Frost mapping using NOAA AVHRR data in the Pampean region, Argentina

On 4 November 1992, the southern and western parts of the Pampean region, Argentina were affected by a frost that caused serious damage to crops and particularly to the wheat in flowering. Frosts were also reported for 5 and 23 November in the northeast and the southeast parts, respectively. In order to assess the frost affected areas, brightness temperatures recorded around 03:00 am from the NOAA-AVHRR sensor were examined for the three dates and the whole area (500 000 km(2)). Qualitatively, the temperature spatial distribution derived from AVHRR data agreed with the reports on the affected areas. To retrieve the actual surface temperature, in theory more appropriate than minimum air temperature (T-n) for frost mapping, 12 split-window algorithms were performed. However, the uncertainties on the surface temperatures retrieved on the one hand and the non-optimal overpass time of the satellite on the other led us to examine how well AVHRR temperatures could predict the T-n registered by the meteorological network. The objective was hence to interpolate T-n taking advantage of AVHRR spatial coverage. The split-window algorithms did not significantly improve the T-n prediction obtained using brightness temperatures. Regressions of brightness temperatures against T-n gave correlations of 0.81, 0.44 and 0.90 and standard deviations of 1.4, 1.9 and 1.7 degrees C for 4, 5 and 23 November, respectively (using 41, 31 and 21 stations). Apart from the interdate variability, these correlations are satisfactory if we consider the different 'nature' of both temperatures and also the size of the study area. The introduction of geographical variables (latitude, longitude and altitude) did not significantly improve the correlations found. The posterior examination of AVHRR NDVI time profiles over affected and non affected areas permitted to confirm and complete the mapping of the damaged area.