Measurements and Analysis of Ice Nuclei Relevant to West Coast U.S. Precipitation

Abstract

Improved understanding of the interactions of natural and human-produced aerosols with clouds, and ultimately their influences on precipitation and climate, is a key science need, and a goal of the DOE ASR program. This study focuses on characterizing the numbers and types of ice nucleating particles (INPs), the rare particles in air that possess properties to trigger the formation of ice crystals in cold clouds, which are the initial seeds for wintertime snowfall and for rainfall in many parts of the Earth (in the absence INPs, clouds can supercool by 38 degrees C below freezing). Thus, these particles can determine the phase of cold clouds as being predominately liquid or ice, impacting their properties to scatter light, their lifetimes, and their precipitation development. This study seeks to analyze data collected in field studies to advance numerical descriptions of INP numbers, types, and spatial distributions feeding winter clouds on the West Coast of the U.S., so that regional-scale and climate model simulations performed by other DOE-funded scientists can be improved.
Winter storms along the West Coast are vital for annual precipitation in this region. We will use data collected during studies between 2011 and 2015 to test the hypothesis that INPs from desert mineral dust/biological aerosol layers transported intercontinental distances at higher altitudes provide an important control over precipitation efficiency in winter cloud systems over the Sierra Nevada Mountains. We will also determine if, in the absence of such seeding or mixing of INPs into lower levels, marine air lifted by winds over the Sierra often supports deep and highly supercooled cloud layers due to a relative deficiency of INPs. Thus, a secondary and important focus of the investigations will be to comprehensively characterize the variability of INP collected from marine air over broad regions feeding the West Coast of the U.S. and develop a novel database to be used to improve parameterizations of marine and other INP types. Parameterizations will be shared with collaborators involved in regional- and global-scale numerical modeling studies of clouds, precipitation, and climate.
Specific objectives and approaches for the proposed work include:

1. Analyze and categorize the number concentration, and chemical and biological composition, of INPs collected during DOE aircraft flights in CalWater-1 (2011) and ACAPEX (2015), in combination with surface (coastal site) and ship-based INP collections and aerosol data from winter 2014 and 2015, to quantify their influences on California winter clouds. Online and offline INP measurements will be used to quantify (over 7 orders of magnitude) the fraction of different particle types (dust, biological particles, sea spray particles, etc.) capable of freezing cloud droplets, in the most relevant range from -5 to -30°C.
2. Perform a case study and conceptual winter storm analyses of INP-cloud interactions using the INP, aerosol, and cloud particle data sets from all sites and platforms, and develop aerosol-specific INP parameterizations for use in collaborator-led numerical modeling studies.
3. Determine the latitudinal, seasonal, and inter-annual variability of marine boundary layer INPs affecting the U.S. West Coast through analyses of ship-based filter samples, including summer 2013 samples collected during the DOE-ARM MAGIC study, the DOE ACAPEX ship studies, and from other cruises over subtropical source regions from which wintertime Atmospheric Rivers emanate. These latter studies will seek to develop novel relations between INP and marine biological
   community populations using genomic analyses that may be applicable over oceanic regions globally.

The proposed work advances the science of the interactions of aerosols, clouds, and precipitation, with direct application to improve representation of such interactions for clouds in regional and global climate models.