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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| True Color Product : Difficult to Discern Any Contrails | Contrail Product: Contrails Emerge As Linear Features |
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| Condensation trails (contrails) can provide potentially very useful information on the location and movement of aircraft. The MODIS contrail product is a qualitative tool designed to enhance these contrails and provide the analyst with a first-order assessment of aircraft activity in those regions conducive to contrail formation. Contrails appear as linear features in the enhanced imagery. | |
| Provided the right atmospheric temperature and moisture conditions, upper
tropospheric water vapor will condense upon the small exhaust particles of
aircraft to form a distinct cloud trail. Under favorable conditions for
contrails, the structures may persist for several hours and even evolve
into naturally occurring cirrus, while in other cases only a short-lived
contrail (or none at all) will form. The contrail enhancement invokes a tri-spectral (8.5, 11.0, and 12.0 micrometer channels) approach which effectively decouples liquid and ice phase clouds. The different particle size distributions occurring in contrails compared to any naturally occurring cirrus in the same scene results in a spectral signature when performing differencing between the above-mentioned channels. It is this signature that allows contrails to be enhanced in the false-color imagery. |
| The main advantage of this product is its ability to draw attention to features that are otherwise too subtle to detect in conventional visible/infrared satellite imagery. Knowledge of areas favorable for contrail formation may also provide pilots with valuable guidance pertaining to minimizing the chances for visual detection. On the other hand, information on the movement of potentially hostile aircraft may also be revealed. |
| Example of Line Striping Artifacts in Current Product |
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| One must remember that the atmosphere is always in motion, and the position
of an old contrail may be displaced tens to hundreds of kilometers downwind
of where it was formed. Wind directional and speed sheer can distort the
features of contrails. In some cases one can identify the leading edge of the contrail an use this as an indicator of actual aircraft position at the time of satellite image collection, however caution must be exercised to not be mislead by the possibility that the aircraft simply departed a region of favorable contrail formation. Nearby contrails can sometimes be used to determine this likelihood. The current product performs best in the vicinity of thin cirrus, although stand-alone contrails are also enhanced when present. Owing to subtle calibration differences between the detectors aboard MODIS, a line-striping artifact is often observed in the contrail product. These artifacts are usually easy to pick out and differentiate from the contrails, as they are equally spaced and are all oriented slightly diagonal in the imagery (normal to the flight track). The example above illustrates the striping effect in the presence of actual contrails. |
| A Shield of Cirrus Gives No Indications of Contrails Present | The Contrail Enhancement Reveals Distinct Flight Pattern |
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| The contrail product can reveal flight pattern information if contrails are formed in regions of weak shear effects. In the example above, a race-track flight pattern often associated with aircraft refueling tankers is revealed. | |
| Contrails in the Arabian Gulf |
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| Contrail activity in and around Iraq in advance of a potential conflict with Iraq increased during March, 2003. Unusual (with respect to commercial) contrail patterns appear in the image above, including aircraft formation flying and various tight radius manuevers. |
Author: Steve Miller Last Updated: Mon Mar 10 11:55:47 2003 Produced by: The Composer (Ver: 1.4 ) |
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