Fig. 2.: Another example of dust (yellow enhancement) in the desert southwest portions of the United States, as observed by Aqua-MODIS on 5 April 2010 at 2100 Z. Here, mineral dust from the Painted Desert region of Arizona is seen lofted in the strong pre-frontal southwesterly winds of a baroclinic system. The dust later affected the air quality and visibility near Grand Junction, and deposited on parts of the snow covered San Juan Mountains to the northeast.
The Blowing Dust Enhancement product is designed to simplify the detection of any significant dust regions present in a potentially complex scene. We do this by gathering together all the ‘dust discriminators’ as outlined in Section 2 of this training module, and present it in a visual form that helps the dust stand out from other constituents of the scene. The end result is imagery that is ‘false color’ (i.e., it no longer looks ‘real’ in comparison to true color imagery) but provides a less ambiguous depiction of where dust resides.
What to look for: Regions of pink/orange against dark green backgrounds (or in the case of the yellow-dust variant, look for regions of yellow against dark blue backgrounds). Sometimes dust plumes will display sharp boundaries (as in Fig. 1, and most often along the leading edge of a dust front), but more often possess a diffuse appearance and may ‘fan out’ like cirrus clouds.
Fig. 5: Example of cold terrain effects. Here, mountainous regions in the southwestern U.S. appear red.
What to watch out for: Cloud shadows, coastlines (both ocean and lake), and cold terrain (especially mountain ranges and elevated plateaus during the winter months; see Fig. 5) occasionally appear ‘enhanced.’ If used regularly, analysts will quickly become adept at identifying and discarding these residual artifacts of the algorithm. Dry lake beds may also appear bright pink in this enhancement. These lake beds often serve as point-sources for lofted dust.
Other Considerations: While major dust outbreaks do occur typically a few times per year over the southwestern deserts of the United States, they are far less prolific than the expansive deserts of Africa and Eurasia. A more common variety of dust storm to the United States is associated with the cold-pool outflow of thunderstorm complexes. Called “Haboobs” in the Middle East, these storms form literal walls of dust on scales of several miles (compared to hundreds or even thousands of miles for major dust storms). These storms can reduce visibilities at the surface to near zero in a matter of minutes. Oftentimes this variety of dust storm is missed by a polar-orbiting sensor like MODIS due either to time sampling (catching the event in progress from one or two snap-shot observations) or their being obscured by overriding clouds associated with the parent thunderstorm(s) at the time of observation. With the advent of GOES-R ABI, detection of these storms will be improved significantly.
Note: The current version of the Blowing Dust Enhancement product requires information from sunlight reflection, and thus is only valid for daytime observations. A future version this product that uses additional infrared bands that will be available from GOES-R ABI will provide a 24-hr capability. A variant on this product is being developed for the MSG-SEVIRI sensor.
The main advantage of Blowing Dust Enhancement imagery over conventional single-channel imagery is the ability to make a rapid assessment of dust in the scene. Its advantage over other dust enhancements is the ability to do so while suppressing (in color/brightness) the non-dust components of the scene. Another advantage of imagery-based enhancements (vs. pure quantitative products such as a ‘dust mask’) is that imagery retains the meteorological context of the scene. Dust is often lofted as a result of characteristic wind flow patterns (e.g., in the cold sector of a baroclinic system, terrain-locked circulations, or even as the result of thunderstorm outflows as often occurs in the desert southwest of the United States) that can often be inferred from the meteorological cloud field.
The main limitation is the depiction of the product in AWIPS as an 8-bit color (256 colors) image. Since the Blowing Dust product is in fact a 24-bit red/green/blue composite, there will be some degradation in quality when attempting to represent a broad color palette (256^3 = almost 17 million) in a reduced number of color tones. This challenge will go away in AWIPS II, which is advertised to have the capability to display 24-bit imagery.