Idealized Large-Eddy Simulations of Stratocumulus Advecting over Cold Water. Part I: Boundary Layer Decoupling

• Published in: Journal of the atmospheric sciences Vol. 78; no. 12; pp. 4089 - 4102
• Main Authors: Zheng, Youtong, Zhang, Haipeng, Rosenfeld, Daniel, Lee, Seoung-Soo, Su, Tianning, Li, Zhanqing
• Format: Journal Article
• Language: English
• Published: Boston American Meteorological Society 01.12.2021
• Subjects: Advection; Atmospheric models; Boundary layer; Boundary layers; Buoyancy; Buoyancy flux; Climate change; Clouds; Cold; Cold water; Convection; Convective circulation; Cooling; Cooling rate; Cumulus clouds; Decks; Decoupling; Downdraft; Entrainment; ENVIRONMENTAL SCIENCES; Fog; Heat exchange; Homogeneity; Large eddy simulation; Large eddy simulations; Mathematical models; Meteorology & Atmospheric Sciences; Mimicry; Moisture effects; Numerical models; Oceanic eddies; Physics; Radiative cooling; Sea surface; Sea surface temperature; Stratocumulus clouds; Sunlight; Surface cooling; Surface temperature; Temperature; Temperature inversion; Temperature inversions; Thermodynamics; Turbulence; Updraft; Vortices; Water; Water temperature
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/JAS-D-21-0108.1

We explore the decoupling physics of a stratocumulus-topped boundary layer (STBL) moving over cooler water, a situation mimicking warm-air advection (WADV). We simulate an initially well-mixed STBL over a doubly periodic domain with the sea surface temperature decreasing linearly over time using the System for Atmospheric Modeling large-eddy model. Due to the surface cooling, the STBL becomes increasingly stably stratified, manifested as a near-surface temperature inversion topped by a well-mixed cloud-containing layer. Unlike the stably stratified STBL in cold-air advection (CADV) that is characterized by cumulus coupling, the stratocumulus deck in the WADV is unambiguously decoupled from the sea surface, manifested as weakly negative buoyancy flux throughout the subcloud layer. Without the influxes of buoyancy from the surface, the convective circulation in the well-mixed cloud-containing layer is driven by cloud-top radiative cooling. In such a regime, the downdrafts propel the circulation, in contrast to that in CADV regime for which the cumulus updrafts play a more determinant role. Such a contrast in convection regime explains the difference in many aspects of the STBLs including the entrainment rate, cloud homogeneity, vertical exchanges of heat and moisture, and lifetime of the stratocumulus deck, with the last being subject to a more thorough investigation in Part II. Finally, we investigate under what conditions a secondary stratus near the surface (or fog) can form in the WADV. We found that weaker subsidence favors the formation of fog whereas a more rapid surface cooling rate does not.