A New Index for Low Cloud Cover and Interpretation of Low Cloud Feedback

Estimated Inversion Strength (EIS; Wood and Bretherton 2006), which is a sophisticated modification of Lower Tropospheric Stability (LTS; Klein and Hartmann 1993), is an extremely skillful index for low cloud cover that has been used in many studies. EIS represents the temperature gap at the top of the boundary layer but does not depend on the vertical structure of humidity. We have developed a new index for low cloud cover, which is derived from a cloud top entrainment (CTE) criterion. The Estimated Cloud Top Entrainment Index (ECTEI; Kawai et al. 2017, J. Clim.) takes into account vertical profiles of both temperature and humidity. Shipboard observational data confirm that the ECTEI is slightly more strongly correlated with low cloud cover than EIS for present day spatial-seasonal cloud variations.

We can use the ECTEI to interpret low cloud feedbacks on climate change. It has been shown in many GCMs that low cloud cover decreases in future climate simulations despite increases in EIS. This appears to be inconsistent with the well-known relationship between low cloud cover and EIS for present-day variations. Changes in low cloud cover cannot be fully determined by changes in EIS alone, but can be better determined from changes in both EIS (positive correlation) and SST (negative correlation). We show that the competing effects of EIS and SST on low cloud cover can be understood in terms of their effects on the temperature and humidity structure and its impact on the cloud cover via the ECTEI. This provides insight into the mechanism whereby low cloud cover can decrease even when EIS increases under a warmer SST climate.