Another impact of the viewing geometry is parallax - the apparent shift of position of an elevated feature with respect to its real location, resulting from a tilted view of the feature. The effect is explained in the next figure (Fig. 2a):
For a full size view, click on the image.
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| Figure 2a. Parallax shift of an elevated object, as observed from geostationary orbit. The small red circle at top of the imaginary "overgrown" storm represents e.g. an overshooting top here. The two red dotted lines indicate apparent location of this feature as observed from two different places in space - one representing a nadir view (e.g. from a satellite in polar orbit, with the storm at its swath center), and the other one a view from a geostationary satellite. While the nadir view places the feature properly, at its real location, slant view from the geostationary position shifts the apparent location of the feature significantly northward (up in this drawing) with respect to the real position of the feature. This difference in apparent location of the feature (indicated by the thick red double arrow) is called the "parallax shift". |
In the drawing above we used as an example of the nadir view, a polar-orbiting satellite, but actually this can be any other observing tool which displays the feature properly with respect to its real location. It can be e.g. any ground data (weather station, hail pad, ...), ground weather radar, lightning detection system, or similar. From the drawing it is obvious, that for the geostationary observations the parallax shift will grow with growing distance of the object from the center of the globe as viewed by the satellite (from its sub-satellite point, its nadir). Also, the higher the object (feature) is above the ground, the larger is the parallax shift.
For a full size view, click on the image.
For a full size view, click on the image.
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Figure 2b. Parallax shift for a satellite at polar orbit. The two red dotted lines indicate the edges of the image swath, the longer dotted blue line its center (nadir view, center of the satellite swath). The thick red double arrow indicates the parallax shift for a storm further away from the swath center. |
As is shown in Fig. 2b, the parallax shift also affects observations from polar-orbiting satellites and their swath-scanning instruments (such as AVHRR or MODIS). In the cases above, the parallax shift will grow with increasing distance of the storm from the image swath center, and again also with the storm height.
One should keep in mind the possible impacts of the parallax shift namely when comparing various satellite data, or when combining these with other, non-satellite data (such as weather radars). Also, some of the satellite derived products (such as cloud mask, or satellite rainfall estimates) may be located improperly if the data is not corrected for the parallax shift - e.g. resulting in clouds apparently precipitating elsewhere than where it really rains ...
The parallax shift is not an issue in regions nearby the area close to the sub-satellite point, but has to be considered at larger distances from the nadir - e.g. at mid-latitudes of central Europe for storm tops at about 15 km the shift reaches ~ 25 km.
More about the parallax shift can be learned here: http://essl.org/cwg/?page_id=165 (including precomputed tables with the parallax shift values for the entire globe as seen by Meteosat satellites, and for storm tops at various heights), or in a recorded webcast here: http://www.essl.org/cwg/res/html/parallax_shift.html.
