Abstract
In order to clarify the influence of orography and land-sea distribution on the large scale atmospheric circulation, numerical experiments are performed by using a two-level geostrophic spectral model under winter conditions. The numerical time integrations are made for the model with mountains and oceans, the model with mountains and no ocean, the model with oceans and no mountain and the model without mountains and oceans. The results of the numerical simulations of the atmospheric general circulation performed by these four models are analyzed and compared to reveal the relative im. portance of the dynamical effect of mountains and the thermal effect of land-sea distribution in determining the large scale features of the atmospheric circulation.
The results thus obtained are summarized as follows:
1) The atmospheric and surface temperatures are h igher by 10° to 15°C in all latitudes in models with the ocean than without, because of the heat release from the ocean to the atmosphere in the winter season.
2) The latitudinal gradients of t h e atmospheric and surface temperatures are smaller in models with the ocean than without the ocean. This is caused by the fact that the amount of heat released from the ocean to the atmosphere is relatively large in middle latitudes.
3) I n models with mountains, the mountain torque dissipates the angular momentum on the hemispheric average. This lack of angular momentum is compensated for by the generation of it due to the surface torque in order to maintain the angular momentum balance. In models without mountains, the generation of angular momentum due to the surface torque vanishes on the hemispheric average.
4) Standing eddies grow in models with mountains. On the other hand, in models with the ocean and no mountain, the standing eddies are very weak.
The computed spectral distribution of eddy kinetic energy i n dicates a maximum at the wave number 2 for standing eddies and at the wave number 5 to 6 for transient eddies. The spectrum of total eddy kinetic energy as seen in the actual atmosphere (existence of the maximum at the wave number 2) is reproduced qualitatively by models including mountains.
5) As for the computed eddy conversion of available potential energy to kinetic energy, the maximum conversion is found at the wave number 2 for standing eddies and at the wave number 5 to 6 for transient eddies. The spectrum for models with mountains is in qualitative agreement with the observed (the primary maximum being at the wave numbers 5 to 6 and the secondary maximum at the wave number 2).
6) Through non-linear interaction, kinetic energy is transferred from eddies to zonal currents in each model. The spectrum of this energy transfer is in qualitative agreement with the observed (the maximum being at the wave number 2) in models with mountains. In this case, standing eddies play an important role.
In models with mountains, the energy exchange between e ddies and zonal currents is performed through mountain effect as well as through non-linear interaction. The flow of energy is from zonal currents to eddies in the energy exchange through mountain effect.
7) In models with mountains, the available potential energy and kinetic energy of standing eddies are transferred to those of transient eddies, respectively, as seen in the actual atmosphere.
8) The large scale features of the atmospheric circulation in the winter are determined primarily by orographic effect. Especially, it should be noted that the Siberian high and the American continental high over the continents and the Aleutian low and the Icelandic low over the oceans are simulated in qualitative agreement with the observed by the model with mountains and no ocean.
