Surface Heat Balance Observation in the Western Tibet

To estimate the surface heat balance in the western part of Tibetan Plateau, two sets of Automatic Weather Station (AWS) have been installed in the western Tibet and continued observation since the end of September in 1997. One of the sites is Gaize (N32‹18f, E84‹03f) which is located at the middle of western Plateau. The other site is Shiquanhe (N32‹30f, E80‹05f) which is located in the western edge of Tibetan Plateau. The ground surface conditions of both sites are bare soil, few grasses and no tree.

The AWS data were collected until December 1998. The following results are obtained through the data analysis. Both sites had snows in the winter of 1997 to 1998. It took about two weeks at the longest when the snow disappeared. The albedo well showed the snow events. The minimum air temperature below -35 degree Centigrade appeared for 5 days at Gaize in snow conditions. Net radiation is positive even in winter season with no snow. On the other hand, once it snowed, net radiation became negative due to large albedo. Soil heat flux changes the direction during February. Its amplitude of yearly variation is about 10W/m². Average soil water content from the surface to 15cm in depth is below 10% in dry season and above 15 % in rainy season.

Net radiation at Gaize is larger than at Shiquanhe in July and August 1998. This is because larger amount of rainfall at Gaize increases the evaporation from the ground surface then the evaporation suppresses the rise of ground surface temperature. Therefore net radiation became larger at Gaize. Sensible and latent heat fluxes were estimated from Bowen ratio method and water balance method with net radiation and soil heat fluxes. The difference between the two methods is about 10W/m². Sensible heat flux accounts for the most part of the heat balance equation. Latent heat flux increases in the rainy season. The yearly averaged Bowen ratio at Gaize and Shiquanhe are about 5.5 and 13 respectively. Accumulated rainfall from September in 1997 till August in 1998 is about 100mm at Gaize and 50mm at Shiquanhe.