A195-07
A Simple Model to Estimate the Extent and Radiative Effect of Stratocumulus Decks Under a Cleaner Atmosphere

Tuesday, 15 December 2020: 10:24
Virtual
Tom Goren, NOAA/CIRES, Boulder, CO, United States; University of Leipzig, Leipzig, Germany, Graham Feingold, NOAA ESRL CSL, Boulder, CO, United States, Edward Gryspeerdt, Imperial College London, Space and Atmospheric Physics Group, London, SW7, United Kingdom and Johannes Quaas, University of Leipzig, Leipzig Institute for Meteorology, Leipzig, Germany
Recorded Presentation
Abstract:
Satellite images often show solid decks of marine stratocumulus clouds (Sc) that break up in the remote oceans farther from the coasts. The breakup occurs when the Sc are deep enough to start precipitating, leading to a lower cloud radiative effect due to the lower cloud cover and optically thinner clouds. Aerosol have been shown to delay the breakup of solid Sc by postponing the onset of precipitation.

We will present a simple model that links between the critical liquid water path (LWPc, defined at the onset of precipitation, roughly when re reaches 15 μm), and droplet concentration (Nd). The difference between the observed LWP and LWPc (ΔLWP) is introduced as a proxy for how far the clouds are from the precipitation initiation stage, in terms of LWP. It follows the logic that the shallower the clouds, the more buildup of LWP is needed to reach LWPc (large and negative ΔLWP). Similarly, for precipitating clouds, the deeper the clouds, the stronger is the precipitation (large and positive ΔLWP). We will show how ΔLWP successfully diagnoses solid and broken Sc decks in satellite observations.

The model is further used to estimate the extent of the Sc solid decks in a counterfactual cleaner atmosphere, but with the same meteorology. Because LWPc is linearly related to Nd (a cloud with a given LWP will precipitate sooner when Nd is lower), ΔLWP is expected to diagnose the portion of the solid Sc deck that would have precipitated assuming a lower Nd. Analysis of several years of satellite data shows that the climatological Sc breakup fronts shift eastward (i.e., closer to the coast) along with a hypothetical reduction of Nd (assuming meteorology remains the same). Building on this idea, we present a relationship between ΔLWP and the cloud radiative effect, which allows us to estimate the global cloud radiative effect in response to the lower coverage of the solid Sc decks in this counterfactual, lower Nd world. The results demonstrate the strong effect of aerosol on the cloud radiative effect via delayed breakup of Sc decks.

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