Testing the coherence of cirrus microphysical and bulk properties retrieved from dual-viewing multispectral satellite radiance measurements

In this paper the coherence of retrieved cirrus microphysical and bulk prop erties using data from a satellite-based dual-viewing and multispectral ins trument is tested using different ice crystal models. Radiance data from th e dual-viewing Along Track Scanning Radiometer (ATSR-2) instrument is used to show that coherent retrievals are possible between nonabsorbing (visible ) and infrared wavelengths if an appropriate ice crystal model is employed. The dominating crystal habit is estimated by finding the ice crystal model that best fits the dual-view 0.87 mu m reflectance data. The ice crystal m odels tested are hexagonal plates, hexagonal columns, bullet-rosettes (six branched), and randomized polycrystals, all of which are assumed to be rand omly oriented in space. Given the best fit crystal shape other cirrus prope rties, such as optical depth, crystal maximum dimension, and an estimate of ice water path, are retrieved by contrasting reflectance data at the wavel engths of 0.55 and 1.6 mu m. To demonstrate probable retrieval errors in te rms of optical depth and crystal maximum dimension, if the wrong crystal ha bit is applied, a tropical convective case in the western Pacific Ocean is used as a typical example. It is found that the more complex particles as r epresented by the bullet-rosette and randomized polycrystals best fit the A TSR-2 radiance data, while the "pristine" geometries represented by the hex agonal plate and column do not. These results indicate that phase functions that are relatively flat at backscattering angles should be employed in sa tellite remote sensing of cirrus. Moreover, if hexagonal plates or columns were assumed as the habit in the radiative transfer model, then this would lead to retrieval errors of about a factor of 2 for optical depth(overestim ate) and crystal maximum dimension(underestimate). To test if the retrieved optical depth at the wavelength of 0.55 mu m is coherent, the extinction o ptical depth at the wavelength of 10.8 mu m is also retrieved to test for a one-to-one relationship between the two wavelengths. To validate this proc edure the same test is applied to a near coincident aircraft and ATSR-2 mid latitude case study.