Evaluation of surface shortwave spectral fluxes at Uccle produced by the ECMWF ecRad radiation scheme (v1.5.0) embedded in the MAR regional model (v3.14) and prediction of UV indices

The ecRad scheme is the latest radiative transfer scheme provided by the European Centre for Medium-range Weather Forecast (ECMWF), and is notably operational in the ECMWF’s Integrated Forecasting System (IFS) since 2017. Recent developments of ecRad enable it both to run using ecCKD high resolution gas-optics models and to produce surface shortwave spectral fluxes. The combination of both features allow ecRad to produce fine surface shortwave spectral fluxes, e.g., over a 310–315 nm spectral band.

We assessed this capability after embedding ecRad (v1.5.0) in the MAR (Modèle Atmosphérique Régional) regional climate model (v3.14). For this purpose, we used ground-based spectral observations captured by the Royal Belgian Institute for Space Aeronomy at Uccle (Belgium; 50.797° N, 4.357° E) from 2017 to 2020, in the 280–500 nm range and with a precision of 0.5 nm. After carefully tuning both MAR and ecRad and configuring fine spectral bands over the 280–500 nm range, we ran a MAR simulation over Belgium for the same period as the Uccle spectral observations.

After integrating the spectral observations on the same bands as configured in our MAR/ecRad simulation, we compared both time series of spectral fluxes at Uccle. Our evaluation yielded correlation coefficients ranging from 0.9 to 0.93 for all bands above 295 nm and low biases for all bands. As our spectral fluxes cover the ultraviolet (UV) range, we tried to predict UV indices with MAR/ecRad spectral fluxes. The UV index is a metric used to inform the public about how much harmful ultraviolet radiation reaches the Earth’s surface at a given time, and consists of a weighted integral of spectral fluxes over the UV range. Our model-based and observations-based UV indices are in very good agreement, though the former falls short of finding the highest UV indices of the latter, due to the ozone mixing ratios in MAR not varying on a daily basis.

Author's note: this research work is detailed in the paper "Inclusion of the ECMWF ecRad radiation scheme (v1.5.0) in the MAR model (v3.14), regional evaluation for Belgium and assessment of surface shortwave spectral fluxes at Uccle" currently available on EGUsphere in preprint (awaiting topic editor decision after referee comments and subsequent revision).