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Modelling climatic impacts of low clouds over tropical waters

31 Jan 2024 | Science Notes

From the CO2Science Archive: Writing that “regions of marine stratocumulus clouds play an important role in maintaining Earth's global energy balance,” Rapp (2015) introduced her study of the subject by noting that “the sensitivity of regimes dominated by low clouds has been identified as the largest contributor to uncertainties in tropical cloud feedback estimates in climate models.” And, therefore, she went on to describe how Atmospheric Model Intercomparison Project [AMIP] simulations of low cloud responses to sea surface temperature (SST) compare with satellite observations in the southeastern Pacific subsidence region.

Paper reviewed: Rapp, A.D. 2015. Cloud responses in the AMIP simulations of CMIP5 models in the southeastern Pacific marine subsidence region. International Journal of Climatology 35: 2908-2921

This work revealed, as she reported, that AMIP models have considerable difficulty in simulating (1) the annual cycle in the cloud radiative effect (CRE), (2) cloud fraction, and (3) liquid water path (LWP), likely due in part to (4,5) “underestimation of the strength of lower tropospheric stability and the depth of the boundary layer,” but also noting that (6) stratocumulus clouds “are not sensitive enough.” In addition, Rapp notes that (7,8) “not only do the seasonal amplitudes disagree but also some models have an annual cycle that is nearly 180 degrees out of phase with the observations.”

Consequently, Rapp concluded that the results she reported “show that climate models still have difficulty reproducing the observed cloud and radiative sensitivities in a low cloud regime, even when forced with climatological SSTs.”

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