Thursday, May 16, 2019

ERA5 Data for Alaska - Including Download Link

Back in November I took an initial look at the new ERA5 reanalysis data set from the world-leading ECMWF weather modeling and forecasting center in Europe.  At the time, the ERA5 data was only available for 2000-2017, but the reanalysis now extends back to 1979, and a further extension to 1950 will soon come online.  The high quality of the data assimilation and modeling framework that's used to produce the reanalysis makes this a real treasure trove of historical climate data.

It's an interesting exercise to compare the ERA5 data for Alaska to NOAA's climate division data, produced by NCEI.  For many years the climate division data was only available for the lower 48, but in 2015 the data set was expanded to include 13 climate zones in Alaska; here's a map.

To facilitate a direct comparison, I calculated area-averages for several ERA5 variables within each of Alaska's climate divisions.  For example, there are 605 ERA5 grid cells that at least partially intersect the Southeast Interior division; so I calculated the area of the intersection for each grid cell and added up the fractional contributions to the total area of the Southeast Interior zone.

Here's a chart showing the mean temperatures for January and for July in the Southeast Interior (which includes Fairbanks).  Aside from a modest cold bias in the ERA5 values in January, the performance is outstanding.

The situation is not quite as good in the North Slope division, which is not surprising as the observing network is more sparse, and moreover weather analysis and forecasting models (like the ECMWF model that underpins ERA5) often have a more difficult time with atmospheric physics in the Arctic.

Interestingly the 1979-2018 linear temperature trends are similar for January, but the ERA5 trend is much smaller than NCEI's trend for July in the North Slope division.

Looking at precipitation, ERA5 does fairly well for the Southeast Interior in both January and July, but again the agreement is not as good for the North Slope.  Precipitation is always a major challenge for reanalysis, and so these results are pretty good.

Finally, I did a quick comparison of ERA5 solar radiation to the CERES gridded data for the Southeast Interior, and again I used an area average for both data sets.  The results show a very close correspondence for the month of March, but there is only modest agreement in July.

We could of course keep going with all sorts of comparisons between ERA5 and other data sets, and between ERA5 and historical climate observations around the state, but there's no doubt that ERA5 is a very high quality reanalysis.  Beyond the pure fidelity of the data, however, the real value of the reanalysis is that it's spatially and temporally complete; and ERA5 even includes uncertainty estimates, although I haven't looked at that aspect yet.

For readers who might like to take a look at the Alaska data themselves, the following link provides the area-averaged data for the 13 climate divisions, including mean temperature, precipitation, solar radiation, and 10m wind speed.

No comments:

Post a Comment