The WRF ARW version 3.1.1 was coupled with the Spectra-Bin Microphysics (SBM) part of the HUCM [Khain et al., 2011], and called the WRF-SBM. Cloud hydrometeors are categorized into one-water and six-ice classes, i.e., water droplets, ice crystals (plate, column, dendrite), snow aggregates, graupel, and hail. The relationships between the particle bulk densities and radii of mass equivalent sphere with the bulk density assumed in the SBM are shown in Fig. 1. Snow aggregates, graupel, and hail are assumed to be fluffy spheres when calculating their microphysical processes [Khain and Sednev, 1995]. The discrete PSDs of the hydrometeor classes are represented on a grid containing 43 doubling mass bins covering particles mass sizes in a range of 3.35 × 10-11 g < mass < 1.47 × 102 g (2 micron < radius < 32.8 mm in terms of the radii of droplets or melted-ice). The supplementary mass size distributions representing a liquid part on melting ice particles and a rimed part on snow aggregates are also added into the sets of 43 bins. The size distribution of condensation nuclei (CN) is discretized into a mass grid containing 13 bins with a radius range from 10-3 micron to 1 micron. The use of a smaller number of CN bins than that of the original HUCM SBM is to improve the efficiency of computation.



This version of SBM is the full package that is different from the Fast-SBM described in Lynn et al., [2005a; 2005b] and Khain et al., [2009; 2010], our WRF-SBM [Iguchi et al. 2012a, 2012b] deal with the complete treatment of microphysics described in Khain et al., [2011, and previous papaers] in the 3D regional model. In this framework, the SBM calculates the nucleation of droplets and ice crystals, both condensation and deposition growth, evaporation, sublimation, droplet freezing, riming, melting, shedding, coalescence growth, and breakup of the categorized hydrometeor particles. The nucleation of droplets is calculated on the basis of the Köhler equation using grid-scale supersaturation with respect to water. All CN larger than a critical radius determined by the supersaturation value are converted to cloud droplets of the radius. Additional detailed discussion is found in Iguchi et al. [2012a]. This special version of the WRF is called the WRF-SBM, designed for special support for the GPM mission and detailed study of microphysics with in-situ observations.
Typical WRF-SBM experiments utilize nested grid simulations to downscale synoptic-scale dynamics into storm-resolving dynamics (1-km horizontal grid spacing and 60 vertical layers). For each 24hr period, total output size of the WRF-SBM is up to 2~4Tb due to 43-bin PSD parameters. For a practical WRF-SBM simulation requires about several hundreds to a few thousands of CPUs. Because of this computational demand we are currently running the WRF-SBM only in the NAS Pleiades super computer.
This special microphysics model is designed to study far most frontier of the microphysics or remote sensing applications. Simulated 3D PSDs can be compared with detailed in-situ aircraft or ground measurements. Also, they are quite useful tool for studying remote sensing algorithm verifications.
So far, we have completed 8 sets of 24-hr simulations for precipitation events over various GPM Ground Validation (GV) sites. Three main GV sites (C3VP, LPVEx, and MC3E) focus on mid- to high-latitude rain and snowfall systems. We have identified two unique “golden-day” cases for each GV case. For these cases, we have compiled and created collections of value-added in-situ observations. These are microphysics measurements of ground-based polarimetric radar reflectivity, ground (from 2D Video Distrometer, Parsivel Disdrometer, bucket gauge) as well as aircraft (from 2DP, 2DC, RICE, CVI) sampling of snow images, ice water content, snow effective density, dimension, and DSDs for model evaluation. In addition, the HMT database are simulated.




 Iguchi T., T. Matsui, J. J. Shi, W.-K. Tao, A. P. Khain, A. Hou, R. Cifelli, A. Heymsfield, and A. Tokay (2012a), Numerical analysis using WRF-SBM for the cloud microphysical structures in the C3VP field campaign: Impacts of supercooled droplets and resultant riming on snow microphysics, Journal of Geophyiscal Research, 117, D23206, doi:10.1029/2012JD018101. 


Iguchi, T., T. Matsui, A. Tokay, P. Kollias, and W.-K. Tao (2012b), Two distinct modes in one-day rainfall event during MC3E field campaign: Analyses of disdrometer observations and WRF-SBM simulation. Geophysical Research Letters, 39, L24805, doi:10.1029/2012GL053329.




Takamichi Iguchi (takamichi.iguchi@nasa.gov)