The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol

Alamirew, Netsanet K, Todd, Martin C, Ryder, Claire L, Marsham, John H and Wang, Yi (2018) The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol. Atmospheric Chemistry and Physics, 18. pp. 1241-1262. ISSN 1680-7316

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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 variabil- ity in the SHL remain uncertain, although water vapour has been implicated as of primary importance. Here, we quan- tify 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 Al- geria (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 simi- lar 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 ef- fects of dust and water vapour surge events. However, the details of the processes differ. Dust generates substantial ra- diative cooling at the surface (∼ 11 W m−2 per standard devi- ation of dust AOD), presumably leading to reduced sensible heat flux in the boundary layer, which is more than com- pensated 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 wa- ter vapour in kg m−2 . Net effects involve a pronounced net atmospheric radiative convergence with heating rates on av-
erage of 0.5 K day−1 and up to 6 K day−1 during synop- tic/mesoscale dust events from monsoon surges and convec- tive 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. De- pending 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 representa- tion of dust and water vapour and quantification of associ- ated radiative impact in models is thus imperative to further understand the SHL and related climate processes.

Item Type: Article
Keywords: Climate Aerosols Dust Desert Africa Sahara
Schools and Departments: School of Global Studies > Geography
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
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Depositing User: Martin Todd
Date Deposited: 05 Feb 2018 14:03
Last Modified: 05 Feb 2018 14:03

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