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Mixed-phase orographic cloud microphysics during StormVEx and IFRACS

Citation

Lowenthal, Douglas et al. (2021), Mixed-phase orographic cloud microphysics during StormVEx and IFRACS, Dryad, Dataset, https://doi.org/10.5061/dryad.b5mkkwhd2

Abstract

Wintertime mixed-phase orographic cloud (MPC) measurements were conducted at the Storm Peak Laboratory (SPL) during the Storm Peak Lab Cloud Property Validation Experiment (StormVEx) and Isotopic Fractionation in Snow (IFRACS) programs in 2011 and 2014, respectively. The data include 92 h of simultaneous measurements of supercooled liquid cloud droplet and ice particle size distributions (PSDs). Average cloud droplet number concentration (CDNC), droplet size (NMD), and liquid water content (LWC) were similar in both years, while ice particle concentration (Ni) and ice water content (IWC) were higher during IFRACS. The consistency of the liquid cloud suggests that SPL is essentially a cloud chamber that produces a consistent cloud under moist, westerly flow during the winter. A variable cloud condensation nuclei (CCN)-related inverse relationship between CDNC and NMD strengthened when the data were stratified by LWC. Some of this variation is due to changes in cloud base height below SPL. While there was a weak inverse correlation between LWC and IWC in the data as a whole, a stronger relationship was demonstrated for a case study on 9 February 2014 during IFRACS. A minimum LWC of 0.05 g m−3 showed that the cloud was not completely glaciated on this day. Erosion of the droplet distribution at high IWC was attributed to the Wegener–Bergeron–Findeisen process as the high IWC was accompanied by a 10-fold increase in Ni. A relationship between large cloud droplet concentration (25–35 µm) and small ice particles (75–200 µm) under cold (<-8 ∘C) but not warm (>-8 ∘C) conditions during IFRACS suggests primary ice particle production by contact or immersion freezing. The effect of blowing snow was evaluated from the relationship between wind speed and Ni and by comparing the relative (percent) ice particle PSDs at high and low wind speeds. These were similar, contrary to expectation for blowing snow. However, the correlation between wind speed and ice crystal concentration may support this explanation for high crystal concentrations at the surface. Secondary processes could have contributed to high crystal concentrations but there was no direct evidence to support this. Further experimental work is needed to resolve these issues.

Methods

The Storm Peak Lab Cloud Property Validation Experiment (StormVEx) was conducted from 15 November 2010 to 25 April 2011 at the Desert Research Institute’s (DRI) Storm Peak Laboratory (SPL). The Isotopic Fractionation in Snow (IFRACS) study was conducted at SPL from 20 January to 27 February 2014. Cloud droplet number concentrations and particle size distributions (PSDs) were measured with an aspirated Particle Measurement Systems (PMS), Inc. (Boulder, CO), FSSP-100 forward scattering spectrometer probe that was electronically modified by Droplet Measurement Technologies (DMT), Inc. (Boulder, CO). Ice particle PSDs were measured with a DMT CIP (Cloud Imaging Probe) optical array probe (OAP).  The 2-D CIP images from StormVEx and IFRACS were processed using the Optical Array Shadow Imaging Software (OASIS) program developed at the University of Manchester and marketed by DMT (http://www.dropletmeasurement.com/optical-array-shadow-imaging-software-oasis). Snow and super-cooled cloud water samples were collected in bags and on cloud sieves and analyzed for their water stable isotopic composition. Water vapor concentration and isotopic composition were measured during IFRACS with a Picarro L2130-i water vapor isotopic analyzer. Meteorological variables were measured continuously with the SPL weather station.

Funding

U.S. Department of Energy, Award: DE-SC0014304

National Science Foundation, Award: AGS-1260462