The nature of the upwelling light field is of significant interest to oceanographers for a number of different applications. The Q factor, Eu/Lu, provides information about the angular distribution of the upwelling light field. This factor is commonly used in semi-analytical reflectance inversion models to exploit the theoretical relationship between the irradiance reflectance R (Eu/Ed) and the inherent optical properties, specifically bb/a. Despite the wide use of Q in converting from R to Rrs, few measurements have been made in attempts to validate theoretical predictions as to the behavior of Q.
Presented here is a theoretical background and conceptual description of the factors influencing the variability we expect to see in Q. Also presented are results from a field program conducted in conjunction with the Friday Harbor Labs "optical oceanography" course which took place on 8_3, 8_4, and 8_5 1998. Among the suite of measurements made during this field program were measurements of Lu and Eu, made with a Satlantic OCR-100 radiometers. Several profile casts and short time series measurements were made each day. Preliminary results are shown in figures 1-4. The plots of Q as a function of wavelength for a series of depths ranging from .5 to 7m shows a slight spectral variation of Q at the surface, and the spectral shape of Q changing with each depth. There are several interesting features of these plots, which can be seen during all three days. 1.) At the surface Q is larger at 650nm than at 443nm and the value at the blue and this initial spectral slope tends to flatten out with depth. 2.) Q at 683 decreases dramatically toward a value, indicating a near isotropic light field. 3.) At 490 510 and 555 the Q values becomes non-changing with a change in depth (Seen primarily on 8_3). The first feature of a change in spectral slope between red and blue wavelengths may be explained by the ratio of backscattering of water and the backscattering of particles. Bbw will dominate in the surface waters before significant interactions with particles are able to occur. The rapid decrease in Q at 683 may be explained by fluorescence, which is an inelastic process that emits light isotropically and thus would result in a Q value equal to p in the absence of other sources. The other feature on 8_3 of non- changing Q values with a change in depth may indicate that light at these wavelengths is approaching an asymptotic value. This final hypothesis may be tested by looking at the profiles of the mean cosine, and Ku values with depth.
Overall shifts in the spectrally averaged Q¹s from day to day seems to be correlated with the b/c ratio seen in plot c of all four figures. Each day had only a slightly different solar zenith angle. Description of the daily variance in Q has yet to be carried out. It has been shown theoretically that the variance in Q can largely be explained by, changes in the solar zenith angle, variations in the single scattering albedo, b/c, or the ratio of the bbw to bbp. It will be very interesting to determine the relative importance of each of these parameters in explaining the observed variance in Q spectrally, spatially, and temporally.