Increasing Atmospheric Temperature in the Upper Troposphere and Cumulus Convection over the Eastern Part of the Tibetan Plateau in the Pre-Monsoon Season of 2004

Abstract

In the spring of 2004, we implemented intensive field observations in the eastern part of the Tibetan Plateau. In this study, we use the field observation data, as well as satellite observation data and reanalysis data, to examine increases in diurnal potential temperature and variations in atmospheric conditions over the eastern part of the Tibetan Plateau. The results of intensive radiosonde observations reveal a significant temperature increase attaining to the tropopause in April. The depth of the mixing layer, estimated from the vertical profile of potential temperatures, was much smaller than the height of the temperature increase. This result indicates that the temperature rise was not solely caused by thermally induced dry convection. During the observation period, strong cloud activity was observed. Cloud-top heights derived from GOES-9 and radiosonde observations indicate that active cumulus convection coincides with the temperature rise in the upper troposphere. For small increases in temperature, lower cloud-top heights are observed.Numerical simulations were also conducted using a regional atmospheric circulation model and radiosonde observation data as initial conditions. The results of the numerical simulations with wet and dry conditions and ideal wind direction (no meridional wind component) reveal that in the wet case air is more efficiently warmed in the upper troposphere with cloud activity. By introducing the observed wind direction as an initial condition, the simulation predicts strong cloud convection over a wide region of the model domain and a greater increase in potential temperature; this result is comparable to observed data.In previous studies, sensitive heat transfer by thermally induced dry convection was thought to be the main cause of pre-monsoon atmospheric heating over the Tibetan Plateau; however, the results of the observation analysis and numerical simulations described in the present study suggest that strong cloud convection plays an important role, even during the period prior to the rainy season.

Diurnal Variation of Water Vapor Mixing between the Atmospheric Boundary Layer and Free Atmosphere over Changwu, the Loess Plateau in China

“…Thus, the potential temperature increased in the free atmosphere above around 4 km under the condition of cumulus convection (figure not shown). Such atmospheric heating in the upper troposphere was also observed in spring at the Tibetan Plateau (Taniguchi and Koike 2007). In contrast to this case, the vertical winds were weak and a cumulus cloud was not observed on DOY 158 in 2005.…”

“…A strong vertical wind, which can reach the free atmosphere, develops in the ABL over the plateau in early summer . Over the Tibetan Plateau, dry convection in the ABL and cumulus convection have been shown to play significant roles in heating the upper troposphere in spring (Taniguchi and Koike 2007). Thus, the developments of the ABL and clouds are critical factors in atmospheric dynamics and water cycles.…”

A China-Japan Cooperative JICA Atmospheric Observing Network over the Tibetan Plateau (JICA/Tibet Project): An Overviews

“…The first intensive in situ observations during early spring upon the Plateau were performed in April 2004 under the framework of the Coordinated Enhanced Observing Period (CEOP) (Koike 2004). Based on in situ and satellite observations, and numerical simulations, Taniguchi and Koike (2007) reported the importance of cumulus activity in terms of increases in atmospheric temperature in the upper troposphere, even in April. In the boreal winter, the change of the heat source across the Plateau may breed out abnormal zonal winds over the equatorial Pacific Ocean, which in turn may result in an abnormal sea surface temperature that would eventually a¤ect ENSO events (Chen et al 2001).…”

Role of Patchy Snow Cover on the Planetary Boundary Layer Structure during Late Winter Observed in the Central Tibetan Plateau

“…From the point of view of cloud convections, Fujinami and Yasunari (2001) found an active convective season over the plateau from March to April by satellite images, and Oku and Ishikawa (2004) also indicated a robust decrease of the cloud free ratio in the same season. According to an intensive sonde observation in April by Taniguchi and Koike (2007), mid-tropospheric warming was associated with cumulous convections, and a numerical simulation demonstrated the necessity of condensation processes to initiate the convection. Those studies indicated that the heating process of the plateau surface has already started in early spring, when the retreat of snow covers and development of topsoil melting layer are activated under the rich subtropical solar radiation.…”