Microphysics Changes to Address Precipitation Biases in the 4-km Operational NAM Model

The original Ferrier microphysics was replaced in August 2014 with the Ferrier-Aligo (F-A) microphysics in order to improve the vertical structure of deep convective storms in the National Centers for Environmental Prediction (NCEP) North American Mesoscale Forecast System (NAM). To reduce a high bias in heavy precipitation in the NCEP NAM CONUS nest, various modifications to the (F-A) microphysics were tested. One modification tested an improved closure for the rime factor (RF) by allowing the density of rime collected on precipitating ice particles to vary with temperature and cloud droplet size based on laboratory results. Preliminary results indicated this change reduced the RF leading to smaller ice densities and smaller fall speeds, resulting in lower rain rates in the convective regions of mesoscale convective systems (MCSs). Other experiments tested changing the maximum number concentrations assumed for large, heavily rimed (RF>10) ice to determine the impact on QPF and on forecast radar reflectivity. Another set of experiments looked at the forecast impact on upper-level ice clouds of changing the assumed closure for determining the number of small ice crystals from larger precipitation ice particles. While the changes specifically targeted cloud ice concentrations near the tops of deep convection, forecast impacts were seen in the area coverage of upper-level ice clouds, and in the distribution of stratiform and convective rainfall.