Evaluation of downward and upward solar irradiances simulated by the Integrated Forecasting System of ECMWF using airborne observations above Arctic low-level clouds

Abstract

Abstract. The simulations of upward and downward irradiances by the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts are compared to broadband solar irradiance measurements from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign. For this purpose, offline radiative transfer simulations with the ecRad radiation scheme using the operational IFS output were performed. The simulations of the downward solar irradiance agree within the measurement uncertainty. However, the IFS underestimates the reflected solar irradiances above sea ice significantly by −35 Wm−2. Above open ocean, the agreement is closer with an overestimation of 29 Wm−2. A sensitivity study using measured surface and cloud properties is performed with ecRad to quantify the contributions of the surface albedo, cloud fraction, ice and liquid water path and cloud droplet number concentration to the observed bias. It shows that the IFS sea ice albedo climatology underestimates the observed sea ice albedo, causing more than 50 % of the bias. Considering the higher variability of in situ observations in the parameterization of the cloud droplet number concentration leads to a smaller bias of −27 Wm−2 above sea ice and a larger bias of 48 Wm−2 above open ocean by increasing the range from 36–69 cm−3 to 36–200 cm−3. Above sea ice, realistic surface albedos, cloud droplet number concentrations and liquid water paths contribute most to a bias improvement. Above open ocean, realistic cloud fractions and liquid water paths are most important to reduce the model-observation differences.

Land use change and coastal water darkening drive synchronous dynamics in phytoplankton and fish phenology on centennial timescales

“…To get the daily integrated PAR just below the surface ( I 0 , μmol photons m −2 day −1 ), we started by obtaining hourly values of (downward) shortwave sea surface radiation ( I surf , W m −2 ) from the ERA‐20CM product (Hersbach et al., 2015), provided by the European Center for Medium‐range Weather forecast (ECMWF), which in ice‐free regions are found to match airborne observations (Müller et al., 2023). For each day with hydrographic observations in the period 1936–2021, I 0 was calculated according to$$$$ {I}_0=\frac{\sum \limits_1^{24}\left({I}_{surf}\right)}{24}\bullet 86400\bullet {cf}_w\bullet {cf}_{surf}\bullet {cf}_{PAR} $$$$where 86,400 is the number of seconds in a day, cf w = 4.57 is a conversion factor from W m −2 to μmol photons m −2 s −1 (Thimijan & Heins, 1983), cf surf = 0.6, accounts for light reduction at the surface due to albedo and absorption, and cf PAR is the fraction of I surf that is PAR and is set to 0.47, which is the mean of published correlations between the PAR photon flux density and daily broadband solar irradiance (Jacovides et al., 2004).…”

Evaluating the Representation of Arctic Cirrus Solar Radiative Effects in the IFS with Airborne Measurements

“…The IFS forecast is fed into version 1.5.0 of ecRad, which is run in an offline mode 155 to allow for sensitivity studies by exchanging specific IFS forecast variables and vary the applied ice optics parameterization. This approach is adapted from Wolf et al (2020) and Müller et al (2023). Instead of the operational Monte Carlo integration of the Independent Column Approximation (McICA, Pincus et al, 2003), the Speedy Algorithm for Radiative Transfer through Cloud Sides (SPARTACUS, Hogan et al, 2016;Schäfer et al, 2016) is used as a solver because it provides the spectral irradiances at all model levels and has tools available to parameterize 3D radiative effects.…”

“…It is therefore important to test the capabilities of the current IFS simulations to realistically represent the influence of cirrus on solar radiative transfer in the Arctic. One approach to evaluate the performance of the IFS, and ecRad in particular, has been shown by Wolf et al (2020) and Müller et al (2023), who used airborne observations of solar irradiance and remote sensing data to evaluate the representation of ice-topped clouds over the North Atlantic and Arctic low-level clouds, respectively. Here, we build on this method to quantify the performance of ecRad concerning Arctic cirrus.…”