On the interactions of longwave radiation, microphysics, and turbulence in boundary layer clouds

A detailed model for longwave radiative transfer in general cloudy atmospheres has been developed and applied to microphysical and macrophysical studies of the radiation regime within typical boundary layer clouds. The model uses high vertical and spectral resolution to provide broadband flux divergence and cloud cooling rates, using LOWTRAN 6 and Chou and Arking\u27s radiative transmission functions and Wiscombe\u27s Mie code to provide the cloud droplet optical characteristics. The model is applicable to clouds with either homogeneous or inhomogeneous liquid water distributions. High radiative cooling rates of about 50 K/hr are calculated at the top of homogeneous clouds. In the presence of a finite cloud-top liquid water gradient these values are reduced to 5-20 K/hr. When a temperature inversion occurs above the cloud the maximum cooling rates are further reduced, and the position of the maximum cooling is shifted downward. The spectral resolution of the model is important in determining cooling rates at the cloud top, especially for arbitrary temperature inversions. The substantial cloud-top cooling rates destabilize the cloud convectively, and an analysis of the relative time constants for radiative and convective processes confirms that convection occurs on a much shorter time scale. As a result the cloud temperature and bulk liquid water profiles change very slowly in response to the cloud-top radiative loss, and the radiative destabilizing forcing mechanism is sustained for a longer period of time. A mixed layer model version is used to show, at first order, that an accurate representation of the vertical distribution of radiation is important in calculating turbulent fluxes and entrainment rates. A grey-cloud parameterization is proposed and shows reasonably good agreement with the mixed-layer calculations that use detailed radiative profiles, while larger discrepancies are found using the bulk radiative approaches proposed by Lilly (1968) and Stage and Businger (1981). This mixed layer version also indicates that the integrated effects of droplet sedimentation influence the turbulent structure of the boundary layer, reducing the entrainment rates by up to 20%. The longwave radiative budget of individual droplets is also determined, and shows a strong dependence on droplet radius. (Abstract shortened with permission of author.