Cloud mapping from the earth’s surface using an infrared radiance contrast technique

The assumptions and performance of an infrared radiance contrast method for mapping clouds from the earth's surface are evaluated, with a view to using the technique for objective cloud observation. The sky is observed using a narrow view infrared window radiometer and a scanning mirror system. The radiometer signal is discriminated into 3 cloud categories (low, middle and high) and clear sky, using the assumption that cloudbase radiance decreases unambiguously with height. The discrimination radiances are obtained by modelling clouds representative of marginal conditions, in a range of atmospheres. A multilayer wavenumber specific infrared radiative transfer model is used. Once discriminated, the radiance data are mapped in polar coordinates and compared with simultaneous all-sky photographs both qualitatively and quantitatively. The latter involves gridding both maps and photos and comparing them using contingency matrices. Cloud amount, integrated by height class over the sky hemisphere, is also used for map/photo comparison. Cloud maps for 5 days in July 1978 indicate the mapping technique shows promise under a wide range of sky conditions (including cirrus, cumulus and layered cloud situations) but that certain problems remain to be solved. On some days large map errors may occur because of rapid changes in the clear sky radiance (which affect the values of the discrimination radiances). The quality of the maps degrades rapidly with time after their creation because of the effects of cloud motions during the scanning of the sky. This loss of quality does not in general make the maps useless for synoptic analysis purposes, but the time interval at which they must be made for such use is about 20 minutes, 1/3 the current standard hourly interval for network cloud observations. The only rejected assumption of the technique (as stated in Werner (1972, 1973a,b)) is that clear sky radiances need only be computed twice a year for a given location. Changes in water vapour mixing ratio over time scales from hours to weeks cause changes in the clear sky radiance over these same scales. To adequately map high and middle clouds, and improve the mapping of low clouds, these changes must be included in the mapping methodology. Some options to implement this suggestion are examined here, others are suggested for future work.Arts, Faculty ofGeography, Department ofGraduat