Stephan Nyeki

A new approach to the long-term reconstruction of the solar irradiance leads to large historical solar forcing

Context. The variable Sun is the most likely candidate for the natural forcing of past climate changes on time scales of 50 to 1000xa0years. Evidence for this understanding is that the terrestrial climate correlates positively with the solar activity. During the past 10u2009000xa0years, the Sun has experienced the substantial variations in activity and there have been numerous attempts to reconstruct solar irradiance. While there is general agreement on how solar forcing varied during the last several hundred years – all reconstructions are proportional to the solar activity – there is scientific controversy on the magnitude of solar forcing. Aims. We present a reconstruction of the total and spectral solar irradiance covering 130xa0nm–10 μ m from 1610 to the present with an annual resolution and for the Holocene with a 22-year resolution. Methods. We assume that the minimum state of the quiet Sun in time corresponds to the observed quietest area on the present Sun. Then we use available long-term proxies of the solar activity, which are 10 Be isotope concentrations in ice cores and 22-year smoothed neutron monitor data, to interpolate between the present quiet Sun and the minimum state of the quiet Sun. This determines the long-term trend in the solar variability, which is then superposed with the 11-year activity cycle calculated from the sunspot number. The time-dependent solar spectral irradiance from about 7000xa0BC to the present is then derived using a state-of-the-art radiation code. Results. We derive a total and spectral solar irradiance that was substantially lower during the Maunder minimum than the one observed today. The difference is remarkably larger than other estimations published in the recent literature. The magnitude of the solar UV variability, which indirectly affects the climate, is also found to exceed previous estimates.We discuss in detail the assumptions that lead us to this conclusion.

Aerosol and cloud effects on solar brightening and the recent rapid warming

[1]xa0The rapid temperature increase of 1°C over mainland Europe since 1980 is considerably larger than the temperature rise expected from anthropogenic greenhouse gas increases. Here we present aerosol optical depth measurements from six specific locations and surface irradiance measurements from a large number of radiation sites in Northern Germany and Switzerland. The measurements show a decline in aerosol concentration of up to 60%, which have led to a statistically significant increase of solar irradiance under cloud-free skies since the 1980s. The measurements confirm solar brightening and show that the direct aerosol effect had an approximately five times larger impact on climate forcing than the indirect aerosol and other cloud effects. The overall aerosol and cloud induced surface climate forcing is ∼+1 W m−2 dec−1 and has most probably strongly contributed to the recent rapid warming in Europe.

Long‐term trend analysis of aerosol variables at the high‐alpine site Jungfraujoch

[1]xa0This study reports the first long-term trend analysis of aerosol optical measurements at the high-alpine site Jungfraujoch, which started 10.5 years ago. Since the aerosol variables are approximately lognormally distributed, the seasonal Kendall test and Sens slope estimator were applied as nonparametric methods to detect the long-term trends for each month. The yearly trend was estimated by a least-mean-square fit, and the number of years necessary to detect this trend was calculated. The most significant trend is the increase (4–7% yr−1) in light-scattering coefficients during the September to December period. The light absorption and backscattering coefficients and the aerosol number concentration also show a positive trend during this time of the year. The hemispheric backscattering fraction and the scattering exponent calculated with the smaller wavelengths (450 and 550 nm), which relate to the small aerosol size fraction, decrease except during the summer, whereas the scattering exponent calculated with the larger wavelengths (550 and 700 nm) remains constant. Generally, the summer months at the Jungfraujoch, which are strongly influenced by planetary boundary layer air masses, do not show any long-term trend. The trends determined by least-mean-square fits of the scattering and backscattering coefficients, the hemispheric backscattering fractions, and the scattering exponent are significant, and the number of years necessary to detect them is shorter than 10 years. For these variables, the trends and the slopes estimated by the seasonal Kendall test are therefore confirmed by the least-mean-square fit results.

The PREMOS/PICARD instrument calibration

PREMOS is a space experiment scheduled to fly on the French solar mission PICARD. The experiment comprises filter radiometers and absolute radiometers to measure the spectral and total solar irradiance. The aim of PREMOS is 1. to contribute to the long term monitoring of the total solar irradiance, 2. to use irradiance observations for nowcasting the state of the terrestrial middle atmosphere and 3. to provide long term sensitivity calibration for the solar imaging instrument SODISM on PICARD. In this paper we describe the calibration of the instruments. The filter radiometer channels in the visible and near IR were characterized at PMOD/WRC and the UV channels were calibrated at PTB Berlin. The absolute radiometers were compared with the World Radiometric Reference at PMOD/WRC and a power calibration relative to a primary cryogenic radiometer standard was performed in vacuum and air at NPL.

Cloud observations in Switzerland using hemispherical sky cameras

We present observations of total cloud cover and cloud type classification results from a sky camera network comprising four stations in Switzerland. In a comprehensive intercomparison study, records of total cloud cover from the sky camera, long-wave radiation observations, Meteosat, ceilometer, and visual observations were compared. Total cloud cover from the sky camera was in 65–85% of cases within ±1u2009okta with respect to the other methods. The sky camera overestimates cloudiness with respect to the other automatic techniques on average by up to 1.1u2009±u20092.8 oktas but underestimates it by 0.8u2009±u20091.9 oktas compared to the human observer. However, the bias depends on the cloudiness and therefore needs to be considered when records from various observational techniques are being homogenized. Cloud type classification was conducted using the k-Nearest Neighbor classifier in combination with a set of color and textural features. In addition, a radiative feature was introduced which improved the discrimination by up to 10%. The performance of the algorithm mainly depends on the atmospheric conditions, site-specific characteristics, the randomness of the selected images, and possible visual misclassifications: The mean success rate was 80–90% when the image only contained a single cloud class but dropped to 50–70% if the test images were completely randomly selected and multiple cloud classes occurred in the images.

Ground‐based aerosol optical depth trends at three high‐altitude sites in Switzerland and southern Germany from 1995 to 2010

[1]xa0Ground-based aerosol optical depth (AOD) climatologies at three high-altitude sites in Switzerland (Jungfraujoch and Davos) and Southern Germany (Hohenpeissenberg) are updated and re-calibrated for the period 1995–2010. In addition, AOD time series are augmented with previously unreported data, and are homogenized for the first time. Trend analysis revealed weak AOD trends (λ = 500 nm) at Jungfraujoch (JFJ; +0.007 decade−1), Davos (DAV; +0.002 decade−1) and Hohenpeissenberg (HPB; −0.011 decade−1) where the JFJ and HPB trends were statistically significant at the 95% and 90% confidence levels. However, a linear trend for the JFJ 1995–2005 period was found to be more appropriate than for 1995–2010 due to the influence of stratospheric AOD which gave a trend −0.003 decade−1 (significant at 95% level). When correcting for a recently available stratospheric AOD time series, accounting for Pinatubo (1991) and more recent volcanic eruptions, the 1995–2010 AOD trends decreased slightly at DAV and HPB but remained weak at +0.000 decade−1 and −0.013 decade−1 (significant at 95% level). The JFJ 1995–2005 AOD time series similarly decreased to −0.003 decade−1(significant at 95% level). We conclude that despite a more detailed re-analysis of these three time series, which have been extended by five years to the end of 2010, a significant decrease in AOD at these three high-altitude sites has still not been observed.

Indirect evidence of the composition of nucleation mode atmospheric particles in the high Arctic

Previous long-term observations have shown that nanoparticle formation events are common in the summer-time high Arctic and linked to local photochemical activity. However, current knowledge is limited with respect to the chemical precursors of resulting nanoparticles and the compounds involved in their subsequent growth. Here we report case-study measurements during new particle formation (NPF) events of the particle size distribution (diameter > 7 nm) and for the first time the volatility of monodisperse particles having diameter ≤40 nm, providing indirect information about their composition. Volatility measurements provide indirect evidence that a predominant fraction of the 12 nm particle population is ammoniated sulfates in the summertime high Arctic. Our observations further suggest that the majority of the sub-40 nm particle population during NPF events does not exist in the form of sulfuric acid but rather as partly or fully neutralized ammoniated sulfates.

A new absolute reference for atmospheric longwave irradiance measurements with traceability to SI units

Two independently designed and calibrated absolute radiometers measuring downwelling longwave irradiance were compared during two field campaigns in February and October 2013 at Physikalisch Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC). One absolute cavity pyrgeometer (ACP) developed by NREL and up to four Integrating Sphere Infrared Radiometers (IRIS) developed by PMOD/WRC took part in these intercomparisons. The internal consistency of the IRIS radiometers and the agreement with the ACP were within ±1u2009Wu2009m−2, providing traceability of atmospheric longwave irradiance to the international system of units with unprecedented accuracy. Measurements performed during the two field campaigns and over the past 4u2009years have shown that the World Infrared Standard Group (WISG) of pyrgeometers is underestimating clear-sky atmospheric longwave irradiance by 2 to 6u2009Wu2009m−2, depending on the amount of integrated water vapor (IWV). This behavior is an instrument-dependent feature and requires an individual sensitivity calibration of each pyrgeometer with respect to an absolute reference such as IRIS or ACP. For IWV larger than 10u2009mm, an average sensitivity correction of +6.5% should be applied to the WISG in order to be consistent with the longwave reference represented by the ACP and IRIS radiometers. A concerted effort at international level will need to be implemented in order to correct measurements of atmospheric downwelling longwave irradiance traceable to the WISG.

Comparison of GPS and ERA40 IWV in the Alpine region, including correction of GPS observations at Jungfraujoch (3584 m)

[1]xa0The 31 stations in the Global Positioning System (GPS) network of Switzerland span an altitude range of 330 to 3584 m. The highest station in the network, Jungfraujoch, suffers from a constant negative bias in the Integrated Water Vapor (IWV) due to a protective radome. We compared Jungfraujoch GPS IWV measurements with coincident Precision Filter Radiometer (PFR) observations and showed that the bias in the GPS is fairly constant with respect to the time of year and to the PFR IWV value. A correction was developed for the GPS data and validated by comparison with coincident Raman lidar observations. The IWV observations from nine GPS stations, including Jungfraujoch, were then compared with the IWV field of the ECMWF 40 year reanalysis (ERA40) data. Altitude differences between the ERA40 surface level and the GPS stations resulted, as expected, in a positive bias in the ERA40 IWV. A fairly linear relationship, with an intercept of −0.3 mm, was found between this bias and the difference between the ERA40 surface pressure and the surface pressure at the GPS station. The ERA40 reanalysis captured water vapor variations on timescales of several days very well, as evidenced by an r2 correlation greater than 0.9 where the altitude difference between ERA40 and the GPS station was less than 1000 m. A comparison between ERA40 and GPS at Davos showed that the reanalysis underestimates IWV during winter temperature inversions.

The GAW‐PFR aerosol optical depth network: The 2008–2013 time series at Cape Point Station, South Africa

A ground-based aerosol optical depth (AOD) climatology is presented for Cape Point (CPT) station, South Africa, for the 2008–2013 period. CPT is part of the Global Atmosphere Watch–Precision Filter Radiometer network which conducts long-term AOD measurements at remote background sites. AOD (λu2009=u2009500u2009nm) and Angstrom exponent (368 to 862u2009nm; α368–862) averages for the entire period were 0.059 and 0.68, displaying only a weak seasonality. Based on an established method for air mass classification using the in situ wind direction and 222Rn concentration, the following four air mass types were used to further investigate AOD: background marine, marine, mixed, and continental. AOD was similar for all types, but α368–862 was distinctly lower (0.43) for background marine and higher (1.07) for continental air masses, illustrating the presence of coarse mode and anthropogenic aerosols, respectively. Trajectory cluster analysis of 5u2009day back trajectories confirmed/augmented this classification. AOD for background marine and marine air mass types were consistent with ship-based (Maritime Aerosol Network) and island (AErosol RObotic NETwork) measurements, suggesting that CPT is a suitable site to monitor pristine conditions in the South Atlantic and Southern Oceans when 222Rn concentrations are <u2009100u2009mBqu2009m−3.