Mixed-phase orographic cloud microphysics during StormVEx and IFRACS

Douglas H Lowenthal,Robert O David,Gerald G Mace,A Gannet Hallar,Randolph D Borys,Ian B Mccubbin

doi:10.5194/acp-19-5387-2019

Douglas H Lowenthal, Robert O David + Show 4 more

Open Access

https://doi.org/10.5194/acp-19-5387-2019

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Abstract

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.

Highlights

Aerosols and their effects on cloud microphysical properties have been shown to alter precipitation formation and distribution over complex terrain (e.g., Pruppacher and Klett, 1997; Borys et al, 2003; Rosenfeld and Givati, 2006; Lowenthal et al, 2011; Saleeby et al, 2013)
This paper examines microphysical properties of wintertime orographic mixed-phase orographic cloud (MPC) at Storm Peak Laboratory (SPL) using data collected during StormVEx and IFRACS
Studies of orographic MPCs were conducted at SPL in northwestern Colorado in January and February during StormVEx (2011) and IFRACS (2014)

Summary

Introduction

Aerosols and their effects on cloud microphysical properties have been shown to alter precipitation formation and distribution over complex terrain (e.g., Pruppacher and Klett, 1997; Borys et al, 2003; Rosenfeld and Givati, 2006; Lowenthal et al, 2011; Saleeby et al, 2013). Higher concentrations of cloud condensation nuclei (CCN) produce more numerous but smaller cloud droplets (Twomey et al, 1984; Peng et al, 2002; Lowenthal et al, 2002). This leads to decreased riming efficiency and decreased precipitation on the windward slope (Borys et al, 2000, 2003) and has been shown to redistribute precipitation over mountain barriers in modeling studies (Saleeby et al, 2009, 2013). Lowenthal et al.: Mixed-phase orographic cloud microphysics during StormVEx and IFRACS

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