Simulation Experiments with a 12-Layer Stratospheric Global Circulation Model. I. Dynamical Effect of the Earth's Orography and Thermal Influence of Continentality

Abstract

To evaluate the role of the earth's orography in the stratospheric general circulation, we conducted numerical experiments simulating January climate with and without mountains in a 12-layer NCAR global circulation model, with the top of the atmosphere extending to 36 km. The model has a spherical horizontal mesh increment of 5° in longitude and latitude and a vertical grid increment of 3 km. The model includes various physics: solar and terrestrial radiation, surface and planetary boundary layers, large-scale precipitation and cloud calculations in the troposphere, surface temperature calculations over land and snow-ice surfaces, convective adjustment, and subgrid-scale transport of heat, water vapor and momentum. The continentality of the earth's surface is incorporated by specifying the ocean surface temperature and a realistic earth's orography for the mountain case and a flat surface over land for the no-mountain case. Mean January conditions of simulation are specified by the declination of the sun, the ocean surface temperature, and the latitude-height distribution of ozone. No photochemical processes are included in the experiments. The differences of two January simulation cases?with and without mountains?are analyzed in terms of the geographical distributions of atmospheric-state variables as well as the zonal mean climatological statistics. A notable difference 15 found in the enhanced vertical transport of planetary wave energy, particularly wavenumber 1, in the stratosphere in the mountain case. The semi-permanent anticyclone over the Aleutian region in the stratosphere during winter was simulated only in the mountain case. This suggests that the earth's orography is very important in influencing the stratospheric general circulation. Another problem examined in detail is the characteristics of vertically propagating wave modes in the tropical stratosphere. Two modes of wave motions?mixed Rossby-gravity waves with a period of about 5 days and atmospheric Kelvin waves with a period of about 15 days?are identified. The slope of these waves with height agrees well with those of the observed indicating that the vertical transport of wave energy is also upward in the model tropical stratosphere. The detailed analyses of momentum balance and energetics in the stratosphere will be described in Part II.