Low-level cold biases in both NCEP Meso Eta and Nonhydrostatic Meso Models (e.g., NMM) have been frequently observed at many locations in the past winter. Although these cold biases usually persisted over an extended period of time, the worst cold biases occurred during the night time when atmosphere was relatively stable. While the two models have very similar physics package, the cold biases were more pronounced in the NMM than in the Eta Models.

Tests have begun which will investigate the sources of cold biases in both models. The model-predicted surface energy budget components, i.e. radiation, sensible heat, latent heat, and ground heat fluxes, as well as land surface fields that affect these fluxes are compared against observations at the locations where cold biases existed. In addition, a comparison between the Eta and NMM models is made to investigate why the cold biases are greater in the NMM. The results of these cold bias diagnoses and of the experiments that are carried out to correct the cold biases will be presented.