Impacts from Free Troposphere Humidity and Temperature on Marine Stratocumulus: Still Kappa?

Cloud top stability parameter κ, which is derived from the Cloud Top Entrainment Instability (CTEI) theory, is regarded as one of the most important factors influencing the properties of marine stratocumuli (MSc). A three-dimensional, high-resolution Large Eddy Simulation (LES) model coupled with bin-microphysics is employed to study how MSc behave with a same κ but different meteorological settings. Since κ depends on the humidity and temperature jumps across the inversion, we further investigate the impact on MSc by varying free tropospheric humidity and temperature, keeping the thermodynamic profiles below the inversion unchanged.

Starting from a base case (κ = 0.73, liquid-water potential temperature jump Δθ_l = 5 K, total water mixing ratio jump Δq_t = 7.5 g kg^-1), we first perform a group of runs with the same κ at an aerosol concentration of 50 mg^-1. Results show that cloud liquid water path (LWP) can vary from 58 g m^-2 (κ = 0.73, Δθ_l = 6 K, Δq_t = 9 g kg^-1) to 94 g m^-2 (κ = 0.73, Δθ_l = 4 K, Δq_t = 6 g kg^-1), which indicates that MSc below a moister free troposphere have a much higher LWP even when the free troposphere is accordingly warmer. To isolate the effects of free tropospheric humidity and temperature, we next perform two groups of runs that vary only Δq_t (Δθ_l = 5 K, Δq_t = 9, 6 g kg^-1) and Δθ_l (Δq_t = 7.5 g kg^-1, Δθ_l = 6, 4 K). We confirm that the influence from free tropospheric humidity is more important than temperature. To investigate the possible influence from the aerosol indirect effect, we repeat our experiments under various aerosol conditions (Na = 25, 100, 200, 500 mg^-1). The results are consistent with our findings for Na = 50 mg^-1, although high sensitivity of LWP to aerosol concentration is present.