Measurements of water vapor and high clouds over the Tibetan Plateau with the Terra MODIS instrument

The seasonal variations of water vapor and cirrus clouds over the Tibetan Plateau are investigated using the recently available Level 3 monthly-mean atmospheric data products with a 1degrees x 1degrees latitude-longitude grid. The data products are derived from the multichannel imaging data acquired with the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra Spacecraft. It is shown that the water vapor concentration over the Tibetan Plateau is normally low, whereas high clouds (mainly cirrus clouds) over the Plateau occur quite frequently. On an annual scale, the water vapor concentration reaches its maximum in July and its minimum in January. During the summer season, the southeastern part of Tibetan Plateau, which can be affected by moistures originating from the Bay of Bengal and southeastern Asia, is slightly moister than the other parts of the Plateau. This observation is in agreement with the previous surface meteorological measurements by Chinese scientists from the 1950s to mid-1970s. The mean high-cloud reflectance over the Plateau reaches its maximum in April and minimum in November. This feature of high clouds over the Plateau has not been reported previously. The special channel centered at 1.375-mum on the MODIS instrument has allowed the observation. We present a plausible mechanism to explain the seasonal variations of high clouds over the Plateau. We expect that the water vapor and high-cloud measurements with MODIS can be used to improve the model initialization and validation for climate models involving the Tibetan Plateau and the nearby regions in Asia.