Record high solar irradiance in Western Europe during first COVID-19 lockdown largely due to unusual weather

Spring 2020 broke sunshine duration records across western Europe. The Netherlands recorded the highest surface irradiance since 1928, exceeding the previous extreme of 2011 by 13 %, and the diffuse fraction of the irradiance measured a record low percentage (38 %). The coinciding irradiance extreme and a reduction in anthropogenic pollution due to COVID-19 measures triggered the hypothesis that cleaner-than-usual air contributed to the record. Based on analyses of ground-based and satellite observations and experiments with a radiative transfer model, we estimate a 1.3 % (2.3 W m$^{-2}$) increase in surface irradiance with respect to the 2010-2019 mean due to a low median aerosol optical depth, and a 17.6 % (30.7 W m$^{-2}$) increase due to several exceptionally dry days and a very low cloud fraction overall. Our analyses show that the reduced aerosols and contrails due to the COVID-19 measures are far less important in the irradiance record than the dry and particularly cloud-free weather.Comment: 21 pages, 12 figures, submitted to Communications Earth and Environmen

High resolution solar irradiance variability climatology dataset part 2: classifications, supplementary data, and statistics

Dataset description: High resolution surface solar irradiance series classification, cloud shadow and enhancement statistics, and satellite observations for studying intra-day surface solar irradiance variability. Part 2 of 2: This dataset is the derived from the 1 Hz observational record of direct, diffuse, and global horizontal irradiance measured by the Baseline Surface Radiation Network station at Cabauw, the Netherlands. More information about the observational site Cabauw can be found at the Ruisdael Observatory website. Methodology: An extensive dataset description is currently being written for Earth System Science Data. In the mean time, a more condensed description is available in preprint at Arxiv. Processing scripts are published at this Zenodo release. Dataset contents: This dataset contains daily time series with the following data, from 2011-02 until 2020-12-31: Cloud shadow and cloud enhancement time series classifications (see methodology) Overcast, clear-sky and variable time series classifications (see methodology) CAMS McClear for clear-sky global horizontal irradiance (version 3.5) CAMS McClear atmospheric composition input (aerosols, ozone, and total column water vapour) Solar elevation and azimuth angles (calculated using PySolar) Quality flags: non-official 1 Hz and official 1-minute (from BSRN at PANGAEA) Cabauw observatory tower wind speed and direction Additional satellite data time series from 2014-01 until 2016-12: MSGCPP satellite data for an area over central Netherlands (CLAAS2 source) Post processed timeseries of cloud types over Cabauw derived from this MSGCPP satellite data A nubiscope + satellite derived validation dataset for overcast and clear-sky classifications Statistics files: Cloud shadow and cloud enhancement event detection and event statistics based on the time series for 2011-2020 Daily radiation statistics for 2011-2020 And finally, for all days there are quicklooks available that visualize the irradiance time series, classification, and if available satellite data

Modeling total and polarized reflectances of ice clouds: evaluation by means of POLDER and ATSR-2 measurements

International audienceFour ice-crystal models are tested by use of ice-cloud reflectances derived from Along Track Scanning Radiometer-2 (ATSR-2) and Polarization and Directionality of Earth's Reflectances (POLDER) radiance measurements. The analysis is based on dual-view ATSR-2 total reflectances of tropical cirrus and POLDER global-scale total and polarized reflectances of ice clouds at as many as 14 viewing directions. Adequate simulations of ATSR-2 total reflectances at 0.865 µm are obtained with model clouds consisting of moderately distorted imperfect hexagonal monocrystals (IMPs). The optically thickest clouds (tau > ~16) in the selected case tend to be better simulated by use of pure hexagonal monocrystals (PHMs). POLDER total reflectances at 0.670 µm are best simulated with columnar or platelike IMPs or columnar inhomogeneous hexagonal monocrystals (IHMs). Less-favorable simulations are obtained for platelike IHMs and polycrystals (POLYs). Inadequate simulations of POLDER total and polarized reflectances are obtained for model clouds consisting of PHMs. Better simulations of the POLDER polarized reflectances at 0.865 µm are obtained with IMPs, IHMs, or POLYs, although POLYs produce polarized reflectances that are systematically lower than most of the measurements. The best simulations of the polarized reflectance for the ice-crystal models assumed in this study are obtained for model clouds consisting of columnar IMPs or IHMs

Reconciling Observations of Solar Irradiance Variability With Cloud Size Distributions

Clouds cast shadows on the surface and locally enhance solar irradiance by absorbing and scattering sunlight, resulting in fast and large solar irradiance fluctuations on the surface. Typical spatiotemporal scales and driving mechanisms of this intra-day irradiance variability are not well known, hence even 1 day ahead forecasts of variability are inaccurate. Here, we use long-term, high-frequency solar irradiance observations combined with satellite imagery, numerical simulations, and conceptual modeling to show how irradiance variability is linked to the cloud size distribution. Cloud shadow sizes are distributed according to a power law over multiple orders of magnitude, deviating only from the cloud size distribution due to cloud edge transparency at scales below ≈750 m. Locally cloud-enhanced irradiance occurs as frequently as shadows, and is similarly driven mostly by boundary layer clouds, but distributed over a smaller range of scales. We reconcile studies of solar irradiance variability with those on clouds, which brings fundamental understanding to what drives irradiance variability. Our findings have implications not only for weather and climate modeling, but also for solar energy and photosynthesis by vegetation, where detailed knowledge of surface solar irradiance is essential

Downscaling daily air-temperature measurements in the Netherlands

High-resolution, regularly gridded air-temperature maps are frequently used in climatology, hydrology, and ecology. Within the Netherlands, 34 official automatic weather stations (AWSs) are operated by the National Met Service according to World Meteorological Organization (WMO) standards. Although the measurements are of high quality, the spatial density of the AWSs is not sufficient to reconstruct the temperature on a 1-km-resolution grid. Therefore, a new methodology for daily temperature reconstruction from 1990 to 2017 is proposed, using linear regression and multiple adaptive regression splines. The daily 34 AWS measurements are interpolated using eight different predictors: diurnal temperature range, population density, elevation, albedo, solar irradiance, roughness, precipitation, and vegetation index. Results are cross-validated for the AWS locations and compared with independent citizen weather observations. The RMSE of the reference method ordinary kriging amounts to 2.6 °C whereas using the new methods the RMSE drops below 1.0 °C. Especially for cities, a substantial improvement of the predictions is found. Independent predictions are on average 0.3 °C less biased than ordinary kriging at 40 high-quality citizen measurement sites. With this new method, we have improved the representation of local temperature variations within the Netherlands. The temperature maps presented here can have applications in urban heat island studies, local trend analysis, and model evaluation.Atmospheric Remote Sensin

Analysis of high frequency photovoltaic solar energy fluctuations

Characterizing short-term variability of generated solar power is important for the integration of photovoltaic (PV) systems into the electrical grid. Using different kinds of high frequency, in-situ observations of both irradiance and generated PV power, we quantify insights on temporal averaging effects on the highest observed peaks and ramp rates, which closely relate to grid stability. We use measurements obtained at three specific spatial scales; a single point pyranometer, two household PV systems and a PV system typical for small medium businesses. We show that the 15-minute time resolution typically used for grid calculations significantly underestimates key dynamics at high temporal resolutions, such as ramp rates and maximum power output, at the local grid level. We find that absolute power peaks in the order of seconds are up to 18% higher compared to a 15-minute resolution for irradiance and up to 22% higher for a household PV system. For the largest PV system, the increase is limited to 11%. Furthermore, we find that the highest peaks solely occur under mixed-cloud conditions. Additionally, we show that the time interval-dependency of the largest power ramps is similar for all systems under research, ranging from ~20% at a 5-second interval to stabilizing at 70–80% between 5 and 10 min, which we can explain based on meteorological arguments