Abstract
Abstract. The radiative effect of long-range-transported Saharan air layers is investigated on the basis of simultaneous airborne high-spectral-resolution and differential-absorption lidar measurements in the vicinity of Barbados. Within the observed Saharan air layers, increased water vapor concentrations compared to the dry trade wind atmosphere are found. The measured profiles of aerosol optical properties and water vapor mixing ratios are used to characterize the atmospheric composition in radiative transfer calculations, to calculate radiative effects of moist Saharan air layers and to determine radiative heating rate profiles. An analysis based on three case studies reveals that the observed enhanced amounts of water vapor within Saharan air layers have a much stronger impact on heating rate calculations than mineral dust aerosol. Maximum mineral dust short-wave heating and long-wave cooling rates are found at altitudes of highest dust concentration (short wave: +0.5 K d−1; long wave: −0.2 K d−1; net: +0.3 K d−1). However, when considering both aerosol concentrations and measured water vapor mixing ratios in radiative transfer calculations, the maximum heating/cooling rates shift to the top of the dust layer (short wave: +2.2 K d−1; long wave: −6.0 to −7.0 K d−1; net: −4.0 to −5.0 K d−1). Additionally, the net heating rates decrease with height – indicating a destabilizing effect in the dust layers. Long-wave counter-radiation of Saharan air layers is found to reduce cooling at the tops of the subjacent marine boundary layers and might lead to less convective mixing in these layers. The overall short-wave radiative effect of mineral dust particles in Saharan air layers indicates a maximum magnitude of −40 W m−2 at surface level and a maximum of −25 W m−2 at the top of the atmosphere.
Highlights
Each year during the northern hemispheric summer months from June to August, large amounts of Saharan mineral dust particles are transported from the African continent towards the Caribbean islands (Moulin et al, 1997)
The aim of this study is to investigate the radiative effect of long-range-transported Saharan air layers (SALs) and to study their impact on the subjacent boundary layer
Results obtained from measurements during SALTRACE (Weinzierl et al, 2017) showed that the size distribution of long-range-transported mineral dust does not change significantly compared to the distributions measured at source regions
Summary
Each year during the northern hemispheric summer months from June to August, large amounts of Saharan mineral dust particles are transported from the African continent towards the Caribbean islands (Moulin et al, 1997). Previous field campaigns (e.g., the Puerto Rico Dust Experiment (PRIDE) in 2000) have shown that especially during the winter months dust transport occurs at lower atmospheric levels down to the sea surface (Reid et al, 2002). Such transport would modify the aerosol composition inside the boundary layer and would potentially impact the boundary layer state
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