Dependence of Cloud and Precipitation Properties over the South-East Atlantic on Aerosol Concentrations Above and Below Clouds
Authors: Gupta Siddhant , McFarquhar Greg , O'Brien Joseph , Poellot Michael , Dzambo Andrew 
Published: March 2020

&lt;p&gt;Instances of contact and separation between biomass-burning aerosols and marine stratocumulus have been observed over the South-East Atlantic Ocean. In-situ measurements of aerosol and cloud properties were made onboard the NASA P-3 aircraft during the ObseRvations of Aerosols above Clouds and their intEractionS ORACLES field campaign in Sep. 2016, Aug. 2017 and Oct. 2018. Variations in vertical profiles of droplet concentration N&lt;sub&gt;c&lt;/sub&gt;, effective radius R&lt;sub&gt;e&lt;/sub&gt;, and precipitation susceptibility S&lt;sub&gt;o&lt;/sub&gt; (change in rain rate R with N&lt;sub&gt;c&lt;/sub&gt;) as a function of cloud thickness H, were determined for varying above- and below-cloud aerosol concentration N&lt;sub&gt;a&lt;/sub&gt;. The data were sorted into 4 regimes according to whether a prominent aerosol layer (N&lt;sub&gt;a&lt;/sub&gt; &gt; 500 cm&lt;sup&gt;-3&lt;/sup&gt;) was in contact or separated from cloud tops, and whether the clouds occurred within a clean or dirty boundary layer (BL).&lt;/p&gt;&lt;p&gt;The N&lt;sub&gt;c&lt;/sub&gt; and R&lt;sub&gt;e&lt;/sub&gt; were calculated using the number distribution function n(D) for 3 &lt; D &lt; 1280 &amp;#956;m from the Cloud and Aerosol Spectrometer and the 2D&amp;#8208;Stereo probe. N&lt;sub&gt;a&lt;/sub&gt; was calculated using n(D) for 0.1 &lt; D &lt; 3 &amp;#956;m from the Passive Cavity Aerosol Spectrometer Probe, and rain rate R was derived using droplet mass m(D), fall speed u(D) and n(D) for 50 &lt; D &lt; 1280 &amp;#956;m. Across the 3 campaigns, a total of 359 vertical profiles were flown through clouds and for 181 contact, or C-cases (N&lt;sub&gt;a&lt;/sub&gt; &gt; 500 cm&lt;sup&gt;-3&lt;/sup&gt; within 100 m above cloud), the mean N&lt;sub&gt;c&lt;/sub&gt; was 77 cm&lt;sup&gt;-3&lt;/sup&gt; higher and mean R&lt;sub&gt;e&lt;/sub&gt; 1.78 &amp;#956;m lower, compared to 178 separated, or S-cases (N&lt;sub&gt;a&lt;/sub&gt; &lt; 500 cm&lt;sup&gt;-3&lt;/sup&gt; within 100 m above cloud). Within clean BLs (N&lt;sub&gt;a&lt;/sub&gt; &lt; 400 cm&lt;sup&gt;-3&lt;/sup&gt; within 100 m below cloud), mean N&lt;sub&gt;c&lt;/sub&gt; for C-cases was 42% higher than S-cases. Within dirty BLs (N&lt;sub&gt;a&lt;/sub&gt; &gt; 400 cm&lt;sup&gt;-3&lt;/sup&gt; within 100 m below cloud), mean N&lt;sub&gt;c&lt;/sub&gt; for C-cases was 53% higher, and the N&lt;sub&gt;c&lt;/sub&gt; change increased from 20% near cloud base to 75% near cloud top. Although cloud-top entrainment increased N&lt;sub&gt;c&lt;/sub&gt; throughout the cloud layer, its effect was weaker (stronger) near cloud base (top) within dirty BLs.&lt;/p&gt;&lt;p&gt;For all data combined, the average S&lt;sub&gt;o&lt;/sub&gt; was positive (0.94) implying R decreased with an increase in N&lt;sub&gt;c&lt;/sub&gt;. For C- and S-cases, the average S&lt;sub&gt;o&lt;/sub&gt; was 0.60 and 1.01, respectively, with the difference arising from low S&lt;sub&gt;o&lt;/sub&gt; (-0.06) for thin (H &lt; 131 m) C-case clouds. When the thin, C-case clouds were further classified into clean &amp; dirty BL cases, S&lt;sub&gt;o&lt;/sub&gt; was 0.67 and -0.57, respectively. Condensational growth is limited by low H in thin clouds and increasing N&lt;sub&gt;c&lt;/sub&gt; in clean conditions decreased R&lt;sub&gt;e&lt;/sub&gt;, hindering collision-coalescence. However, in dirty conditions with low R&lt;sub&gt;e&lt;/sub&gt;, increasing N&lt;sub&gt;c&lt;/sub&gt; only increased the number and collision efficiency of small droplets. The mean N&lt;sub&gt;c&lt;/sub&gt; (average S&amp;#173;&lt;sub&gt;o&lt;/sub&gt;) in the cleanest and dirtiest conditions (S-case &amp; clean BL and C-case &amp; dirty BL) was 84 cm&lt;sup&gt;-3&lt;/sup&gt; and 206 cm&lt;sup&gt;-3&lt;/sup&gt; (1.17 and 0.43), respectively. When condensational growth is no longer limited by H, R&lt;sub&gt;e&lt;/sub&gt; decreases with N&lt;sub&gt;c&lt;/sub&gt; and clean clouds with the highest R&lt;sub&gt;e&lt;/sub&gt; are the most susceptible to precipitation suppression.&lt;/p&gt;