MMM Research Highlight: March, 2010
Toward comprehensive parameterizations of cloud microphysics for weather and climate models
Representation of cloud microphysical processes in models of various complexity (from small-scale to global) remains one of the most challenging aspects of numerical weather prediction and climate modeling. This is mostly because of the disparity between scales at which cloud microphysical processes operate (millimeters and centimeters) and scales resolved by models and observations. With the advent of convection-permitting numerical weather prediction using the Weather Research and Forecasting (WRF) model and application of the superparameterization approach to climate modeling (Grabowski and Smolarkiewicz 1999; Randall et al. 2003) representation of cloud microphysics emerges as the next "key problem", similarly to the "convection parameterization problem" in the past. The superparameterization approach to climate modeling is the focus of the NSF's Science and Technology Center for Multiscale Modeling of Atmospheric Processes (CMMAP) at Colorado State University. Several NCAR scientists are part of the CMMAP team and actively involved in the CMMAP research.
MMM continued development, improvement, and application of various cloud microphysics schemes, both bulk and bin. Further enhancements were included into the novel comprehensive two-moment bulk microphysics scheme to represent warm-rain and ice processes (Morrison and Grabowski 2007, 2008a, 2008b). A new multicomponent bin microphysics scheme was also developed to serve as a benchmark for developing and testing the bulk scheme (Morrison and Grabowski 2010). The bulk and bin schemes were compared in a kinematic flow model mimicking precipitating shallow cumulus. The bulk scheme has been implemented in both WRF and the Eulerian-Lagrangian cloud model (EULAG). Work is in progress to test the scheme against observations using these models for a range of case studies, including the Tropical Warm Pool - International Cloud Experiment (TWP-ICE); the latter effort is part of Grabowski and Morrison's contribution to the DOE ARM (Atmospheric Radiation Measurement) Program. A key aspect of these studies is to document sensitivity to various microphysics parameters and settings, and compare this with other model sensitivities (e.g., model dimensionality, resolution, etc.). Preliminary work suggests the importance of interactions between the ice phase and the supercooled liquid water (i.e., the growth by riming) on simulated deep convection and associated anvil clouds. Together with French colleagues a novel approach was developed to model warm-rain processes that merges techniques used in bin (spectral representation) and bulk (saturation adjustment) schemes. The new hybrid bulk-bin scheme was included in EULAG (Grabowski et al. 2009).
Work has also continued on testing and further development of the two-moment Morrison microphysics scheme currently available in WRF (Morrison et al. 2009). The two-moment scheme was compared against a one-moment version of the same scheme to assess the impact of predicting particle number concentration. It was found that prediction of rain number concentration was important in simulating the trailing stratiform region of an idealized mid-latitude squall line (see Figure). This effort is being extended to examine the role of various rain microphysics parameters and processes (breakup, evaporation, size distribution shape) on the characteristics of organized deep convection. This effort will also compare the Morrison scheme against other bulk microphysics schemes (e.g., Thompson, Milbrandt-Yau) for idealized and detailed observationally-based case studies.
References:
Grabowski, W. W., and P. K. Smolarkiewicz, 1999: CRCP: A Cloud Resolving Convection Parameterization for Modeling the Tropical Convecting Atmosphere. Physica D, 133, 171-178.
Grabowski, W. W., O. Thouron, J.-P. Pinty, and J.-L. Brenguier, 2009: A hybrid bulk-bin approach to model warm-rain processes. J. Atmos. Sci. (accepted).
Morrison, H., and W. W. Grabowski, 2007: Comparison of bulk and bin warm rain microphysics models using a kinematic framework. J. Atmos. Sci., 64, 2839-2861.
Morrison, H., and W. W. Grabowski, 2008a: Modeling supersaturation and subgrid-scale mixing with two-moment bulk warm microphysics. J. Atmos. Sci., 65, 792-812.
Morrison, H., and W. W. Grabowski, 2008: A novel approach for representing ice microphysics in models: description and tests using a kinematic framework. J. Atmos. Sci., 65, 1528-1548.
Morrison, H., G. Thompson, and V. Tatarskii, 2009: Impact of cloud microphysics on the development of trailing stratiform precipitation in a simulated squall line: Comparison of one- and two-moment schemes. Mon. Wea. Rev., 137, 991-1007.
Morrison, H., and W. W. Grabowski, 2010: A new multicomponent detailed bin microphysics scheme: Toward an improved representation of rimed snow and graupel, J. Atmos. Sci. (in press)
Randall, D., M. Khairoutdinov, A. Arakawa, and W. Grabowski, 2003: Breaking the cloud-parameterization deadlock. Bull. Amer. Meteor. Soc., 84, 1547-1564.
X-height plot of simulated radar reflectivity at t = 6 hr for an idealized 2D squall line using the one-moment microphysics scheme (top) or two-moment scheme (bottom) (Morrison et al. 2009). Note the significantly larger extension of the trailing stratiform precipitation in the two-moment scheme.