Mountain ranges and high plateaus influence atmospheric circulation patterns on all scales, ranging from ultralong planetary waves to small turbulent eddies. Some of these effects are brought about simply by orographic obstacles acting as barriers to the flow. Of equal importance, however, are the thermal effects of elevated land masses, which can generate considerable baroclinicity. Various time scales have to be considered in the thermal forcing of the atmosphere by large elevated land masses. Diurnal variations of the heating and cooling cycle have been shown to be prominent factors over Tibet. On time scales from days to weeks, the Northern Hemisphere plateaus seem to influence the monsoon circulations. There are strong indications that interseasonal “memory” exists in the heat balance of plateaus that might affect seasonally abnormal monsoon behavior. Such “memory” could be caused by feedback between thermal effects of land masses and “near-resonant” planetary waves.
In order to assess the thermal impact of mountains and plateaus, we need considerably more detailed knowledge of the energy transfer processes between the valley atmosphere, the yet poorly delineated planetary boundary layer over mountains, and the “free atmosphere.” To achieve such knowledge, experimental and theoretical studies involving micro-, meso-, and macroscales will have to intermesh more closely than in the past.
1 Invited paper presented at the Second AMS Conference on Mountain Meteorology, 9–12 November 1981, Steamboat Springs, Colo. Research reported here was supported partly by NSF Grant ATM 80 16867, DOE Contract DE-AS02-76EV01340, and NASA Grant NAG 5-136. Valuable help by Donna Tucker and Dengyi Gao in preparing some of the analyses is gratefully acknowledged.