High Resolution Temperature and Precipitation Downscaling for Glacier Modelling

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 modelling of alpine glaciers, where fine-scale topographic detail generates sharp gradients in temperature and precipitation. Thus, we present techniques for downscaling the two variables of primary interest for glacier mass balance - temperature and precipitation. For precipitation, we rely on 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 DEM. The regions of focus for our study are southern British Columbia and northern Washington state where observational station density is high. We next perform downscaling over these regions to generate a high resolution 29 year climatology of temperature and precipitation for the years 1979 through 2007. We have validated our downscaling against Environment Canada's observational data, the Global Summary of the Day data set, and snow pillow data for validating precipitation in mountainous terrain. Our downscaling shows good agreement with annual precipitation amounts and mean annual temperatures and captures daily 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 orographic precipitation model significantly improves downscaled precipitation fields in high altitude, mountainous regions. Overall, our downscaling agrees with measured data and can be considered a reliable driver for regional glacier modelling (e.g. Jarosch, A. H. et al., Session C20, AGU FM 2008).