Analyses Of Land Atmosphere Interaction Over Semiarid Region

In this paper, semiarid sites with different land surfaces were analyzed to understand differences and similarities of land-atmosphere interaction between different land surfaces under similar climatic background and geographical environment. Based on the observation data provided by AmeriFlux observation network, including 3 semiarid sites US-Srm, US-Whs, US-Wkg(January 2010 to December 2011) and a semi humid site US-Bkg(January 2008 to December 2009). Characteristics of basic meteorological factors, surface turbulent fluxes, radiation and energy balance were analyzed and compared with the Semi-arid Climate and Environment Observatory of Lanzhou University(SACOL) and Tongyu Semi-arid Climate and Environment Observatory(the degraded grassland station and the farmland station) in China to reveal the features of land-atmosphere interaction in different semiarid land surfaces.(1) Air temperature, wind speed and relative humidity demonstrated obvious diurnal and annual variations. TA and RH diurnal variations had a peak and a valley value, wind speed had a single peak, TA and wind speed reached peak at afternoon when RH reached valley value. Among semiarid sites, US-Srm had the largest TA monthly mean values, US-Wkg wind speed monthly mean values were greatest, precipitation appeared mainly in July-September and winter, total annual precipitation at US-Srm was larger than other 2 sites. US-Bkg TA monthly mean values were lower than semiarid sites and precipitation concentrated from April to November.(2) Turbulent fluxes had distinct diurnal, seasonal and annual variation. ? and heat fluxes diurnal variations had a single peak which appeared at afternoon, ? and H annual variations curves presented “M” shape. Among semiarid sites, H occupied the dominant position except few months, US-Srm had greatest change ranges of ?(diurnal, seasonal and annual variation) and H(diurnal and annual variation); LE had greater values during precipitation concentration months(July to September), US-Whs had greater annual LE change range than other 2 sites. CO2 flux had similar diurnal variations in spring, summer and autumn, wintertime CO2 flux revealed an opposite diurnal variation, Fc diurnal variation differences between sites were obvious, US-Srm Fc change range was greatest and US-Whs had similar Fc diurnal curves in all seasons. At US-Bkg, ? reached Peak value earlier than semiarid sites and monthly LE mean values were greater than H after April.(3) Net radiation Rn, solar radiation Si, upward shortwave radiation So, upward long wave radiation Lo had similar diurnal variations with a peak value in all sites, downward long wave radiation Li did not show an obvious diurnal variation. Surface albedo had obvious diurnal variations and daytime values were less than nighttime and minimums appeared at about midday. Among 3 semiarid sites, α diurnal variations presented different characteristics. In daytime, variation curves presented “U” shape at US-Wkg, “一” shape at US-Whs and “w” shape at US-Srm except summer. US-Wkg showed weak seasonal and annual variations in comparison to other semiarid sites. At US-Bkg, differences of α diurnal variation between different seasons was obvious, surface albedo demonstrated significant seasonal and annual variations.(4) Soil temperature and heat flux had evident diurnal and annual variations, soil water content had obvious annual variations but the diurnal variation curves did not fluctuate apparently. Among semiarid sites, soil heat fluxes seasonal averages were positive in spring and summer, negative in autumn and winter. Soil temperature, change range of soil heat flux and peak value were greater at US-Whs than other 2 sites. At US-Bkg, soil temperature was lower and soil water content was greater than semiarid sites, peak values and change ranges of soil heat flux in all seasons were less than other sites.(5) Energy balance components had clear diurnal and annual variations and energy was not closed at all sites. The unclosed rates were 21%, 19% and 27% at US-Srm, US-Whs and US-Wkg. Among semiarid sites, energy loss was least at shrub land surface(US-Whs) and greatest at grassland surface(US-Wkg). At US-Bkg, the unclosed rate was 32%.(6) Characteristics of turbulent fluxes, surface albedo and energy balance closed rates at US-Srm, US-Whs and US-Wkg showed differences compared with semiarid SACOL and Tongyu in China.