Fig. 2: Kriging interpolation of air-temperature measurements of 25 ANETZ-stations located over the Central
Plains of Switzerland for May 1, 1989, 5:00 UTC. All temperatures in degrees Celsius, coordinates are in
meters. ANETZ-stations are denoted with „+“ and an internal number above and the measured grass
temperature below. The Jura mountains are in the upper-left, the Swiss Alps in the lower-right of the rectangle.
In these areas, the interpolation is not valid du to topographic effects.
23 Linear combination of SSM/I brightness temperatures
The third approach uses the SSM/I 19 GHz brightness temperatures to estimate the physical
land-surface temperature. The advantage of this method is that these data are spatially and temporally
coincident with the microwave data. Neglecting cosmic background radiation and the atmospheric effect for
microwave radiation at 19 GHz, the microwave brightness temperature Tb f v of an object at frequency f and
polarisation p (p = vertical or horizontal) is proportionally related to its emissivity e fp and physical
temperature T:
' n >f,P = e f,P T (1)
Ground measurements of the emissivity ej p at frequencies 4.9, 10.4, 21, 35 and 94 GHz of
19 different surface types (including open water, vegetation and snow-free and snow-covered areas) shows
(MATZLER, 1994) that the relationship
l _e f, v “ e f,v -e fji ®
is valid for frequencies below 37 GHz and small amounts of open water and snow within the observed area.
Prom (2) it follows that
2e f,v-ef4,“ConsL
Assuming that Eq. (2) indicates a proportionality we get instead of (3) the more general form, namely
(3)
521
