Directional effect on radiative surface temperature measurements over a semiarid grassland site

In this study, an experimental design was conceived, as part of the Semi-Ar id-Land-Surface-Atmosphere (SALSA) program, to document the effect of view angle Variation on surface radiative temperature measurements. The results indicated differences between nadir and off-nadir radiative temperature of up to 5 K. The data also illustrated that, under clear sky and constant veg etation conditions, this difference is well correlated with surface soil mo isture. However, the correlation decreased when the same comparison was mad e under changing vegetation conditions. To investigate the possibility of d eriving component surface temperatures (soil and vegetation) using dual-ang le observations of directional radiative temperature, two radiative transfe r models (RTM) with different degrees of complexity were used. The results showed that despite their differences, the two models performed similarly i n predicting the directional radiative temperature at a third angle. In con trast to other investigations, our study indicated that the impact of ignor ing the cavity effect term is not very significant. However, omitting the c ontribution of the incoming long-wave radiation on measured directional rad iance seemed to have a much larger impact. Finally, sensitivity analysis sh owed that an accuracy of better than 10% on the plant area index (PAI) was required for achieving a precision of 1 K for inverted vegetation temperatu re. An error of 1 K in measured directional radiative temperature can lead to an error of about 1 K in the soil and vegetation temperatures derived by inverting the RTM. (C) 2001 Elsevier Science Inc. All rights reserved.