the rain rate as observed during a period of 10 days has been reported earlier 
(Schanda and Mätzler, 1981). 
GLOBAL SNOW MAPS RETRIEVED FROM NIMBUS-7 
SCANNING MULTICHANNEL MICROWAVE RADIOMETER DATA 
The Nimbus-7 satellite, launched into a sun-synchroneous orbit on 
October 24, 1978, carries a five frequency (6.6, 10.7, 18, 21, 37 GHz) dual 
polarized, conical scan microwave radiometer (SMMR) with correspondipg surface 
resolution (cell sizes) of approximately 150 x 150 km^ to 30 x 30 km^. The I 
SMMR operates on alternate days due to space-craft power limitations. One full I 
coverage of the entire earth is completed every 5 days with many overlapping Hl 
cells in the near to polar regions. SMMR was designed for the global determi- | 
nation of environmental parameters of weather and oceans. | 
| 
For snow detection combinations of the outputs of the various 
frequency channels can be selected to achieve a significant discrimination 
and quantitative determination of a few important parameters of the snow cover. 
In a recent investigation (Kuenzi et al. 1981 and 1982) with the aid of an 
interactive image display system the retrieval algorithms for the extent of 
dry snow, for the onset of snow melt and for the determination of the snow ih 
depth have been developped. SMMR data sets of two periods October/November Ii 
1978 and February/March 1979 have been used. 
The algorithm for calculating the extent of dry snow makes use of the 
decrease of brightness temperatures with increasing frequency : T, (37 GHz) - | 
Tg (18 GHz) « D, where the value D can be adjusted to obtain correspondence to | 
the ground truth data. 
36 GHz 
ue to 
  
Figure 6 presents a map of snow cover of the northern hemisphere of 
the dates November 11, 12, 13, 1978. The dark grey level corresponds to 
Tg (37) - Tg (18) » - A Kc and- isiddentified snow-free land surface, the values 
- 5 K to - 8 K are presented as light grey areas corresponding to thin (« 10 cm) 
or patchy snow, while all values below - 9 K, presented as white areas correspond i 
to a cover of dry snow of more than ~ 10 cm thickness, it should be noted here Il 
that this algorithm is valid only for snow cover over land and not over ice Hil 
shields (Greenland) or sea ice. We remind here to the fact that moist snow And 
exhibits a brightness temperature spectrum like Fig. 2 while the same cover of | 
dry snow behaves like Fig. 1. If now within the days of observation due to 
sunshine over day or due to warm air masses a large area of the snow cover 
becomes moist, the resulting change of the spectrum can be detected and the 
regions of melting snow can be delineated. Figure 7 presents à snow cover map 
of Northern America of March 1, 3, 5, 1979. Additionally to the algorithm for 
mapping of dry snow (as applied in Fig. 6) a supplementary algorithm utilizes | 
here the temporal change of the T, (37) - Tg (18) values. Areas for which this | 
change during the three days surpässed a given limit (> 12 K) were identified | 
as melting snow and are presented in light grey. A more detailed explanation 
of algorithms used and a more thorough discussion of the results achieved wi th 
the Nimbus-7 SMMR data is given in the above-mentioned publications. It is worth 
mentioning that all these results have been verified by the ground truth made 
available by a large number of snow observing stations in Canada, Finland and 
the USSR. This was particularly important for the estimation of the snow depth. 
  
  
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