Using sun and aureole measurements to check MODIS COD and Reff retrievals for cirrus
Description
Cirrus is an important element in the atmospheric climate system. In situ measurement of cirrus is difficult, as evidenced by need for the SPARTICUS and MACPEX programs. Remote measurement is also difficult because of the complex shapes and strong forward-scattering properties of cirrus ice crystals. Sun and aureole measurement provides a new tool for validating remote retrieval of cirrus optical properties. Several examples are shown illustrating the use of sun and aureole measurements to check optical properties retrieved by the MODIS MOD06 algorithm. Monte Carlo scattering calculations of the solar aureole and disk radiance profile using the MODIS-retrieved optical depth and the phase function corresponding to the retrieved effective particle size are compared with radiance measurements at 670 nm made with Visidyne’s SAM (Sun and Aureole Measurement) imaging sun photometer. The comparisons suggest that (1) the MODIS phase functions should be reviewed and (2) the strength of forward scattering needs to be properly accounted for in cirrus property retrievals involving transmittance measurements.
