Sensitivity to Rimed Ice Species


Despite a number of studies dedicated to the sensitivity of deep convection simulations to the properties of rimed (covered with ice) ice species in microphysics schemes, no consensus has been achieved on the nature of the impact. Considering the need for improved quantitative precipitation representation and forecasts, it is crucial that the cloud modeling community better understands the reasons for the differing conclusions among previous studies and knows the relevance of these sensitivities for the numerical weather prediction. Research conducted at Brookhaven National Laboratory examined the role of environmental conditions and storm type on the sensitivity of precipitation simulations to the nature of the rimed ice species (soft [graupel] or hard hail). Idealized three-dimensional simulations of supercells and squall lines were performed in varying thermodynamic environments. These studies showed that, for simulation periods of sufficient length (> 2h), graupel-containing and hail-containing storms produce domain-averaged surface precipitation more similar than many earlier studies suggest. While graupel is lofted to higher altitudes and has a longer residence time aloft than hail, the simulations suggest that most of this graupel eventually reaches the surface and the surface precipitation rates of hail- and graupel-containing storms converge. However, environmental conditions play an important role in the magnitude of this sensitivity. Storms in large-CAPE environments (typical of storms in the midwestern United States) are more sensitive than their low-CAPE counterparts (typical of storms in Europe) to the nature of rimed ice species in terms of domain-average surface precipitation. Supercells are more sensitive than squall lines to the nature of the rimed ice species in terms of spatial precipitation distribution and peak precipitation (disregarding of the amount of CAPE).


K. Van Weverberg. 2013. “Impact of Environmental Instability on Convective Precipitation Uncertainty Associated with the Nature of the Rimed Ice Species in a Bulk Microphysics Scheme,” Monthly Weather Review, DOI: http://dx.doi.org/10.1175/MWR-D-13-00036.1.