ABSTRACT: Downscaling temperature and precipitation from NARR reanalysis for glacier modelling.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The North American Regional Reanalysis (NARR) provides a high-quality retrospective on climate in the Northwest hemisphere by providing atmospheric variables at 3 hour intervals on a roughly 32 km grid spacing with 29 vertical levels. Despite this fairly high spatial resolution, the NARR data are too-widely spaced to directly drive spatially distributed alpine glacier models. We present techniques for downscaling the two variables of primary interest for glacier mass balance - temperature and precipitation. We then apply those data to a glacier surface mass balance model for the North American Cordillera. For precipitation, we apply a linear model of orographic precipitation with modifications for air mass tracking and dynamic calculation of nucleation and fallout timescales. Our temperature downscale calculates free-air and inversion lapse rates and uses these to adjust mid-tropospheric temperature to ground-level on the high- resolution target DEM. The regions of focus for our study are southern British Columbia and northern Washington state where observational station density is high and validation of our downscale is possible. Our downscaling efforts produce a high resolution 30 year climatology of temperature and precipitation for the years 1979 through 2008. We have validated our downscaling against Environment Canada's observational data, British Columbia and NRCS SNOTEL snow pillow data for validating precipitation and temeprature in mountainous terrain. We also compare our results to the PRISM gridded precipitation and temperature products. Our downscaling shows good agreement with annual precipitation amounts and mean annual temperatures and captures shorter time-scale events as well. Downscaled temperature shows best performance in summer likely due to simpler boundary layer structure than winter in which cold air pooling can predominate. The physics-based precipitation downscale significantly improves precipitation fields in high altitude, mountainous regions relative to the raw NARR data. Our temperature downscaling yields similar performance to the PRISM product but at a higher spatial and temporal resolution. Using the downscaled temperature and precipitation yields seasonal specific glacier mass balance with RMSE of 0.70 m w. eq. for summer balance and 0.6 m w. eq. for winter mass balance when compared to measurements made on glaciers throughout our domain.

Bibliography of Canadian Geomorphology