摘要:
本文利用锡林郭勒草原2008年春季近地层涡旋相关系统和铁塔的风、温平均梯度观测资料,分析了总体输送系数随梯度Richardson数的变化特征,建立了动量总体输送系数随大气稳定度、近地层风速以及感热总体输送系数随大气稳定度和近地层气温的关系.中性条件下,半干旱草原植被下垫面动量总体输送系数与近地层大气动力状态之间存在明显的相互作用,总体输送系数与近地面层风速之间满足二次曲线拟合关系;风速较小时,大气动力特征对地表粗糙度长度的改变不是很明显,动量总体输送系数随气流增强而增大;而当风速较大时,强风速会使植被高度发生改变,动量总体输送系数随气流增强而减小.另外,感热总体输送系数与近地层气温之间也存在二次曲线关系.动量总体输送系数与近地层风速之间的关系、感热总体输送系数与近地层气温之间关系的建立为总体输送系数参数化提供了重要途径,同时该方案避免了对动力学粗糙度长度和热力学粗糙度长度的求解.
Abstract:
The momentum and heat transfer coefficients used in the atmospheric numerical models usually are taken as constants. In fact, the bulk transfer coefficients change not only with atmospheric stabilities, but also with the air motion and thermodynamic properties of the surface boundary layer. In this paper, the bulk transfer coefficients of momentum and sensible heat are determined by using the data observed by the eddy correlation system and those of the average wind velocity and temperature gradients over the Xilin Guole semiarid grassland in May, 2008. The relations between the bulk transfer coefficients and the gradients in Richardson numbers are analyzed, and the relationship between the bulk transfer coefficients and atmospheric stabilities is also studied. Finally, the changes of momentum bulk transfer coefficients with wind speed and the relations between sensible heat transfer coefficients and temperature are determined. Comparison with the eddy correlation method for observing transfer coefficients, there is a great difference between the computational values of typical Businger-Dyer similarity functions. The revised Monin-Obukhov similarity functions could significantly improve the accuracy of the transfer coefficients. Under the near neutral stratification, there was an obvious interaction between the land surface and the flow condition over semiarid regions. The momentum bulk transfer coefficient varies with the mean wind speed at 10 m according to the quadratic curve laws. Under the low wind velocity condition, the influence of the air flow on the roughness was not significant and the momentum transfer coefficient will increase with increasing wind velocity. While at the high wind speed, the momentum transfer coefficient will decrease with increasing wind velocity, for the air flow changes the roughness height of surface boundary layer. Under the near neutral stratification, there was a quadratic curve law between the temperature and the sensible heat transfer coefficient, which provides a useful parameterization scheme for sensible heat transfer coefficient. The new schemes could parameterize the turbulent fluxes with average wind velocity and temperature gradients data, and does not need to compute the roughness.
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