A High-Resolution, Dynamically Downscaled ERA5 Reanalysis Dataset for Alaska

The need for high-resolution, multi-decadal gridded meteorological data over Alaska has been frequently articulated by science users and the wider stakeholder community. With the increase in frequency of extreme weather events in the light of climate change, which render the task of fire, land, infrastructure, and emergency managers more challenging, this type of data is in high demand. Climatological analysis and interpretation, too, requires long-term records of local conditions across an entire study region. Past efforts, such as dynamically downscaled ERA-Interim temperature and precipitation data at 20 km spatial resolution (Bieniek et al., 2016) have enjoyed much use in particular for the study of extreme weather events (for example, Bieniek et al., 2018; Lader et al., 2020). But even higher spatial resolutions would be desirable.

With ERA-Interim now discontinued and replaced by the much improved ERA5, we have undertaken a new effort to generate a downscaled dataset based on ERA5. To this end we used ERA5 to initialize the Weather Research and Forecasting Model (WRF) version 4.3.3. over a set of two nested domains (see Figure, a)): The outer domain is downscaled to a spatial resolution of 12 km and covers all of Alaska with the exception of some of the most western Aleutian islands, plus the Yukon Territory in Canada. The inner domain is focussed on the central Alaska land mass, and immediately neighboring Canada, thereby capturing virtually all of the wildfire prone land. We decided to retain a wide selection of variables, including temperature, atmospheric pressure, water vapor mixing ratio, wind and cloud fraction at seven pressure levels, as well as sea surface temperature, sea ice cover, albedo, precipitation and soil moisture content. The data is produced in 2-day runs, each with a 6 hour spin-up phase, and spectral nudging enabled. Output data is generated in hourly intervals, to be able to study diurnal processes. Ultimately, the entire ERA5 dataset, covering the period of 1940 to present, is targeted for downscaling.

To mitigate the choice made in ERA5 to fill the snow water equivalent variable over glaciated areas with a constant 10 m value, we have generated a composite snow product to initialize the snow fields. It gap-fills the masked ERA5 raster using data from the JRA55 reanalysis, which is available from 1959 to present; this corresponds to the current, initial downscaling period. Since JRA55 provides realistic snow values, this approach allows us to avoid temperature values adjacent to glaciers to be forced towards excessive cold biases.

We illustrate the quality of the downscaled 4 km and 12 km ERA5 variables and their ability to accurately capture to locally measured data using notable extreme weather events that have taken place in Alaska over recent years, including:

• The September 2021 ex-Typhoon Merbok, which caused extensive flooding along the southwestern Alaska coast (see Figure, b))
• The December 2022 rain-on-snow event, an ice storm which led to substantial road ice conditions in the Fairbanks region
• Extreme fire weather occurring in various parts of interior Alaska throughout the 2000s, 2010s and 2020s

The tasks that remain to be completed on this project consist in a comprehensive assessment of bias and bias-correction of the downscaled 4 km and 12 km data, and completion of the 1940 to 1958 period, for which no JRA55 snow data is available.

References

Bieniek, P. A., Bhatt, U. S., Walsh, J. E., Rupp, T. S., Zhang, J., Krieger, J. R., & Lader, R. (2016). Dynamical Downscaling of ERA-Interim Temperature and Precipitation for Alaska. Journal of Applied Meteorology and Climatology, 55(3), 635–654. https://doi.org/10.1175/JAMC-D-15-0153.1

Bieniek, P. A., Bhatt, U. S., Walsh, J. E., Lader, R., Griffith, B., Roach, J. K., & Thoman, R. L. (2018). Assessment of Alaska Rain-on-Snow Events Using Dynamical Downscaling. Journal of Applied Meteorology and Climatology, 57(8), 1847–1863. https://doi.org/10.1175/JAMC-D-17-0276.1

Lader, R., Walsh, J. E., Bhatt, U. S., & Bieniek, P. A. (2020). Anticipated changes to the snow season in Alaska: Elevation dependency, timing and extremes. International Journal of Climatology, 40(1), 169–187. https://doi.org/10.1002/joc.6201