Abstract
Abstract. Aerosol size distribution has major effects on warm cloud processes. Here, we use newly acquired marine aerosol size distributions (MSDs), measured in situ over the open ocean during the Tara Pacific expedition (2016–2018), to examine how the total aerosol concentration (Ntot) and the shape of the MSDs change warm clouds' properties. For this, we used a toy model with detailed bin microphysics initialized using three different atmospheric profiles, supporting the formation of shallow to intermediate and deeper warm clouds. The changes in the MSDs affected the clouds' total mass and surface precipitation. In general, the clouds showed higher sensitivity to changes in Ntot than to changes in the MSD's shape, except for the case where the MSD contained giant and ultragiant cloud condensation nuclei (GCCN, UGCCN). For increased Ntot (for the deep and intermediate profiles), most of the MSDs drove an expected non-monotonic trend of mass and precipitation (the shallow clouds showed only the decreasing part of the curves with mass and precipitation monotonically decreasing). The addition of GCCN and UGCCN drastically changed the non-monotonic trend, such that surface rain saturated and the mass monotonically increased with Ntot. GCCN and UGCCN changed the interplay between the microphysical processes by triggering an early initiation of collision–coalescence. The early fallout of drizzle in those cases enhanced the evaporation below the cloud base. Testing the sensitivity of rain yield to GCCN and UGCCN revealed an enhancement of surface rain upon the addition of larger particles to the MSD, up to a certain particle size, when the addition of larger particles resulted in rain suppression. This finding suggests a physical lower bound can be defined for the size ranges of GCCN and UGCCN.
Highlights
Clouds play a key role in the Earth’s climate system
A Nafion dryer was installed before the SMPSOPC, which reduced the sampled air relative humidity (RH) to below ∼ 35 %, below the efflorescence point for NaCl (Gupta et al, 2015); we considered Dp to be dry
We propose that the threshold diameter for which the surface rain yield is enhanced be defined as the lower bound for GCCN, and the threshold diameter for which surface rain begins to be suppressed as the lower bound for UGCCN
Summary
Clouds play a key role in the Earth’s climate system. By scattering and absorbing solar and terrestrial radiation, clouds influence the radiative balance. The study of giant CCN (GCCN) and ultragiant CCN (UGCCN) and their effects on warm clouds and precipitation has been the subject of various works (Beard and Ochs III, 1993; Feingold et al, 1999; Khain et al, 2000; Yin et al, 2000b; Dagan et al, 2015a). Their size definition is loose, as the lower threshold of GCCN has been defined within a wide range of mean particle diameter (Dp) of 2–10 μm (Feingold et al, 1999; Yin et al, 2000a), while particles with Dp > 20 μm are usually defined as UGCCN
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