| Popis: |
Knowledge of sea surface emissivity is an important factor in measuring valid thermal IR radiometric temperatures from viewing positions both near the sea surface and from satellite platforms. In the latter case, we find that the effect of as little as a 0.01 change in emissivity from a blackbody assumption may create an increase of as much as 1.0°C in recovered temperature in dry atmospheres where recovered temperatures are radiometric temperatures obtained by applying the Planck function to radiances received at the satellite. As atmospheric moisture increases to around 5 g/cm2, variations in emissivity of the same order have negligible effect on recovered sea surface temperatures. Laboratory measurements of fresh (tap) and sea water samples with a Barnes 8–14 μm PRT-5 radiometer show distinctive differences in the behavior of emissivity with changes in suspended sediment concentrations for both organic and inorganic materials. Tap water emissivity remains essentially invariant at 0.980 over a wide range of concentrations. In contrast, however, sea water emissivity values show an immediate but steady decrease from 0.975 to a value 0.970, with increasing suspended sediment loading up to around 100 mg/L, where emissivity levels off until it falls again to a value of 0.962 at concentrations of 10,000 mg/L. Consequently, emissivity variations should not be neglected in making thermal measurements of coastal waters. |
