Chapter 2 Precipitation幻灯片.ppt

Chapter 2 Precipitation 2.1 atmosphere 2.2 precipitation 2.3 distribution of precipitation 2.4 point precipitation 2.5 areal precipitation 2.6 probable maximum precipitation 2.7 gross and net precipitation 2.8 precipitation frequency analysis 2.9 snow accumulation Objectives The purpose of this chapter is Define precipitation, discuss its forms, describe its spatial temporal attributes Illustrate techniques for estimating areal precipitation amounts for specific storm events for maximum precipitation-generating conditions Principal form of precipitation Rain (main form in China) Snow (polar region) Hydrological modeling water resources assessments depend on the form amount of precipitation 2.1 atmosphere Troposphere (对流层): The lower layer of the atmosphere, 10km Contains almost all of the atmosphere moisture Water in moist air 4%. The atmosphere depends for its heat content on the radiant energy from the sun. One-sixth of solar energy absorbed by atmos. one third reflected into space one half absorbed by the earth’s surface Heat from the earth’s surface to atmos. by: Conduction(传导)－greater to less energy Convection(对流)－vertical interchange Radiation(辐射)－long wave radiation Radiation: is important link in the energy balance of atmos. The greenhouse effect: the increase of atmospheric temperature as a result of a decrease in the long wave radiation Reason: man-produced gases－CO2,CH4, N2O obstruct(阻挡) the outgoing radiation energy accumulated high temperature occurs Convection: vertical interchange of masses of air A parcel of air moves vertically(垂直) expands due to a decrease of the external pressure energy needed for expansion cause T to fall Lapse rate(温度垂直梯度): the rate of change of air temperature with height the average lapse rate is 0.65℃ per 100m rise The pressure The pressure of air: 1013mbar, 1.013bar MSL The atmospheric pressure decreases with the height For the lower atmosphere de