Dust emission and transport over Iraq associated with the summer Shamal winds
Dust emission and transport over Iraq associated with the summer Shamal winds
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DOI：
10.1016/j.aeolia.2016.11.001
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发表时间：
2017-02-01
期刊：
影响因子：
3.3
通讯作者：
Oolman, L.
中科院分区：
文献类型：
--
作者：
Francis, D. Bou Karam;Flamant, C.;Oolman, L.
在这项研究中，我们利用地面和空间观测资料以及使用中尺度模式meso-NH进行的数值模拟，研究了夏季Shamal风（一种准永久低空西北风特征）的日演变及其在伊拉克沙尘排放和运输中的作用。来自伊拉克天气观测站的6年数据集可以确定夏季近地面强风与能见度降低之间的显著联系。在一个给定的、有代表性的案例研究（2010年6月25日至7月3日）的中尺度北半球模拟的基础上，探讨了Shamal事件中起作用的详细过程。沙马勒地表风和对流层低层风（通常高于地面500米）之间表现出非相位关系，地表最大风速在白天观测，而高空最大风速在夜间观测。白天的近地面风是尘埃排放的源头，与夜间低空急流向地面的动量向下转移有关，这是由于每天太阳加热开始后湍流混合造成的。利用自旋增强型可见光和红外成像仪、中分辨率成像光谱仪和模拟得到的气溶胶光学深度，首次估算了与伊拉克夏季夏马尔事件相关的沙尘量。沙尘量的日变化很大，在0600 UTC左右日最小值为1 Tg，在1500 UTC左右日峰值为2.5 Tg以上，受近地面风速日循环的驱动。伊拉克夏季沙马尔沙尘暴的日沙尘负荷峰值与乍得博德勒洼地下游的模拟结果相同，后者被认为是世界上最大的沙尘源。(C) 2016 Elsevier B.V.版权所有
In this study, we investigate the diurnal evolution of the summer Shamal wind (a quasi-permanent low-level northwesterly wind feature) and its role in dust emission and transport over Iraq, using ground based and space-borne observations together with a numerical simulation performed with the mesoscale model meso-NH. A 6-year dataset from the synoptic stations over Iraq allows establishing the prominence of the link between strong near surface winds and reduced visibility in the summer. The detailed processes at play during Shamal events are explored on the basis of a meso-NH simulation for a given, representative case study (25 June-3 July 2010). The Shamal exhibits an out-of-phase relationship between the surface wind and winds in the lower troposphere (typically 500 m above ground level), the maximum surface wind speeds being observed during the day while in altitude the maximum wind speeds are observed at night. The daytime near surface winds, at the origin of dust emission, are associated with the downward transfer of momentum from the nocturnal low-level jet to the surface due to turbulent mixing after solar heating commences each day. For the first time, an estimate of the dust load associated with summer Shamal events over Iraq has been made using aerosol optical depths derived from the Spinning Enhanced Visible and Infrared Imager, the Moderate Resolution Imaging Spectroradiometer, and the simulation. The dust load exhibits a large diurnal variability, with a daily minimum value of 1 Tg around 0600 UTC and a daily peak of 2.5 Tg or more around 1500 UTC, and is driven by the diurnal cycle of the near surface wind speed. The daily dust load peak associated with the summer Shamal over Iraq is in the same order of magnitude as those derived from simulations downstream of the Bodele depression in Chad, known to be the world's largest dust source. (C) 2016 Elsevier B.V. All rights reserved.