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

Chin-Hoh Moeng National Center for Atmospheric Research, Boulder, Colorado

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Don H. Lenschow National Center for Atmospheric Research, Boulder, Colorado

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David A. Randall Colorado State University, Fort Collins, Colorado

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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 method is proposed to separate the cloud-top radiative and evaporative cooling contributions to downdraft acceleration, which drives the boundary-layer circulations. Analysis of these three flow fields shows that cloud dissipates totally only in the case that the evaporative feedback dominates. When the radiative feedback 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 positive feedback loop, two controlled experiments have been conducted—one with evaporative cooling turned off and the other with radiative cooling turned off—after the cloud was brought into the unstable regime with 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 without radiative cooling (in which circulations are driven only by evaporative cooling) dissipates rapidly.

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 method is proposed to separate the cloud-top radiative and evaporative cooling contributions to downdraft acceleration, which drives the boundary-layer circulations. Analysis of these three flow fields shows that cloud dissipates totally only in the case that the evaporative feedback dominates. When the radiative feedback 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 positive feedback loop, two controlled experiments have been conducted—one with evaporative cooling turned off and the other with radiative cooling turned off—after the cloud was brought into the unstable regime with 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 without radiative cooling (in which circulations are driven only by evaporative cooling) dissipates rapidly.

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