The relationship between the diurnal variation of the water vapor and topography over the Tibetan Plateau

The Tibetan Plateau has a extensive area and high altitude properties, and it is well recognized that the plateau act ac a heat source for the broad scall monsoon circulation. There is a obvious diurnal cycle in the surface meteorological element over the plateau and its vicinity through the year. (Yeh and Gao,1979;Yanai et al. 1992; Murakami,1981).

Yanai et al.(1992) analyzed the atmospheric heat source Q1 and moisuture sink Q2 over the plateau using with the FGGE and QXPMEX-79 data set. They found that there is a remarkable diurnal variation in the planetary boundary layer and virtical circulation. It is also noted that water vapor is not uniform in the vertical while the deep mixed layer was observed. Furthermore water vapor is larger in the morning than in the evening over the plateau. Yanai and Li (1994) found that the dry horizontal advection is important for water vapor budget in the plateau boundary layer.

Recently Kimura and Kuwagata (1995) examined the thermally induced circulation over the complex terrain. They pointed out that the atmospheric water vapor in the valley transport to the mountain reagion and its intensity has a maximum around critical scale of the topography. The tibetan plateau is consisted with many mountain ranges and deep valleys and upper-air station over the plateau are almost established in valley/basin. It is suggested that the upper-air soundings may not be represent synoptic scale condition and the estimation of the atmospheric heat source and moisture sink could be affected by the heat and moisture transport by the thermally local circulation.

In the present study, the caracteristics of the diurnal variation of the water vapor is described and examine the relationship between the diurnal variation of the water vapor and the thermally induced local circulation over the plateau and its vicinity using QXPMEX-79 data set in the summer of 1979.

We defined that the normalized precipitable water difference as NWD=(W07BST-W19BST)/(0.5*(W07BST+W19BST))*100. The precipitable water is larger in the moring than that in the evening at 42 stations. On the contrary to the distribution of the precipitable water vapor in the morning, the large positive variation, it means that the water vapor decreased in the daytime, was observed over the central and western plateau and its value exceeded 20%. The relationship between the NWD and valley/basin dimension over the station above 3000m MSL over the plateau shows that there is a linear relationship between NWD and valley/basin width. On the other hand there is weak correlation between NWD and valley/basin depth. These results suggested that the thermally induced circulation over the Tibetan plateau is quit important for the heat and moisture transport in diurnal cycle.