Investigation of surface fluxes using ground observations and NOAA/AVHRR satellite measurements

The synergy of satellite and ground-based observations allows us to characterize surface heat exchange processes over large areas. NOAA/AVHRR (Advanced Very High Resolution Radiometer) Global Area Coverage (GAC) data were combined with conventional ground observations in order to close the surface energy balance equation and describe time series of surface heat fluxes over two target areas with different vegetation.Ground observations included daily minimum and maximum air temperature, daily mean soil temperature at 2 cm depth, wind speed, global radiation and cloudiness from the 1994-1998 period. AVHRR data were taken from a specially collected dataset including raw counts of original 4-km resolution GAC data of daily (day- and night) observations for a five-year period over two 50-km areas in Hungary, consisting of agricultural, forested and marshland surfaces. After the calibration and cloud detection 10-day maximum NDVI (normalized difference vegetation index) and land surface skin temperature (LST) were derived. Surface heat fluxes were derived based on ground measurements, and satellite-derived NDVI and LST. The observed differences over the two target areas can be attributed to differences in the dominant vegetation cover. Examination of the time series of energy fluxes shows consistence with observed meteorological conditions and vegetation development. Empirical relationships are being investigated between NDVI, daily maximum air temperature and the surface thermophysical properties. These relationships will be applied to AVHRR time series within the NOAA/NESDIS GVI (Global Vegetation Index) multiyear multisatellite dataset to investigate if any changes in the variability of energy fluxes occurred during the last decade and whether they are related to global warming phenomenon.