A comparison of cloud albedo and cloud fraction retrievals from long-term surface based shortwave radiation measurements

Description

Atmospheric Radiation Measurement (ARM) has been collecting radiation measurements for about two decades; a method that utilizes the high-resolution, long term radiation measurements to retrieve cloud properties such as cloud fraction and cloud albedo is highly desirable. Cloud albedo and cloud fraction are intimately related, and separate retrievals often suffer from mutual contamination of errors. Here a new analytical approach is presented that simultaneously retrieves cloud albedo and cloud fraction from the total and direct SW radiative fluxes measured at the surface and thus minimizes the potential mutual error contamination of separate retrievals. A comparison of sixteen years (1997-2012) measurements at ARM Southern Great Plains (SGP) site shows that although the new retrievals have an overall good agreement to the existing retrievals, there are some noticeable differences. The cloud fractions derived with the new algorithm have generally smaller values, perhaps due to the use of retrieved cloud albedo. Compared to the determination of cloud albedo using the relation between cloud albedo, cloud fraction and relative cloud radiative forcing (RCRF), the new retrieval algorithm tends to produce greater and smaller cloud albedos for thin and thick clouds, respectively. The potential of using the new retrievals for evaluating SCMs in a consistent framework is also explored.