Microphysics complexity in squall line simulations. As high-resolution climate modelsincreasingly turn towards an explicit representation of convection, the researchspotlight quickly shifts to the parameterization of cloud microphysics. Over the pastdecade or so, these parameterizations have rapidly gained complexity. With thisincrease in complexity also comes an increased computational burden so it isimportant to understand what level of complexity is appropriate for thecomputationally expensive high-resolution climate simulations. DOE scientists atBrookhaven National Laboratory and collaborators, through sensitivity studies of anidealized squall line with the Weather Research and Forecasting model (WRF), showedthat there is a benefit of using two prognostic variables (2-moment schemes) todescribe the size distribution evolution of all hydrometeors. Adding hydrometeor types(by adding hail to a scheme containing rain, snow and graupel) seemed less beneficial,since a large sensitivity was introduced to an arbitrary threshold to convert graupel intohail. Most importantly, it was shown that two equally complex microphysics schemes(Milbrandt and Yau 2005 [MY] and Morrison et al. 2009 [MTT]), still behave verydifferently in terms of surface precipitation and moist processes aloft. Thesedifferences could be entirely related to different treatment of collisional raindropbreakup and depositional growth. Over the past years, the focus in microphysicsmodeling often has been on the role of size distribution assumptions in state-of-the-artschemes, but this study has identified that an equally large variability is associated withprocesses such the ice initiation, growth processes and the breakup of raindrops.
Reference: K. Van Weverberg, A. Vogelmann, H. Morrison, and J. Milbrandt, 2012:“Sensitivity of idealized squall line simulations to the level of complexity used in two-moment bulk microphysics schemes,” Mon. Wea. Rev., 140, 1883-1907.
Motivation
●It is important to understand what level ofcomplexity in the parameterization ofcloud microphysics is appropriate forcomputationally expensive high-resolutionclimate simulations.
Approach
●Idealized simulations of a 2D squall linewere performed with the WRF model,systematically using different levels ofmicrophysics complexity.
Result
●Adding prognostic moments to a schemeprovides a clear benefit, while addinghydrometeor types increases thesensitivity to little understood processes.The difference between two equallycomplex parameterizations could be tiedto specific processes such as ice initiation,depositional growth rates and collisionaldrop breakup.
Sensitivity of idealized squall line simulations to the level ofcomplexity used in two-moment bulk microphysics schemes
K. Van Weverberg, A, Vogelmann, H, Morrison, and J, Milbrandt, 2012: “Sensitivity of idealized squall line simulations to the level of complexityused in two-moment bulk microphysics schemes,” Mon. Wea. Rev., 140, 1883-1907
Sensitivity of surface precipitation to drop breakupparameterization. The spatial distribution over a 2Ddomain of accumulated surface precipitation for a full timeintegration of 5 hours are given for: the Morrison et al.(2009) scheme (MTT-BASE), the Milbrandt and Yau (2005)scheme (MY-BASE), and a hybrid scheme (MTT-MYBREAKUP) that is identical to MTT-BASE except that theMY raindrop breakup parameterization was used.
