Measurements of UV irradiance within the area of one satellite pixel

Abstract

Abstract.A measurement campaign was performed in the region of Vienna and its surroundings from May to July 2007. Within the scope of this campaign erythemal UV was measured at six ground stations within a radius of 30 km. First, the homogeneity of the UV levels within the area of one satellite pixel was studied. Second, the ground UV was compared to ground UV retrieved by the ozone monitoring instrument (OMI) onboard the NASA EOS Aura Spacecraft. During clear-sky conditions the mean bias between erythemal UV measured by the different stations was within the measurement uncertainty of ±5%. Short term fluctuations of UV between the stations were below 3% within a radius of 20 km. For partly cloudy conditions and overcast conditions the discrepancy of instantaneous values between the stations is up to 200% or even higher. If averages of the UV index over longer time periods are compared the difference between the stations decreases strongly. The agreement is better than 20% within a distance of 10 km between the stations for 3 h averages. The comparison with OMI UV showed for clear-sky conditions higher satellite retrieved UV values by, on the average, approximately 15%. The ratio of OMI to ground measured UV lies between 0.9 and 1.5. and strongly depends on the aerosol optical depth. For partly cloudy and overcast conditions the OMI derived surface UV estimates show larger deviation from the ground-based reference data, and even bigger systematic positive bias. Here the ratio OMI to ground data lies between 0.5 and 4.5. The average difference between OMI and ground measurements is +24 to +37% for partly cloudy conditions and more than +50% for overcast conditions.

Spatial and temporal UV irradiance and aerosol variability within the area of an OMI satellite pixel

“…For the specific campaign the number of days with variable or overcast cloud conditions was limited. Similar work of Weihs et al (2008) showed under partly cloudy conditions the satellite UV-values are, on average, in the order of 30% higher than the ground observations. The ratio OMI to ground UV becomes even larger for overcast conditions.…”

“…Satellite ozone and UV data were exhaustively validated by means of GB measurements, regarding TOMS; (Kalliskota et al, 2000;McKenzie et al, 2001;Krotkov et al, 2001;Fioletov et al, 2002;Meloni et al, 2005;Arola et al, 2005;Kazantzidis et al, 2006) and lately OMI; Ialongo et al, 2008;Weihs et al, 2008;Kazadzis et al, 2009;Buchard et al, 2008). All these validation studies converge to the following conclusions:…”

“…The GB measurements are representative of only a small area, whereas satellite measurements are representative of a large region (OMI minimum pixel at nadir 13×24 km 2 ), thus further uncertainties may arise from the cloud or aerosol variability, parameters that both affect UV irradiance, inside the satellite pixel area. The influence of ground inhomogeneity on the UV retrieval as well as the representativeness of the derived UV at one OMI pixel is an issue that only few studies have approached (Weihs et al, 2008). In the work of Weihs et al (2008) measurements within one OMI satellite pixel were used to investigate the inhomogeneity of UV and so to give hints concerning possible UV fluctuations.…”

“…The influence of ground inhomogeneity on the UV retrieval as well as the representativeness of the derived UV at one OMI pixel is an issue that only few studies have approached (Weihs et al, 2008). In the work of Weihs et al (2008) measurements within one OMI satellite pixel were used to investigate the inhomogeneity of UV and so to give hints concerning possible UV fluctuations. Their results for UV erythemal dose variability within one OMI satellite pixel showed deviations up to ±5%, ±10% and ±50% for cloudless, partly cloudy and overcast conditions, respectively.…”

The Austrian radiation monitoring network ARAD – best practice and added value

“…In the 1970s and 1980s, surface radiation studies in Austria were focused on establishing radiation climatologies and increasing the process understanding based on the available station data (e.g., Neuwirth, 1980Neuwirth, , 1983. From the mid-1990s until today, work related to ultraviolet (UV) radiation and health risks (e.g., Blumthaler et al, 1996;Rieder et al, 2010;Weihs et al, 2008Weihs et al, , 2013, to the role of aerosols in alpine valleys (e.g., Blumthaler et al, 1997;Wuttke et al, 2012) and studies dealing with anthropogenic influences on radiative fluxes and solar energy (e.g., Weihs et al, 2015) predominated in Austria.…”