Equilibrium States Simulated by Cloud-Resolving Models

Abstract

Recently, several cloud-resolving models (CRMs) were used to study the tropical water and energy cycles and their role in the climate system. They are typically run for several weeks until modeled temperature and water vapor fields reach a quasi-equilibrium state. However, two CRMs produced different quasi-equilibrium states (warm and humid versus cold and dry) even though both used similar initial thermodynamic profiles, horizontal wind, prescribed large-scale vertical velocity, and fixed sea surface temperature. Sensitivity tests were designed to identify the major physical processes that determine the equilibrium states for the different CRM simulations. Differences in the CRM simulated quasi-equilibrium state can be attributed to how the atmospheric horizontal wind was treated throughout the integration. The model that had stronger surface wind produced a warmer and more humid thermodynamic equilibrium state. The physical processes responsible for determining the modeled equilibrium states can be identified by examining the differences in the modeled water vapor, temperature, and moist static energy budget between warm/humid and cold/dry states. One of the major physical processes responsible for the warmer and more humid equilibrium state is larger latent heat fluxes from the ocean (due to stronger surface wind). The moist static energy budget further indicates that the large-scale forcing in water vapor is another major physical process responsible for producing the warmer and more humid thermodynamic equilibrium state. The model results also indicated that the advective forcing in temperature (cooling) and water vapor (moistening) by the imposed large-scale vertical velocity was larger (smaller) for the warm and humid (cold and dry) equilibrium state. This is because the domain mean thermodynamic state is more unstable and has a stronger vertical gradient of water vapor for those experiments that produced a warmer and more humid climate. Specified minimum wind speed in the bulk aerodynamic formulas and initial soundings on the modeled thermodynamic equilibrium state are also discussed.