Diurnal variation of water vapor over the central Tibetan Plateau during summer

The diurnal variation of water vapor over the central Tibetan Plateau during summer are investigated by use of various observation data and a numerical model. A systematic diurnal cycle of the precipitable water was found in the rawinsonde data. That is, the precipitable water decreased in the daytime at the valley area having lower topographical height, while that increased during daytime over the large-scale mountain range. The horizontal distribution of the diurnal cycle is examined by global objective analysis datasets and GMS-5 satellite data, and the results are found to be consistent with above topographic dependence of the diurnal cycle. The systematic diurnal cycle of the water vapor amount is interpreted to be caused by the water vapor transport due to the thermally induced circulation over complex terrain. The distribution of the water vapor accumulation over the central Tibetan Plateau in the late afternoon can be qualitatively simulated by the two-dimensional numerical model. According to the numerical simulation assuming a simple two-dimentional valley, the effective horizontal topographic scale (the valley width) for which the water vapor accumulation over the mountain region becomes largest increases not only with the depth of valley, but also with decreasing the atmospheric density. The horizontal scale of large-scale topography of the central Tibetan Plateau is 200 to 300 km, which is close to the effective horizontal topographic scale favorable for the water vapor transport, estimated by the numerical model.