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Calculation of near-surface attenuation coefficients: the influence of wave focusing

The influence of small waves (<2 to 10 cm in height) in in-ground tanks on the vertical attenuation coefficient for downwelling irradiance (Kd) calculated from either the mean irradiances over a 5 minute run or the average of point-to-point measurements made at 10 and 20 Hz is investigated. Waves of mean frequencies of about 1 Hz were introduced in the tanks that produced corresponding frequencies in the power spectrum of measured irradiances. Coherence was greater than 0.7 between spectra of irradiance and waves when small waves were introduced, but was less than 0.3 under ambient conditions. Both increases and decreases in Kd values were observed when comparing Kd calculated from the point-to-point values and that obtained using the mean irradiances. When large waves (5-10 cm) were introduced, the methods deviated by up to a factor of two, presumably due to the large fluctuations in the point-to-point Kd values relative to the fluctuations in irradiance averaged over the 5 minute run. There were changes of 30% in Kd, calculated from mean irradiances, as wave height was increased.