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

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.

Characteristics and thermodynamics of Sahelian heatwaves analysed using various thermal indices

“…There is a considerable uncertainty in many meteorological fields between various reanalyses (Roberts et al 2014(Roberts et al , 2017 notably for some of the key radiative quantities of water vapour, clouds and dust. Dust is a crucial component of the Sahelian atmosphere (Knippertz and Todd 2012) and its radiative contribution to the atmospheric heat budget (Alamirew et al 2018) may then be an important parameter for heatwaves. Furthermore there is a clear need for improved observations across the Sahel to support assimilation into reanalyses and direct process analysis.…”

Synoptic-scale controls of fog and low-cloud variability in the Namib Desert

“…Future work should also focus on understanding the local and possibly synoptic-scale drivers of the vertical structure of FLCs in the Namib region on diurnal to seasonal scales and the day-to-day variability in (marine) boundary-layer humidity. As FLCs in the Namib are clearly connected to marine stratus/stratocumulus clouds, findings of recent and ongoing field campaigns over the southeastern Atlantic (Zuidema et al, 2016;Formenti et al, 2019) and related insights concerning the aerosol-cloud-meteorology system of the Namibian stratocumulus cloud field (e.g., Adebiyi and Zuidema, 2018;Andersen and Cermak, 2015;Diamond et al, 2018;Formenti et al, 2019;Fuchs et al, 2017Fuchs et al, , 2018Gordon et al, 2018) are relevant to fully understand FLCs in the Namib Desert.…”

Later Wet Seasons with More Intense Rainfall over Africa under Future Climate Change

“…Water vapor is a greenhouse gas, leading to further temperature increases (Evan et al 2015;Vizy and Cook 2017). Variations in dust aerosol have also been linked with variations in the strength of the SHL (Alamirew et al 2018) and Sahel precipitation (Konare et al 2008;Solmon et al 2008).…”