Using the objectively analyzed FGGE II-b upper-air data, the large-scale circulation, heat sources and moisture sinks over the Tibetan Plateau and surrounding areas are examined for a 9-month period from December 1978 to August 1979. In addition to the FGGE data, special soundings obtained during the Chinese Qinghai-Xizang (Tibet) Plateau Meteorological Experiment (QXPMEX) from May to August 1979 are also used in the objective analyses. The evolution of the large-scale flow patterns, temperature, outgoing longwave radiation (OLR) and vertical circulation is described in order to identify the distinct seasonal changes from winter to summer that lead to the onset of the Asian summer monsoon. The Tibetan Plateau maintains a large-scale thermally driven vertical circulation which is originally separated from the planetary-scale monsoon system. The rising motion exists only on the western Plateau in winter and then spreads to the whole Plateau as the season progresses. The monsoon onset over Asia is an interaction process between the Plateau-induced circulation and the circulation associated with the principal rainbelt migrating northward. During winter the Plateau is a heat sink, but it is surrounded by regions of more intense cooling. In spring the Plateau becomes a heat source, but the cooling in the surrounding areas continues. The sensible heat flux from the surface provides the major source of heating on the Plateau. However, additional contribution from condensation heating is observed in the western Plateau during all seasons and, more significantly, in the eastern Plateau during summer. The sensible heating of the elevated Plateau surface and the radiative cooling in the environment maintain the horizontal temperature contrast that drives the thermally direct vertical circulation. The detailed examination of the warming process of the upper troposphere during two transition periods, i, e., the onset of the Southeast Asian monsoon in May and that of the Indian monsoon in June, reveals that the temperature increase over the eastern Plateau during the first onset was mainly the result of diabatic heating, whereas that over the Iran-Afghanistan-western Plateau region leading to the second onset was caused by intense subsidence. There are large diurnal variations in the boundary layer and vertical circulation over the Plateau. As a result of diurnal heating of the surface, a deep mixed layer of nearly uniform potential temperature exists over the Plateau in the evening (1200 UTC), suggesting the role of thermal convection in the upward transport of heat. However, moisture is not well mixed vertically and there is a large horizontal temperature gradient in the boundary layer. From late spring to summer the boundary layer becomes more stable for dry convection. On the other hand, the vertical distributions of equivalent potential temperature in late spring and afterwards show a conditionally unstable stratification for moist convection with the increase of moisture of surface air.

https://www.jstage.jst.go.jp/article/jmsj1965/70/1B/70_1B_319/_pdf

Seasonal Heating of the Tibetan Plateau and Its Effects on the Evolution of the Asian Summer Monsoon

Recent climate changes over the Tibetan Plateau and their impacts on energy and water cycle: A review

Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May, then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs. Fina...

Tibetan Plateau Forcing and the Timing of the Monsoon Onset over South Asia and the South China Sea

Precipitation over the central Andes in South America exhibits a marked annual march, with most of the rainfall concentrated during the austral summer season (December‐February), when the atmospheric circulation favors the uplifting of moist air from the lowlands to the east of the mountain range. Within its rainy season, the central Andes experiences week-long rainy and dry episodes. The large-scale and local conditions during these episodes are investigated using satellite imagery, reanalyzed atmospheric fields, and in situ data. Despite the deep layer of conditional instability prevalent during most summertime afternoons, deep convection can occur only on those days in which the mixing ratio within the local boundary layer exceeds some threshold (; 7gk g 21), yielding saturation of near-surface air parcels rising more than 600 m above ground. Convective cloudiness anomalies over the central Andes extend southeastward and tend to be concurrent with anomalies of opposite sign over the eastern part of the continent. Rainy (dry) episodes are also associated with anticyclonic (cyclonic) anomalies centered over subtropical South America that extend through the depth of the troposphere, accompanied by easterly (westerly) wind anomalies over the central Andes. These anomalies are presumably forced by planetary waves originating in the Southern Hemisphere extratropics. To gain insight into the regional processes linking the large-scale and local conditions, The Pennsylvania State University‐National Center for Atmospheric Research Mesoscale Model Version 5.2 was used to simulate contrasting rainy and dry episodes. The most marked and relevant differences are the strength and extent of diurnally varying flow over the eastern slope of the Andes. During the rainy simulation, strong easterly winds reach the upper part of the slope by midmorning, initiating an intrusion of warm and moist air (high ue air originating in the eastern lowlands) into the central Andes. In the dry case, the moisture transport from the east is restricted to the eastern slope of the Andes, and the central Andes is flooded by low ue air from the western foothills that cannot support deep convection even in the presence of localized updrafts. The momentum balance based on the model output indicates that turbulent momentum mixing from aloft (determined by the large-scale anomalies of the upper-level flow) into the convective boundary layer is the leading term causing the differences in the daytime upslope flow (and hence moisture transport) over the upper part of the eastern side of the Andes between rainy and dry simulations.

http://www.dgf.uchile.cl/rene/PUBS/Altiplano_multi_MWR.pdf

Multiscale Analysis of the Summertime Precipitation over the Central Andes

The Monsoon Himalayan Precipitation Experiment (MOHPREX) occurred during June 2001 along the south slopes of the Himalayas in central Nepal. Radiosondes were launched around the clock from two sites, one in the Marsyandi River basin on the eastern footslopes of the Annapurna range, and one farther to the southwest near the border with India. The flights supported rainfall and other hydrometeorological observations (including surface winds) from the Marsyandi network that has been operated in this region since the spring of 1999. The thermodynamic profiles obtained from the soundings support the observed nocturnal maximum in rainfall during the monsoon, with total column moisture and instability maximized just before rainfall peaks. Coinciding with the appearance of a monsoon depression over central India, the onset of the monsoon in this region was characterized by a weeklong weakening of the upper-level westerlies, and an increase in moisture and convective instability. The vertical structure of...

Monitoring the Monsoon in the Himalayas: Observations in Central Nepal, June 2001

Estimates of the time and space variability of the atmospheric heat source over Tibet are presented for the summer of 1979. These estimates rely on new data from the People's Republic of China allowing a better assessment of the surface heat fluxes, and on new satellite data from Nimbus-7 giving the radiation balance at the top of the atmosphere. The estimates of the atmospheric heat source turned out to be considerably smaller than those provided earlier in the literature, mainly because of different assumptions of the drag coefficient. The atmospheric heat source over Tibet is mainly modulated by the release of latent heat. Over the southeastern and southwestern plateau regions the heat source appears to be in phase with the precipitation yield of the Indian summer monsoon, whereas central Tibet reveals an out-of-phase behavior. Over western Tibet there appears to be hardly any net import of moisture from outside the region, whereas the maintenance of the hydrological cycle over eastern Tibet requires moisture flux convergence from outside the region of up to 40 percent of the mean rainfall, in agreement with what is known about the surface hydrology of Tibet.

Longxun Chen

Papers

The Atmospheric Heat Source over the Tibetan Plateau: May–August 1979