Brightness temperature difference (BTD) values are calculated for selected Geostationary Operational Environmental Satellite (GOES-6) channels (3.9, 12.7 µm) and Advanced Very High Resolution Radiometer channels (3.7, 12.0 µm). Daytime and nighttime discrimination of particle size information is possible given the infrared cloud extinction optical depth and the BTD value. BTD values are presented and compared for cirrus clouds composed of equivalent ice spheres (volume, surface area) versus randomly oriented hexagonal ice crystals. The effect of the hexagonal ice crystals is to increase the magnitude of the BTD values calculated relative to equivalent ice sphere (volume, surface area) BTDs. Equivalent spheres (volume or surface area) do not do a very good job of modeling hexagonal ice crystal effects on BTDs; however, the use of composite spheres improves the simulation and offers interesting prospects. Careful consideration of the number of Legendre polynomial coefficients used to fit the scattering phase functions is crucial to realistic modeling of cirrus BTDs. Surface and view-angle effects are incorporated to provide more realistic simulation.

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Subvisual cirrus is defined as optically thin cirrus with visible optical depth 0.05. This includes threshold subvisual cirrus as defined in the literature. Subvisual cirrus is associated with at least five different dynamical phenomenon in the atmosphere. These associations are: 1) equatorial, 2) frontal, 3) jet stream, 4) orographic and 5) other types. Characteristic properties of each cirrus association derived from observations reported in the literature are provided. Rough estimates of the effects of subvisual cirrus on vertical profiles of atmospheric radiances show that these clouds have the potential to significantly affect sensor performance. DEFINITION OF SUBVISUAL CIRRUSThe term "cirrus" refers to a principal cloud type with morphologically distinct features. Cirrus is defined as detached clouds that appear as thin (without selfshadowing), white, fibrous tufts usually showing delicate filaments, often with considerable vertical extent. Cirrus is primarily composed of ice crystals and its optical morphology is a result of glaciation, ice crystal microphysics and the interaction of falling ice crystals with atmospheric winds'.A class of cirrus, termed subvisual cirrus, is of interest to the scientific community due to their effect on sensors and potential impact on the climate through interactions with the radiative heat budget of the upper troposphere. The existence of subvisual cirrus has been documented by numerous investigators27. The term "subvisual" or "subvisible" has only recently come into wide use. Prior terminology used to describe these optically thin cirrus clouds includes: • cirrus haze • semi-transparent cirrus • subvisible cirrus veils • low density clouds • a field of ice aerosols • invisible cirrus • anvil cirrus • zenith-subvisual cirrus • high altitude tropical (HAT) cirrus. We define subvisual cirrus as high, thin ice clouds with visible optical depth, ext 0.05. This simple definition incorporates threshold-subvisible cirrus clouds8. Subvisual cirrus is 68 /SP!E Vol. 2578 O-8194-1942-7/95/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/15/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

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