Dependence of Precipitation Susceptibility in Marine Stratocumulus on Vertical Structure of Overlying Absorbing Aerosols: Results from ORACLES

Precipitation susceptibility (S_o) can be calculated using the measured droplet concentration (N_c) or an aerosol proxy for N_c such as the Aerosol Optical Depth or aerosol concentration (N_a). In addition, the value of precipitation rate (R) used to calculate S_o can be determined using measured liquid water path and N_c, radar reflectivity, or the number distribution function (n(D)) and droplet fall speed. This study examines the sensitivity of S_o to calculation techniques using observations of marine stratocumulus made over the South East Atlantic Ocean where biomass-burning aerosols overlaid the clouds with varying vertical distances.

The in‐situ measurements and remotely sensed retrievals of aerosol and cloud properties were obtained during the ObseRvations of Aerosols above Clouds and their intEractionS (ORACLES) field campaign in September 2016 and August 2017. A NASA P‐3 aircraft was equipped with a Cloud Droplet Probe (CDP) sizing particles between 2 and 50 μm, a Cloud and Aerosol Spectrometer (CAS) sizing between 0.51 and 50 μm, a 2D‐Stereo probe (2D-S), nominally sizing between 10 and 1280 μm, a Cloud Imaging Probe (CIP), from 25 to 1600 μm, and a High Volume Precipitation Sampler (HVPS‐3), from 150 μm to 1.92 cm for measuring n(D), along with a King hot-wire probe for measuring bulk liquid water content. An Ultra-High Sensitivity Aerosol Spectrometer (UHSAS) measured the aerosol n(D) for 0.06 < D < 1 μm and a Passive Cavity Aerosol Spectrometer Probe (PCASP) for 0.1 < D < 3 μm. The Airborne Precipitation and Cloud Radar Third Generation (APR-3) measured radar reflectivity using the Ku (14 GHz) and Ka (35 GHz) bands operating in scanning mode with a +/- 25° swath, and the W (95 GHz) band in scanning and nadir modes.

Forty cloud profiles flown during six ORACLES-2016 research flights were analyzed. For cases where a prominent aerosol layer with N_a > 100 cm^-3 was within 100 m above cloud top, there was an increase of 100 to 150 cm^-3 in N­_c along with a decrease of 1.5 to 2 µm in effective radius throughout the cloud deck, relative to cases without such a layer above cloud top, with little difference in average liquid water content between cases. In this study, the calculation of S_o using the ORACLES measurements is presented in order to determine the following: 1) the effects of different ways of determining R and N_c on S_o; and 2) the impact of different meteorological conditions on S_o as opposed to the effects of aerosol-induced changes in cloud microphysics associated with the mixing of the aerosol plume into the cloud deck. Consequently, the relative importance of aerosol and meteorological effects on precipitation susceptibility is determined.