Control of Stability and Relative Humidity in the Radiative-Convective Equilibrium Model Intercomparison Project

The Radiative-Convective Equilibrium Model Intercomparison Project (RCEMIP) exhibits a large spread in the simulated climate across models, including in profiles of buoyancy and relative humidity. Here we use simple theory to understand the control of stability, relative humidity, and their responses to warming. Across the RCEMIP ensemble, temperature profiles are systematically cooler than a moist adiabat, and convective available potential energy (CAPE) increases with warming at a rate greater than that expected from the Clausius-Clapeyron relation. There is higher CAPE (greater instability) in models that are on average moister in the lower-troposphere. To more explicitly evaluate the drivers of the intermodel spread, we use simple theory to estimate values of entrainment and precipitation efficiency (PE) given the simulated values of CAPE and lower-tropospheric relative humidity. We then decompose the intermodel spread in CAPE and relative humidity (and their responses to warming) into contributions from variability in entrainment, PE, the temperature of the convecting top, and the inverse water vapor scale height. Model-to-model variation in entrainment is a dominant source of intermodel spread in CAPE and its changes with warming, while variation in PE is the dominant source of intermodel spread in relative humidity. We also decompose the magnitude of the CAPE increase with warming and find that atmospheric warming itself contributes most strongly to the CAPE increase, but the indirect effect of increases in the water vapor scale height with warming also contribute to increasing CAPE beyond that expected from Clausius-Clapeyron. Idealized model simulations of the tropical atmosphere disagree on profiles of temperature and humidity. Here we use simple theory to understand the intermodel spread in humidity and in a measure of instability to convection (small-scale rising motion). There is greater instability in models that are on average moister. We find that model-to-model variations in small-scale turbulent mixing between cloudy and clear air cause the intermodel spread in instability. Model-to-model variations in the fraction of cloud water that reaches the surface as rainfall cause the intermodel spread in humidity. The instability increases substantially with warming, which is driven by a warmer and moister lower atmosphere. A stability-relative humidity phase space is used to diagnose theory-implied entrainment and precipitation efficiency in RCEVariations in entrainment explain intermodel spread in CAPE but variations in precipitation efficiency explain spread in humidityIncreases in the water vapor scale height with warming lead to CAPE increases with warming that are faster than Clausius-Clapeyron scaling