Observational Study Of Surface Energy Balance Over Semi-Arid Region Of Loess Plateau

In this study, the variation of surface radiation, energy balance and albedo over Semi-Arid region of Loess Plateau are analyzed by using the land-surface radiation data, turbulence fluxes and soil data observed over Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) during April 2006 to April 2008. The daily, seasonally, yearly cycle of them are studied. The relationship between synoptic condition, soil volumetric water content and surface radiation, energy balance, albedo are discussed as well. We also discuss the difference between Jingtai and SACOL by using the land-surface radiation data observed over JingTai during March to April 2008. The main results are listed in the following:1. The mean diurnal cycle of surface radiation is close to which are under clear sky condition. It suggests, the diurnal cycle of surface radiation under clear sky condition dominate on the behalf of climatology. The phases of solar radiation, upward shortwave radiation and net radiation are similar to the surface radiation, which the maximal value is at about 13:00 (Beijing Time). However, variations of upward and downward longwave radiation lag by the solar radiation, and the response time are one hour and two hours, respectively.2. The range of annual cycle of solar radiation, upward shortwave, downwardlongwave, upward longwave and net radiation are 300, 75, 230, 270, 253.3 W/m~2, respectively. And monthly values of them are 5916.5, 1389.9, 8875.2, 11587.5, 1813.5 MJ/m~2, respectively. There are two transformed periods of net radiation in annual cycle. One is the value of net radiation change from positive to negative, when the upward longwave radiation are the main part of energy balance in wintertime. And the other one is the value of net radiation change from negative to positive, when the solar radiation are the main part of energy balance.3. The surface albedo is greatly influenced by synoptic condition. The surface albedo under clear sky condition takes on "U" shape, which is low in noon and high in nightfall. And the value of surface albedo is higher in the morning, and lower in nightfall before the snow didn't melt completely. However, the value of surface albedo is lower in the morning, and higher in the nightfall after the rainfall. The mean diurnal cycle of surface albedo is similar to which is under the clear sky ("U" shape). The snowfall in wintertime may increase the surface albedo and rainfall may decrease the value. Furthermore, the relationship between surface albedo and soil volumetric water content (VWC) takes on exponential attenuation, that is, along with the soil VWC increases the surface albedo decreases gradually, when the soil VWC increases to some value, the value of albedo is changeless.4. The sensible heat flux is the main part of the available energy except that in the growing season over Loess Plateau area. In the growing season the latent heat flux has the same order with the sensible heat flux. And the phase variation between sensible heat flux and latent heat flux is negative.5. The diurnal cycle of solar radiation and downward longwave radiation in Jingtai are similar to SACOL during March to April 2008. And the maximum value of solar radiation is 800 W/m~2 in both March and April. The variation range of downward longwave radiation in April is higher than that in March, and the variation range of downward longwave radiation in Jingtai is more than that in SACOL. It means, the variation of cloud in Jingtai is more than that in SACOL during this period. The phase variation between air temperature and relative humidity is negative.6. The sand particles can heat up the air in near-ground layer during the daytime. And this action also can heat up the water vapor along with the sand storm and decrease the relative humidity. However, when sand storm occurs in the nighttime, because there is no solar radiation and this heating action disappear. So the air temperature in this layer decreases at all times, and the relative humidity increases to some extent and gradually decreases.