The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol
The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol
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DOI：
10.5194/acp-18-1241-2018
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发表时间：
2017-05
影响因子：
6.3
通讯作者：
Netsanet K. Alamirew;M. Todd;C. Ryder;J. Marsham;Yi Wang
中科院分区：
文献类型：
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作者：
Netsanet K. Alamirew;M. Todd;C. Ryder;J. Marsham;Yi Wang
抽象。撒哈拉热低压（SHL）是西非气候系统的一个关键组成部分，也是西非季风在一系列时间尺度上变化的重要驱动因素。驱动SHL变化的物理机制仍然不确定，尽管水蒸气被认为是最重要的。在这里，我们量化的独立影响的灰尘和水蒸气的变化对辐射收支和大气加热的区域使用的辐射传输模型配置的观测输入数据从Fennec现场活动在阿尔及利亚南部的Bordj Badji Mokhtar（BBM）的位置（21.4 N，0.9 E），接近SHL核心2011年6月。总的来说，我们发现沙尘气溶胶和水蒸气在驱动大气层顶（TOA）辐射收支的变化方面具有相似的重要性，因此SHL上的柱积分加热（每标准差沙尘气溶胶光学厚度- AOD为1.7W m −2）。因此，我们推断，SHL的强度很可能是类似的增强的影响，灰尘和水汽激增事件。然而，这些过程的细节不同。尘埃在表面产生大量的辐射冷却（每标准偏差为1.11 W m −2），可能导致边界层中的显热通量减少，这是由尘埃边界层中的尘埃吸收短波（SW）的直接辐射加热所补偿的。相比之下，水蒸气在地表引起的辐射变暖为每千克米-2柱积分水蒸气的标准差为1.66瓦米-2。净效应包括在季风潮和对流冷池外流（“haboobs”）产生的天气/中尺度沙尘事件期间，平均0.5 K day −1和高达6 K day −1的加热率的明显净大气辐射辐合。在此基础上，我们作出推论的过程驱动的变化与辐射和平流加热/冷却的SHL。根据该区域的天气背景，驱动变率的过程既包括水汽和沙尘的独立影响，也包括沙尘和水汽共同变化的补偿事件。预测模型通常在柱积分水蒸气中有高达2 kg m −2的偏差（相当于2.6 W m −2 TOA净通量的变化），并且通常缺乏灰尘的可变性，因此预计这些耦合很难代表。因此，为了进一步了解SHL和相关的气候过程，必须改进模型中灰尘和水蒸气的表示以及相关辐射影响的量化。
Abstract. The Saharan heat low (SHL) is a key component of the west African climate system and an important driver of the west African monsoon across a range of timescales of variability. The physical mechanisms driving the variability in the SHL remain uncertain, although water vapour has been implicated as of primary importance. Here, we quantify the independent effects of variability in dust and water vapour on the radiation budget and atmospheric heating of the region using a radiative transfer model configured with observational input data from the Fennec field campaign at the location of Bordj Badji Mokhtar (BBM) in southern Algeria (21.4 ∘ N, 0.9 ∘ E), close to the SHL core for June 2011. Overall, we find dust aerosol and water vapour to be of similar importance in driving variability in the top-of-atmosphere (TOA) radiation budget and therefore the column-integrated heating over the SHL ( ∼ 7 W m −2 per standard deviation of dust aerosol optical depth – AOD). As such, we infer that SHL intensity is likely to be similarly enhanced by the effects of dust and water vapour surge events. However, the details of the processes differ. Dust generates substantial radiative cooling at the surface ( ∼ 11 W m −2 per standard deviation of dust AOD), presumably leading to reduced sensible heat flux in the boundary layer, which is more than compensated by direct radiative heating from shortwave (SW) absorption by dust in the dusty boundary layer. In contrast, water vapour invokes a radiative warming at the surface of ∼ 6 W m −2 per standard deviation of column-integrated water vapour in kg m −2 . Net effects involve a pronounced net atmospheric radiative convergence with heating rates on average of 0.5 K day −1 and up to 6 K day −1 during synoptic/mesoscale dust events from monsoon surges and convective cold-pool outflows (“haboobs”). On this basis, we make inferences on the processes driving variability in the SHL associated with radiative and advective heating/cooling. Depending on the synoptic context over the region, processes driving variability involve both independent effects of water vapour and dust and compensating events in which dust and water vapour are co-varying. Forecast models typically have biases of up to 2 kg m −2 in column-integrated water vapour (equivalent to a change in 2.6 W m −2 TOA net flux) and typically lack variability in dust and thus are expected to poorly represent these couplings. An improved representation of dust and water vapour and quantification of associated radiative impact in models is thus imperative to further understand the SHL and related climate processes.