Stefan Wacker

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 ±1 okta with respect to the other methods. The sky camera overestimates cloudiness with respect to the other automatic techniques on average by up to 1.1 ± 2.8 oktas but underestimates it by 0.8 ± 1.9 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.

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 ±1 W m−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 4 years have shown that the World Infrared Standard Group (WISG) of pyrgeometers is underestimating clear-sky atmospheric longwave irradiance by 2 to 6 W m−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 10 mm, 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.

A method to calculate cloud-free long-wave irradiance at the surface based on radiative transfer modeling and temperature lapse rate estimates

We demonstrate a method to improve the performance of commonly used parameterizations to calculate the cloud-free down-welling long-wave radiation at the surface. The method uses a monthly climatology of the effective radiating temperature of the atmosphere instead of the instantaneous screen-level temperature. The climatology of the effective radiating temperature can be derived from pyrgeometer measurements and was incorporated into two commonly used schemes. We compared the calculated cloud-free down-welling long-wave irradiances to high-quality pyrgeometer measurements from four Swiss sites. The discrepancies between observations and modified schemes can be reduced by up to 35 %, resulting in a model uncertainty close to 5 W m−2 which corresponds to the measurement uncertainty of pyrgeometers. Furthermore, we introduce a new long-wave model which is based on radiative transfer calculations in the 8–14-

Longwave irradiance measurements using IRIS radiometers at the PMOD/WRC-IRS

\upmu

Revising shortwave and longwave radiation archives in view of possible revisions of the WSG and WISG reference scales: methods and implications

m wavelength range. In the remaining long-wave spectrum, the radiation is calculated using the Planck function with the effective radiating temperature of the atmosphere. The performance of this new model is consistent with the modified parameterizations.

Trends in surface radiation and cloud radiative effect over Switzerland in the past 15 years

Measurements between a newly developed windowless longwave irradiance radiometer IRIS at PMOD/WRC and commercially available pyrgeometers based on thermopile detectors show seasonal variations of up to 6 Wm−2 between different instrument groups which are likely due to the spectral mismatch of the dome transmissions of the respective pyrgeometers. The measurements at PMOD/WRC suggest that Eppley PIR and PRE2003CG4 (Kipp & Zonen CG4 Pyrgeometers manufactured before 2003) require a longwave irradiance correction of about −0.5 Wm−2mm−1 IWV when the atmospheric integrated precipitable water vapor (IWV) falls below 10 mm, while POST2003CG4 do not show these discrepancies and give consistent results with respect to the IRIS Radiometers. The IRIS radiometers measure on average 4.6+2.5 Wm−2 higher longwave irradiances than the WISG (for IWV>10 mm), which needs to be confirmed by independent measurements.

Effective atmospheric boundary layer temperature from longwave radiation measurements

A large number of radiometers are traceable to the World Standard Group (WSG) for shortwave radiation and the interim World Infra-red Standard Group (WISG) for longwave radiation, hosted by the Physikalisch Meteorologisches 10 Observatorium Davos/World Radiation Centre (PMOD/WRC, Davos, Switzerland). The WSG and WISG have recently been found to overand underestimate radiation values, respectively (Fehlmann et al., 2012; Gröbner et al., 2014), although research is still ongoing. In view of a possible revision of the reference scales of both standard groups, this study discusses the methods involved, and the implications on existing archives of radiation time-series, such as the Baseline Surface Radiation Network (BSRN). Based on PMOD/WRC calibration archives and BSRN data archives, the downward longwave 15 radiation (DLR) time-series over the 2006 – 2015 periods were analysed at four stations (polar and mid-latitude locations). DLR was found to increase by up to 3.5 and 5.4 W m-2, respectively, after applying a WISG reference scale correction and a minor correction for the dependence of pyrgeometer sensitivity on atmospheric integrated water vapour content. Similar increases in DLR may be expected at other BSRN stations. Based on our analysis, a number of recommendations are made for future studies. 20

Cloud effect of persistent stratus nebulosus at the Payerne BSRN site

We analyzed 15 years (1996-2010) of high quality observations of surface down-welling short-wave and longwave radiation from four Swiss sites. Down-welling short-wave radiation at Locarno-Monti has significantly increased by 9 Wm−2 in the 1996-2010 period, whereas no significant trends at the 95% confidence level are observed at the other three stations. In addition, long-wave radiation has not significantly changed in the corresponding period. The cloud radiative effect was determined using radiative transfer calculations for the cloud-free short-wave radiation and an empirical scheme for the cloud-free long-wave radiation. Results indicate that the net cloud radiative effect has decreased by up to 7.5 Wm−2 which implies a reduction in fractional cloud cover over the four Swiss sites.