The 3.9 µm channel at night over very cold cloud tops

May 17th, 2018 |

GOES-16 ABI Infrared Imagery from 3.9 µm (Upper Left), 10.3 µm (Upper Right), 8.5 µm (Lower Left) and 12.3 µm (Lower Right), 0747 – 0832 UTC on 15 May 2018 (Click to enlarge)

When cloud top temperatures are very cold, the 3.9 µm imagery will have characteristics that suggest a noisy signal.  The 45-minute animation above shows a cold cloud top east of Florida in 4 different infrared channels:  3.9 µm (Upper Left), 10.3 µm (Upper Right), 8.5 µm (Lower Left) and 12.3 µm (Lower Right).  That the 3.9 µm image shows noise is not a new problem, as it was present in legacy GOES imagery as explained here.  At very cold temperatures the relationship between radiance (detected by the satellite) and temperature is highly non-linear, because of the character of the Planck function for that wavelength, meaning a very small change in radiance — within the noise — causes a large change in temperature (Compare the first two figures at this link for legacy GOES, for example).

Examine the two figures for GOES-16 below. They show the Planck curves for Band 14 (11.2 µm) and Band 7 (3.9 µm). Two things are apparent. Band 7 (3.9 µm), by design, covers a larger range of temperatures. In addition, very small changes in detected radiance (“counts”) at cold temperatures cause very big changes in the 3.9 µm brightness temperature. The relationship between detected radiance and very cold temperatures is much smoother at 11.2 µm.  The 3.9 µm band lacks precision compared to the other window channels, such as the 11.2 µm, for very cold temperatures. 

Plot of discrete values of Radiance vs. 11.2 µm brightness temperatures (190 K to 420 K) according to the Planck Relationship (Click to enlarge)

Plot of discrete values of Radiance vs. 3.9 µm brightness temperatures (190 K to 420 K) according to the Planck Relationship (Click to enlarge)

A zoomed-in view for cold brightness temperatures between 190 and 230 K (-83.15º C to -43.15º C) is shown below. If a true temperature of 208 K is being sensed by the satellite at the two wavelengths, it will be well-resolved at 11.2 µm, but the 3.9 µm detection will jump between 205 K and 210 K: the nature of the relationship between radiance and brightness temperature is such that there is less precision at the colder end at 3.9 µm. In the 30 K range from 197-227 K, just 12 possible bits are available in the 3.9 µm band (12 out of 2^14 — 16,384; recall that Band 7 on ABI has the highest bit depth of all the channels).  A change of just one count is a large difference in 3.9 µm brightness temperature.

Users need smarter ways to enhance the coldest 3.9 µm to prevent the flashing pixels evident in common traditional color and black-and-white enhancements.  Consider creating a color enhancement that shows only one color at temperatures colder than, say, -40º C, because the detector does not precisely distinguish between the coldest temperatures.  In other words, don’t highlight the noise!  Conversely, don’t use the 3.9 µm imagery at night to discern cloud-top features.   During the day, solar radiation at 3.9 µm reflected off cloud tops causes an increase in apparent brightness temperature so this quantization noise does not occur.

Plot of discrete values of Radiance vs. 11.2 µm brightness temperatures (190 K to 230 K) according to the Planck Relationship (Click to enlarge)

Plot of discrete values of Radiance vs. 3.9 µm brightness temperatures (190 K to 230 K) according to the Planck Relationship (Click to enlarge)

As noted above, this is not a new problem. An image (produced using McIDAS-X) of an Mesoscale Complex over the Great Plains of the United States from GOES-16 is here at 10.3 µm and here at 3.9 µm; the same image from GOES-15 is shown here at 10.7 µm and here at 3.9 µm. In both shortwave images, speckling at very cold cloud top temperatures is apparent.

(Thanks to Mat Gunshor, CIMSS, and Tim Schmit, NOAA, for figures and comments on this entry)

Fires in Saskatchewan

May 15th, 2018 |

GOES-16 ABI Band 1 (“Blue Visible”, 0.47 µm, top), Band 2 (“Red Visible”, 0.64 µm, middle) and Band 7 (“Shortwave Infrared”, 3.9 µm, bottom) from 1345 to 2230 UTC on 15 May 2018 (Click to animate);  Note that the yellow enhancement in the shortwave infrared starts at 305 K.

Fires that developed over the plains of Saskatchewan, near Meadow Lake in west-central Saskatchewan, and near Prince Albert in central Saskatchewan, showed up well in Visible and Infrared imagery, shown above.  A wind shift that occurred as the fires burned changed the direction of the smoke plume.  Prince Albert had visibilities that dropped to 3 statute miles.  Meadow Lake had visibilities down to 4 statute miles.

True-Color imagery (Source: (Link), imagery provided by Paul Ford, ECC Canada), also shows the distinct smoke plumes from the fires.

True-Color imagery over Saskatchewan, 1730-2000 UTC

The Dual-Pol S-band radar at Radisson captured the plume north of Prince Albert at 1900 UTC (See below; click here for the satellite imagery at that time).  Very small Cross-Correlation coefficients are apparent in the smoke plume. The radar at 2010 UTC (link) suggests 3 separate fires, which agrees with the satellite imagery. (Click here for 2015 UTC Satellite Imagery).

Cross-Correlation Scan from the dual pol, S-band at Radisson, Saskatchewan, 1900 UTC on 15 May 2018 (Click to enlarge)

Many Thanks to Paul Ford, ECC Canada, for the radar imagery, and for alerting us to this event. Saskatchewan fires can be tracked at this website. Most of Saskatchewan is currently under a fire ban.


========== ADDED ============
AWIPS imagery of this fire were collected. Click here to see the towns of the region. Full-disk imagery is available from GOES-16 at 15-minute increments. The 3.9 µm imagery is shown from 1200 to 2345 UTC, followed by the Fire RGB Imagery. The Fire RGB image combines the 3.9 µm (Red), 2.2 µm (Green) and 1.6 µm (Blue) imagery. The wavelength of the radiation emitted by the fire decreases as the temperature of the fire increases; a relatively cool fire will emit mostly 3.9 µm energy and will be red in the RGB. A very hot fire will emit all three wavelengths and will appear whiter in the RGB.

GOES-16 ABI Band 7 (“Shortwave Infrared”, 3.9 µm) from 1200 to 2345 UTC on 15 May 2018 (Click to enlarge)

GOES-16 ABI Fire RGB, combining 3.9 µm, 2.2 µm and 1.6 µm imagery, from 1200 to 2345 UTC on 15 May 2018 (Click to enlarge)

The imagery below is zoomed in on the region of the three fires.  (Map).  The 3.9 µm is shown first, then the Fire RGB.

GOES-16 ABI Band 7 (“Shortwave Infrared”, 3.9 µm) from 1200 to 2345 UTC on 15 May 2018 (Click to enlarge)

GOES-16 ABI Fire RGB, combining 3.9 µm, 2.2 µm and 1.6 µm imagery, from 1200 to 2345 UTC on 15 May 2018 (Click to enlarge)

 

The RGB — like many — gives an excellent qualitative estimate of the fire.  Quantitative estimates are available that more define the fire more comprehensively. The 1845 UTC Fire RGB suggests a very hot fire (the 3.9 µm imagery at 1845 UTC suggests the same thing). What do the Baseline fire products show? The Fire Temperature, Fire Power, and Fire Area products for 1845 UTC are shown below.  (Animations are here:  Fire Temperature, Fire Power, Fire Area)   Hotter fire pixels are apparent at 1745 and 2015 UTC.    Click for toggles of Band 7 (3.9 µm), Fire RGB and Baseline Fire Temperature at 1745 UTC, 1845 UTC, and 2015 UTC.  These products might facilitate resource allocation in a way that single channels or RGB combinations cannot.

GOES-16 Baseline Fire Temperature Product 1845 UTC on 15 May 2018 (Click to enlarge)

GOES-16 Fire Power Baseline Product, 1845 UTC on 15 May 2018 (Click to enlarge)

GOES-16 ABI Fire Area Baseline Product at 1845 UTC on 15 May 2018 (Click to enlarge)

Strong Thunderstorms move through Washington DC.

May 14th, 2018 |

GOES-16 ABI Channel 13 “Clean Window” (10.3 µm) at 1-minute time-steps from 1607-2359 UTC on 14 May 2018 (Click to animate)

A GOES-16 Mesoscale Sector produced 1-minute imagery as a strong thunderstorm complex approached Washington DC late in the afternoon/early evening of 14 May 2018.  The (150-megabyte (!!)) animated gif above shows overshooting tops quickly developing and decaying as the complex moved over the Potomac Basin.  Winds in excess of 60 knots were reported around the Washington DC metropolitan area, with widespread tree damage. (Smaller MP4 animations with plots of SPC storm reports are also available: Infrared | Visible)

NOAA/CIMSS ProbSevere All Hazards (Source), below, showed very high ProbHail and ProbWind with this cell as it approached Washington DC.

NOAA/CIMSS ProbSevere All Hazards, 2200 UTC on 14 May 2018 (Click to enlarge)

GOES-16 Geostationary Lightning Mapper (GLM) data from Real Earth (Link for animation), below, shows an increase in electrical activity to the storms as they moved through Washington DC.

CONUS Hybrid Radar Reflectivity overlain with GLM observations, 2200-2330 UTC 14 May 2018 with 15-minute timestep.

Hail-producing storm on the High Plains of Colorado and Kansas

May 14th, 2018 |

GOES-16 Red Visible (0.64 µm) from 1212 on 14 May 2018 through 0037 UTC on 15 May 2018 (Click to animate)

A Thunderstorm complex moved through eastern CO into western Kansas on 14 May 2018, producing 2 to 3 inch hail in Kit Carson and Cheyenne Counties in east-central Colorado and in Wallace County in northwest Kansas (SPC Storm Reports). Visible animation (0.64 µm) from GOES-16, above, shows the storms initiating near metropolitan Denver before moving eastward across the Plains.

GOES-16 ABI Clean Window imagery (10.3 µm), below, shows very cold overshooting  tops associated with these storms, with brightness temperatures colder than -60º C.  The area of coldest cloud tops shows a pronounced southeastward motion.

GOES-16 ABI “Clean Window” Infrared Imagery (10.3 µm), 1212 UTC 14 May 2018 to 0037 UTC on 15 May 2018 (Click to animate)

The GOES-16 ABI “Snow/Ice” Channel (at 1.61 µm) is important in diagnosing cloud-top properties in convection, in particular because glaciated clouds absorb solar energy at 1.61 µm (rather than reflecting it).  Thus, glaciated cloud tops will look dark.  That is the case with this system, shown below.  Note that above-anvil cirrus banners are apparent in this animation as well towards the end, stretching west-southwest to east-northeast.  These above-anvil banners are very well correlated with severe weather. This link shows a toggle between the Visible (0.64 µm) and Snow/Ice (1.61 µm) bands at 2302 UTC on 14 May, during the time when hail was occurring in eastern Colorado.

GOES-16 ABI “Snow/Ice” Near-Infrared Imagery (1.61 µm), 1212 UTC 14 May 2018 to 0037 UTC on 15 May 2018 (Click to animate)

One of the Derived Products available from GOES-16 is Lifted Index. This animation, from 1212 UTC on 14 May through 0037 UTC on 15 May shows widespread Lifted Indices of -3º to -5º in the inflow into this thunderstorm. (The clear-sky only Lifted Index is plotted on top of the Snow/Ice 1.61 µm Imagery). Note also that dewpoints over the High Plains of Colorado and Kansas were fairly high for that region: 40s and 50s Fahrenheit. (Click to view 2007 UTC “Veggie” Band 0.86 µm imagery with surface metars plotted).

NOAA/CIMSS ProbSevere All Hazards showed very high probabilities for this cell at 2330 UTC, when it was over eastern Colorado, as shown below (Source).

NOAA/CIMSS ProbSevere All Hazards, 2330 UTC on 14 May 2018 (Click to enlarge)

As noted elsewhere (link, link), hail deposited by this storm in central Colorado (in Douglas and Elbert counties) was widespread enough to be visible from satellite, below.  The hail appears white in the visible (0.64 µm) imagery and dark in the 1.61 µm Snow/Ice imagery because ice strongly absorbs energy with wavelengths of 1.61 µm.

GOES-16 Band 2 (“Red Visible”, 0.64 µm) and Band 5 (“Snow/Ice”, 1.61 µm) imagery at 2217 UTC on 15 May 2018 showing hail on the ground in Douglas and Elbert counties, Colorado (Click to enlarge)

GOES-16 Band 2 (“Red Visible”, 0.64 µm) and Band 5 (“Snow/Ice”, 1.61 µm) imagery at 2217 UTC on 15 May 2018 showing hail on the ground in Douglas and Elbert counties, Colorado (Click to enlarge)

Once the severe convection moved closer to the Colorado/Kansas border, a second hail swath was later seen to the east-northeast, below.

GOES-16 “Red” Visible (0.64 µm, top) and Near-Infrared

GOES-16 “Red” Visible (0.64 µm, top) and Near-Infrared “Snow/Ice” (1.61 µm, bottom) images, with SPC storm reports (red) and hourly plots of surface reports [click to play MP4 animation]