RGB Imagery Evaluation for Aviation and Cloud Analysis Underway at SR NWS Coastal Offices…

The RGB Imagery for Aviation and Cloud Analysis got underway on December 1st with SPoRT’s collaborative coastal NWS offices in Southern Region.  These offices include Corpus Christi, Houston, Slidell, Mobile, Melbourne, and Miami.  A separate evaluation with AK offices and CA/OR coastal offices has also begun recently.  The SR coastal office evaluation will run through the end of January, where offices will be evaluating the VIIRS and MODIS Nighttime Microsphysics RGB imagery, VIIRS Day-Night Band Reflectance and Radiance RGBs, in addition to the hybrid GOES/MODIS/VIIRS 11-3.9 um looped product.  SPoRT personnel have conducted training for the offices and will be helping during the evaluation with questions and/or technical issues.  Although RGBs have been used in the European forecast community for years, they are quite new to most U.S. forecasters.  However, and importantly, the imagery available from the Aqua/Terra satellites (MODIS imager) and the Suomi NPP satellite (VIIRS imager) are a part of GOES-R and JPSS Proving Ground activities and will serve as educational tools for forecasters before the GOES-R and JPSS eras.  As a part of the evaluation, forecasters will answer a short survey about the operational impact of these imagery on aviation forecasts in particular, but may of course include impacts for other operational products (i.e. advisories, fire weather, public, hydrologic, etc).

While many potential positive impacts to various forecast products have been related on this blog, I’ll be watching and posting those which forecasters at these offices (and myself) observe during the evaluation period (time permitting of course).  Take the following case from yesterday, December 9th, for example…

Image 1.  SUOMI NPP VIIRS Nighttime Microphysics RGB valid 9 DEC 2013 0736 UTC.

Image 1. SUOMI NPP VIIRS Nighttime Microphysics RGB valid 9 Dec 2013 0736 UTC.

In the image above, notice the swath of light purple colors that extend across a good portion of the TX Gulf Coast.  Further north, in north central and northeastern TX extending to include portions of Oklahoma, Missouri and Arkansas, an area of low clouds with colors closer to dull reds to greenish-white are apparent.  In Image 1, a small area near Corpus Christi, TX has been sampled, with the contributions from Red (183), Green (132) and Blue (209) included in the image (This was sampled in Microsoft Paint).  At about the same time, observations across this region of coastal TX were nearly uniform.  Ceilings were around 300-400 ft from Houston, to Port Lavaca and Corpus Christi, with visibilities ranging from 1.5 to 2.5 SM.

Image 2.  GEOES IR image (730 UTC) with Ceiling (AGL) and Visibility observations (0800 UTC) 9 Dec 2013.

Image 2. GEOES IR image (730 UTC) with Ceiling (AGL) and Visibility observations (0800 UTC) 9 Dec 2013.

Thus, the colors represented by the shades of light purple represented an extensive low stratus/fog deck encompassing the area.  Notice that a swath of this color/cloud type also extended into northern Louisiana and Mississippi.  Low visibilities ranging from 2.5 to 3 SM and low ceilings around 400 ft were observed in both areas.

Herein lies the power of the RGB imagery.  Since the combination of colors are related to several physical characteristics (i.e. red – optical depth, green – particle phase and size, blue – temperature), then it is easier to make assessments about cloud homogeneity or inhomogeneity.   While other satellite observations generally just relate one physical characteristic (usually temperature), or in the case of the standard 11-3.9 channel (particle phase/size), they don’t have the ability to tie together several physical characteristics together in one image like the RGBs can.  It is thus much easier, with RGB imagery, to assess locations where cloud characteristics are the same and make inferences about the similarity of ceilings and visibility in areas without direct observations.

This next image shows a sample of the color taken from the Texarkana site in NE Texas at the same time, underneath the area of low stratus containing more dull red colors.

Image 3.  Suomi NPP VIIRS Nighttime Microphysics RGB valid Dec 9 2013 0736 UTC.

Image 3. Suomi NPP VIIRS Nighttime Microphysics RGB valid 9 Dec 2013 0736 UTC.

As the difference in colors suggests, the cloud characteristics are different here than in SE coastal TX.  Referring to image 2, the ceiling and visibility at Texarkana were 1100 ft and 6 SM, respectively.  Ceilings were still relatively low, but were higher than in coastal TX, as was the visibility.  Essentially, this was a slightly higher stratus deck.  The red color contibutions were very similar in each location, suggesting clouds of similar depth.  However, differences in green and blue are clearly discernible.  The cloud deck near the coast certainly contained more blue, indicating warmer temperatures, which makes physical sense.  The clouds in NE TX contained more green however, which would suggest smaller water particle size. But, emssions in the 3.9 channel from the surface beneath the low/thin cloud deck near the coast may also be contributing to less green color there.  Taking a look at proximity soundings in this area of clouds from Forth Worth (FWD) and Little Rock (LZK), cloud tops decreased during the 00-12 UTC period, but contained super-cooled water droplets by the 12 UTC sounding.

As an aside, forecasters have expressed the desire (including myself) to have the specific values from the red, green, blue color contributions available in AWIPS when sampling the imagery.  This is a valuable part of the feedback and evaluation process.  Unfortunately, this is not possible in AWIPS I, but will be in AWIPS II.

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