Land surface hot-spot observed by MODIS over Central Africa

The hot-spot is the term used to describe the increased directional re ectance that occurs over most land surfaces when the solar and view directions coincide. Under these viewing conditions, shadows cast on the surface are minimized, resulting in increased re ectance. This phenomenon has been observed in laboratory measure-ments (Sandmeier et al. 1998), airborne data (Vermote et al. 1997) and multi-angular satellite sensor data by systems such as POLDER (Hautecoeur and Leroy 1998). Figure 1 and cover illustrates the hot-spot eVect in MODIS land surface re ectance data (Vermote et al. 2002) sensed over dense forest in the Democratic Republic of Congo, 8:43 am GMT, 14 March 2001. MODIS has seven re ective bands for land studies, two 250m bands and ve 500m bands. Figure 1 shows bands 0.645 mm (250m), 0.555 mm (500m) and 0.469 mm (500m) as red, green and blue respectively to give a quasi-true colour representation. The 500m 0.555 mm and 0.469 mm bands are sharpened to 250m by multiplying them with the ratio of the 250m and 500m 0.645 mm data. An area of approximately 115 km×160 km is shown with the top of the image sensed before the bottom of the image as MODIS passed from approxi-mately north to south in a polar orbit inclined at approximately 98°. The solar and viewing geometry change between pixels. MODIS senses 10 km along track with each rotation of its scan mirror, sensing forty 250m scanlines and twenty 500m scanlines per scan (Wolfe et al. 1998). Hot-spots are evident in successive MODIS scans as the solar and view directions coincide and observed forest canopy shadows decrease. Figure 2 shows a 20 km transect along a scan line through one of the central hot-spots. The 0.645 mm, 0.555 mm, and 0.469 mm land surface re ectance are plotted as a function of view zenith angle. The scattering angle, de ned in equation (1), is also shown.