Fog at Sunrise with RGBs using Visible Imagery

Fog at Sunrise with RGBs using Visible Imagery

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Nighttime Microphysics RGB via GOES-16 at 1122 UTC, 13 August 2018 over the Southeast U.S.

During the early morning of 13 August 2018, clear skies resulted in wide spread low clouds and fog over the East/Southeast.  The image above is the Nighttime Microphysics (NtMicro) RGB via GOES-16 at 1122 UTC or 7:22 and 6:22 AM for Eastern and Central times respectively.  At this time the low clouds and fog in shades of cyan are still apparent, but soon this coloring will fade as solar reflectance at sunrise will influence the shortwave IR used in the RGB and therefore, the NtMicro will be rendered ineffective (see mp4 animation).  Typically, visible imagery is used at sunrise to continue to monitor fog in small-scale valleys, often with a lack of in situ observations.  The new capabilities of GOES-16 provide new RGBs for daytime use that include the 0.64 micron visible channel.  The Natural Color RGB, originally developed by EUMETSAT is available within AWIPS (as ‘Day Land Cloud’), and it uses the visible channel in it’s blue component.  Below is a slide show of the NtMicro, Natural Color and Visible RGBs just after sunrise (1222 UTC).  Note that the Natural Color RGB (also see mp4 animation) shows the fog and water clouds in gray while ice clouds are in cyan.  The Natural Color RGB can be used through the day to monitor the microphysics of cloud tops due to the use of the 1.61 micron channel, and it also provides qualitative land surface information via the 0.87 micron channel.   A legacy ‘Visible’ RGB (also see mp4 animation) that uses the visible in the red and green components (‘Day Land Convection’ within AWIPS), also provides value to monitor fog after sunrise as it depicts warm clouds in yellows and cold clouds in grays to white in daytime.

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VIIRS Day-Night Band (DNB) RGB Imagery assisted by Nighttime Microphysics RGB

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VIIRS DNB Reflectance RGB within AWIPS/D2d for 17 February 2014 at 0905 UTC over the Colorado, Wyoming, Nebraska, Kansas region

The VIIRS Day-Night Band (DNB) RGB imagery from SPoRT uses the DNB channel for both the red and the green components of the RGB, and then the single channel 11 micron band for the blue component.  So warm, reflective clouds have both red and green and result in a shade of yellow while cold, reflective clouds appear in shades of blue to white.  The image above from 17-February-2014 has a mixture of yellow, blue, and white objects, but the question is: Are all of these areas clouds?  Some of the city lights appear to be distinct, sharp yellow points on the ground with little diffusion of light through clouds even though they are surrounded by shades of yellow.  Notice the area at the intersection of the CO, WY, and NE borders as well as some of the inter-mountain regions of western Colorado and how the city lights in these regions are not blocked, nor spread over a large area due to scattering by the clouds.

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VIIRS Nighttime Microphysics RGB within AWIPS/D2d for 17 February 2014 at 0905 UTC over the Colorado, Wyoming, Nebraska, Kansas region

The Nighttime Microphysics RGB imagery from VIIRS is provided above as a comparison to the DNB.  Using this RGB one can identify areas of clouds and their type.  Several of the areas in question turn out to be surface features as opposed to clouds.  This realization that clouds do not exist in some of the yellow shaded areas of the DNB as well as the fact that city lights are not scattered in these regions, leads one to conclude that the DNB is showing snow on the ground.  Several areas are labeled as “Snow Cover”.  However, note that some clouds do exist.  In fact some low clouds in the inter-mountain west of Colorado are evident in yellowish-green tones, and low- and mid-level clouds are highlighted in northwest Kansas and northeastern Colorado.  The Nighttime Microphysics RGB also hints at the potential of fog in southeast Colorado with dull gray to shades of aqua.  Perhaps snow cover has melted to some extent to provide a moist ground with clear skies overnight that resulted in some very thin, to scattered fog or low clouds. Below are images of the same time but over a wider area in order to provide greater perspective.  Note that much of the yellow shaded areas in the DNB RGB are the result of snow cover vs low cloud features.  Hence VIIRS demonstrates valuable insight to both clouds and surface features at night via reflected moonlight.

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VIIRS DNB RGB within AWIPS/D2d for 17 February 2014 at 0905 UTC over the western U.S.

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VIIRS Nighttime Microphysics RGB within AWIPS/D2d for 17 February 2014 at 0905 UTC over the western U.S.

Fog and Moisture Boundary Analysis from VIIRS

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A wide VIIRS swath covering parts of the Southeast U.S. provides a Night-time Microphysics RGB image showing wide fog in the valleys of Tennessee, Georgia, and North Carolina (see yellow outline in upper center of image).  Also of note is the difference in coloring between the areas above and below the white line stretching east-west from Mississippi to South Carolina.  The brighter purple coloring indicates drier air compared to the darker shades south of the line where the dewpoints were relatively high (in the +70F range).  In this case the red component of the RGB (12-11 micron difference), which is related to optical thickness, contributes less color in the areas of higher moisture; this results in a darker color of purple compared to the relatively drier air to the north.  The red decreases in the southern area because the difference in the 12-11 micron channels decreases from the values in the drier air.  Looking at the individual channels, this change is fairly small, on the order of 1-2 K; but the range of the channel difference for the red component is limited to 6 K total.  Hence a noticeable change in the red occurs and it brings out the change in air density.

Night-time Microphysics RGB from VIIRS – Where is the fog?

The SPoRT program is providing the Night-time Microphysics RGB product from VIIRS and MODIS using the EUMETSAT receipe of channels/differences.  A unique webpage has been created for part of the southeast U.S. to allow products related to fog and low clouds to exist in one place regardless of instrument source:

SPoRT Real-time Data Fog and Low Cloud

Here is an example from a VIIRS pass this morning.  Can you see areas of fog without looking at a surface obs plot first?  Can you differentiate fog from low clouds?

Post comments with your guess of where the fog actually is and check back for the answer in a future post.

VIIRS Night-time Microphysics RGB

VIIRS Night-time Microphysics RGB derived from multiple VIIRS channels. Resolution is 1km. This image is at the native resolution if you click on it to see the original size. Image on the post has been made to fit the page. This is from the early morning of October 24, 2012.

Night-time Microphysics shows low clouds at LYS

The MODIS “fog” or 11-3.9 channel difference product for 2012, April 20 at 0431Z shows northern New Mexico dotted with yellow pixels indicating low clouds and/or fog.  The MODIS Night-time Microphysics RGB at this time narrows the area of concern.  The colors of light to dull aqua (nearly gray & violet) show a small area near and north of Las Vegas NM where fog or low clouds are likely occurring.  Surrounding this area in the RGB imagery, the cloud features are more yellow, tan, and orange, indicating a mixture of mid-level clouds (colder, thicker) that are likely further above the surface.  The observation at Las Vega (KLVS) shows 10 miles visibility but a ceiling at 600 feet.  Luckily this observation verifies what we see in the imagery, but what if the observation was not there?  The imagery can help fill the gap between observations as low clouds and fog can be very mesoscale.

2012 April 20 at 0431UTC. MODIS 11-3.9 channel difference over northern New Mexico.

2012 April 20 at 0431UTC. MODIS Night-time Microphysics RGB over northern New Mexico.

Night-time Microphysics RGB Imagery for Fog

2012 April 19 – Night-time microphysics RGB imagery from MODIS centered on WFO Charleston, WV. States in the image include OH, PA, WV, MD, KY, VA, NC, TN.

After a weak cold front passage earlier this week through the Tennessee and Ohio valleys and the precipitation associated with a shortwave during the mid-week, fog and low clouds developed in the wake of these events due to moist ground and calm winds overnight into Thursday (4/19) morning.  Can you see the fog and low cloud areas in the Night-time Microphysics RGB (NTmicro) imagery above from MODIS?  Take a look below at the MODIS 11-3.9 difference product used for fog and low clouds. Wide spread areas of yellow indicate low clouds and fog, but with such a large area it can be hard to differentiatethe two.  The METAR ceiling and visibility observations help to interpret the MODIS 11-3.9 product.

2012, April 19 – 11-3.9 channel difference from MODIS with METAR observations

Take another look below at the NTmicro RGB from MODIS with the METAR observations now overlaid. Note some of the color variations and associated observations.  First, note that the NTmicro areas in the lower right portion of the swath (VA, NC) have some tan coloration which indicates a thicker cloud, likely stratus.  Observations in these areas typically show multi-layer clouds, often with ceiling and visibility criteria within the VFR range (but not always; could have fog or low ceiling under the stratus).  Next look for the bright areas of aqua and light blue in the region.  These areas correspond to low clouds where visibility conditions are often VFR, but ceilings of thick stratus are low enough to cause IFR conditions.  Lastly, look in the western portions of the RLX WFO (Charleston, WV) county warning area and parts of the JKL and MRX WFOs futher to the southwest.  These have very dull aqua colored regions that appear nearly gray to dull purple.  Several observations show 1/4 mile visibility with ceilings of 100 feet.  It’s as if some of the violet color representing the land surface is showing through the fog and mixing with the aqua.  In fact, the surface emissions are likely influencing the 3.9 channel to be warmer than surrounding low clouds.  Hence the 11-3.9 difference used in the green component of the RGB will be smaller than areas of low, thick stratus and therefore contribute less green to the RGB.

Same as first image, but with observations of ceiling, visibility and present weather overlaid.

Fog event 9/27 – multi-state, post frontal

MODIS spectral difference and Night-time Microphysics RGB

MODIS spectral difference (11-3.9 micron, left) and Night-time Microphysics RGB (multi-channel composite, right)

Light precipitation on 9/26 and subsequent clearing overnight (post frontal) setup a wide spread fog event that stretched from SE Mississippi to central Ohio. The MODIS spectral difference product (11-3.9 micron) can help identify areas of low clouds and fog.  The MODIS RGB night-time microphysics product (derived from EUMETSAT’s SEVIRI standard RGB suite) uses the same spectral difference, but only for the green channel.  Two additional inputs are provided to create the night-time microphysics RGB: 12.0-11 micron (relating to cloud thickness) for the red channel and 11 micron (related to thermal emission) for the blue channel. The images have similar areas denoted for fog (right click to open in new window and click in new window for full image).  The night-time microphysics image for 0657 UTC shows varying shades of light blue corresponding to fog thickness because it includes the additional channels.  Thicker clouds have more contribution from the red channel, and the warmer temperatures of the low level fog and clouds provides a brighter blue channel (colder temperatures are actually closer to black).  Pixels with thin fog are influenced by some of the thermal emission of the land surface being transmitted through the fog layer; hence the light violet color of the land surface bleeds through the thin fog. This makes it easier for the forecaster to see areas of thick vs. thin fog. The night-time microphysics RGB indicates a slightly wider area of thin fog affecting the central and southwestern portions of the BMX CWA than indicated by the spectral difference product at this time.  Other mid and high level clouds (not fully shown here) are more easily identified with the RGB product in order to not confuse them with other low cloud features. Similar channels will be available on GOES-R’s ABI instrument to provide a geostationary version of this product.