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Microphysics complexity in squall line simulations.As high-resolution climate models increasingly turn towards an explicit representation of convection, the research spotlight quickly shifts to the parameterization of cloud microphysics. Over the past decade or so, these parameterizations have rapidly gained complexity. With this increase in complexity also comes an increased computational burden so it is important to understand what level of complexity is appropriate for the computationally expensive high-resolution climate simulations. DOE scientists at Brookhaven National Laboratory and collaborators, through sensitivity studies of an idealized squall line with the Weather Research and Forecasting model (WRF), showed that there is a benefit of using two prognostic variables (2-moment schemes) to describe 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 into hail. Most importantly, it was shown that two equally complex microphysics schemes (Milbrandt and Yau2005 [MY] and Morrison et al. 2009 [MTT]), still behave very differently in terms of surface precipitation and moist processes aloft. These differences could be entirely related to different treatment of collisional raindrop breakup and depositional growth. Over the past years, the focus in microphysics modeling often has been on the role of size distribution assumptions in state-of-the-art schemes, but this study has identified that an equally large variability is associated with processes 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.

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Sensitivity of idealized squall line simulations to the level of complexity used in two-moment bulk microphysics schemes

  • Motivation
  • It is important to understand what level of complexity in the parameterization of cloud microphysics is appropriate for computationally expensive high-resolution climate simulations.
  • Approach
  • Idealized simulations of a 2D squall line were performed with the WRF model, systematically using different levels of microphysics complexity.
  • Result
  • Adding prognostic moments to a scheme provides a clear benefit, while adding hydrometeor types increases the sensitivity to little understood processes. The difference between two equally complex parameterizations could be tied to specific processes such as ice initiation, depositional growth rates and collisional drop breakup.

Sensitivity of surface precipitation to drop breakup parameterization. The spatial distribution over a 2D domain of accumulated surface precipitation for a full time integration 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 the MY raindrop breakup parameterization was used.

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