Surface-atmosphere decoupling prolongs stratocumulus persistence under warm advection
Submitter:
Li, Zhanqing — University of Maryland
Area of research:
Cloud Processes
Journal Reference:
Science
Enhancing the understanding of stratocumulus (Sc) cloud responses to meteorological factors could help reduce uncertainties in climate projections. Among various factors (lower-tropospheric stability, subsidence rate, wind speeds, etc.), the influence of horizontal temperature advection (Tadv), particularly warm-air advection (WADV), on Sc remains the least understood. This study elucidates the physical mechanism of the responses of Sc cloud lifetime to WADV and highlights the crucial role of WADV-induced reduction in entrainment drying in cloud lifetime.
Impact
There are two contradictory mechanisms regarding the Sc response to Tadv: 1) with increasing Tadv (i.e., from cold-air advection [CADV] to WADV), enhanced surface-atmosphere decoupling reduces moisture supply to Sc clouds, decreasing cloudiness; 2) Meanwhile, suppressed surface fluxes in WADV help sustain cloud decks by weakening entrainment drying. The findings reconcile these two mechanisms by using the idealized large-eddy simulations, demonstrating that the entrainment-drying-oriented mechanism dominates the moisture-deficit mechanism, contributing to understanding the observed relationships between low-cloud fraction and Tadv.
Summary
An initially well-mixed Sc deck can stay overcast for several tens of hours under WADV conditions, even though the moisture supply is cut off from the ocean due to surface-atmosphere decoupling. A set of idealized large-eddy simulations were performed to investigate the physical mechanism of how WADV impacts the evolution of a pre-existing Sc deck. To mimic warm-air advection, we decrease the sea surface temperature linearly over time in a doubly periodic domain. Given the identical initial conditions, the Sc deck is more persistent when experiencing WADV than CADV. As shown in Figure 1a-b, this persistence is caused by reduced cloud-top entrainment drying due to decoupling, which outweighs decoupling-induced cutoff of moisture supply. This mechanism is more notable when the free troposphere becomes more humid (Figure 1c-d), due to the larger sensitivity of entrainment drying to free-tropospheric moisture in WADV.
Figure 1. (a-b) Schematic diagrams illustrating the physical mechanism of the longer persistence of cloud layers from cold-air to warm-air advection given the same initial conditions. Panels (c-d) show the same case but in a free troposphere with higher humidity. Red circles represent cloud-top entrainment, and blue ones represent the boundary-layer turbulence. The strength of the entrainment or turbulence is denoted by the circle size and thickness.