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CHAPTER 10. THEORETICAL BACKGROUND
10.4.3
217
Water vapor retrieval
A water vapor retrieval can be included after the aerosol retrieval because the aerosol retrieval
does not use water vapor sensitive spectral bands, but the water vapor algorithm (employing bands
around 940 or 1130 nm) depends on aerosol properties. The water vapor retrieval over land is
performed with the APDA (atmospheric precorrected differential absorption) algorithm [81]. In its
simplest form, the technique uses three channels, one in the atmospheric water vapor absorption
region around 940 or 1130 nm (the ”measurement” channel), the others in the neighboring window
regions (”reference” channels). The depth of the absorption feature is a measure of the water vapor
column content, see figure 10.15.
In case of three bands the standard method calculates the water vapor dependent APDA ratio as :
RAP DA (ρ, u) =
L2 (ρ2 , u) − L2,p (u)
w1 (L1 (ρ1 ) − L1,p ) + w3 (L3 (ρ3 ) − L3,p )
(10.85)
where the index 1 and 3 indicates window channels (e.g. in the 850-890 nm region and 1010-1050
nm region), respectively. Index 2 indicates a channel in the absorption region (e.g., 910-950 nm).
L and Lp are the total at-sensor radiance and path radiance, respectively. The symbol u indicates
the water vapor column. The weight factors are determined from
w1 = (λ3 − λ2 )/(λ3 − λ1 )
and
w3 = (λ2 − λ1 )/(λ3 − λ1 )
(10.86)
Figure 10.15: Reference and measurement channels for the water vapor method. The at-sensor radiance
is converted into an at-sensor reflectance.
The problem is the estimation of the surface reflectance ρ2 in the absorption band (eq. 10.85). The
technique tries to estimate the reflectance ρ2 with a linear interpolation of the surface reflectance
values in the window channels (ch. 1, 3) that are not or only slightly influenced by the water vapor
content: Therefore, the reflectance ρ2 is calculated as
ρ2 = w1 ρ1 + w3 ρ3
(10.87)
Then equation (10.85) can be written as
RAP DA (u) =
ρ2 τ2 (u)Eg2 (u)
τ2 (u)Eg2 (u)
=
ρ2 τ2 (u = 0)Eg2 (u = 0)
τ2 (u = 0)Eg2 (u = 0)
(10.88)