Modified Microphysics for Use in High-Resolution NAM Forecasts

The Ferrier-Aligo (FA) microphysics scheme is a modified version of the National Centers for Environmental Prediction (NCEP) Ferrier microphysics scheme, and was developed to improve forecasts of deep convection in the 4-km North American Mesoscale Forecast System (NAM) Contiguous United States (CONUS) nest and 1.33-km NAM fire weather nest. The new formulations in the FA microphysics were developed through extensive model sensitivity experiments of the 29-30 June 2012 derecho and eight additional cases using the Non-hydrostatic Multi-scale Model on the B-grid (NMMB) at a horizontal grid spacing of 4-km. Evaluation of the experimental output was done in close collaboration with the Storm Prediction Center (SPC). Changes were made in how the ice-phase hydrometeors were advected, which allowed improved coupling with the variations in the density of ice that resulted from different amounts of liquid water riming. The modified microphysics eliminated the need for a constant maximum number concentration of large ice, and instead it is assumed to vary between relatively low values in the convective region and relatively high values in the stratiform region, which increased the reflectivity in the convective region and improved the vertical structure of the derecho. Increasing the radar backscatter from wet, melting ice and supercooled liquid coexisting with ice also led to higher reflectivities in the convective region. NMMB model runs of the derecho using the Ferrier-Aligo microphysics produced a system that moved too slow, however, reflectivity in the convective region was higher than that using the original Ferrier microphysics, and more closely matched observations.

Improved storm structure was also seen in simulations of the May 20, 2013 Moore, OK tornado outbreak using the FA microphysics, which showed a more accurate depiction of the convective mode compared to the original Ferrier microphysics. The FA scheme produced more surface hail accumulation compared to the original Ferrier microphysics, and vertical cross sections through the axis of the most intense convection showed the higher-density ice particles were confined to the convective region with small amounts reaching the ground. Other examples of improved guidance from NMMB runs using the FA microphysics will be shown.