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

Abstract

Abstract. Low level clouds over the Western North Atlantic show a seasonal cycle in cloud properties which anticorrelates to aerosol concentrations. To determinate the impact of dynamic and aerosol processes within 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 in 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 are mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts (w > 3 m s-1), NC are impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol limited and for high pollution 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 pollution levels due to altering processes, whereas unimodal distributions prevail in summer with a significant difference in their aerosol concentration and composition. The increase in pollution level 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.

Dimethylamine in cloud water: a case study over the northwest Atlantic Ocean

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Icing Wind Tunnel Measurements of Supercooled Large Droplets Using the 12 mm Total Water Content Cone of the Nevzorov Probe

“…The CCP used at Collins was provided by Embraer. For the measurements at RTA we use DSDs derived from data of an FCDP (Glienke and Mei, 2020;Kirschler et al, 2022), a 2D-S (Lawson et al, 2006), a PIP (Baumgardner et al, 2017), a CAPS that was provided by DLR and a Malvern Spraytec probe provided by the tunnel operator (Ferschitz et al, 2017).…”

Organic enrichment in droplet residual particles relative to out of cloud over the northwestern Atlantic: analysis of airborne ACTIVATE data

“…AMS measurements were conducted downstream of an isokinetic double diffuser inlet (Brechtel Manufacturing Inc.) in cloudfree conditions and downstream of a counterflow virtual impactor (CVI) inlet (Brechtel Manufacturing Inc.) in clouds (Shingler et al, 2012). For classification of data as cloud and cloud-free, we use a liquid water content (LWC) threshold of 0.05 g m −3 based on data from the Fast Cloud Droplet Probe (FCDP; D p ∼ 3-50 µm) (SPEC Inc.; Kirschler et al, 2022). This LWC threshold has been used in recent work using ACTIVATE data (Dadashazar et al, 2021a).…”

“…We determine whether flights occurred during cold air outbreaks (CAOs) leveraging methods in recent ACTIVATE studies (Seethala et al, 2021;Corral et al, 2022). Briefly, Visible Infrared Imaging Radiometer Suite (VIIRS) imagery (NASA Worldview) is used to visually identify cloud streets that are characteristic of CAOs.…”

“…9), which included two consecutive flights (RFs 17 and 18) based out of Hampton, Virginia profiling aerosol and cloud properties in CAO conditions. These two flights were investigated in past work showing enhanced new particle formation in ACT legs (Corral et al, 2022) and that entrainment of free tropospheric air dilutes MBL CCN concentrations (Tornow et al, 2022). The other four flights (Fig.…”