The most fundamental characterization of cloud optics is the optical thickness, τ. The optical thickness depends on the integral of the cloud drop-size distribution through the cloud. The simplest bulk characterizations of cloud microphysics are the effective droplet radius, re, and the column-integrated liquid water path, W. These quantities are linked through the equation
τ=(3W)/(2ρwre)
where ρw is the density of water. If we assume a log-normal distribution of cloud droplets, then we can relate τ to W and N, the number of drops per volume.
We have examined relationships between τ and cloud properties using data from LegII of EPIC2001 taken aboard the NOAA ship Ronald H. Brown. The data include in situ measurements of downward solar radiative flux, microwave radiometer estimates of W, and ceilometer and cloud radar estimates of cloud base and top heights. Using a model for clear sky solar flux, we have computed the time series of cloud solar transmission coefficient, Tr, during the daytime for periods when a cloud was directly over the ship (as determined by ceilometer). Several different parameterizations of Tr as a function of τ have been used to deduce τ and N from these observations. Previous EPIC investigators have noted the strong diurnal variations in W and cloud top height. Our analysis shows similar strong variations in τ, from a maximum of about 20 just after sunrise to a minimum of about 5 just after noon (note, these values do not average in clear periods). The number of cloud droplets varies only weakly about a nominal value of 50 cm-3. This is consistent with the concept of a weak diurnal variation in the cloud condensation nucleus (CCN) spectrum but strong variations in W associated with diurnal variations in cloud thickness (about 500 m at sunrise and 250 m just after noon). Our analysis also indicates a small negative bias (about 20 g/m2) in the microwave values for W and that the clouds are very close to adiabatic.