A new normalized difference cloud retrieval technique applied to landsat radiances over the Oklahoma ARM site

The authors propose a new cloud property retrieval technique that accounts for cloud side illumination and shadowing effects present at high solar zen ith angles. The technique uses the normalized difference of nadir reflectiv ities (NDNR) at a conservative and an absorbing (with respect to liquid wat er) wavelength. It can be further combined with the inverse nonlocal indepe ndent pixel approximation (NIPA) of Marshak et al. that corrects for radiat ive smoothing, thus providing a retrieval framework where all 3D cloud effe cts can potentially be accounted for. The effectiveness of the new techniqu e is demonstrated using Monte Carlo simulations. Real-world application is shown to be feasible using Thematic Mapper (TM) radiance observations from Landsat-5 over the Southern Great Plains (SGP) site of the Atmospheric Radi ation Measurement (ARM) Program. For the moderately oblique (45 degrees) so lar zenith angle of the available Landsat scene, NDNR gives similar regiona l statistics and histograms when compared with standard independent pixel a pproximation (IPA), but significant differences at the pixel level. Inverse NIPA is also applied for the first time on observed high-resolution radian ces of overcast Landsat subscenes. The dependence of the NIPA-retrieved clo ud fields on the parameters of the method is illustrated and practical issu es related to the optimal choice of these parameters are discussed. It is natural to compare novel cloud retrieval techniques with standard IPA retrievals. IPA is useful in revealing the inadequacy of plane parallel th eory in certain situations and in demonstrating sensitivities to parameter choices, parameterizations, and assumptions. For example, it is found that IPA has problems in matching modeled and observed band-7 (2.2 mum) reflecta nce values for similar to6% of the pixels, most of which are at cloud edges . For simultaneous cloud optical depth-droplet effective radius retrievals (where a conservative and an absorptive TM band are needed), it is found th at the band-4 (0.83 mum)-band-7 pair was the most well behaved, having less saturation, smaller changes in nominal calibration, and better overall con sistency with modeled values than other bands. Mean values of optical depth , effective radius, and liquid water path (LWP) for typical IPA retrievals using this pair are tau = 22, r(e) = 11 mum, and LWP = 157 g m(-2), respect ively. Inclusion of aerosol scattering above clouds results in similar to8% decrease in mean cloud optical depth for an aerosol optical depth of 0.2. Degradation of instrument resolution up to similar to2 km has small effects on the optical property means and histograms, suggesting small actual clou d variability at these scales and/or radiative smoothing. Comparisons with surface instruments (microwave radiometer, pyranometer, and pyrgeometer) ve rify the statisitical adequacy of the IPA retrievals. Last, cloud fractions derived with a simple threshold method are compared with those from an aut omated procedure called Automatic Cloud Cover Assessment now in operational use for Landsat-7. For the northernmost 2000 scanlines of the scene, the c loud fraction Ac is 0.585 from thresholding, as compared with A(c) = 0.563 for the automated procedure, and the full scene values are A(c) = 0.870 and A(c) = 0.865, respectively. This suggests that the Landsat-7 automated pro cedure will likely give reliable scene-averaged cloud fractions for moderat ely thick clouds over continental U.S. scenes similar to SGP.