Abstract
Postfrontal cloud regime transitions often result from strong aerosol‐cloud‐precipitation interaction. Using ACTIVATE measurements from 40 flights, we show a diurnal pattern of evolving aerosol particle size distributions (PSD) in the marine boundary layer upwind of clouds. A nucleation mode grows in size between 12 and 17 LT, likely from new particle formation (NPF), and an accumulation mode increases in concentration during daytime. Selecting the day of strongest size growth, we use quasi‐Lagrangian large‐eddy simulations to examine cloud impacts. With a morning PSD, moderate aerosol hygroscopicity aligns best with satellite and field campaign observational targets. Using a PSD near 17 LT, greater cloud droplet number concentrations delay the precipitation onset and keep albedo elevated for longer, primarily driven by increased accumulation mode concentrations, which facilitate an updraft‐limited regime and inhibit activation of smaller modes despite their growth after NPF. These results constitute a caveat for NPF‐related activation under polluted conditions.
Cloud breakup in the postfrontal sector of extratropical cyclones is often the result of precipitation. Once precipitating, the marine boundary layer (MBL) stabilizes as partial evaporation of precipitation cools and moistens the sub‐cloud layer. In addition, aerosol available as cloud condensation nuclei decreases in number during precipitation‐forming collisional events. Here we use multiple years of flight measurements collected over the Northwest Atlantic. Inside the MBL we reveal a diurnal pattern of evolving aerosol properties before clouds are forming: very small particles (diameters smaller 10 nm), so called nucleation mode aerosol, grow in diameter over the course of each day while medium‐sized particles (diameters greater 80 nm), so called accumulation mode aerosol, increase in number. In order to understand if grown nucleation mode particles may impact clouds, we perform large‐eddy simulations that follow an MBL air mass. Using aerosol prior to growth, we find that their ability to attach condensate is generally moderate, evidenced by a better agreement to field and satellite data. When switching to the time period after aerosol growth, simulations show brighter clouds that begin precipitating later, which we attribute to increased accumulation mode concentrations that are numerous enough to inhibit activation of grown nucleation mode aerosol.
Upwind of postfrontal clouds we identify a daytime increase in nucleation mode aerosol diameter and accumulation mode number concentration Observational constraints based on ACTIVATE aircraft data and satellite retrievals indicate a moderate aerosol hygroscopicity ( = 0.3) Increased afternoon accumulation mode concentrations facilitate an updraft‐limited regime that curbs activation of Aitken mode aerosol