A long record of cloud optical thickness and effective particle radius retrieved from cloud reflectance exists with no comparable dataset retrieved from cloud transmittance. This is due to a lack of sensitivity to the effective radius in cloud transmittance. A new algorithm that uses spectrally resolved cloud transmittance observations to retrieve optical thickness and effective radius is presented. The algorithm relies on the spectral slope of the normalized transmittance between 1565 nm and 1634 nm and on cloud transmittance at a visible wavelength. Using the spectral slope rather than the transmittance itself enhances the sensitivity of transmittance observations with respect to the effective radius. This is demonstrated by applying the algorithm to hyperspectral data from two field sites. The liquid water path is derived and compared to the simultaneous observations from a microwave radiometer and the optical thickness and effective radius are compared to MODIS retrievals. The algorithm was applied to ship-based observations in another field campaign, CalNex, which featured a day, 16 May 2010, of coordinated observations from a research ship and aircraft, providing the opportunity to compare retrievals from surface-based radiometers, an airborne radiometer, a satellite imager, and in-situ cloud probes. A statistical look at the cloud properties is presented and compared to previous studies. The retrievals and cloud transmittance are used to make observations of cloud transmittance susceptibility for the first time. Cloud transmittance susceptibility quantifies the change in cloud transmittance for a change in cloud droplet number concentration, thereby representing a possible change in cloud transmittance due to a change in aerosol burden.;The results of the two initial case studies showed that, in general, the effective radius uncertainties were much larger for the standard retrieval than for the spectral retrieval, particularly for thin clouds. When defining 2 μm as upper limit for the tolerable uncertainty of the effective radius, the standard method returned only very few valid retrievals for douds with an optical thickness below 25. At one field site (mean optical thickness 23), the spectral method provided valid retrievals for 84% of the data (24% for the standard method). At the other (mean optical thickness 44), both methods provided a high return of 90% for the spectral method and 78% for the standard method. The CalNex comparisons for 16 May 2010 showed that the agreement between the retrievals increased as the difference between the sampling volumes of the instruments decreased. The average in-situ reff (7.7 μm) fell between the average reff retrieved using the Atlantis-based SSFR radiance (5. 7 μm) and irradiance (9.5 μm). The statistical study of all clouds during CalNex showed a diurnal pattern observed in previous studies of marine boundary layer clouds. The climatology of cloud optical thickness and liquid water path was shown to be represented by a gamma distribution, consistent with previous studies of high cloud fraction marine boundary layer clouds. Model calculations of transmittance susceptibility showed that clouds are more susceptible as effective radius increases and less susceptible as optical thickness increases. The observations of cloud transmittance susceptibility show that the clouds encountered during CaiN ex were not highly susceptible. Comparisons to previous studies of cloud reflectance susceptibility also show this. Comparisons of observations in northern California and southern California show that the least susceptible clouds were in the south, where aerosol concentrations were higher at the surface, effective radius was smaller, optical thickness was larger than the observations in the north.
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