Mixed-Phase Cloud Microphysics during StormVEx and IFRACS

 

Authors

Douglas Lowenthal — Desert Research Institute
Gannet Hallar — University of Utah
Ian B. McCubbin — Desert Research Institute

Category

Microphysics (cloud, aerosol and/or precipitation)

Description

The Storm Peak Lab Cloud Property Validation Experiment (StormVEx) was conducted from November, 2010 to April, 2011 (Marchand et al., 2013). The Isotopic Fractionation in Snow (IFRACS) study was also conducted at Storm Peak Laboratory (SPL) from January to February, 2014 (Lowenthal et al., 2016). Cloud microphysical observations from these studies were used to better understand snow formation mechanisms in mixed-phase orographic clouds. Cloud microphysical measurements were made using the same instruments during StormVEx and IFRACS. Cloud droplet particle size distributions (PSDs) from 2-47 µm were measured with an aspirated Droplet Measurement Technologies (DMT), Inc. SPP-100 forward scattering spectrometer probe. Ice particle PSDs were measured with a DMT CIP (Cloud Imaging Probe [25-1600 µm]). The cloud probes were calibrated and serviced prior to each field campaign. The cloud probes and sonic anemometers were mounted on a rotating wind vane (to orient them into the wind) located on the west (upwind) railing of the roof approximately 6 m above the snow surface. High concentrations of small ice were observed during both field campaigns, compared with those observed aloft. Different mechanisms were explored to explain the concentrations observed. First, the hypothesis of blowing snow was investigated and dismissed, as there was no relative enhancement of small ice crystals at higher compared to lower wind speed. Next, mechanisms for enhanced ice nucleation near the surface were investigated. At SPL, temperatures less than -15°C are rarely observed, thus deposition nucleation is most likely not active. Several studies have reported liquid-dependent ice formation in mixed-phase clouds with proposed mechanisms including contact and immersion freezing (e.g., Lance et al., 2011). We will present relationships between large cloud droplet concentration and small ice particle concentration, segregated by temperature. This segregation is meant to contrast colder temperature process such as immersion freezing from warmer temperature mechanisms such as droplet shattering during riming which may occur at temperatures between -3 and -8°C (Hallett and Mossop, 1974) or the involvement of biogenic IN. Moderate to strong relationships were observed between large drops and small ice particles at low temperature. Whereas, there was a moderate correlation at warmer temperatures.