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[1] A 12 year archive of the Tropical Rainfall Measuring Mission (TRMM) rain rate is used to document the regionality of diurnal rainfall cycle in the Yangtze River Valley (YRV). The regional rain peaks, local phase shifts, rain event's behavior, and related seasonal change from March to August are examined. In the middle reach of YRV, rainfall appears mainly in early morning and displays a distinct local shift of diurnal phase. Such features are well established at each presummer (from May to June) and result from the eastward migrating events with a late night growth in size. They are supported by the low-level convergence that moves from the east slope of the Tibetan Plateau to the middle reach of YRV, as the deviated wind vector rotates clockwise to enhance southerlies at late night and southwesterlies in the morning. In the lower reach of YRV, however, one observes an eruption of morning rainfall with less local difference in diurnal phase. Morning rainfall is active in presummers of some years but suppressed in some others, contributing greatly to the variance of rainfall budget and resulting in anomalous wet/dry seasons. It is found to arise from a local growth of rain events rather than the migrating events from the middle reach. A majority of these organized convections prefer to form and develop in a belt-shaped zone where the nocturnal southwesterlies of warm/moist air impinge on the Meiyu front in the lower troposphere.

[1] The generation processes of mesoscale convective systems (MCSs) that occurred over the Sichuan Basin in China were revealed in the early 2008 monsoon season. MCS occurrences detected in METEOSAT geostational satellite images are associated with the traveling of midlatitude troughs after the onset of the Indian monsoon. Three episodes of large-scale MCSs were generated under synoptic conditions of merging southwesterly low-level monsoon flows with a northerly midlatitude air mass following the trough in the leeward of the Tibetan Plateau without direct migration of the vortex generated from the plateau surface. Numerical simulations using a weather research and forecast (WRF) model showed that the MCS was triggered in the evening by strengthening of the low-level wind convergences with horizontal shear between the southerly monsoon flow, with large convective available potential energy, and the northerly dry intrusion. A sudden increase in the northerly winds was confirmed by sonde observation data in the western basin, and the winds were simulated as intrusions passing over the Qinling Mountains when the daytime clouds over the mountain were diminished. Sensitivity experiments by a WRF simulation revealed that the topography of the Sichuan Basin was able to capture the dry intrusion at the bottom to prevent the propagation of disturbances from the plateau and to cause the sudden onset of MCSs apart from the plateau with a heavy precipitation zone.

[1] This study revealed the formation processes of mesoscale convective systems (MCSs) over the eastern Tibetan Plateau (TP), associated with the establishment of subplateau-scale convergences affected by the longitudinal surface wetness gradient. Composites of reanalysis data and satellite infrared images showed that large MCSs occurred in the afternoon under the condition of the eastward expansion of upper tropospheric anticyclone with the enhancement of near-surface low pressure in the western plateau. The low-level convergence was systematically formed through the eastward propagation of a thermally induced cyclonic circulation formed in the north central plateau before the day of the MCS genesis. A numerical model successively simulated the MCS occurrence processes with the diurnal evolution of convergences, namely, (1) the formation of horizontal wind shear between southwesterlies and northeasterlies over the strong heated land-surface causing a thermal low in the northwest, (2) the eastward propagation of the vorticity due to intensification of upper westerlies in the night, and (3) the MCS genesis by low-level convergences behind the migrated vortex with a convective instability condition over the eastern wet land surface. Numerical sensitivity experiments confirmed that eliminating either the western surface sensible heat flux or the eastern surface latent heat flux prevented the development of MCS. Namely, the enhanced southeast-northwest gradation of the plateau-scale soil moisture distribution could effectively form the MCSs in the eastern plateau during the monsoon season.

[1] Surface air temperature, observed in the eastern Nepal Himalayas, showed large intraseasonal and year-to-year variability during the nonmonsoon season. A significant negative correlation was found in the monthly outgoing longwave radiation (OLR) between the area over the Himalayas and tropical areas between 80 and 150°E, which was confirmed by in situ observation data. The correlation was stronger in the months of December, March, and April with different correlative areas in the tropics. From comparison of data observed over 10 years with OLR and global precipitation data, the variability of monthly local surface air temperature in winter was primarily attributed to the indirect effect of convective activities in the tropics that caused a variation in the precipitation/snow cover condition at high elevations.

[1] The characteristics of atmospheric circulations in Asia causing snowfalls over the central Tibetan Plateau (TP) were investigated during the nonmonsoon season. During 1994–2004, a clear pattern of a barotropic pressure anomaly was found over northeast Asia by means of composite analysis. NCEP reanalysis data and METEOSAT images were examined for snowfall events detected by in-situ surface albedo measurements during Nov.–May, 1998/99, and the possible mechanisms causing precipitation in the TP and the influence of mid-latitude large-scale meandering flow were discussed.

[1] Land-atmosphere interactions on the Tibetan Plateau are important because of their influence on energy and water cycles on both regional and global scales. Flux variance and eddy covariance methods were used to measure turbulent fluxes of heat, water vapor, and momentum over a Tibetan shortgrass prairie during the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) in 1998. Under unstable conditions during the monsoon period (July-September), the observed standard deviations of temperature and specific humidity (normalized by appropriate scaling parameters) followed the Monin-Obukhov theory. The similarity constants for heat C T and water vapor C q in their dimensionless functions of stability under a free convection limit were both 1.1, unlike the differences (i.e., C T ≤ C q ) reported in other studies. While the transfer efficiency of heat and water vapor exchange generally agreed with the prediction from the Monin-Obukhov theory, momentum exchange was less efficient than predicted. In comparison with the eddy covariance data, the flux variance method (with C T = Cq = 1.1) underestimated both heat and water vapor fluxes by