Aerosol liquid water content in the moist southern West African monsoon layer and its radiative impact

Konrad Deetz, Heike Vogel, Sophie Haslett, Peter Knippertz, Hugh Coe, Bernhard Vogel
2018 Atmospheric Chemistry and Physics  
<p><strong>Abstract.</strong> Water uptake can significantly increase the size and therefore alters the optical properties of aerosols. In this study, the regional-scale model framework COSMO-ART is applied to southern West Africa (SWA) for a summer monsoon process study on 2–3 and 6–7 July 2016. The high moisture and aerosol burden in the monsoon layer makes SWA favorable to quantify properties that determine the aerosol liquid water content and its impact on radiative transfer. Given the
more » ... fer. Given the marked diurnal cycle in SWA, the analysis is separated into three characteristic phases: (a) the Atlantic inflow progression phase (15:00–02:00<span class="thinspace"></span>UTC), when winds from the Gulf of Guinea accelerate in the less turbulent evening and nighttime boundary layer, (b) the moist morning phase (03:00–08:00<span class="thinspace"></span>UTC), when the passage of the Atlantic inflow front leads to overall cool and moist conditions over land, and (c) the daytime drying phase (09:00–15:00<span class="thinspace"></span>UTC), in which the Atlantic inflow front reestablishes with the inland heating initiated after sunrise. This diurnal cycle also impacts, via relative humidity, the aerosol liquid water content. We analyzed the impact of relative humidity and clouds on the aerosol liquid water content. As shown by other studies, accumulation-mode particles are the dominant contributor of aerosol liquid water. We find aerosol growth factors of 2 (4) for submicron (coarse-mode) particles, leading to a substantial increase in mean aerosol optical depth from 0.2 to 0.7. Considering the aerosol liquid water content leads to a decrease in shortwave radiation of about 20<span class="thinspace"></span>W<span class="thinspace"></span>m<span class="inline-formula"><sup>−2</sup></span>, while longwave effects appear to be insignificant, especially during nighttime. The estimated relationships between total column aerosol liquid water and radiation are <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">305</mn><mo>±</mo><mn mathvariant="normal">39</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="ad1727355ada26fa5f9ca13291c0575c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-14271-2018-ie00001.svg" width="52pt" height="10pt" src="acp-18-14271-2018-ie00001.png"/></svg:svg></span></span><span class="thinspace"></span>W<span class="thinspace"></span>g<span class="inline-formula"><sup>−1</sup></span> (shortwave in-cloud), <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">114</mn><mo>±</mo><mn mathvariant="normal">42</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="f142b242062d9bb6115a1e81fae4b77d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-14271-2018-ie00002.svg" width="52pt" height="10pt" src="acp-18-14271-2018-ie00002.png"/></svg:svg></span></span><span class="thinspace"></span>W<span class="thinspace"></span>g<span class="inline-formula"><sup>−1</sup></span> (shortwave off-cloud) and about <span class="inline-formula">−10</span><span class="thinspace"></span>W<span class="thinspace"></span>g<span class="inline-formula"><sup>−1</sup></span> (longwave). The results highlight the need to consider the relative humidity dependency of aerosol optical depth in atmospheric models, particularly in moist tropical environments where their effect on radiation can be very large.</p>
doi:10.5194/acp-18-14271-2018 fatcat:a5x7y6k5szbc7i3ykfycjprcau