A ridge of high pressure moved into the desert southwest in the wake of a strong winter storm system that impacted the region last weekend. Strong low level inversions beneath the ridge and melting snow cover increased the potential for an extended period of low clouds and freezing fog. The MODIS Nighttime Micropyhsics product valid at 0501 UTC November 29, 2013 showed low clouds and possible areas of freezing fog along the I-70 corridor over and west of Grand Junction. Observations at Grand Junction verified IFR low clouds at 900′ with visibilities above 6 miles. The following VIIRS Nighttime Microphysics product at 0904 UTC November 29, 2013 showed low clouds and freezing fog expanding eastward into the Colorado, Gunnison, and Uncompahgre River Basins and the Grande Valley south of Grand Junction. Visibilities have been significantly reduced to 1/4 mile at Montrose and farther south at Cortez. The ceiling at Montrose is 200′ with visibility at 3/4 mile. The nighttime microphysics product shows exceptional detail within the narrow valleys along Roan Creek to the northeast of Grand Junction. Notice the fog and low cloud deck is very close to Rifle at 0904 UTC but the observation still suggests clear skies. 31 minutes later at 0935 UTC the observation at Rifle dropped to LIFR at 400′ however visibilities remained at or above 6 miles.
Archive for the ‘VIIRS’ Category
A strong winter storm system that impacted NM over the weekend delivered the first round of significant widespread snowfall for the 2013-2014 winter season. Major impacts to travel were reported across much of the area along with record to near record cold temperatures. The VIIRS DNB Reflectance imagery after the storm showed several interesting features over the area. Crisp city lights over central and eastern NM on top of areas of light yellowish reflectance values contrast the more blurred city lights over western NM with the same light yellowish reflectance. The VIIRS Nighttime Microphysics from the same time provides much better insight into what is happening over the region. Thick low clouds are detailed very well over western NM while central and eastern NM remain clear.
The MODIS Snow Cloud RGB product dives even deeper into the story. Much of central and eastern NM are snow covered and several well defined snow bands can be seen. There are even some low clouds still hanging tough over northwest NM. The composite radar reflectivity from the 24th shows how these snow bands set up over the area. Minor impacts to aviation occurred over western NM as MVFR conditions impacted Farmington and Gallup with 12 to 24 hours of broken ceilings.
During the late night hours of November 11th through early morning hours of November 12, 2013, a strong cold front raced through the eastern plains of New Mexico. The front was shallow, and therefore ran into some obstacles – namely higher terrain areas. Frequent updates of the MODIS-VIIRS Nighttime Microphysics RGB imagery helped NWS Albuquerque resolve with high precision, the obstacles that the low clouds behind the front developed around. The following set of images were looped in a Graphicast on the NWS Albuquerque webpage.
Cloud heights generally ranged from 200 ft to 1500 ft. Additionally, areas of fog, freezing fog, drizzle, and light rain were also noted during the overnight hours.
The nighttime microphysics (NTmicro) RGB comparison below provides some insight into the traditionally difficult challenge of differentiating low clouds from fog. The RGB product enhances the regularly employed 10.8-3.9 channel difference (Green) to include an additional 12.0-10.8 channel difference (Red) and repeat use of the 10.8 thermal channel (Blue). The NTmicro RGB valid at 311am MDT on October 17, 2013 showed an area of cloud cover developing in the Pecos River Valley from Carlsbad north to Artesia and just east of Roswell (aqua shades). Observations within this cloud cover indicated a low cloud deck developing toward the north with ceilings in the 1,500-2,000′ level. The hybrid 11-3.9 imagery indicated the low clouds continuing to expand north toward Roswell through 11Z (yellow areas). The aviation forecaster anticipated that low clouds would move into Roswell and thus amended the TAF to include IFR ceilings for several hours. Fog was not added to the TAF however a brief period was observed. The NTmicro RGB valid the following day at 251am MDT on October 18, 2013 showed a significantly more widespread area of cloud cover impacting much of southeast NM and the Permian Basin of west TX (purple shades). Observations within this cloud cover indicated a very low cloud deck of 100-200′ and numerous reports of fog. The hybrid 11-3.9 imagery indicated the low clouds were expanding north toward Roswell however remaining to the east of the site through 12Z (yellow areas). The aviation forecaster did not expect fog and/or low clouds to impact Roswell based on this imagery and the clear sky forecast was maintained. Without the support of observations across the region on the 18th the forecaster may have been able to distinguish shallow/thin fog from the low clouds on the 17th. The green contribution has been reduced because the 3.9micron channel is seeing some emission from not only the shallow/thin fog but the surface as well. The 3.9 channel is thus warmer and the difference from the 10.8 channel is less, resulting in less green color. Greater amounts of red and blue lead to the strong purple shades.
The early season snow storm across the Front Range had both large amounts and a large area of snow coverage. With the sub-surface temperatures still relatively warm at this point and an early fall sun angle, the MODIS and VIIRS Snow-Cloud RGB imagery provides a 1km resolution look at the snow cover and subsequent melt from image to image. The series of 4 images here spans the afternoon from 1830-200 UTC and changes in the snow cover can be seen even with just an hour or two different between images. While the view here is a large domain (via SPoRT website), users receiving data from SPoRT can use their AWIPS/D2d systems to view a more mesoscale region. Even at this wide view the increase in bare ground (more aqua coloring) in southeast WY is apparent. And, parts of the snow cover (red coloring) in western WY and northwest CO could also be seen decreasing. Even southern MT shows changes in snow cover. Compare the 1831 UTC MODIS image and the 2102 VIIRS image to more easily see the overall change. Monitoring of the snow cover change has been applied to potential flood forecasts, given the depth of snow is relatively known.
A beautiful aurora erupted across the skies along the U.S. / Canadian border on the overnight hours of October 1st-2nd. Following is a VIIRS Day/Night Band Radiance image from the Suomi NPP satellite, which captured part of the event.
And now, another look at the Radiance RGB image valid at the same time.
Here is a photograph of the event taken from Boulder Lake, MN around midnight local CDT. (photo courtesy Dan Miller, SOO NWS Duluth, MN)
And, here’s a photograph closer to the time of the aurora closer to the time of the VIIRS pass…
I’m a big fan of RGBs, especially the Nighttime Microphysics RGB, as the imagery is fantastic for delineating between various cloud types and highlighting clouds that can have some of the largest impacts…especially fog (cumulonimbus aside). I noticed the other day, however, that the Day/Night Band RGB can be superior for the detection of low clouds in certain situations, particularly when relatively warm, low clouds are observed against a similarly warm ocean background state, or through thin cirrus. For example, take a look at the Nighttime Microphysics RGB image below, centered on the Gulf of Mexico at 0658 UTC Sep 22. Notice the fairly solid area of low and mid-level clouds in the NW part of the Gulf along and near the TX and LA coasts. Much colder, thick convective clouds are also observed in the central NW Gulf (red/orange/yellow colors). Elsewhere, mostly high cirrostratus (or altostratus) and cirrus dominate…although a few low clouds can be observed interspersed around the cirrus canopy in some spots (pinkish-aqua colors).
Now, take a look at the VIIRS Day/Night Band RGB in the same region and valid at the same time (image 2). I’ve annotated where low clouds are clearly more observable.
Here’s a loop of both images, demonstrating how well the low clouds stand out in the Day/Night Band imagery vs the Nighttime Microphysics image.
The Nighttime Microphysics RGB contains a recipe of Infrared channels only, and cold, thin cirrus clouds become very dark blue (near black) in the imagery due to small contributions from red (thin optical depth), green (composed of ice particles) and blue (cold temperatures) colors. The resulting color of these cloud types then masks the underlying low cumulus. Meanwhile, the similar characteristis of warm, liquid-water clouds and the ocean’s surface make these features barely indistinguishable. Thin, cold cirrus are more transparent in visible wavelengths however, which are contained in the Day/Night Band Radiance RGB recipe. Also, the whitish color of clouds in the visible is contrasted against the dark ocean surface at night, making these types of warm, low clouds more distinguishable.
So, why is this important? Well, in sub-tropical latitudes, this may be less relevant. However, relatively low cumuls in the tropics or in very warm, moist, near-tropical environments can produce precipitation. As a forecaster once in the deep tropics (central Pacific) myself, I observed that small bands of low cumulus within the trade winds could produce precipitation once cloud depth exceeded about 5 kft. The assosicated cloud top temperatures were only slightly cooler than the surrounding ocean surface, leading to great difficulty when trying to observe these clouds in standard IR imagery, or even some types of RGB imagery, as demonstrated above. Additionally, thin cirrus clouds, which are often present in these environments due to nearby convection, can mask the underlying low cumulus. However, as shown here, Day/Night Band RGBs show promise in helping to locate low clouds at night that can be important for precipitation probability forecasts, particularly in the tropics or in very warm/moist environments.
The Nighttime Microphysics RGB helped again to delineate small areas of locally dense fog on the late evening of the 21st and the early morning hours of the 22nd. The Terra MODIS image below (image 1) valid at 0347 UTC shows fog (white-aqua colors) already beginning to develop in the narrow valleys cutting through the far southern end of the Cumberland Plateau in northeastern Alabama, as high clouds in association with a front boundary cleared the region (red-colored clouds). Fog shows up especially well in the Paint Rock River Valley in NE Alabama…a valley which is prone to fog formation during about any month of the year, particularly following rain events. Notice the extent of the fog though in the valleys of eastern Kentucky, which is much larger in extent. This first image was disseminated to collaborative offices before midnight local time, and would have been available for midnight shift forecasters, allowing them to easily see locations of developing fog. Due to the resolution of the imagery and the effiiciency it provides for fog detection, it could be helpful for limiting the extent of areas under advisory, and allowing for more descriptive impacts in advisory products (i.e., fog mainly in valley locations, fog across portions of Hwy 72 north of Scottsboro, etc).
The fog became locally dense in some, if not most, of these locations. Observations from Fort Payne during the morning indicated visibility at or below 1/4 statue miles from 0500 UTC through 1300 UTC. Interestingly, as the loop of available VIIRS and MODIS images shows that morning (image 2), the fog did not become widespread, but was confined mainly to the narrow river valleys of the region. Also, the subdued nature of the colors in the DeKalb Valley (Ft Payne) suggests the fog was probably quite shallow.
Notice the the fog could also be observed in the VIIRS Day/Night Band imagery valid at 0658 UTC in most of the valley locations, but becomes difficult to discern in the DeKalb Valley near Ft. Payne due to the presence of the city lights.
With the RGB Imagery for Aviation and Cloudy Analysis evaluation underway, we’re already getting some good feedback from our end-users. From WFO Morristown – “I looked at the Nighttime Microphysics product in hindsight to see how fog around TRI was depicted this morning. The (RGB) product did an outstanding job of clearly showing areas of fog vs. clouds, even though there were some thin midlevel clouds over the fog areas.” Below is a loop of the available MODIS (Aqua and Terra) and Suomi NPP VIIRS images from the southern Appalachain region early from late last evening through early this morning (click the image to see the loop).
The image shows the early production of fog (ligher, aqua colors) in the Cumberland and Allegheny Plateau region of eastern Kentucky and SW Virginia by the time of the first MODIS pass at 0359 UTC. Notice how the fog spreads to include other valley locations in East Tennessee by the time of the last images at 0736 UTC and 0810 UTC.
Meanwhile, the fog is clearly not as apparent using the standard 11-3.9 µm imagery (and standard color curve) even with the 1km Aqua MODIS image inserted (image 2, ~0810 – 0815 UTC).
…and is even less noticable in the 4km GOES-East image alone a little later at 0832 UTC (image 3).
Fog in the narrow valleys in the region shows up quite well in the VIIRS Day/Night Band Radiance RGB, developed by SPoRT (image 4). The forecaster noted that, “the DNB Radiance RGB showed fog clearly as well, but maybe not quite as well as the Nighttime Microphysics product.” I would agree, however I was encouraged by the detail and the relative ease with which fog was discernible in the image.
The typical issues with the timeliness of the polar-orbiting imagery, for opertional considerations, appears to be the largest concern for forecasters at this point in the survey process. Of course, this won’t be an issue in the GOES-R era, and acquanting forecaters with these types of imagery before the next generation of GOES satellites is launched is an important step in the learning process.
As a part of ongoing GOES-R and JPSS Proving Ground activities, NASA SPoRT is conducting an evaluation of various RGB imagery with some of our collaborative NWS offices. Day/Night Band RGBs, produced by SPoRT and available from the VIIRS instrument onboard Suomi NPP, include both the Radiance RGB and Reflectance RGB products. Nighttime Microphysics RGBs from the VIIRS and MODIS (Aqua and Terra satellites) instruments will also be evaluated. The offices participating in the evaluation span a broad area, from Huntsville, Morristown, Nashville and Raleigh in the Southeast, to Albuquerque in the Southwest, and Great Falls at the Front Range of the Northern Rockies. Through late September and into October, forecasters will be evaluating the utility of these types of imagery from an operational perspective. The imagery have been made available to WFOs for viewing within their AWIPS I systems, so that they may overlay with other data and/or imagery. However, imagery are also available online or through ftp download (kmz format). A plug-in is still required for viewing in AWIPS II, so WFO Huntsville will be utilizing the online option for now.
A forecaster at WFO Huntsville remarked that “the Nighttime Microphysics imagery…did show the low clouds really well (bright aqua) over north GA this morning and confirmed in greater detail what we were seeing with the 11-3.9 um channel GOES output.” Other surveys have already been received by staff at the Albuquerque and Raleigh WFOs, so the evaluation is well underway!