Comparison of Two-Moment Bulk Microphysics Schemes in Idealized Supercell Thunderstorm Simulations

Abstract

Idealized three-dimensional supercell simulations were performed using the two-moment bulk microphysics schemes of Morrison and Milbrandt–Yau in the Weather Research and Forecasting (WRF) model. Despite general similarities in these schemes, the simulations were found to produce distinct differences in storm structure, precipitation, and cold pool strength. In particular, the Morrison scheme produced much higher surface precipitation rates and a stronger cold pool, especially in the early stages of storm development. A series of sensitivity experiments was conducted to identify the primary differences between the two schemes that resulted in the large discrepancies in the simulations. Different approaches in treating graupel and hail were found to be responsible for many of the key differences between the baseline simulations. The inclusion of hail in the baseline simulation using the Milbrant–Yau scheme with two rimed-ice categories (graupel and hail) had little impact, and therefore resulted in a much different storm than the baseline run with the single-category (hail) Morrison scheme. With graupel as the choice of the single rimed-ice category, the simulated storms had considerably more frozen condensate in the anvil region, a weaker cold pool, and reduced surface precipitation compared to the runs with only hail, whose higher terminal fall velocity inhibited lofting. The cold pool strength was also found to be sensitive to the parameterization of raindrop breakup, particularly for the Morrison scheme, because of the effects on the drop size distributions and the corresponding evaporative cooling rates. The use of a more aggressive implicit treatment of drop breakup in the baseline Morrison scheme, by limiting the mean–mass raindrop diameter to a maximum of 0.9 mm, opposed the tendency of this scheme to otherwise produce large mean drop sizes and a weaker cold pool compared to the hail-only run using the Milbrandt–Yau scheme.

Uncertainty from the choice of microphysics scheme in convection-permitting models significantly exceeds aerosol effects

“…1 indicates the wide range of differences that may be expected when comparing bulk against bin schemes. A significant body of work has shown that two-moment bulk microphysics schemes generally represent cloud and precipitation characteristics more realistically than single-moment schemes (most recently Morrison et al, 2009;Wu and Petty, 2010;Weverberg et al, 2013Weverberg et al, , 2014Igel et al, 2015), and thus our study is restricted to the comparison of two five-class, double-moment schemes commonly used in WRF and shown by Cintineo et al (2014) to perform well against satellite observations of cloud in North America: that described by Morrison et al ( , 2009) and Morrison and Milbrandt (2011) (hereafter Morrison, or abbreviated to MORR), and that described by Thompson et al (2004Thompson et al ( , 2008 (hereafter Thompson, or THOM). Both schemes are twomoment in rain and ice (prognostic mass and number), while the Morrison scheme is also two-moment in snow and graupel.…”

“…McFarquhar et al, 2006), the description of dense precipitating ice as hail or graupel categories (e.g. Morrison and Milbrandt, 2011;Bryan and Morrison, 2012) and changes in thresholds or rates for converting between ice categories (e.g. Morrison and Grabowski, 2008).…”

The role of precipitation size distributions in km‐scale NWP simulations of intense precipitation: evaluation of cloud properties and surface precipitation

“…We modified the intercept and density assumptions for hail used in ExpH to those typical of soft graupel (Table 2), but kept the terminal fall velocity formulation following Wisner et al (1972). Note that the choice between graupel and hail as the rimed ice species is an issue in one-moment (Gilmore et al, 2004) and multi-moment schemes (Morrison and Milbrandt, 2011). Schemes that include both species (Milbrandt and Yau, 2005) still have considerable uncertainty on how to implement the graupel-to-hail conversion, and simulations strongly depend on which species prevails (Van Weverberg et al, 2012).…”

“…Further, modifying the snow size distribution assumptions (ExpGS) again hardly impacts the domain-average surface precipitation ( Figure 8 and Table 5), but the domain-maximum precipitation overestimation is further reduced to 27%. In contrast, numerous idealized case-studies of deep convection suggest that replacing large hail by small graupel tends to reduce surface precipitation significantly (Gilmore et al, 2004;van den Heever and Cotton, 2004;Morrison and Milbrandt, 2011). Those idealized studies typically do not include boundary layer and radiation processes and, for supercell simulations, also do not capture the full life cycle of the convection.…”

“…For frontal stratiform precipitation events, surface precipitation was sensitive to the representation of the snow size distribution and fall velocity (Thompson et al, 2004;Colle et al, 2005;Serrafin and Ferretti, 2007;Woods et al, 2007). For convective precipitation events, the size and nature of the largest rimed ice species (either hail or graupel) have been shown to have a significant impact on surface precipitation (Gilmore et al, 2004;van den Heever and Cotton, 2004;Cohen and McCaul, 2006;Morrison and Milbrandt, 2011). In operational NWP, a single model set-up is required to satisfactorily simulate stratiform and convective precipitation events.…”