Radiative effects of long-range-transported Saharan air layers as determined from airborne lidar measurements

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.

Long-term characterisation of the vertical structure of Saharan dust outbreaks over the Canary Islands using lidar and radiosondes profiles: implications for radiative and cloud processes over the subtropical Atlantic Ocean

“…4 (b), one can see how the real Summer-SAL radiative impact is characterised by a strong cold anomaly near the SAL's top which cannot be attributed to the solely radiative effect of mineral dust. This cold anomaly, also observed by other authors (Dunion and Marron, 2008;Wong et al, 2009;Chen et al, 2010;Gutleben et al, 2019Gutleben et al, , 2020, are consistent with the temperature difference of about -2 K (Fig. 3 (a)), and also by the 190 m descent from isozero height level (Tables 1 and 2).…”

“…These scenarios, called summer-clean* and winter-clean*, are not real but can be used to separate contributions of mineral dust and water vapour to the radiative forcing in the atmospheric column. This approach is similar to that carried out in previous studies (Kim et al, 2004;Gutleben et al, 2019Gutleben et al, , 2020.…”

“…6. Several papers in the literature have been focused on determining the SAL radiative effects in summer (Carlson and Benjamin, 1980;Kim et al, 2004;Wong et al, 2009;Chen et al, 2010;Gutleben et al, 2019Gutleben et al, , 2020. These authors found that, under clear sky conditions, the net radiative effect, a combination of LW and SW interactions, results in maximum net heating located slightly below the maximum dust concentration level, and also a minor net heating observed near the surface.…”

Atmospheric radiative profiles during EUREC<sup>4</sup>A

“…ERA5 values are used above the highest level measured by each sonde to extend the observed soundings vertically to 0.1 hPa and account for the effect of high-altitude thermodynamic variability on the radiative cooling profiles below. To obtain the lower boundary condition, we linearly interpolate the ERA5 sea surface skin temperature (SST skin ) also at hourly and 0.25 • resolution (Hersbach et al, 2020) to the time, longitude, and latitude and where the sounding was launched.…”

“…Similarly, we do not directly represent the radiative effect of mineral dust aerosols. The dominant aerosol radiative effect in this region has been shown to result from the covariance of aerosols with water vapor such that aerosols tend to be associated with elevated moisture layers (Gutleben et al, 2019(Gutleben et al, , 2020. Dust aerosol layers are, moreover, more common in the summer than in winter (Lonitz et al, 2015).…”