Comparison of UV and Broadband Irradiance
Jääskeläinen, Emmihenna1; Hovila, Jari1; Manninen, Terhikki1; Tamminen, Johanna1; Jay, Herman2
1Finnish Meteorological Institute, FINLAND; 2JCET University of Maryland, UNITED STATES
For optical satellite data the atmospheric correction is of paramount importance. However, especially for older data sets (such as NOAA/AVHRR), there is typically not enough information about the atmospheric conditions during the satellite overpass to reliably determine the surface reflectance. Hence it would be useful, if additional satellite data sets could be used in the atmospheric correction procedure. This study is motivated by the long time series of satellite based measurements in the UV wavelength range. If a strong enough relationship can be found between the UV and visible radiation characteristics of the atmosphere, the UV range satellite products can be used as additional information for the atmospheric correction of the visible and NIR channels of the NOAA/AVHRR time series. NASA has developed a ground-based spectrometer called Pandora, whose main purpose is to calculate concentrations of various trace gases (for example NO2 and O3). The instrument has many routines and our main interest is in those which do the measurements in UV wavelength range. The instrument does its calculations in several steps and one of the quantities measured is proportional to the solar irradiance. In our research we have used Pandora-instrument's (so far uncalibrated) quantity which has been measured in UV wavelength range as UV irradiance and as for broadband irradiance we have collected pyranometer data. Both instruments locate at the Finnish Meteorology Institute in Helsinki, Finland. Only bright days and bright moments of the days are included. Also only the periods corresponding to sun zenith angle values smaller than 70 degree are included in the studied data set. When comparing the UV and broadband irradiance values, a strong correlation was found between them. For very bright days (meaning no clouds) the coefficient of determination is almost 1. This encouraged to try a regression, where the independent variables were UV irradiance and sun zenith angle and the dependent variable was broadband irradiance measured by pyranometer. For example for a bright day in October, this regression produced a coefficient of determination of almost unity.