Comparison of Simulated Polarimetric Signatures in Idealized Supercell Storms Using Two-Moment Bulk Microphysics Schemes in WRF

Abstract

Microphysics parameterization becomes increasingly important as the model grid spacing increases toward convection-resolving scales. The performance of several partially or fully two-moment (2M) schemes within the Weather Research and Forecasting (WRF) Model, version 3.5.1, chosen because of their well-documented advantages over one-moment (1M) schemes, is evaluated with respect to their ability in producing the well-known polarimetric radar signatures found within supercell storms. Such signatures include the ZDR and KDP columns, the ZDR arc, the midlevel ZDR and ρHV rings, the hail signature in the forward-flank downdraft, and the KDP foot. Polarimetric variables are computed from WRF Model output using a polarimetric radar simulator. It is found that microphysics schemes with a 1M rimed-ice category are unable to simulate the ZDR arc, despite containing a 2M rain category. It is also found that a hail-like rimed-ice category (in addition to graupel) may be necessary to reproduce the observed hail signature. For the microphysics schemes that only contain a graupel-like rimed-ice category, only very wet graupel particles are able to reach the lowest model level, which did not adequately reduce ZDR in this signature. The most realistic signatures overall are found with microphysics schemes that are fully 2M with a separate hail category.

Systematic Bias in the Prediction of Warm‐Rain Hydrometeors in the WDM6 Microphysics Scheme and Modifications

“…It is located at 45-50 μm, which is close to the upper limit of Dmc (50 μm) set in the WDM6 scheme, indicating that the spectra of cloud droplets are wide. Dmr is mainly concentrated between 0.1 and 0.2 mm regardless of the position relative to the melting layer, which is similar to the results of Johnson et al (2016) showing a distinct bias toward small raindrops when compared to other schemes. The frequency of larger Dmr values increases progressively as raindrops fall to the ground, indicating the capability for raindrop size-sorting forecasts in the WDM6 scheme.…”

“…Adams‐Selin et al () found that in bow echo simulation, the WDM6 scheme produced higher mixing ratios and number concentrations of raindrops than other schemes. Johnson et al () found that the WDM6 scheme produced higher frequencies of small raindrops than the other four double‐moment schemes in idealized supercell storms. Morrison et al () found that the WDM6 scheme produced higher rain mixing ratios compared with the other eight schemes and underestimated the mean volume diameter of raindrops and radar reflectivity for a squall line case.…”

A Balloonborne Particle Size, Imaging, and Velocity Probe for in Situ Microphysical Measurements

“…Doing so however requires the use of microphysical parameterization schemes to represent modeled cloud and precipitation particles. Bin microphysical schemes are physically very detailed, but are computationally expensive (Johnson et al 2016). The more commonly used alternative bulk microphysics schemes simplify the microphysics via single, double, or triple moment prediction equation approximations of microphysical processes that act to form the various hydrometeor size distributions.…”

“…While there has been recent focus on the improvements resulting from using higher-moment schemes over the simpler one-moment schemes, questions remain regarding which schemes are more representative (Morrison et al 2009;Morrison and Milbrandt 2011;Weverberg et al 2012;Johnson et al 2016). Specifically, many of these studies have found that how individual schemes represent graupel and hail can have a major impact on simulated airflow through impacts on total buoyancy from diabatic heating and cooling and precipitation loading.…”

In Situ Microphysical Observations of the 29–30 May 2012 Kingfisher, OK, Supercell With a Balloon‐Borne Video Disdrometer

“…Though much of the recent focus of in situ microphysics has been on the improvements resulting from using higher‐moment schemes over the simpler one‐moment schemes, questions remain regarding which schemes are more representative (Johnson et al, ; Morrison & Milbrandt, ; Morrison et al, ; Weverberg et al, ). The manner in which these schemes handle graupel and hail has a significant impact on the ability of a model to maintain an accurate storm mode and can even effect the amount, type, and location of surface precipitation.…”