Convective Mixing, Lower Tropospheric Stability and Low Cloud Feedback

Thursday, 18 December 2014
Hui Su, Jonathan H. Jiang, Chengxing Zhai and Tsae-Pyng J Shen, Jet Propulsion Lab, Pasadena, CA, United States
A recently published study by Sherwood et al. (2014) suggested that convective mixing is a key process that determines the inter-model spread in low cloud feedback and equilibrium climate sensitivity (ECS). The parameterized sub-grid scale convective mixing (S) is represented by the temperature and humidity differences between the lower free troposphere, 700 hPa, and the boundary layer, 850 hPa. Although a strong correlation between convective mixing strengths averaged over a deep convective region and ECSs among the models was shown, how convective mixing relates to the distribution and variation of low clouds was not examined in the study. Lower tropospheric stability (LTS, the potential temperature difference between 700 hPa and 1000 hPa) is known to have a strong correlation with low cloud fraction (Klein and Hartmann 1993). How S is related to LTS and whether model simulations of LTS contribute to the model spread in low cloud feedback are of great scientific interest. Combined AIRS and CloudSat/CALIPSO observations along with CMIP5 model simulations are analyzed to address the relationships between S, LTS, and cloud variations in the past decade and in the future climate model projections under the doubling of CO2. The implications for climate sensitivity are explored.