Analysis Of Turbulent Flux On Different Underlying Surfaces Over Semi-arid Area

Under the background of the global warming,the land-atmosphere interaction absorbs more and more attention.The interaction between the land and the atmosphere has a great influence on local weather and even the global climate by the general atmospheric circulation.In order to know more deeply about the land-atmosphere interaction under different conditions,especially on the complex terrain,we calculate the turbulent flux on three kinds of underlying surfaces by eddy covariance method,compare the characters of the diurnal and seasonal variations of turbulent heat flux and momentum flux and analyse the influences of different types of land surface to the turbulence by using the turbulent data and conventional meteorological elements data from January 2007 to December 2008 which comes from Semi-arid Climate and Environment Observatory of Lanzhou University(SACOL)and the Tongyu Observation Station(includes grassland and cropland station).With the help of the mesoscale numerical model WRF,we discuss the characters of the low-level jets and their influences on turbulence over the Loess Plateau in the stable boundary layer at night.These work lay the foundation of researching the land-atmosphere interaction over the semi-arid area,reveal the characters of the low-level jets and the role they play in the land-atmosphere interaction.(1)At the SACOL,grassland station and cropland station,the diurnal variations of sensible and latent heat flux both show a typical single-peak structure.The peak value of latent heat flux(LvE)is 40% ~75% of which of sensible heat flux(Hs),and the peak value of Hs in SACOL,grassland and cropland are 146.8 W·m-2,149.9 W·m-2 and 147.2 W·m-2.The annual maximum bowen ration is between 1.0 to 2.5 that is less 10 times than which over arid area.The turbulent heat flux mainly locate in-22.6 W·m-2 to 24.2 W·m-2 at night,Hs is always negative with the positive LvE,which indicates there are temperature inversion layers on different underlying surfaces that make the heat transferring from atmosphere to the ground and the transferring of water has the opposite direction of heat.(2)The diurnal variation of the momentum flux(?)is significant,but because of the wind speed and wind directions,the annual mean value of daily peak in Tongyu is 0.25 kg·m-2·s-1,which in SACOL is 0.08 kg·m-2·s-1.And the diurnal variation of ? in Tongyu has obvious differences in four seasons,the daily range in spring and autumn is larger than other seasons.(3)The low-level jets(LLJ)over the Loess Plateau at night in summer are intermittent and formed by the south wind,the present interval of which is 30 minutes to 60 minutes.Compared with winter,the low-level jets in summer occur more frequently,the jet axises and wind speeds are higher,the former mainly locates in 200.0 m to 700.0 m and the latter is about 6.00 m·s-1 to 17.40 m·s-1;the duration is shorter,which is 1 hour approximately.(4)The low-level jets formed below 500.0 m have a significant effect on wind speeds especially.The horizontal wind speed is 7.67 m·s-1 to 10.64 m·s-1 with the vertical wind speed is 0.18 m·s-1 to 0.34 m·s-1 when the jets happen.Without the jets,the wind speeds drops to 4.78 m·s-1 and 0.13 m·s-1 respectively.Moreover,the time of the influences of the jets are about 30 minutes,which is decided by the height of jet axises,the maximum and vertical gradient of wind speeds.(5)The low-level jets can strengthen the turbulence intensity and increase the turbulent kinetic energy,makes the vertical turbulence intensity increases 11.2%,the turbulent kinetic energy focuses on 2.0 m2·s-2 ~ 3.5 m2·s-2.The dynamic effect between the jets axises and ground can make 0.0023 < ? < 0.0065.Although the air jets have no effects on the turbulent heat flux,they can accelerate the transportation of momentum.(6)The size of horizontal turbulent eddy is mainly located in 60.0 m and 250.0 m that is larger than which in forest.And the size of vertical turbulent eddy is under 10.0 m that is 10 times smaller than which of horizontal eddy.(7)The turbulent power spectrum in the inertial subrange is up,especially for the vertical power spectrum.The “-2/3 power law” doesn?t suit well over the Loess Plateau.(8)When there are no LLJs,the turbulent power spectrum in buoyant subrange is significant.If there are LLJs,the buoyant subrange will disappear.The shear force will broke the lengthways turbulent eddy,with the downward momentum transportation can increase the energy of the large eddy and restrain the development of the small-scale eddy.