Comparing reanalysis anomalies with those from observed values

Anomalies WRT 1981-2010 LTA

When viewed as mean temperature anomalies from reanalysis data for the first sixteen days of August 2020, the recent heatwave in the southeast of England looks a little less dramatic. The core of the greatest warmth stretched up from Africa and on through eastern Spain into southern Scandinavia. It was always a bit further east over northern France and northwest Germany (+4°C) than SE England, not forgetting the fact that temperatures in the first week were slightly closer to average. Temperatures across the central north Atlantic were again below average (-1°C) and Greenland has been anomalously cold do far (-5°C).

If you take a more detailed look at the anomalies using SYNOP for the first nineteen days of August (chart below) you’ll notice that the anomalies are much higher, in fact in the warmest areas there are anomalies of 6°C and higher. This is because the daily reanalysis data that I work with is the average of the each days 00, 06, 12 and 18 UTC temperature that I take from a very coarse 2.5° x 2.5° grid, rather that the actual daily extreme maximum and minimum I can extract from the SYNOP data, it also for use a different LTA Period that will slightly lower the anomaly as well.

1-19 August 2020
Mean Anomaly chart from SYNOP Observations
Anomalies calculated WRT the 1961-1990 LTA
Heathrow Temperatures
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