Dynamical Effects of Mountains on the General Circulation of the Atmosphere: III. Effects on the General Circulation of the Baroclinic Atmosphere

Abstract

In order to investigate dynamical effects of mountains on the general circulation of the baroclinic atmosphere, the model developed in Part I is integrated for about 150 days. A simplified diabatic heating function and the Newtonian cooling evaluated from observational data are adopted for the thermal process in the model. The 6.5*K/km convective adjustment is used to parametarize the vertical convective mixing of heat. The nonlinear viscosity and the vertical momentum diffusion are also included. Time integrations are performed with and without an idealized steep mountain like the Tibetan Plateau. The model without the mountain simulates' successfully the gross features of the general circulation similar to the real atmosphere. That is, the westerly wind in the middle latitudes, the easterly wind in the tropics and the meridional three cell circulation are well simulated. Baroclinic unstable waves are well developed, though a little less active than the real ones. However, there are some defficiency in the model atmosphere. For example, the subtropical jet is too strong and its position is too south compared with the real atmosphere due to the large latitudinal gradient of the diabatic heating near 20*latitude. The active region of baroclinic waves is limited to relatively higher latitudes. Incorporating the mountain does not change these features largely. The most remarkable change is the northward shift of the subtropical high. It is located at 30* latitude in the model without the mountain while it is at 45* latitude in the model with the mountain. The region of the downward motion in the subtropics also expands northward. This is due to the geostrophic adjustment of the pressure field to the westerly wind weakened by the blocking effect of the mountain. This phenomenon may be one of the causes of the northward shift of the dry area in the Eurasian continent. We may also speculate that this process contributes to the formation of the Siberian high to some extent. The mountain does not excite stationary planetary waves so much as expected from the observation and other studies, because of the weak westerly wind over the mountain. In addition, a zonally symmetric circulation is obtained. Though there is a large difference between it and the real atmosphere, it is considerably similar to the real atmosphere compared with the results of Hunt (1973) who also investigated a zonally symmetric circulation. It is supposed that one of the causes of the difference between the two results is the difference of the scheme used for the nonlinear horizontal viscosity in the two models.

Hadley Circulations and Penetrative Cumulus Convection

“…Those for *=*(s) are shown in Fig. 15c and d. No distinct meridional circulation is found in the mid-and high latitudes in both runs, as in Nakamura (1978). Hadley circulation is confined within low latitudes, though their poleward limit is not so clear.…”