Observing short-timescale cloud development to constrain aerosol–cloud interactions

<p>The aerosol impact on liquid water path (LWP) is a key uncertainty in the overall climate impact of aerosol. However, despite a significant effort in this area, the size of the effect remains poorly constrained, and even the sign is unclear. Recent studies have shown that the relationship between droplet number concentration (<span class="inline-formula"><i>N</i>_d</span>) and LWP is an unreliable measure of the impact of <span class="inline-formula"><i>N</i>_d</span> variations on LWP due to the difficulty in establishing causality. In this work, we use satellite observations of the short-term development of clouds to examine the role of <span class="inline-formula"><i>N</i>_d</span> perturbations in LWP variations.</p> <p>Similar to previous studies, an increase followed by a general decrease in LWP with increasing <span class="inline-formula"><i>N</i>_d</span> is observed, suggesting an overall negative LWP response to <span class="inline-formula"><i>N</i>_d</span> and a warming LWP adjustment to aerosol. However, the <span class="inline-formula"><i>N</i>_d</span> also responds to the local environment, with aerosol production, entrainment from the free troposphere and wet scavenging all acting to modify the <span class="inline-formula"><i>N</i>_d</span>. Many of these effects act to further steepen the <span class="inline-formula"><i>N</i>_d</span>–LWP relationship and obscure the causal <span class="inline-formula"><i>N</i>_d</span> impact on LWP.</p> <p>Using the temporal development of clouds to account for these feedbacks in the <span class="inline-formula"><i>N</i>_d</span>–LWP system, a weaker negative <span class="inline-formula"><i>N</i>_d</span>–LWP relationship is observed over most of the globe. This relationship is highly sensitive to the initial cloud state, illuminating the roles of different processes in shaping the <span class="inline-formula"><i>N</i>_d</span>–LWP relationship. The nature of the current observing system limits this work to a single time period for observations, highlighting the need for more frequent observations of key cloud properties to constrain cloud behaviour at process timescales.</p>

Abstract licence: CC BY

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