Short-Wave Solar Radiation in the Earth’s Atmosphere Part 9 pot

246 Analysis of Radiative Observations in Cloudy Atmosphere

Fig. 7.7a,b. Spectral dependence of single scattering co-albedo 1 – ω0 retrieved from the

ground observation data: a in Arctic, 1979 and b in St. Petersburg suburb (city Petrodvorets),

1996

Fig. 7.8. Spectral dependence of optical thickness τ0 retrieved from the data of the ground

observations in Arctic: experiment 11 – 13 August 1979 and experiment 12 – 08 October

1979

7.3.2

Data Processing of Satellite Observations

Optical thicknessτ0 and single scattering co-albedo 1−ω0 for extended clouds

were obtained with inverse asymptotic formulas [(6.13), (6.28)]. The approx￾imate accounting of the horizontal inhomogeneity including the scattering of

radiation by the upper atmospheric layers was accomplished with (6.36) and

(6.39). Multidirectional reflected radiance measurements with the POLDER

Optical Parameters from Ground and Satellite Observations 247

instrument were processed for the retrieval of cloud optical parameters. The

pixels with the cloud amount exceeding 0.5 were only considered.

The following sequence of the procedures for every pixel is proposed for

processing POLDER data:

1. At the first step the angular dependent functions are calculated.

2. The next step includes the calculation of the approximate optical thick￾ness for every viewing direction with the simple formula, assuming

the conservative scattering. The obtained values show the degree of the

shadowing influence (or the influence of the cloud top deviation from

the plane) and give the possibility to evaluate parameter r with (6.39).

Besides, they allow choosing the pairs of viewing directions where the

optical thickness is approximately equal.

3. The third stage consists of the parameters

2 retrieval from the radiances at

each pair of viewing directions with the equal optical thickness [(6.13)].

If the optical thickness defined at the previous stage without accounting

of the absorption is more than 100, parameter s

2 is obtained according

to (6.16). Then the averaging over all pairs of the viewing directions is

accomplished, and the relative mean square deviation is estimated.

4. At the fourth stage optical thickness τ0 is calculated for every viewing

direction, assuming the true absorption, and the results are averaged.

5. Then, the similar procedure is repeated for every available wavelength.

6. At the sixth stage the results are prepared for mapping (inserting the

missed pixels; inserting the values averaged over the neighbor pixels

to the missed pixels or to the pixels with only one viewing direction;

rejecting the edge pixels). The uncertainties are calculated for every

pixel using the formulas similar to (6.46).

7. Finally, the images of the single scattering co-albedo and optical thick￾ness are figured with the GRADS editor. The space distribution of single

scattering co-albedo (1 − ω0) is shown in Fig. 7.9, optical thickness τ0

is shown in Fig. 7.10 (Melnikova and Nakajima 2000a,b). The values of

(1 − ω0) are in the range 0.001–0.010; the optical thickness is about

15–25 and can reach 100 in the Tropics. Black gaps in the images cor￾respond to the pixels with the cloud amount less than 0.5. Four images

are presented in Figs. 7.9 and 7.10, the upper picture join three images

registered during the successive satellite pass with time interval about

one hour (i. e. these images are presenting one cloud field). Figure 7.11

demonstrates the values of (a) – single scattering co-albedo (1 − ω0),

and (b) – optical thickness τ0 and shadow parameter r multiplied by 102

in three spectral channels versus pixel numbers. The latter turns not to

depend on wavelength, and in contrast the spectral dependence of the

optical thickness decreases with wavelength for all (!) processed pixels.

Please remember that the processing has been accomplished for every

wavelength independently. The size of every pixel is about 60 km.

248 Analysis of Radiative Observations in Cloudy Atmosphere

Fig. 7.9. Images of single scattering co-albedo (1 − ω0) of the cloud pixels, retrieved from

POLDER data

Fig. 7.10. Images of optical thickness τ0 of the cloud pixels, retrieved from POLDER data