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

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Version 2 2023-06-12, 08:48
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journal contribution
posted on 2023-06-12, 08:48 authored by Netsanet Alamirew, Martin ToddMartin Todd, Claire L Ryder, John H Marsham, Yi WangYi Wang
The Saharan heat low (SHL) is a key component of the West African climate system and an important 10 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 (0.9E, 21.4N), close 15 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 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), 20 presumably leading to reduced sensible heat flux into the boundary layer, which is more than compensated by direct radiative heating from 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/meso-scale dust events from monsoon surges and convective cold 25 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, 30 and so are expected to poorly represent these couplings. An improved representation dust and water vapour and quantification of associated radiative impact is thus imperative in quest for the answer to what remains to be uncertain related with the climate system of the SHL region.

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Publication status

  • Published

File Version

  • Published version

Journal

Atmospheric Chemistry and Physics

ISSN

1680-7316

Publisher

European Geosciences Union

Issue

2

Volume

18

Page range

1241-1262

Department affiliated with

  • Geography Publications

Research groups affiliated with

  • Sussex Sustainability Research Programme Publications

Full text available

  • Yes

Peer reviewed?

  • Yes

Legacy Posted Date

2018-01-10

First Open Access (FOA) Date

2018-01-10

First Compliant Deposit (FCD) Date

2018-01-10

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