Are Stratocumulus Radiative Cooling, Turbulence, Entrainment, and Cloud Base Updraft Speeds Always Correlated?

Stratocumulus cloud radiative cooling, turbulence, entrainment, and cloud base updraft speeds are considered to be correlated. Consequently, empirical or model-based relationships between stratocumulus cloud radiative cooling and cloud base updraft speeds could facilitate satellite retrieval of cloud condensation nucleus concentrations at cloud base, and parameterization of cloud base updraft speeds in climate models. In this work the relationship between stratocumulus cloud radiative cooling, turbulence, entrainment, and cloud base updraft speeds is investigated using large eddy simulations. It is shown that stratocumulus cloud radiative cooling, turbulence, cloud top entrainment, and cloud base updraft speeds are not always correlated. Both a constant as well as a decreasing cloud radiative cooling are capable of driving stratocumulus turbulence, entrainment, and cloud base updraft speeds that increase with time. The underlying mechanism is identified as boundary layer cooling which (a) increases latent heat release and uptake at cloud level and thereby promotes cloud turbulence and entrainment, and (b) drives an increasing surface heat flux which accelerates cloud base updrafts. This mechanism governs stratocumulus turbulence in nighttime conditions at high cloud water paths, when cloud longwave cooling is nearly saturated. During daytime, the diurnal insolation cycle governs cloud radiative cooling, boundary layer temperature, turbulence, and entrainment. Cloud radiative cooling and cloud base updraft speeds are then correlated. Day- and nighttime relationships between cloud radiative cooling and cloud base updraft speeds are derived and compared with empirical results. The relationships are found to depend on meteorological conditions.