratocumulus clouds over the oceans play an important role for Earth’s climate owing to their large areal coverage and the high contrast in albedo between the bright clouds and the dark sea underneath (Wood, 2012)
Open-cell cloud formation, optical depth, and cloud water path, followed by rain water path will be established as predictors of the spatial distribution of surface air temperature and surface sensible heat flux, but not of surface water vapor nor surface latent heat flux
Cloud formation is, not a good predictor of surface water vapor and surface latent heat flux, with comparably low lagged correlation coefficients (Table 3a). We propose that this is due to the aforementioned discordant action of surface wind speed and enhanced water vapor in driving the cold pool surface latent heat flux (Sect. 4.1), and the formation of cold pool water vapor halos (Sect. 4.2)
Summary
Stratocumulus clouds over the oceans play an important role for Earth’s climate owing to their large areal coverage and the high contrast in albedo between the bright clouds and the dark sea underneath (Wood, 2012). A multitude of mechanisms can lead to the development of rain in closed cells, and to open-cell formation, including perturbations in boundary layer moisture, temperature, aerosol concentration, and surface sensible and latent heat fluxes (Wang et al, 2010). The investigation is motivated by the presence of cold and moist pools with surface wind divergence in the open-cell state, which have the potential to modify the surface flux of sensible and latent heat and of sea salt aerosol, relative to the closed-cell state. Applying the ARW dynamical core to the residual wind field in this manner is akin to moving the model domain with the geostrophic wind, its geographic location remains fixed This approach facilitates the analysis of horizontal spatial structures such as closed and open cells, which remain stationary with respect to the domain. In the original implementation of ARW, the closed and open cells would be advected through the domain by the geostrophic wind
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