Bimodal cloud droplet distributions in marine stratus clouds observed in VOCALS 2008
Author
Gunnar I. Senum — Brookhaven National Laboratory
Category
Cloud Properties
Description
A bimodal cloud droplet distribution, the small-sized distribution at 13.9 µ (diameter) and the larger at 18.9 µ. Both have a relative dispersion of about 0.07. This a 100-meter long sample taken on 10/18/2008 near cloud top at 1320 feet at -18.5 lat and -75.3 long. The LWC was 0.54 g/m3. The bimodal distribution regions of the marine stratus clouds extend up to 12 km in spatial extent.
The spectral or relative dispersion (ε) of cloud droplet distributions affects the formation of precipitation, cloud lifetime, and cloud albedo. Presently there are many differing models relating the relative dispersion to these cloud microphysical properties, but none with bimodal distributions. Bimodal cloud droplet distributions (as shown in the figure) were observed in many instances in the marine stratus clouds in the east Pacific off the coast of Chile during the VOCALS 2008 campaign. These bimodal distributions were measured by a CAPS probe with an improved particle-by-particle (PbP) feature. This allowed the size of every cloud droplet to be measured, supplanting the binned data channel of the CAPS probe. The bimodal cloud droplet distributions were discovered by the increased PbP cloud droplet sizing resolution.
These bimodal cloud droplet distributions tended to be present in low turbulence and minimal vertical velocity regions, that is, acquiescent regions, of the marine stratus clouds. These bimodal distributions had individual relative dispersions of about 0.07 in the droplet size maxima distribution. The overall dispersion was more than twice that of the individual peaks. They were observed near cloud top. As the liquid water content (LWC) increases, the smaller-sized distribution decreases in cloud particle number and larger increases until at some LWC, the smaller distribution completely disappears. The opposite occurs with decreasing LWC. The smaller cloud droplet diameter ranged from 13 to 17 µ and the larger from 18 to 23 µ. They also were seen only in regions with LWC between 0.3 and 0.6 g/m3. Other microphysical properties also play a role in the formation and properties of these bimodal distributions, such as interstial aerosol concentration and drizzle.
The formation of these bimodal distributions has been thought to be caused by collisional coalescence and/or entrainment and mixing processes in marine stratus clouds. Further work will examine the various models to see if these bimodal distributions significantly change the formation of precipitation, cloud lifetime, and albedo of marine stratus clouds compared to the previous assumed unimodal cloud particle distributions.
