Simultaneous Remote Sensing of Thin Cirrus and Aerosol Properties from MODIS Data

Tropospheric aerosols and high-level thin cirrus frequently coexist and affect the atmospheric radiation budget and vertical heating rate distribution in different manners associated with their unique optical properties and altitude. However, distinguishing between these two from satellites is extremely difficult because both are optically thin with optical depths generally less than about 0.5. Presently, retrieval of such information as optical thickness and effective particle size by satellite sensors is made separately. That is, aerosols are only examined in presumed clear sky situations while thin cirrus properties are retrieved while assuming a background aerosol size distribution. There is evidence that thin/sub-visible cirrus may exist in greater quantity, especially in the tropics, than is currently being detected, which, in turn, has been leading to large errors in the retrieved aerosol properties. By using new satellite sensors with a greater number of channels and higher spectral resolution, new information is now available which may make it possible to obtain a more accurate estimate of both thin cirrus and aerosol optical properties when they occur in the same field of view. We have developed a procedure to retrieve thin cirrus and aerosol optical depth, as well as cirrus ice crystal effective size, using six Moderate Resolution Imaging Spectroradiometer (MODIS) channels. Theoretical simulations show that these particular channels are sensitive to specific aerosol and cirrus properties. Error analysis reveals that the retrieved optical depths are accurate to within 20%, while ice crystal effective sizes lie within twice that of the real value. Case studies taken from CRYSTAL-FACE indicate that our retrievals compare well with both in-situ measurements and other remotely sensed values. This new retrieval method will increase the total area in which aerosol information can be obtained and reduce the effect of thin cirrus contamination.