General Circulation Experiments with a Six-Layer NCAR Model, Including Orography, Cloudiness and Surface Temperature Calculations

This paper describes the method of incorporating into the NCAR global circulation model the dynamic effect of mountains, the prediction of cloudiness for radiation calculations, and the calculation of ground surface temperature using a heat balance equation. Other aspects of the physics of the model and the finite-difference schemes are very similar to those discussed by the authors in 1967 and 1970. For the simulation of seasonal climate we specify two parameters: the sun's declination and the distribution of ocean surface temperatures. Since the prediction of cloudiness is parameterized in terms of the relative humidity and the vertical motion fields, solar and atmospheric radiation processes interact closely with the dynamics of the atmosphere through variations in the fields of cloudiness, temperature and water vapor. Coupling between radiation and dynamics helps to maintain stronger baroclinic activity in middle latitudes. Although a hydrologic cycle is included in the model atmosphere and the ground surface temperature is computed, a hydrologic cycle in the ground is not taken into account. Instead, it is assumed that the latent heat transport from the ground to the atmosphere and the soil heat transport below the surface are both functions of the sensible heat transport between the ground and the atmosphere. Experiments are conducted to simulate January climate with and without the earth's orography. In both experiments the domain of continents, the January mean ocean surface temperatures, and the sun's declination for mid-January are unchanged during the time integrations. The model has a spherical horizontal mesh spacing of 5° in both longitude and latitude and six vertical layers at 3-km height increments. The time step is 6 min and both cares are integrated up to 80 days starting from an isothermal atmosphere at rest. The results of the 41–70 days of the time integration are analyzed for various diagnostic studies. Synoptic comparisons of the two experiments are made for selective meteorological variables to discuss the relative importance of the thermal and orographic influences upon the large-scale motions of the atmosphere. Detailed studies are made on the balance of momentum, water vapor, heat and energy. The present experiments indicate that the six-layer and 5° mesh model can simulate successfully a January climate and that the earth's orography plays a minor role over the thermal effect of continentality in determining the major features in the transport mechanism of momentum, water vapor, heat and energy in terms of the zonal mean state. However, for the regional aspects of general circulation the effects of orography are significant.