Large Eddy Simulation of Low Cloud Feedback to a 2-K SST Increase

1. Large Eddy Simulation of Low Cloud Feedback to a 2-K SST Increase Anning Cheng1, and Kuan-Man Xu2 1. AS&M, Inc., 2.NASA Langley Research Center, Hampton, VA • Motivation • Low-level clouds are a source of major uncertainties in projecting climate changes due to anthropogenic forcing; • Parameterization of low-level clouds in global climate models is unable to handle the complicated interactions among turbulence, cloud microphysics, radiation, and surface fluxes (e.g., Zhang and Bretherton 2008); • Large-eddy simulation (LES) is a promising tool to study the low-cloud feedback; • No LES has been used to simulate long-term equilibrium states with an imposed increase in sea surface temperature (SST). • Experiment Design • Each experiment is imposed with an SST difference between the tropical warm pool and subtropical cold pool at 20ºN. A range of 4, 6, 8, 10, 12 to 14 K difference is studied. The imposed subsidence is balanced by radiative cooling associated with the different soundings (see Zhang and Bretherton 2008). The latitude is specified to 20ºN. The mean TOA incoming solar radiation is 408.7 W m-2. The sensitivity experiment has SST increased by 2 K, compared to the control experiment. The domain size is 6.4 km by 6.4 km by 12.8 km in horizontal and vertical directions, with grid spacings of 200 m in horizontal, and 30 m near the surface and stretched to 90 m at 2 km, and 156 m at 4 km, respectively. The integration time is 30 days. Fig. 6. Same as Fig. 5 except for liquid water path (a), cloud fraction (b), albedo at the top of the atmosphere (TOA, c), and upward long wave fluxes at TOA (d). Fig. 2. Selected mean profiles averaged over the last 20 days for Experiments 4K and 6K. Fig. 7. Cloud radiative forcing (CRF) averaged from the last 20 days for longwave (a), shortwave (b), and net radiation (c). Negative CRF represents negative cloud feedback. Table 1: Positive (+) and negative (-) cloud feedback mechanisms inferred from pairs of LES experiments with different SSTs. Fig. 3. Same as Fig. 2 except for Experiments 8K, 10K, 12K, and 14K. Results Control Table 2: Major non-cloud characteristics inferred from pairs of LES experiments with different SSTs. + 2 K Fig. 4. Mean profiles averaged over the last 20 days for sensible heat flux (a and c) and latent heat flux (b and d). Experiments 4K and 6K are shown in (a) and (b), while the rest in (c ) and (d). • Summary • The changes in cloud radiative forcing (CRF) of both stratocumulus and shallow cumulus clouds to a 2 K SST increase are negative although the magnitudes of the CRF changes is larger for the stratocumulus clouds than for the shallow cumulus clouds. • The negative SW cloud feedback overwhelms the positive LW cloud feedback for all the cases, but individual cloud-feedback mechanisms contribute differently among the experiments. • For shallow cumulus clouds, the increased cloud amount related to the large surface latent fluxes accounts for the increased TOA albedo, while the increased liquid water path plays a major role for the stratocumulus clouds. • References • Zhang, M.-H., and C. S. Bretherton, 2008: Mechanisms of low cloud climate feedback in idealized single-column simulations with CAM3. J. Climate (in press). Control + 2 K Fig. 1. Time evolution of cloud fraction for all control (labeled CTLnK) and sensitivity experiments (labeled PSSTnK), where n = 4, 6, 8, 10, 12 and 14. Larger cloud fraction and higher cloud top are associated with the sensitivity experiments of 4 K, 6 K and 8 K, Cloud thickness and top are slightly increased in the sensitivity experiments of 10 K, 12 K and 14 K . Fig. 5. Time series of surface latent heat flux (LHF, a), sensible heat flux (SHF, b), height of maximum gradient of potential temperature (c), and the maximum vertical velocity within the LES domain (d) for all experiments.