Numerical investigations of the roles of radiative and evaporative feedbacks in stratocumulus entrainment and breakup

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. 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. To confirm the key role of cloud-top evaporative cooling in this positive feedback loop, two controlled experiments have been conducted - one with evaporative cooling turned off and the other with radiative cooling turned off. -from Authors