New Physical Constraints on Rainfall Formation
The simulation of rain processes in climate models is of crucial importance in determining cloud properties and thus the energy balance of climate models. Many current climate models treat these processes, such as the conditions for conversion of cloudwater to rainwater, as free tuning parameters to adjust cloud properties so that simulated top-of-atmosphere radiative fluxes broadly agree with observations. This empirical tuning has various disadvantages. In particular, there is a lack of explicit physical constraints in the tuning process and the need for retuning at different horizontal resolutions. In this publication, partially funded by DOE, the authors describe a novel method to constrain warm rain processes climate models based on observations. Rainfall formation is linked to cloud and precipitation conditions in a new way that is also independent of model resolution. The method might ultimately help to effectively eliminate these free tuning parameters in climate models. The new method was implemented into the University of Hawaii’s regional climate model iRAM. A series of test integrations were performed at horizontal resolutions ranging from 0.25°x0.25° to 2°x2°. The constrained approach was compared with a conventional approach commonly found in current climate models. Comparisons with an observational climatology of cloud liquid water amount reveal significant improvements, in particular a better consistency between different model resolutions. The study enables improved constraint for determining rain formation in climate models.
Bennartz, R., A. Lauer, and J. L. Brenguier. 2011. “Scale-Aware Integral Constraints on Autoconversion and Accretion in Regional and Global Climate Models,” Geophysical Research Letters 38, L10809. DOI:10.1029/2011GL047618.