cloud and that of the sea surface with the relative contributions of the cloud top 

 and surface temperatures depending on the percentage of cloud within the field of 

 view. 



4. 2. 1 Daytime Cases 



In daytime passes, both Channel 3 (0. 2-5 micron) and Channel 5 (0. 5-0. 7 

 micron) measure the comparatively intense reflection of solar radiation from the 

 cloud surfaces, and thus can serve as a discriminator between cloudy and clear 

 areas. For the TIROS VII data, Channel 5 exhibits somewhat less noisy characteristics 

 than Channel 3, and was used throughout the study. 



Before the Channel 5 data can be used for this purpose, a clear-cloudy threshold 

 value must be determined. Channel 5 data as usually listed represents the irradiance 

 observed by the sensor in the 0. 5-0. 7 micron (visible) spectral region. This observed 

 value, however, depends on the solar zenith angle as well as the cloud reflectivity and 

 in this form can be used only by noting abrupt changes in its relative value. A more 

 significant parameter is obtained when the Channel 5 value is converted to the albedo. 

 The albedo is determined from the equation 



A 



I cos e 

 o 



when 6 is the zenith angle of the sun (which is included on the FMRT for each set of 



data points), I is the radiance measured by Channel 5, and I is the radiance that 



the satellite would measure from a perfectly reflecting isotropic surface illuminated 



by the sun at its zenith and with no intervening absorption or scattering. The value 



of I , which is a constant, may easily be calculated from the solar constant and the 

 o 



known properties of the sensor. In the albedo format, the effect of the solar zenith 

 angle is removed, and thus comparisons from one part of a pass to another can be 

 more easily made. Techniques for the calculation and use of this format were 

 developed midway through the study, and it was used for all subsequent cases. 



To determine the required no cloud-cloud threshold, cases were chosen using 

 satellite photographs, in which a distinct cloud edge running nearly perpendicular to 

 the IR swath lines was observed. It was noted that, over clear ocean areas, the 

 measured albedo ranged from to 10 %, while over overcast areas the albedo ranged 

 from 30 to 70 %, or even higher. Threshold values were chosen in the range of 

 from 10 to 20 % (the exact thresholds used in each case are specified in the subsequent 

 case study discussions). Trade-offs must be considered between the greater safety 



10 



