Abstract
As part of an ongoing study of the regional climate and hydrology of the southwestern United States, in this paper we investigate the systematic biases of two versions of the PSU/NCAR mesoscale model (MM4). These are a standard version and one that includes a more detailed treatment of radiative transfer, surface physics, and soil hydrology. We simulated the period 1–30 January 1979, in which nine Pacific storms moved across the western United States. Results from both model versions are compared to the large scale analysis used to provide initial and lateral boundary conditions. Both models show a lower tropospheric cold bias of 1–3 K near the surface over land and an upper tropospheric warm bias of less than 1 K, which suggest high model stability and reduced vertical mixing. The model atmospheres are wetter than that of the analysis, particularly in the lower troposphere and over the ocean. The wind magnitude bias is positive near the surface (∼1.5–3 m s−1), negative in the upper troposphere (∼−1.5 m s−1) and positive above the jet-level (∼3 m s−1). The wind direction bias is small throughout the model atmospheres except at the top model layer near 10 mb. These results indicate that the model evaporation and nighttime land surface sensible heat fluxes are larger compared to the analysis, while the daytime sensible heat fluxes and surface wind drag are smaller. The biases are generally smaller in the midtroposphere than in the lower troposphere and in the stratosphere. In general, both models capture most regional features of the orographic forcing of precipitation by the western United States topography quite well. Compared to station data, precipitation amounts tend to be overpredicted. Daily precipitation threat scores for various precipitation thresholds vary between 0.315 and 0.385. The threat scores for the 30-day precipitation, more indicative of the model's ability to simulate climatological precipitation averages, are higher, ⩾0.8 for light precipitation to ∼0.5 for moderate to heavy precipitation. Snow depths predicted by the augmented model also show realistic regional features. In general, the inclusion of the new physics package did not strongly affect precipitation prediction or the temperature, moisture, and wind midtropospheric biases. In the boundary layer over land, however, the augmented model was significantly colder and drier than the standard model due to larger nighttime surface sensible heat fluxes and lower evaporation rates. The regional hydrologic budgets simulated by the soil hydrology package of the augmented MM4 appear realistic in many respects, although verification is difficult at the present model resolution.
