Looking at Clouds from Both Sides Now
Ghan, S. J., Pacific Northwest National Laboratory
Cloud Life Cycle
Zhang Z, X Dong, B Xi, H Song, P Ma, S Ghan, S Platnick, and P Minnis. 2017. "Inter-comparisons of marine boundary layer cloud properties from the ARM CAP-MBL campaign and two MODIS cloud products." Journal of Geophysical Research: Atmospheres, 122(4), 10.1002/2016JD025763.
The impact of clouds on the reflection of sunlight to space depends on two key properties: cloud optical depth and droplet effective radius. Satellites provide nearly global coverage of these cloud properties, but the accuracy of the retrievals has not been evaluated. This study used ground-based retrievals to evaluate the satellite retrievals, and found good agreement for cloud optical depth and an understandable bias for droplet effective radius.
The agreement between the retrievals suggests that satellite retrievals can be used to evaluate the global distribution of cloud properties simulated by climate models.
Satellites provide nearly global coverage of cloud measurements, while ground-based instruments provide higher accuracy. Now, researchers at University of Maryland – Baltimore County, University of Arizona, University of North Dakota, Pacific Northwest National Laboratory, NASA Goddard Space Flight Center, and NASA Langley Research Center have used co-located measurements over an Atmospheric Radiation Measurement (ARM) site on an Atlantic island to quantify how well the retrievals from those instruments agree. Their analysis found good agreement between two different sets of retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instrument for cloud optical depth and droplet effective radius, and good agreement between MODIS and ARM retrievals of cloud optical depth. But the MODIS and ARM retrievals of droplet effective radius are consistently different. This offset was attributed to the different viewing angle of the instruments, with the MODIS sensing droplet radius near cloud top and ARM sensing droplet radius near cloud base, where droplets are smaller. As expected, the comparison of the MODIS and ARM retrievals depends on how close the retrievals were to each other.