Effect of Aerosol on Orographic Precipitation in Qinling Mountains
Based on the dataset of observations of precipitation and visibility r ange since 1954 at the top of Mountain Hua, the ratio between the precipitation at Mt. Hua and at the nearby plain stations, which is defined as the orographic enhancement factor Ro, and the relationship between Ro and visibility, were appl ied to quantitatively learn the ways that air pollution aerosols suppress orogra phic precipitation. Ro decreased gradually during the measurement period, and mo st of the decrease occurred after 1980, about 20%, matched with the decreasing v isibility and increasing aerosol, which indicates that enhanced pollution aero sols suppress the orographic precipitation. The decrement of average annual precip itation at Mt. Hua after 1980 was 132mm, 15%, compared with that before 1980, a nd while the decrease was about 16mm, 3%, at the plain stations. The decrease in Ro is mainly caused by days of the light and moderate rain (daily precipitati on of less than 30 mm), but not by days with more than 30 mm, which suggested th at the thin short living orographic clouds are much more susceptible to precipit ation suppression by air pollution aerosols. Under dynamical uplift, the suppres sion of aerosol to clouds on the mountain top is stronger than that to clouds at plains, and causes 20%-30% decrease of precipitation at Mt. Hua from 1980 to 20 04. On the contrary, for the thermo-dynamically driven clouds, the suppression o f aerosol is equivalent to clouds on the mountain top and at plains.