Analyzing Cloud Droplet Spatial Tendencies on the Millimeter and Centimeter Scales in Stratocumulus Clouds.

Abstract:

Up until recent decades it was mostly accepted that droplet spacing within clouds was uniform and not important for calculating the growth of an ensemble of droplets. This must be viewed as tentative as evidence has shown that droplets tend to be clustered at the millimeter and centimeter scales, having direct consequences on microphysical parameterizations (where droplet clustering enhances both the collision kernel and collision efficiency). This research aims to measure droplet clustering within stratocumulus clouds to gain a better understanding of how cloud droplets interact on the smallest scales, along with environmental factors that influence the amount of clustering that is measured.

Cloud droplet-turbulent relationships are derived from the Peruvian stratocumulus deck sampled over 14 flights during the 2008 VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx) by the Center for interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Time stamps (at 10^-4 m spatial resolution) for when each cloud droplet was encountered were measured using the Artium Flight phase Doppler interferometer (PDI). Along with other meteorological variables measured, these data are used to investigate droplet clustering and turbulence within the stratocumuli, particularly, is there any correlation with the amount of clustering and (1) in-cloud height; (2) precipitating vs. non-precipitating regions of cloud; (3) aerosol number concentration, where the pair correlation function (PCF) is used to identify the scale of droplet clustering.

Preliminary results show, for a typical flight day, that the turbulence (turbulent kinetic energy (TKE), TKE dissipation rate) is at a maximum in the middle portion of the stratocumulus layer, where the Stokes number (which depends on TKE dissipation rate and drop size) is known to be correlated with clustering. It is hypothesized that (1) clustering should be enhanced in the middle portion of the cloud, in line with the enhanced turbulence that has been measured; (2) droplet clustering will be less for precipitating regions of cloud as compared to non-precipitating regions due to the fact that precipitation acts to stabilize the stratocumulus layer; (3) Droplet clustering and aerosol number concentration will be positively correlated.