The Change Features And Influence Mechanism Research On Dynamic Roughness Of Summer Monsoon Transition Zone

The summer monsoon system have a great influence on China,further,Summer Monsoon Transition Zone,called SMTZ,is the most sensitive region when summer Monson changes both in intensity and range of motion,with complicated land-atmosphere interaction and regional topography function,the weather conditions are complex and changeable,the ecological environment is fragile,the climate change extremely susceptible.Hail,drought,sandstorm,rainstorm and other weather disasters are very serious,so itis completely necessaryto research the SMTZ on this account,the aerodynamic roughness length is an important parameter of surface process and boundary layer meteorology,the influence of surface roughness element characteristics on aerodynamic roughness is obvious,this is consistent with the researchers' general understanding.more than that,recent studies suggest that actions of atmospheric conditions on aerodynamic roughness are also very significant.The rough elements and subtle variations in atmospheric conditions can be applied to aerodynamic roughness.It is not only sensitive but also highly variable,in numerical model,it is used as the basic parameter to describe the land surface process and near-stratum meteorological state,and can also be used to estimate other flux parameters.in consequence,it is very important to study the change characteristics of aerodynamic roughness length to improve the description the surface process,this paper uses the ‘Northern Collaborative Observation Experiment' from July to September 2008,including grassland YuZhong,cropland DingXi and hybrid underlying surface XingLongShan,and the data from January 2007 to December 2012 under the "Observation And Experimental Study Of The Land Surface Of Loess Plateau",including grassland underlying surface YuZhong,analyze the characteristics of the aerodynamic roughness parameters of the transition zone,draw the following conclusions:(1)The effect of atmospheric aerodynamic factors on aerodynamic roughness is significant.The article utilizes the observation data belong to the northern Chinasynergy observation experiment during July and September of 2008,which embraces YuZhong,DingXi and XingLongShan,locate in Summer Monsoon Transition Zone,represent grassland,cropland and hybrid underlying surface,respectively analyses change rule of aerodynamic roughness length along with wind velocity,wind direction,friction velocity,footprint region and a new influnencing factor.Study indicates that a significant varation trend appear when the footprint area change within the scope of four wind direction section,the taller vegetation,the fluctuanter topography of bullet surface,respectively larger aerodynamic roughness,the bigger footprint region can also result in a larger aerodynamic roughness.Inverse correlation appears in roughness length and velocity,more narrowly,aerodynamic roughness length decreases along with wind velocity increasing accordingly,the relationship reflect a significant difference when station change,to be specific,superior correlation in grassland and hybrid underlying surface,while inferior one in cropland underlying surface.However,changes intricately along with friction velocity,a time at friction velocity,it is far more complex that roughness length increase when friction velocity vary in a small range,assume a positive correlation,on the contrary,roughness length decrease when friction velocity vary in a wider range,show a negative correlation,while superior one occurs in cropland and hybrid underlying surface,inferior one in grassland.At the same time,preferable indexes appear in all the three underlying surface when it is turn to the new influnencing factor with the aerodynamicroughness length,including correlation index and standard deviation index,and fortunately it is verified at YingKe cropland station.The aerodynamic roughness length is negatively correlated with a atmospheric new dynamic factor,similarvariation trend appears in aerodynamic roughness length and wind velocity.Obvious to see that wind velocity perform a more important role when analysing the aerodynamic roughness length change characteristics along with wind velocity,friction velocity.(2)The influence of atmospheric thermodynamic factors on aerodynamic roughness is obvious.The article utilizes the observation data belong to the northern China synergy observation experiment during July and September of 2008,which embraces YuZhong,DingXi and XingLongShan,locate in Summer Monsoon Transition Zone,represent grassland,cropland and hybrid underlying surface.When atmospheric dynamic main factor wind velocity of aerodynamic roughness length is not eliminated,the relationship between aerodynamic roughness length andthermodynamic stability z L represent not clearly.In the case of similaraerodynamic conditions,the effect of atmospheric dynamic factor wind velocity on dynamic roughness is eliminated,when the atmosphere is in neutral layer,the aerodynamic roughness length of the different underlying surface is maximized without exception.In atmospheric stable stratification,obvious negative correlation between the aerodynamic roughness length and atmospheric stability occur,when the atmosphere turn positive into active,obvious negative correlation between the aerodynamic roughness length and atmospheric stability occur again.Thelaw of change is not the same as Zilitinkevich's conclusion by means of theory derivation,to be specific,the tremendous amount of difference is obvious when the atmosphere is unstable.In this paper,the relationship between the theoretical fitting aerodynamic roughness length value of Zilitinkevich and the measured dynamic roughness,the test fitting aerodynamic roughness length value and the measured dynamic roughness are verified by using YingKe cropland data,it is concluded that the experimental fitting value and the experimental dynamic roughness are not only more weakly discrete than Zilitinkevich's theoretical fitting value,but also appropriate for all atmospheric activity conditions,regardless of whether stable,neutral or unstable.The correlation and stability are much more better than theoretical fitting value of Zilitinkevich,can replace the measured aerodynamic roughness value better,the theoretical fitting value of Zilitinkevich is so close to the experimental fitting value in the stable layer that also can take place of the measured aerodynamic roughness value,theoretical fitting value of Zilitinkevich is so hugely different from the experimental fitting value when air stay at the unstable stratification that replacing the measured aerodynamic roughness length value will make the error very large,assume a lack of authenticity?(3)Annual aerodynamic variability of the aerodynamic roughness is so strong.The terrain of the YuZhong SACOL grassland station is complicated,and the topographic relief varies greatly.The dominant wind is the northwest wind and the southeast wind,with obviously affecting factor the terrain near the site,the northwest wind speed of the station is larger and the wind frequency is higher.The aerodynamic roughness length is greatly influenced by different wind direction,and present a big difference.The aerodynamic roughness length is actually a physical parameter that varies with the state of the atmosphere.The difference of the topography and the underlying vegetation result in the different aerodynamic roughness length,theaerodynamic roughness length of SACOL station is mainly distributed within the range of 0.0 to 0.05 m,and we find that the aerodynamic roughness length distribution of many years is similar.In other words,time is not the main factor affecting the aerodynamic roughness.The aerodynamic roughness length of the north wind and the east wind is greater,and the increase of aerodynamic roughness length exist an unconspicuous periodic oscillation.For a non-uniform complex underlying surface similar to the SACOL station.Due to different vegetation characteristics and topography and other factors,the variation of aerodynamic roughness length of different wind direction is significantly different,to be specific,the aerodynamic roughness length of the north wind direction and the east wind direction is larger,the aerodynamic roughness length of the north wind and the east wind is very different,so far as to the annual change trend is quite the opposite.The change trend of north wind to aerodynamic roughness length is related to temperature and precipitation change trend in YuZhong region,but the change of the aerodynamics roughness length of the west wind is similar to that of the stability during the year.