应用压力中心法确定农田空气动力参数

利用小麦、水稻、大豆及玉米作物叶面积密度和冠层内外风廓线等的实测资料,作者首先在大田条件下根据压力中心法计算了上述农田的空气动力参数,并导出了一个计算零平面位移(d)的物理模式。通过与实测资料的比较得出,压力中心法不仅完全可以用于野外的大田作物,而且结果的稳定性基本上不受大气层结的影响,精度在非中性层结下通常高于常用的对数廓线法和拽力系数法。本文结果还表明,通常情况下d,Z0与作物高度(h)之比值是随作物群体结构和大气层结、湍流状况等的不同而变化的,而不是人们所普遍接受的常数值。随群体中最大密度层的相对分布高度和风速递减指数的增大,d/h增大。只有对于矮杆的大田作物,在其群体处于中等密度时,以下关系d≈0.64h,Z0≈0.08h才较稳定。对于高杆作物或密度稀疏的群体,土壤表面湍流通量的增加以及叶子的拽力系数随风速的变化指数的选取都对压力中心法的结果产生一定的影响。

AERODYNAMIC PARAMETERS OF CROP CANOPIES ESTIMATED WITH A CENTER-OF-PRESSURE TECHNIQUE

The Center-of-Pressure technique (CPT),originally proposed by Thom in 1971,is firstly verified in the field,based on the canopy architecture and microclimate data of four crops. The comparison of u.estimated by CPT with that by eddy-correlation method shows that CPT not only works well in the field,but also gives more steady and accurate results,which are hardly affected by atmospheric stratification stability,thannthat of the log-profile fitting method which is frequently used in practice. In addition,a physical model for directly calculating d is approximately developed. The results yield that the ratio of d,zato h,in general,varies with the canopy structure,turbulence intensity and the stability of flow above and within the canopy;d/h increases with the in crease of α(the relative height of the maximum foliage density layer) and Yu(the wind extinction coefficient within canopies).Only for short stem ctops and when their canopies reach the moderate foliage density and stable structure,d and zo approximately and steadily approach to 0.64h and 0.08h,respectively. Besides,concerned with very sparse or tall crops,the fairly large stresses at the soil surface and variability of m undoubtedly influence the accuracy of CPT to a certain extent.