4B.4 The New Ferrier-Aligo Microphysics in the NCEP 3-km NAM Nest

Tuesday, 24 January 2017: 9:15 AM
Conference Center: Tahoma 4 (Washington State Convention Center )
Eric Aligo, EMC/NCEP/NWS/NOAA and I.M. Systems Group, Inc., College Park, MD; and B. S. Ferrier, G. Thompson, J. R. Carley, E. Rogers, and G. DiMego

Handout (1.2 MB)

     A new version of the Ferrier-Aligo (F-A) microphysics, which is part of the version 4 upgrade to the North American Mesoscale system (NAMv4; see Rogers et al., 2017; Carley et al., 2017), will be described that addressed concerns expressed from various centers and from the field.  The following improvements were made based on comparisons with experimental 3-km Nonhydrostatic Multiscale Model on the B-grid (NMMB) runs of deep convective storms using the Thompson microphysics. (i) A new drizzle parameterization was added, which decreased drop sizes and increased number concentrations in clouds where only warm-rain processes were active, such as in thin clouds that often developed near the top of the boundary layer. This change, along with modifying the cloud-to-rain autoconversion process, reduced the occurrence of light (< 20 dBZ) composite reflectivity that was not often observed. (ii) The lack of stratiform reflectivity and associated rainfall was improved by assuming fixed drop sizes below melting layers, which increased the area of  < 40 dBZ reflectivity outside areas of active convection. (iii) A large high bias in heavy QPF in the operational 4-km CONUS nest was dramatically reduced in the 3-km NAMv4 CONUS nest, which involved many changes in the modeling system beyond the physics, but in terms of the microphysics the biases were reduced as a result of fixing a bug in the code that controlled the first-guess mean diameter for snow/graupel, adding a transition zone that adjusted the graupel number concentrations more gradually between convective and stratiform areas, and reducing the fall speeds of light-to-moderately rimed ice. (iv) The assumed number concentrations and mean diameters of snow colder than 0C in stratiform regions were adjusted in a way that led to reduced anvil reflectivities that better matched observations. (v) In very specific conditions associated with hail, the ice fall speeds were increased using the velocity-diameter relationship for graupel from the Thompson scheme.

     Examples from 3-km NMMB simulations will show the impact of these changes for several warm season cases.  

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