NUMERICAL INVESTIGATIONS OF THE ROLES OF RADIATIVE AND EVAPORATIVE FEEDBACKS IN STRATOCUMULUS ENTRAINMENT AND BREAKUP

When the surface buoyancy flux is small and the shear is weak, turbule nce 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 e ntrainment, however. When the entrainment rate increases, the effectiv eness of radiative cooling in driving circulations decreases (a negati ve 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 break up, are examined in this paper through large eddy simulations. Three s tratus cases (with a small surface buoyancy Aux) are simulated: one is stable with respect to the Lilly-Randall-Deardorff cloud-top entrainm ent instability criterion, and the other two are unstable. Only one of the two cloud decks in the unstable regime dissipates totally; the ot her remains nearly solid. A method is proposed to separate the cloud-t op radiative and evaporative cooling contributions to downdraft accele ration, which drives the boundary-layer circulations. Analysis of thes e three flow fields shows that cloud dissipates totally only in the ca se that the evaporative feedback dominates. When the radiative feedbac k dominates, as in one of the unstable cases, the cloud remains nearly solid even though the Lilly-Randall-Deardorff criterion is satisfied. To confirm the key role of cloud-top evaporative cooling in this posi tive feedback loop, two controlled experiments have been conducted-one with evaporative cooling turned off and the other with radiative cool ing turned off-after the cloud was brought into the unstable regime wi th respect to the Lilly-Randall-Deardorff criterion. The cloud without evaporative cooling (for which boundary-layer circulations are driven only by cloud-top radiative cooling) remains solid, while that withou t radiative cooling (in which circulations are driven only by evaporat ive cooling) dissipates rapidly.