The early summertime Saharan heat low: sensitivity of the radiation budget and atmospheric heating to water vapour and dust aerosol
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Published:2018-01-31
Issue:2
Volume:18
Page:1241-1262
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Alamirew Netsanet K., Todd Martin C., Ryder Claire L.ORCID, Marsham John H.ORCID, Wang YiORCID
Abstract
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.
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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