Improvement of Drag Coefficient Calculation Under Near‐Neutral Conditions in Light Winds Over land
Changwei Liu, Yubin Li, Zhiqiu Gao, Hongsheng Zhang, Tongwen Wu, Yixiong Lu, Xiaoye Zhang
Abstract
Abstract By analyzing the data collected over a rice paddy field in China, the occurrence of large drag coefficient in light winds ( U ≤ 1ms −1 ) under near‐neutral conditions (−0.1 ≤ z / L ≤ 0.1) is analyzed in this study. It is found that, with the use of either vector‐ or scalar‐averaged wind speed, the drag coefficient under near‐neutral conditions ( C DN ) always increases with the decrease of wind speed. Such behavior of C DN cannot be interpreted by the change of aerodynamic roughness length z 0 because it would need to be enlarged by an order of magnitude. While in light winds, the nonlocal transport, which is caused by large‐scale eddies and overlooked by traditional Monin‐Obukhov Similarity theory, is found to be the main cause of increased C DN . At last, a fixing term related to nonlocal effects is proposed based on our observation, and its applicability is verified with two AmeriFlux observation sites with a landscape of forest and savanna, respectively.