An Investigation of the Influence of Droplet Number Concentration and Giant Aerosol Particles upon Supercooled Large Drop Formation in Wintertime Stratiform Clouds

Abstract

Supercooled large drops (SLD) can be a significant hazard for aviation. Past studies have shown that warm-rain processes are prevalent, or even dominant, in stratiform clouds containing SLD, but the primary factors that control SLD production are still not well understood. Giant aerosol particles have been shown to accelerate the formation of the first drizzle drops in some clouds and thus are a viable source of SLD, but observational support for testing their effectiveness in supercooled stratiform clouds has been lacking. In this study, new observations collected during six research flights from the Alliance Icing Research Study II (AIRS II) are analyzed to assess the factors that may be relevant to SLD formation, with a particular emphasis on the importance of giant aerosol particles. An initial comparison of observed giant aerosol particle number concentrations with the observed SLD suggests that they were present in sufficient numbers to be the source of the SLD. However, microphysical calculations within an adiabatic parcel model, initialized with the observed aerosol distributions and cloud properties, suggest that the giant aerosol particles were only a limited source of SLD. More SLD was produced in the modeled clouds with low droplet concentrations, simply by an efficient warm-rain process acting at temperatures below 0°C. For cases in which the warm-rain process is limited by a higher droplet concentration and small cloud depth/liquid water content, the giant aerosol particles were then the only source of SLD. The modeling results are consistent with the observed trends in SLD across the six AIRS II cases.

A New Three-Dimensional Visualization System for Combining Aircraft and Radar Data and Its Application to RICO Observations

“…Previous analyses of the RICO radar data by Knight et al (2008) and Reiche and Lasher-Trapp (2010) noted that Z e_max was often not collocated with Z DR_max in the RICO data. In a study of Florida cumuli, Knight et al (2002) speculated that separation of the greatest Z e and Z DR signals in the cloud FIG.…”

“…may result from larger drops appearing in weaker areas of clouds before coalescence begins in earnest, in regions often indistinguishable from Bragg scattering. Knight et al (2008) referred to this phenomenon in the RICO data as size sorting, where the biggest drops fall out of the cloud faster because they have reached sizes too large to be carried higher in the cloud updrafts along with the smaller drops. A possible source of these earliest, very large drops is giant aerosol particles (GA) such as sea salt (e.g., Woodcock 1953;de Leeuw 1986;Lewis and Schwartz 2004), which have diameters exceeding 2 mm and thus enter the cloud as larger deliquesced drops than those having formed on smaller cloud condensation nuclei (CCN).…”

“…A possible source of these earliest, very large drops is giant aerosol particles (GA) such as sea salt (e.g., Woodcock 1953;de Leeuw 1986;Lewis and Schwartz 2004), which have diameters exceeding 2 mm and thus enter the cloud as larger deliquesced drops than those having formed on smaller cloud condensation nuclei (CCN). Several modeling studies have suggested that GA can accelerate the formation of raindrops in cumuli because GA need little or no condensational growth prior to coalescence onset (e.g., Johnson 1982;Laird et al 2000;Lasher-Trapp et al 2001;Knight et al 2002;Blyth et al 2003). However, because smaller droplet number concentrations are present in maritime clouds, raindrop formation can occur quite quickly, even without the aid of GA (Feingold et al 1999;Lasher-Trapp et al 2008;Reiche and Lasher-Trapp 2010), leaving the importance of GA to precipitation formation in maritime clouds questionable.…”

“…Several modeling studies have suggested that GA can accelerate the formation of raindrops in cumuli because GA need little or no condensational growth prior to coalescence onset (e.g., Johnson 1982;Laird et al 2000;Lasher-Trapp et al 2001;Knight et al 2002;Blyth et al 2003). However, because smaller droplet number concentrations are present in maritime clouds, raindrop formation can occur quite quickly, even without the aid of GA (Feingold et al 1999;Lasher-Trapp et al 2008;Reiche and Lasher-Trapp 2010), leaving the importance of GA to precipitation formation in maritime clouds questionable. Observational studies are also in conflict regarding the importance of GA. Blyth et al (2003) found GA were necessary to explain the early radar echo in small Florida cumuli over a range of CCN number concentrations, but other observational studies have shown that low CCN number concentrations may be the explanation for the quick development of precipitation in maritime clouds (Hudson and Yum 2001;Coló n-Robles et al 2006;Gö ke et al 2007;Hudson et al 2009).…”

“…For example, the half-power beamwidth of the S-PolKa radar used in the RICO field campaign is 0.918 (Keeler et al 2000), with 150-m range resolution between samples. The vertical resolution of the radar data and the temporal sampling rate depend upon the scanning strategy used; during RICO, this was typically 3-4 min for a full volume scan of a 908 sector (Knight et al 2008), resulting in 4-8 files per volume scan as large as 3-6 MB each. Although useful in their own right, separate 2D analysis software packages force an investigator to spend significant (and tedious) amounts of time trying to collocate the data and synthesize it into a conceptual picture before being able to use it to test hypotheses.…”