UBC Theses and Dissertations
Evaluation of dynamically downscaled near-surface meteorological variables and energy fluxes at three mountain glaciers in British Columbia Tessema, Mekdes Ayalew
All models of glacier melt, regardless of their complexity, must be forced by observed meteorological fields at or in the vicinity of the glacier in question. In the absence of these observations, the forcing is commonly derived from statistical or dynamical downscaling of low resolution climate reanalysis models. Here we focus on a dynamical downscaling via Weather Research and Forecasting (WRF) model, which has previously showed promising results in simulating a surface energy balance (SEB) at several glacierized terrains. Our goal is to evaluate the WRF downscaling approach at three mountain glaciers in the interior mountains of British Columbia where the automatic weather stations (AWSs) recorded data over several summer seasons. The WRF model, nested within the ERA-Interim global reanalysis produced output fields at 7.5 km and 2.5 km spatial resolution, as well as 1 km resolution for one of the sites. We analyze how closely the WRF model output, at sub-daily and daily temporal resolution, resembles the observed meteorological fields and SEB fluxes needed to assess seasonal surface melt at these glaciers. We find that the model at 2.5 km closely simulates the cumulative seasonal melt (±10% difference) despite large biases in the individual components of the SEB model. Overestimation of the number of clear sky days explains the positive bias in the modeled net shortwave radiation. This positive bias, however, is compensated by a negative bias in the modeled net longwave radiation, and by an underestimation of sensible and latent heat fluxes. The underestimation in the latter two fluxes, calculated from the bulk aerodynamic method, is due to underestimated near-surface wind speeds. Radiative fluxes, which are dominant drivers of seasonal melting, are poorly downscaled with WRF, while successfully simulated by the ERAInterim at the course spatial resolution. Therefore, we advocate that SEB models be directly forced with the output from global climate reanalysis. Finally, simulating turbulent heat fluxes at sloped glacier surfaces remains a major challenge, and the 1-km resolution state-of-the-art WRF model is not yet ready to tackle it.
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