Item: USING HIGH-RESOLUTION VERTICAL TEMPERATURE GRADIENTS TO FORECAST PERSISTENT WEAK LAYER FORMATION
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Title: USING HIGH-RESOLUTION VERTICAL TEMPERATURE GRADIENTS TO FORECAST PERSISTENT WEAK LAYER FORMATION
Proceedings: Proceedings, International Snow Science Workshop, Whistler, BC, Canada, 2026
Authors:
- Seth Waterfall [ Washington State Department of Transportation, USA ]
- John Stimberis [ Washington State Department of Transportation, USA ]
- Rachel Griffith [ University of Manchester, UK ]
Date: 2026-09-28
Abstract: Vertical temperature gradients near the snow surface play a critical role in snowpack metamorphism and atmospheric energy exchange. The near-surface boundary layer strongly influences avalanche formation (Birkeland, 1998; Schweizer and Lütschg, 2001; Slaughter et al., 2009); tracking these micro-climatic shifts provides vital precursor information for avalanche forecasting. However, capturing boundary layer thermal gradients at high temporal and spatial resolutions remains challenging in operational settings. A high-resolution field study conducted at Snoqualmie Pass, WA, USA (921 m) evaluated near-surface energy exchanges above an unbuffered, shallow snowpack. The study site is characterized by exceptionally light ambient airflow and persistently near-saturated relative humidity, creating a highly favorable micro-climate for surface hoar formation (Breyfogle, 1986). To capture these dynamics, a custom vertical thermocouple array measured air temperature profiles at 10-cm increments up to 1.5 m above the snow-air interface at 10-second intervals from December 2025 through March 2026, generating more than 15 million data points. High-resolution profiling captured steep boundary layer temperature inversions co-occurring with relative humidity near saturation (averaging 98.5%–99.6%). A focused analysis identified distinct metamorphic drivers across two primary event windows: (1) a January 15–18 cycle where clear-sky nocturnal radiational cooling drove near-surface temperature gradients exceeding 16.0°C/m, facilitating extensive surface hoar growth; and (2) a mid-February window (Feb 10–13) where strong near-surface temperature gradients (>16.8°C/m) cultivated a secondary surface hoar layer before subsequent storm loading on February 13. Tracking these thermodynamic precursor conditions from growth through surface evolution or storm slab burial demonstrates the operational value of monitoring near-surface boundary layer dynamics. While the January 15–18 surface hoar crystals were destroyed by rain prior to burial, the mid-February surface hoar growth (Feb 10–13) was preserved, forming the structural foundation for a reactive persistent weak layer (PWL) across the forecast zone. Multi-element vertical thermocouple arrays face practical operational deployment constraints across standard automated weather station (AWS) networks—including lower-sensor snow burial and diffuse shortwave daytime noise—results are compared with co-located infrared snow surface temperature sensors to evaluate single-sensor proxies for operational study plots.
Object ID: ISSW2026_P2.18.pdf
DOI: https://doi.org/10.15788/1790098923
Language of Article: English
Presenter(s): Seth Waterfall
Keywords: Avalanche Forecasting, Instrumentation and Remote Sensing, Snowpack Properties, Modelling and Quantitative Forecasting
Page Number(s): 652 - 659
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