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Archive for the ‘RGB’ Category

I’ve written about the operational utility of Day-Night Band (DNB) RGB imagery several times in the SPoRT blog, and here I’m going to take the chance to do that again.  First, just some brief background information in case you’re not familiar with this type of imagery.  The DNB RGB is composed of a long wave IR channel (~10.8 µm), which is assigned to the blue color component of the RGB recipe, while the DNB channel (0.7 µm) is assigned equally to the red/green colors of the RGB.  SPoRT produces two DNB RGB products: Radiance and Reflectance.  I typically prefer the Radiance RGB for operational uses since it is composed of the raw data (emitted and reflected light) from the sensor.  Sure, cities are quite bright in the imagery, but the cloud features also stand out better compared to the reflectance product, where the data are normalized by the available amount of moonlight. Below are a few observations from the Suomi-NPP VIIRS instrument during this most recent full moon cycle.

First, take a look at the images below from the SE half of the CONUS on the early morning of December 7th.  The top image (Image 1) is a Nighttime Microphysics RGB at approx. 0736 UTC, while Image 2 is a DNB Radiance RGB valid at the same time.  While this type of imagery is far superior to legacy IR imagery (even enhanced with fanciful color curves), there are proper operational forecasting/analysis applications that one has to consider.  The Nighttime Microphysics RGB is generally more useful for distinguishing different cloud types (e.g., low stratus vs fog, thin cirrus vs thick cirrus, etc).  After at least a year of viewing the DNB imagery, I think perhaps the best application of these types of imagery (at least with respect to operational forecasting) lies in the ability to view low clouds through cirrus at night.  No other imagery available to forecasters offers this capability currently.  Take for example these first two images below and pay close attention to the cloudy regions stretching from the central Plains into the lower Mississippi Valley.

Image 1.  Nighttime Microphysics RGB 0736 UTC 7 Dec 2014

Image 1. Nighttime Microphysics RGB 0736 UTC 7 Dec 2014.  Ceiling and Visibility observations from some ASOS and AWOS stations also shown in cyan.

Suomi-NPP VIIRS Day-Night Band Radiance RGB 0736 UTC 7 Dec 2014.  Ceiling/Visibility observations are shown in cyan.  Notice that details of the extensive deck of low clouds can be seen more easily than in the Nighttime Microphysics RGB.

Image 2.  Suomi-NPP VIIRS Day-Night Band Radiance RGB 0736 UTC 7 Dec 2014. Ceiling/Visibility observations are shown in cyan. Notice that details of the extensive deck of low clouds can be seen more easily than in the Nighttime Microphysics RGB.

Notice that in the Nighttime Microphysics RGB the expansive deck of low stratus across much of Kansas, southwestern Missouri and Oklahoma is almost entirely obscured by the cold cirrus clouds.  Of course, this is only realized upon looking at the DNB imagery.  Details in the low stratus can also be observed in the DNB imagery, such as the cloud banding stretching SW-NE across much of northern Louisiana and Mississippi.  Since the cloud bases in this imagery were mostly at MVFR and IFR levels with respect to aviation forecast concerns, knowledge about the details and characteristics of the low clouds are very important.

The next series of images from the New England region in the early morning hours of December 9th again demonstrates this application of the DNB imagery.

Image 3.  Suomi-NPP VIIRS IR (~10.8 u m) 0658 UTC 9 Dec 2014.  Ceiling/visibility observations from regional ASOS/AWPS are shown in cyan.

Image 3. Suomi-NPP VIIRS IR (~10.8 µm) 0658 UTC 9 Dec 2014. Ceiling/visibility observations from regional ASOS/AWPS are shown in cyan.

Image 4.  VIIRS Nighttime Microphysics RGB 0658 UTC 9 Dec 2014.  Ceiling/visibility observations shown in cyan.

Image 4. VIIRS Nighttime Microphysics RGB 0658 UTC 9 Dec 2014. Ceiling/visibility observations shown in cyan.

Image 5.  VIIRS DNB Radiance RGB 0658 UTC 9 Dec 2014.  Ceiling/visibility observations shown in cyan.

Image 5. VIIRS DNB Radiance RGB 0658 UTC 9 Dec 2014. Ceiling/visibility observations shown in cyan.

In the images above, notice that the extensive low cloud deck across the region that spans from Maine to at least as far south as northeastern North Carolina cannot readily be observed either in the legacy IR (10.8 µm ) imagery or in the Nighttime Microphysics RGB.  However, more details about the low clouds can be discerned from the DNB imagery.  Sure, cirrus clouds are optically thick enough to prevent viewing of any low clouds in the NY metro area.  Nevertheless, the advantages of the DNB imagery for detecting low clouds beneath thin cirrus can clearly be seen.  Again, as expressed earlier, this type of imagery certainly offers application for aviation forecasting, in particular.

Lastly, here are some observations from just this morning (Dec 10th) over the Rio Grande Valley region.

VIIRS color-enhanced IR (10.8 u m) image 0819 UTC 10 Dec 2014.  Ceiling/visibility observations are shown in cyan.

Image 6.  VIIRS color-enhanced IR (10.8 µm) image 0819 UTC 10 Dec 2014. Ceiling/visibility observations are shown in cyan.

Image 7.  VIIRS DNB Radiance RGB 0819 UTC 10 Dec 2014.  Ceiling/visibility observations are shown in cyan.

Image 7. VIIRS DNB Radiance RGB 0819 UTC 10 Dec 2014. Ceiling/visibility observations are shown in cyan.

In the VIIRS IR image (Image 6) just as in previous IR imagery the cirrus clouds obscure the presence of any clouds beneath.  However, the patchy low clouds in eastern New Mexico can be much more easily seen in the DNB imagery.  In the area between Midland, TX (KMAF) and Fort Stockton (KFST), a forecaster might have made the assumption that the low clouds were continuous based on the observations alone and without the aid of the DNB imagery.  Yet, what becomes noticeable in the DNB imagery is that a gap exists in the low cloud deck.

Of course, with all of this said, the availability of the imagery severely limits its application for operational forecasting and analysis.  Generally, only one or two passes are available over a given location on any night.  Also, due to moonlight limitations, the imagery are only available for about half of the month…at best.  I can only lament that the DNB imagery will not be available on a geo-stationary platform (at least anytime soon).  Nevertheless, understanding the limitations of the imagery while also appreciating its advantages can offer operational utility when applied properly to a forecast challenge.

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So, with the moon now passing into the waning crescent phase, the Day-Night Band imagery is less operationally useful, at least for the detection of fog and other lower level cloud types.  That is, at least until the moon is back into the waxing gibbous phase.  Nevertheless, when cirrus clouds aren’t present, the Nighttime Microphysics RGB has proven to be a very valuable tool for the detection of fog and other low-level clouds.  Just this morning a forecaster at the Huntsville, AL WFO was able to use the imagery not only for the detection of fog, but also to aid in the issuance of a special weather statement about the fog.  The image below valid at ~724 UTC (0224 am CDT) 17 Oct shows the fog (whitish-aqua colors) lying across the valley areas of NE Alabama and adjacent areas of southern Tennessee and NW Georgia.

MODIS Nighttime Microphysics RGB 724 UTC 17 October 2014

 

Around the time of this image, the visibility in the foggy locations had decreased to ~1/4 – 1/2 SM or less.  Notice the fog in the DeKalb Valley is fainter than the fog in areas to the north and west.  Not only is the DeKalb Valley more narrow, but the fog was likely more shallow.  This feature of the imagery can also help to guide forecasters in assessing the longevity of the fog once sunrise breaks.  Over time, forecasters can develop a sense of pattern recognition with the varying degrees of color shading and tailor forecasts to better match the time of dissipation.  In this case, the fog in the DeKalb Valley began to dissipate significantly by about 1430 UTC, while  the deeper and more expansive fog to the north and west lasted about an hour longer.

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I wanted to follow up with this sooner, but I’ve been rather busy preparing for the NWA conference.  Yep, I tend to be one of those “last-minute” people.  Anyway, just last week I posted an example showcasing the advantage of the VIIRS Day-Night Band Radiance RGB in detecting fog through cirrus clouds.  The next day, SPoRT collaborator Doug Schneider (Morristown WFO), followed up with another great example that I wanted to share.  I’m just going to take an excerpt from an email he sent to the Morristown staff…

“I know I’ve been sending out quite a few emails lately about NASA SPoRT products, so please bear with me, but I thought this was a great example of how they can add value.

I’ve mentioned the MODIS/VIIRS Fog product before, but sometimes there are better products available for identifying fog, especially when thin cirrus are present. In the MODISVIIRS Fog product image that is attached, you can see that it is difficult to see the extent of fog. There’s clearly some in the Sequatchie Valley, but there is also some in the NE TN/SW VA area that can’t be easily seen.

The attached MODIS/VIIRS Nighttime Microphysics product also shows fog in the southern areas where it is clear (fog is light blue colors), but cirrus obscures NE sections.

The VIIRS Day/Night Band Radiance RGB product does the best job showing the extent of fog. Fog is clearly identified in the valleys of SW VA and NE TN, despite the presence of cirrus. The extent of fog is also more easily seen in the central and southern valley areas.

The attached menu image shows where you can find the Day/Night Band Radiance RGB product under the Satellite -> NASA SPoRT -> Polar Imager -> MODIS/VIIRS -> SR East menus.

Remember that these products are from a polar-orbiting satellite, and may only be available once a night, usually between 07z and 10z.”

I’ve included the images he referenced below, and circled the specific area of fog in NE Tennessee / SW Virginia that he was mentioning.

Suomi-NPP VIIRS Day-Night Band Radiance RGB, 0724 UTC 10 Oct 2014.  Circled area shows valley fog not detectable in subsequent GOES or VIIRS NT Microphysics images.

Suomi-NPP VIIRS Day-Night Band Radiance RGB, 0724 UTC 10 Oct 2014. Circled area shows valley fog not detectable in subsequent GOES or VIIRS NT Microphysics images.

Suomi-NPP VIIRS 11-3.9 um image, 0724 UTC 10 Oct 2014.  Notice that high cirrus clouds (blue colors) obscure the fog below.

Suomi-NPP VIIRS 11-3.9 um image, 0724 UTC 10 Oct 2014. Notice that high cirrus clouds (blue colors) obscure the fog below.

Suomi-NPP VIIRS Nighttime Microphysics RGB, 0724 UTC 10 Oct 2014.  Fog can be seen in other areas to the south/southwest, but not beneath the cold, cirrus clouds (under the yellow circle) since this RGB recipe contains multi-spectral IR components.

Suomi-NPP VIIRS Nighttime Microphysics RGB, 0724 UTC 10 Oct 2014. Fog can be seen in other areas to the south/southwest, but not beneath the cold, cirrus clouds (under the yellow circle) since this RGB recipe contains multi-spectral IR components.

This is exactly the kind of inter-office sharing SPoRT is looking for from our close partners.  We greatly thank Doug and the Morristown office for their collaborative efforts!

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From September 20 through September 23, 2014, the Ocean Prediction Center (OPC) was monitoring the development of the season’s first hurricane-force extratropical storm in the East Pacific.  Models were suggesting a marginal hurricane-force wind event would unfold well west of the Pacific Northwest, near 140W longitude, north of 40N latitude.  OPC is routinely using satellite data to monitor and forecast these strong ocean storms.  On this particular event, OPC forecaster James Kells collaborated with Michael Rowland and David Kosier on if and when to pull the trigger on the hurricane-force warning.

GOES-15 6.5 um water vapor animation showing the evolution of the hurricane-force low.

GOES-15 6.5 um water vapor animation showing the evolution of the hurricane-force low.

The above animation shows the evolution of the hurricane-force low, with an eye-like feature evident near the end of the loop.  By 1200 UTC on the 23rd, it was forecast to develop hurricane force winds (64 knots or greater) just west of Oregon near 140W.  During the production of the 1200 UTC OPC Surface Analysis, there was question of whether or not the winds had reached hurricane force intensity. The ASCAT pass from ~0600 UTC showed a large area of 50-55 knot winds in the strong cold advection south of the low center, and the GFS model indicated that the system was still developing.  The GFS 0-30m boundary layer winds also indicated a very small area with hurricane force intensity.

Advanced Scatterometers A and B overlaid on GOES-15 Infrared imagery showing storm force winds at ~0600 UTC on 09/23/14.

Advanced Scatterometers A and B overlaid on GOES-15 Infrared imagery showing storm force winds at ~0600 UTC on 09/23/14.

The 1130 UTC MODIS RGB Air Mass product was timelier, and it showed an area of downward momentum south of the low with the deep purple shading. The corresponding water vapor image was less clear with upper level moisture obscuring the downward motion just beneath it.   In addition, there were no surface reports south of the low center as there were no buoys moored nor drifting in that vicinity.  Furthermore, most ships were aware of the danger and navigated away from the region neglecting the possibility of a surface report in the area of question.

Aqua MODIS RGB Air Mass image from 1130 UTC on 09/23/14.

Aqua MODIS RGB Air Mass image from 1130 UTC on 09/23/14.

A cross-section of the 1200 UTC 09/23/14 GFS model potential temperature and dew point temperature was taken through the low center in order to analyze the depth of the stratospheric intrusion, and also to gauge the magnitude of the downward momentum.  It showed a deep stratospheric intrusion to roughly 500 hPa, and it corroborated the strong downward momentum indicated by the imagery.  The RGB Air Mass image showed the intensity of the downward momentum through the red/purple coloring and gave a good indication of the stronger winds aloft mixing down toward the surface.  The imagery increased confidence with classifying the system as a hurricane force low.

The 1200 UTC 09/23/14 GFS vertical cross-section of potential temperature and dewpoint showing the downward transport of drier air associated with the tropopause fold.

The 1200 UTC 09/23/14 GFS vertical cross-section of potential temperature and dewpoint showing the downward transport of drier air associated with the tropopause fold.

The 1200 UTC 09/23/14 OPC surface analysis.

The 1200 UTC 09/23/14 OPC surface analysis.

~ Guest blogger, James Kells (OPC)

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A unique weather event is unfolding this week as Hurricane Odile, now a tropical storm, is impacting Baja California Sur, bringing heavy rain and high winds to the region and causing tourists to evacuate resorts. The National Hurricane Center reports that Odile ties Olivia (1967) as the strongest hurricane to make landfall in the satellite era in Baja California Sur**. NASA SPoRT provides specialized satellite products to National Weather Service Forecast Offices as well as National Centers such as the National Hurricane Center to aid forecasting high impact events such as Hurricane Odile.

Below is an example of Passive Microwave RGB imagery created from the NASA Global Precipitation Measurement (GPM) mission as part of The Core Observatory satellite launched on 27 February 2014. The images are in N-AWIPS (National Centers for Environmental Prediction Advanced Weather Interactive Processing System) format and are an example of products available to forecasters at the National Hurricane Center.  Forecasters use the 89 GHz RGB product to look for areas of strong convection which show up as deep red as seen in Fig. 1 which captures Hurricane Odile a few hours before landfall.

89 GHz RGB 0121 UTC 15 September 2014. Areas of deep convection appear red and can be seen surrounding the eye wall and within the rainbands of Odile in this image a few hours before landfall.

Figure 1. GMI 89 GHz RGB 0121 UTC 15 September 2014. Areas of deep convection appear red and can be seen surrounding the eye and within the rainbands of Hurricane Odile in this image a few hours before landfall.

The 37 GHz can additionally be used to distinguish areas of deep cloudiness (light blue) from more active convection (pink) as well as open water (green) or land (cyan).  Note the areas of pink or active convective in Fig. 2 surrounding the eye and within the rainbands.

odile_37RGB1

Figure 2. GMI 37 GHz RGB 0121 UTC 15 September 2014. Areas of active convection appear pink and can be seen surrounding the eye and within the rainbands of Hurricane Odile in this image a few hours before landfall.

Figure 3 and 4 show similar observations from the legacy NASA Tropical Rainfall Measurement Mission (TRMM) as Hurricane Odile made landfall near Cabo San Lucas around 445 UTC 15 September. TRMM is expected to run out of fuel by February 2016 and will no longer be available to collect valuable observations. We are well prepared for a replacement with GPM in orbit and already collecting observations.

TRMM 89 GHz RGB 0307 UTC 15 September 2014

Figure 3. TRMM 89 GHz RGB 0307 UTC 15 September 2014.  Areas of deep convection appear red and can be seen surrounding the eye and within the rainbands of Hurricane Odile in this image a little over one hour before landfall.

TRMM 37 GHz RGB

Figure 4. TRMM 37 GHz RGB 0307 UTC 15 September 2014.  Areas of active convection appear pink and can be seen surrounding the eye and within the rainbands of Hurricane Odile in this image a little over one hour before landfall.

Additionally the Visible Infrared Imaging Radiometer Suite (VIIRS) Day-Night Band Radiance imagery from the next generation NASA Suomi National Polar-orbiting Partnership (NPP) satellite shows an impressive picture of Hurricane Odile approximately one day before landfall (Fig. 5). Note the city lights that can be seen through the clouds in Fig. 5 as well as lightning within the area of convection in the rainband. This imagery can be used to support disaster response and help emergency managers identify the areas where conditions have caused power outages. Local knowledge of city light patterns can allow users to identify where the most significant power outages are and determine where to begin relief efforts.

VIIRS Day-Night Band Radiance

Figure 5. VIIRS Day-Night Band Radiance 0904 UTC 14 September 2014. City lights and lightning observed approximately one day before Hurricane Odile made landfall.

As the community transitions from legacy instruments such at TRMM and MODIS, NASA SPoRT will continue to develop unique products from Next-Generation missions such as GPM and Suomi NPP to aid National Weather Service Forecast Offices and National Centers in forecasting high impact events such as Hurricane Odile.

**see archived National Hurricane Center forecast discussion at http://www.nhc.noaa.gov/archive/2014/ep15/ep152014.discus.021.shtml?

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It is said that a picture says a thousand words…well in this case let’s just say 434 words, as are contained in this post. Anyway, I’d like to point out six features in this morning’s Nighttime Microphysics RGB.  The image below (MODIS Nighttime Microphysics RGB) showed several features of varying degrees of operational relevance.

MODIS Nighttime Microphysics RGB with annotations valid 0755 UTC 16 July 2014

MODIS Nighttime Microphysics RGB with annotations valid 0755 UTC 16 July 2014

 

A myriad of cloud features can be observed, including fog in the valleys of central Appalachia, deep convective clouds along the Florida coast, patches of thin and thick cirrus over north-central Alabama, and low stratus clouds in Missouri…to name just a few.  Sure, this isn’t an exhaustive list of the potential cloud features to observe, but showcases the ability to contrast effectively between different cloud types.  Of perhaps significant interest is the ability to see the contrasting airmasses displayed across the Southeast region.  Notice the  pinkish colors north and west of the yellow curved line that stretches from central Louisiana to southern Virginia.  This represents a lower relative contribution of blue color, or lesser longwave radiation at the 10.8 µm wavelength, which is indicative of cooler temperatures.  To the south and east of this line, much more blue is apparent, which is thus indicative of warmer temperatures.   Surface observations valid at about the same time have been overlaid with the RGB image to provide temperature data context.  Air and dew point temperatures are around 10 degrees F cooler behind the line/front, but notice that the northerly wind shift is still on the south/east side of the line at such locations as Montgomery, AL and Columbus, GA.  At those locations, dew point temperatures were still 70 and 71 F, respectively, with air temperatures at 72 F.  So, the gradient in temperatures still lingered behind the surface front and is well depicted in the RGB imagery.  This type of information can be valuable to forecasters, as temperature, moisture, and wind characteristics are often complex in the vicinity of surface fronts.  Thus, while wind shifts may be observed initially, as in this case, the imagery shows the location of the temperature gradient much better.

The importance of this type of imagery is that it offers a much more effective assessment of meteorological phenomena than existing GOES imagery.  The only problem currently is the limitation of available imagery to forecasters, since these are from polar-orbiting platforms (Terra, Aqua, Suomi NPP), and thus provide just a few snapshots per night over a given location.  Nevertheless, the imagery form the VIIRS and MODIS instruments offer added value to existing GOES imagery and serve as valuable teaching and preparatory aids for future GOES-R and JPSS missions.

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I didn’t have a chance to make this post last week when the imagery were more time-relevant.  Nevertheless, I wanted to point out another example of the usefulness of MODIS and VIIRS imagery over current GOES imagery and show the usefulness of exciting products and imagery to come!  First, let’s take a look at the color-enhanced GOES-IR image below from the morning (0715 UTC) of June 20th.

Color-enhanced GOES-IR (11um) image valid 0715 UTC 20 June 2014

Image 1.  Color-enhanced GOES-IR (11 µm) image valid 0715 UTC 20 June 2014

 

I’ve placed the yellow circles in the image for a reason, which you’ll see below.  Further down, I’m going to show areas of fog displayed in the MODIS and VIIRS imagery, and granted, this is not the standard GOES channel difference (11-3.9 µm) typically used for making fog assessments.   However, this post is meant to show current (MODIS / VIIRS) and future capabilities (GOES-R / JPSS) that will make fog detection and cloud differentiation much more easy for the operational forecaster.  So, in the image above, fog is nearly unidentifiable as it was in the 11-3.9 µm channel difference image that morning (not shown).  Mainly high cirrus clouds can be observed scattered across the region.  Now, let’s take a look at the MODIS “fog” product, or channel difference (11-3.9 um) product valid at about the same time (Image 2).

Color-enhanced MODIS 11-3.9 u m product valid 0718 UTC 20 June 2014

Image 2.  Color-enhanced MODIS 11-3.9 µm image valid 0718 UTC 20 June 2014

Notice that in the same areas we can now begin to see low clouds (indicated by yellow colors) scattered around the valleys of the southern Appalachian region.  While the GOES-East imager is capable of detecting larger scale fog often in the valleys in the eastern circle, fog in the valleys in the western circle present challenges for the current GOES-East instrument, and is often not shown very well (even in the standard 11-3.9 µm channel difference).    Next, let’s take a look at a VIIRS Nighttime Microphysics RGB valid at about the same time.

VIIRS Nighttime Microphysics RGB valid 0723 UTC 20 June 2014

Image 3.  VIIRS Nighttime Microphysics RGB valid 0723 UTC 20 June 2014

In the RGB imagery it is much easier to detect the extent of the fog, making the operational forecast process much more effective.  Notice also that it is possible to see the fog through the higher clouds around the TN/GA/NC border region.  Not only does the resolution of the VIIRS and MODIS instruments allow for superior fog detection, but the RGBs in particular offer tremendous operational advantages.  As a user of RGBs for about 2 years now, I am convinced that this type of imagery has a relevant and needed place in future operational forecasting.  Of course, it will take time for forecasters to become accustomed and adjust to the new imagery, but it will happen.

 

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