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

Abstract

Abstract. To determine the impact of dynamic and aerosol processes on marine low clouds, we examine the seasonal impact of updraft speed w and cloud condensation nuclei concentration at 0.43 % supersaturation (NCCN0.43%) on the cloud droplet number concentration (NC) 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 NCCN0.43% loadings, and the density functions of NC and w near the cloud bases are compared. For low updrafts (w < 1.3 m s−1), NC in winter is mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts (w > 3 m s−1), NC is impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol-limited, and for high NCCN0.43% 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 NCCN0.43% 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 NCCN0.43% is accompanied with an increase of organic aerosol and sulfate compounds in both seasons. We demonstrate that NC can be explained by cloud condensation nuclei activation through upwards processed air masses with varying fractions of activated aerosols. The activation highly depends on w and thus supersaturation between the different seasons, while the aerosol size distribution additionally affects NC within a season. Our results quantify the seasonal influence of w and NCCN0.43% on NC and can be used to improve the representation of low marine clouds in models.

Spatially-coordinated airborne data and complementary products for aerosol, gas, cloud, and meteorological studies: The NASA ACTIVATE dataset

“…The stitched size distribution for 2D-S/FCDP was explained briefly in sect. 4.5 and described more extensively by Kirschler et al (2022). The 2D-S imagery in Fig.…”

“…The FCDP is a forward scattering probe with a rapid sampling rate of 25 ns to enable single particle detection for all particles. Its fast electronics and other features like a small pinhole for coincidence reduction imply lower uncertainties in particle sizing and counting (Baumgardner et al, 2017;Kirschler et al, 2022;Kleine et al, 2018;Knop et al, 2021;Voigt et al, 2021). Archived FCDP data include aerosol and droplet number size distributions, LWC, effective diameter, and median volume diameter.…”

“…Archived FCDP data include aerosol and droplet number size distributions, LWC, effective diameter, and median volume diameter. Extensive processing and corrections to the FCDP data are described in Kirschler et al (2022). Meanwhile, the CDP and CAS data have not been similarly corrected to date, which may introduce biases particularly for high cloud droplet number environments exceeding 500 cm -3 (Lance, 2012).…”

“…The recorded ensemble of 'slices' obtained rapidly by triggered photodiodes help generated 2D images of particles (Knollenberg, 1970). The 2D-S used on the Falcon is described in detail by Kirschler et al (2022), who note that with the fast response time of 41 ns, the 2D-S has less uncertainty for characterizing spheroids, and is in the middle of the range for ice particles, compared to other optical array probes (Baker and Gurganus and Lawson, 2018;Lawson and Baker, 2006;Bansmer et al, 2018). For data users interested in stitching together 2D-S size distributions with the other probes like FCDP, the method discussed by Kirschler et al (2022) is a suitable option to confront the overlap of the two probes between 16-51.3 μm.…”

Wintertime Synoptic Patterns of Midlatitude Boundary Layer Clouds Over the Western North Atlantic: Climatology and Insights From In Situ ACTIVATE Observations

“…More specifically, cloud droplet number concentration is computed from the droplet size distribution derived from the Fast Cloud Droplet Probe (FCDP, Knop et al., 2021). The FCDP, manufactured by SPEC Inc., is a forward scattering probe that measures droplet size distributions ranging from 1.5 to 50 μm (Kirschler et al., 2022). In addition, aerosol concentration in the range 90–7,500 nm is derived from the TSI Laser Spectrometer (LAS, Moore et al., 2021) model 3340.…”