A Prognostic Parameterization of Cloud Microphysics and Fractional Cloudiness for NWP and Climate Models

Parameterizing the horizontal extent of cloudiness in regional to global scale models is an issue that has continued to dampen our ability to quantify cloud feedbacks in simulations of present and future climates. We have developed a prognostic parameterization of fractional cloudiness coupled with a bulk cloud microphysics scheme for water and ice clouds with the aim to bridging horizontal scales between cloud resolving-, regional-, and global-scale models. A consistent parameterization between the cloud fraction and cloud microphysics processes (condensation, evaporation) is the first step needed to physically based parameterizations of cloud water droplets and ice crystals nucleation in large-scale models.

First, we discuss the results of high-resolution idealized simulations run with the eulerian-mass Weather Research Forecast Model (WRF). We compare the life cycle of our cloud system against that obtained with cloud schemes of higher complexity with the aim to test our cloud microphysics parameterization when clouds fill the entire model grid-box.

Next, we discuss the impact of including fractional cloudiness using a series of numerical experiments run over the Continental United States. The cloud resolving model simulation uses a 5-km horizontal resolution, a scale at which cloud resolving models assume that clouds entirely fill the model grid box. The second simulation uses a 10-km resolution, an upper limit for convection to be numerically resolved. Finally, the last simulation uses a 30-km resolution, a typical horizontal scale for regional and high-resolution global climate models.