Numberical Study On The Atmospheric Water Vapor Budget And Transport Over Arid/Semi Arid Area Of Northwest Of China

Using the mesoscale model WRF, a series of numerical simulations are first performed over the area around Lanzhou City of China. In these numberical experiments the model vegetation cover or the type of the landsurface in different patches of the study area has been arbitrarily changed to analysis the local and non-local responses of the atmosphere to changes in vegetation cover and surface types over a semi-arid region. A group of numerical experiments are further performed to simulate some water vapor budget and transport properties over two distinc areas, i. e., Xinlong mountain and Lanzhou city. The cases are selected when precipitation occurred over Xinlong mountain but have no precipitation over Lanzhou city. The results show that:Changes in vegetation cover have a complex effect on the surface temperature with a significant diurnal cycle. Contrast to a strong and persistent warming caused by urbanization of the area, decreases in vegetation cover only give rise to a weak and intermitted warming depending on the partition of the surface net available energy between sensible heat flux, latent heat flux and soil heat flux.The local temperature anomalies caused by the vegetation and surface type change can propagate out of the source region by background winds with details of the propagation depending on wind fields and underlying topography. Under suitable wind conditions, those temperature signals can pass around a mountain to form a persistent warming region lee of the mountain. A decrease in vegetation cover tends to force a 1.1 km deep secondary circulation. Decreases in vegetation cover also lead to decreases in column total water vapour over the area for one hand, and force secondary circulations due to temperature contrast on the other. The forced secondary circulations tend to transport inward the water vapour outside of the area to compensate the decrease of water vapour input from the surface. The integrated effect of a decrease in vegetation cover is to decrease column total water vapour over the area. Outside of the area, the colume total water vapour tends to increase upwind of the area and decrease downwind of the area. Over Xinglong mountain, the vegetated land-surface supply plenty of water vapor to the atmosphere with obvious seasonal variation. The maximum water vapor input into the atmosphere from the surface occurs in the summer cases. The Xinglong mountain area is mainly affected by the moist airflow from southeast except for the spring cases and has strong exchange of water vapor with the area around it. The area has plenty of cloud water and rain water above, and hence, high possibility of rain; Over Lanzhou city, the water vapor supply from surface to atmosphere is stable and has no evident seasonal variation, The total water vapor over Lanzhou city is much lower than that over Xinglong mountain in summer and autumn cases. Lanzhou city is mostly controlled by the dry airflow from north or northeast and the water vapor transport from outside of the city is quite small. Consequently, the cloud water and rain water as well as the possibility of rain over the city are lower than those over Xinlong mountain.