J. Vivekanandan and scientists from the Naval Research Laboratory (NRL) in Monterey used time-coincident GOES-9 and NOAA satellite datasets, in collaboration with NRL scientists to investigate the capability of GOES-8/9 for estimating the channel 2 (3.9 micrometers) cloud reflectance and its use for a daytime stratus and fog product. The reflectance technique is based upon the sun zenith angle and a lookup table of the cloud anisotropic reflectance factor that can be generated offline. It is computationally efficient and is appropriate for operational GOES usage. By requiring the lookup table of the anisotropic reflectance factor, the reflectance product is tied to radiative transfer models of typical stratus clouds, with the necessary assumptions of the effective radius of the cloud drop size distribution.
To investigate this, time-coincident GOES and National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) data near coastal California were individually processed using lookup tables of the anisotropic reflectance factor. These were generated offline at 1 micrometer increments in the effective radius Re between 1 and 15 micrometers for optically thick clouds. The different satellite zenith angles provide for two independent measurements of a given pixel. With knowledge of the sun zenith angle, the lookup tables are scanned until the minimum difference between the NOAA- and GOES-retrieved reflectance was obtained, which then corresponds to the drop size distribution effective radius Re. Retrieval of low cloud effective radius using data from two satellites (GOES-9 and NOAA-14) on 8 August 1996 near 2200 UTC along the southern California coast is shown in Figure 1. In the situation examined here, along the south-central California coast, effective radii between 7-13 micrometers were obtained in the reflective low clouds. We have no in-situ data against which we may compare the retrieved results. A unique characteristic of the near-IR reflectance is its ability to enhance regions of smaller effective radius when they are surrounded by relatively larger water drops.
