Cloud modulation of zenith sky oxygen photon path lengths over Boulder, Colorado: Measurement versus model

Cloudy sky average photon path lengths were measured using the gamma band o f oxygen near 628 nm on many days over Boulder, Colorado, using a zenith-lo oking spectrograph with a resolution of 0.8 nm. The approach allows accurat e measurement of the average photon path length. Days characterized by rela tively extensive cloud cover are examined here, which exhibit very large pa th length variations as the cloud fields overhead evolve. The ability of a plane parallel line-by-line model with a single uniform cloud layer to pred ict the measured O-2 path lengths has been tested by constraining the model to the (independently) observed surface irradiance. Overall, its performan ce is quite good. which demonstrates that the relationship between the diff use transmission by clouds and the average photon path length is consistent with plane parallel radiative transfer calculations for the conditions stu died. Thus large errors in the cloud radiative transfer of such models, as suggested by some recent cloud absorption measurements, are not supported b y this study. Furthermore, direct observations in the blue and red spectral regions do not support even a 1% differential absorption between these spe ctral regions.