Numerical Investigations of the Roles of Radiative and Evaporative Feedbacks in Stratocumulus Entrainment and Breakup

Abstract

Abstract When the surface buoyancy flux is small and the shear is weak, turbulence circulations within a stratus-topped boundary layer are driven by two buoyancy-generating processes at cloud top: radiative cooling and evaporative cooling. These two processes respond very differently to entrainment, however. When the entrainment rate increases, the effectiveness of radiative cooling in driving circulations decreases (a negative feedback) but the effectiveness of evaporative cooling can increase (a positive feedback). The roles of these two competing feedbacks in determining the entrainment rate, and hence in determining cloud breakup, are examined in this paper through large eddy simulations. Three stratus cases (with a small surface buoyancy flux) are simulated: one is stable with respect to the Lilly–Randall–Deardorff cloud-top entrainment instability criterion, and the other two are unstable. Only one of the two cloud decks in the unstable regime dissipates totally; the other remains nearly solid. A me...