Seasonal updraft speeds change cloud droplet number concentrations in low-level clouds over the western North Atlantic

<p>To determine the impact of dynamic and aerosol processes on marine low clouds, we examine the seasonal impact of updraft speed <span class="inline-formula"><i>w</i></span> and cloud condensation nuclei concentration at 0.43 % supersaturation (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mrow><msub><mtext>CCN</mtext><mrow><mn mathvariant="normal">0.43</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="italic">%</mi></mrow></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="faa6509b9a168c2d20b916afe5c73f85"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-8299-2022-ie00001.svg" width="46pt" height="14pt" src="acp-22-8299-2022-ie00001.png"/></svg:svg></span></span>) on the cloud droplet number concentration (<span class="inline-formula"><i>N</i>_C</span>) of low-level clouds over the western North Atlantic Ocean. Aerosol and cloud properties were measured with instruments on board the NASA LaRC Falcon HU-25 during the ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment) mission in summer (August) and winter (February–March) 2020. The data are grouped into different <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mrow><msub><mtext>CCN</mtext><mrow><mn mathvariant="normal">0.43</mn><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="italic">%</mi></mrow></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="d077d2e88cd5e3c72e44184910e391ad"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-8299-2022-ie00002.svg" width="46pt" height="14pt" src="acp-22-8299-2022-ie00002.png"/></svg:svg></span></span> loadings, and the density functions of <span class="inline-formula"><i>N</i>_C</span> and <span class="inline-formula"><i>w</i></span> near the cloud bases are compared. For low updrafts (<span class="inline-formula"><i>w</i></span> <span class="inline-formula">&lt;</span> 1.3 <span class="inline-formula">m s⁻¹</span>), <span class="inline-formula"><i>N</i>_C</span> in winter is mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts (<span class="inline-formula"><i>w</i></span> <span class="inline-formula">&gt;</span> 3 <span class="inline-formula">m s⁻¹</span>), <span class="inline-formula"><i>N</i>_C</span> is impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol-limited, and for high <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mrow><msub><mtext>CCN</mtext><mrow><mn mathvariant="normal">0.43</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="italic">%</mi></mrow></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="61b3e4ac0d33593da7f02f125a825f99"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-8299-2022-ie00003.svg" width="46pt" height="14pt" src="acp-22-8299-2022-ie00003.png"/></svg:svg></span></span> it is influenced by aerosols and updraft. The aerosol size distribution in winter shows a bimodal distribution in clean marine environments, which transforms to a unimodal distribution in high <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mrow><msub><mtext>CCN</mtext><mrow><mn mathvariant="normal">0.43</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="italic">%</mi></mrow></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1c63eef11f66c585b17f92286e8b12d4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-8299-2022-ie00004.svg" width="46pt" height="14pt" src="acp-22-8299-2022-ie00004.png"/></svg:svg></span></span> due to chemical and physical aerosol processes, whereas unimodal distributions prevail in summer, with a significant difference in their aerosol concentration and composition. The increase of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mrow><msub><mtext>CCN</mtext><mrow><mn mathvariant="normal">0.43</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="italic">%</mi></mrow></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e78540cb8e68457fd53ee7d6295f3936"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-8299-2022-ie00005.svg" width="46pt" height="14pt" src="acp-22-8299-2022-ie00005.png"/></svg:svg></span></span> is accompanied with an increase of organic aerosol and sulfate compounds in both seasons. We demonstrate that <span class="inline-formula"><i>N</i>_C</span> can be explained by cloud condensation nuclei activation through upwards processed air masses with varying fractions of activated aerosols. The activation highly depends on <span class="inline-formula"><i>w</i></span> and thus supersaturation between the different seasons, while the aerosol size distribution additionally affects <span class="inline-formula"><i>N</i>_C</span> within a season. Our results quantify the seasonal influence of <span class="inline-formula"><i>w</i></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>N</mi><mrow><msub><mtext>CCN</mtext><mrow><mn mathvariant="normal">0.43</mn><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="italic">%</mi></mrow></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="c40e2f911b4adbbc557f459c3ccfb9d5"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-8299-2022-ie00006.svg" width="46pt" height="14pt" src="acp-22-8299-2022-ie00006.png"/></svg:svg></span></span> on <span class="inline-formula"><i>N</i>_C</span> and can be used to improve the representation of low marine clouds in models.</p>

Abstract licence: CC BY

A person counts once a day per paper. We keep no cookie and no address: a reader is a hash that changes every day. Any month under five reads shows no number.