Cloud-Resolving Simulation of Low-Cloud Feedback to an Increase in Sea Surface Temperature

• Published in: Journal of the atmospheric sciences Vol. 67; no. 3; pp. 730 - 748
• Main Authors: XU, Kuan-Man, ANNING CHENG, MINGHUA ZHANG
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.03.2010
• Subjects: Aircraft; Atmosphere; Clear sky; Climate; Climate sensitivity; Cloud optical depth; Cloud thickness; Clouds; Control systems; Control theory; Cumulus clouds; Earth, ocean, space; Equilibrium; Exact sciences and technology; External geophysics; Feedback; Flux; Heat flux; Heat transfer; Latent heat; Latent heat flux; Liquid water content; Liquids; Low clouds; Marine; Meteorology; Moisture content; Ocean temperature; Optical analysis; Optical thickness; Parameter sensitivity; Physical properties; Physics of the high neutral atmosphere; Radiation; Radiative equilibrium; Saturation; Sea surface; Sea surface temperature; Simulation; Stratocumulus clouds; Surface temperature; Temperature; Vapor pressure; Vapour pressure; Water; Water content; Water depth; Water vapor; Water vapour; Stratocumulus cloud; Cumulus cloud; Vapor pressure; sensitivity analysis; water pressure; clouds; Sea surface temperature; subtropical zone; feedback; saturation; Equilibrium state; Liquid water path; Clear sky; water vapor; water content; Optical thickness; heat transfer; Cloud top; latent heat; Radiative equilibrium
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/2009jas3239.1

Abstract This study investigates the physical mechanisms of the low cloud feedback through cloud-resolving simulations of cloud-radiative equilibrium response to an increase in sea surface temperature (SST). Six pairs of perturbed and control simulations are performed to represent different regimes of low clouds in the subtropical region by specifying SST differences (ΔSST) in the range of 4 and 14 K between the warm tropical and cool subtropical regions. The SST is uniformly increased by 2 K in the perturbed set of simulations. Equilibrium states are characterized by cumulus and stratocumulus cloud regimes with variable thicknesses and vertical extents for the range of specified ΔSSTs, with the perturbed set of simulations having higher cloud bases and tops and larger geometric thicknesses. The cloud feedback effect is negative for this ΔSST range (−0.68 to −5.22 W m−2 K−1) while the clear-sky feedback effect is mostly negative (−1.45 to 0.35 W m−2 K−1). The clear-sky feedback effect contributes greatly to the climate sensitivity parameter for the cumulus cloud regime whereas the cloud feedback effect dominates for the stratocumulus regime. The increase of liquid water path (LWP) and cloud optical depth is related to the increase of cloud thickness and liquid water content with SST. The rates of change in surface latent heat flux are much higher than those of saturation water vapor pressure in the cumulus simulations. The increase in surface latent heat flux is the primary mechanism for the large change of cloud physical properties with +2 K SST, which leads to the negative cloud feedback effects. The changes in cloud fraction also contribute to the negative cloud feedback effects in the cumulus regime. Comparison of these results with prior modeling studies is also discussed.