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Identifying causes of short-range forecast errors in maximum temperature during recent Central European heatwaves using the ECMWF-IFS ensemble


In the last fewyears, Central Europe faced a number of severe, record-breaking heatwaves. Previous studies focused on predictability of heat-waves on medium-range to subseasonal time scales (5 – 30 days). However, also short-range (3-day) forecasts of maximum temperature (Tmax) can exhibit substantial errors even on larger spatial scales. This study investigates the causes of short-range forecast errors in Tmax over Central Europe for the summers of 2015-2020 using the 50-member ensemble of the operational ECMWF-IFS (ECMWF-ENS). 3-day forecast errors, individually calculated for each ensemble member with respect to a 0-18h control forecast, are fed into a multi-variate linear regression model to study the relative importance of different error sources. Outside of heatwaves, errors in Tmax forecasts are predominantly caused by incorrectly predicted downwelling short-wave radiation, mainly due to errors in low cloud cover. During heatwaves, ECMWF-ENS exhibits a systematic underestimation of Tmax (-0.4K), which is exacerbated under clear-sky and low wind conditions, and other error sources gain importance: the second most important error source is over- or underestimation of nocturnal temperatures in the residual layer. Additional Lagrangian trajectory analysis for the years 2018–2020 (due to limited data availability) suggests a link to accumulating errors in near-surface diabatic heating of air masses associated with forecast errors in residence time over land and cloud cover. Regionally, other physical processes can be of dominant importance during heatwaves. Coastal regions are influenced by errors in near-surface wind whereas errors in soil moisture are more important in southeastern parts of Central Europe.