Characteristic Vertical Profiles of Cloud Water Composition in Marine Stratocumulus Clouds and Relationships With Precipitation. Issue 7 (13th April 2018) - MacDonald, Alexander B.
Abstract: This study uses airborne cloud water composition measurements to characterize the vertical structure of air‐equivalent mass concentrations of water‐soluble species in marine stratocumulus clouds off the California coast. A total of 385 cloud water samples were collected in the months of July and August between 2011 and 2016 and analyzed for water‐soluble ionic and elemental composition. Three characteristic profiles emerge: (i) a reduction of concentration with in‐cloud altitude for particulate species directly emitted from sources below cloud without in‐cloud sources (e.g., Cl − and Na + ), (ii) an increase of concentration with in‐cloud altitude (e.g., NO2 − and formate), and (iii) species exhibiting a peak in concentration in the middle of cloud (e.g., non–sea‐salt SO4 2−, NO3 −, and organic acids). Vertical profiles of rainout parameters such as loss frequency, lifetime, and change in concentration with respect to time show that the scavenging efficiency throughout the cloud depth depends strongly on the thickness of the cloud. Thin clouds exhibit a greater scavenging loss frequency at cloud top, while thick clouds have a greater scavenging loss frequency at cloud base. The implications of these results for treatment of wet scavenging in models are discussed. Key Points: Three characteristic in‐cloud vertical concentration profiles are identified: those that peak at the base, middle, and top of cloud Nonreactive surface‐derived species present the in‐cloud vertical concentration profiles that are most influenced by rainout Rainout loss frequency is greatest at cloud base for moderate‐drizzling thick clouds, and at cloud top for light‐drizzling thin clouds