Introduction

1. Short-­Term Downward Long-Wave Radiation Measurements using Modified CGR3 Pyrgeometers in Switzerland Stephan Nyeki, Stefan Wacker, Julian Gröbner Physikalisch-Meteorologisches Observatorium Davos/World Radiation Centre (PMOD/WRC), Davos, Switzerland Introduction Down-welling short-wave and long-wave radiation (DSR and DLR), clouds and aerosols are key components in the radiative balance of the Earth. Therefore, it is essential to accurately monitor and analyze these variables and their variability in order to detect and predict climate change. Ground-based observations are not only valuable for the long-term analysis of these variables but are also crucial for the validation of the corresponding satellite products and model outputs. In this study, we present down-welling long-wave radiation data for the 8 – 14 mm window from modified CGR3 measurements at four Swiss stations for the 2008 – 2014 period. Measurement Sites and Instruments Modified CGR3 and Other Time-Series • A set of standard CGR3 pyrgeometers (K&Z) instruments were modified by K&Z to have an additional Germanium band-pass filter which changed the spectral range from 4.5 – 42 mm to 8 – 14 mm allowing only the transmittance of radiation in the 8 – 14 μm range (so-called main atmospheric window). Modified CGR3 pyrgeometers are characterisedin the PMOD/WRC black body cavity every 1 – 2 years in order to monitor their stability. This range was chosen as the absorption and emittance of long-wave radiation by greenhouse gases is less effective, hence making the atmosphere more transparent. Down-welling long-wave radiation is therefore reduced in this range, making it mainly dependent on the atmospheric water content. • Available time-series for analysis include: • Broadband DSR and DLR measurements (amongst others). Data available since 1996 at all stations (Wacker et al., 2011). • Narrowband (8 – 14 mm) DLR measurements using a modified CGR3 (Kipp & Zonen) instrument. Quality controlled data available since 2008 at all four stations. • Aerosol Optical Depth (AOD) at 368, 412, 500 and 862 nm using Precision Filter Radiometers (PFR) manufactured by PMOD/WRC. Data at Davos since 1991, and 1994 – 1995 at the other stations (e.g. Nyeki et al., 2012; 2014). • Standard meteorological parameters. Comprehensive radiation, aerosol and meteorological measurements are conducted at four stations in Switzerland: Payerne, Jungfraujoch, Davos and Locarno (see Figure 1 opposite). These sites have instruments belonging to the MeteoSwissSACRaM (Swiss Alpine Climate Radiation Monitoring) network, and to the PMOD/WRC, Davos. Measurements of a number of radiation and aerosol parameters have been conducted for periods longer than 10 years in some cases. MACE HEAD IZANA Application of Modified CGR3 Pyrgeometers Radiation Time-Series at Four Swiss Stations Atmospheric Boundary Layer Temperature and Cloud-Free Long-Wave Models A method to improve the performance of commonly used parameterizations to calculate the cloud-free surface DLR was developed using CGR3 measurements (Groebner et al., 2009; Wacker et al., 2013). A monthly climatology of the effective radiating atmospheric temperature is used instead of the instantaneous screen-level temperature. Calculated cloud-free DLR compared to pyrgeometer measurements shows that the difference can be reduced by nearly 35%, resulting in a model uncertainty ~5 W.m−2 which corresponds to the measurement uncertainty of pyrgeometers. Time-series of monthly average radiation values and short-term trends. Trend analysis was performed using the seasonal Kendall test, a generalization of the Mann-Kendall test, which is a non-parametric test that can be applied to time-series with seasonal cycles and missing values. Values of the trend slope were determined using Sen’s method. The decadal trends are shown in brackets in the legends. Several aspects of the time-series should be noted: 1) CGR3 measurements are only available until 2011 at Payerne, and 2) Although broadband DLR and DSR time-series go back further than 2008, only time-series concurrent to that of the CGR3 are examined here for consistency. Figure 2. Residuals of observed and calculated DLR using the Prataformula with the screen level temperature, T0 (blue line) and a monthly climatology of the effective radiating temperature of the atmosphere, (cyan line), for a) Locarno-Monti, b) Payerne, c) Davos, and d) Jungfraujoch. The green line represents the residuals of the PMOD model. The illustrated cloud-free periods occurred in 2011. After Wacker et al., 2013. Figure 2. Time-series of 1-month average radiation values at four Swiss stations. DSR = downward short-wave radiation, DLR = downward long-wave radiation, and DLR8-14 = downward long-wave radiation for the 8 – 14 mm range. Lines represent the short-term trends, and numbers in brackets the decadal trends (W.m-2/decade). Results of the Trend Analysis DLR (8 – 14 mm) The current decadal trends are negative at three stations, however, none are statistically significant at the 5 or 10% level. DLR (broadband) The current decadal trends are mixed at three stations, however, none are statistically significant at the 5 or 10% level. DSR (broadband) The current decadal trends are positive at three stations. Trends at PAY and LOC are statistically significant at the 10% level. References Gröbner, J., S. Wacker, L. Vuilleumier, N. Kämpfer, (2009), Effective atmospheric boundary layer temperature from longwave radiation measurements, J. Geophys. Res. 114, D19116, doi:10.1029/2009JD012274. Nyeki, S., C. H. Halios, W. Baum, K. Eleftheriadis, H. Flentje, J. Gröbner, L. Vuilleumier, and C. Wehrli, (2012), Ground-based aerosol optical depth trends at three high-altitude sites in Switzerland and southern Germany from 1995 to 2010, J. Geophys. Res., 117, D18202, doi:10.1029/2012JD017493. Nyeki, S., C. H. Halios, J. Gröbner, N. Kouremeti, S. Wacker, and C. Wehrli, (2014), Ground-based aerosol optical depth trends at baseline GAW-PFR stations from 2000 – 2013, to be submitted to J. Geophys. Res. Wacker, S., J. Gröbner, K. Hocke, N. Kämpfer, and L. Vuilleumier, (2011), Trend analysis of surface cloud-free downwellinglong-wave radiation from four Swiss sites J. Geophys. Res. 116, D10104, doi:10.1029/2010JD015343. Wacker, S., J. Gröbner and L. Vuilleumier, (2013), A method to calculate cloud-free long-wave irradiance at the surface based on radiative transfer modeling and temperature lapse rate estimates, Theor. Appl. Climatol., doi: 10.1007/s00704-013-0901-5.