Satellite Liaison Blog

Have you noticed stray light contamination in visible, near-IR and SWIR imagery at night (~0400 – 0600 UTC) recently? Missing pieces of images as well? Odd GLM behavior? This is due to light from the sun directly entering the imaging sensors during a period known as the eclipse season. The eclipse season occurs in the Northern Hemisphere during the Spring (late Feb to mid April) and Fall (late Aug to mid-Oct) every year. During this period, the satellite enters the shadow of the Earth during the late night hours. Sunlight enters the sensors and impacts the VIS/NIR/SWIR imagery and GLM as the satellite enters and leaves the Earth’s shadow. More details on this phenomenon can be found on the NESDIS/OSPO webpage here. According to NESDIS for the current eclipse season:

“Stray light contamination and truncated swaths during the GOES-East Fall 2018 eclipse season will be apparent on ABI imagery: Starting August 8, 2018 from approximately 0415 UTC – 0545 UTC each day in the Northern Hemisphere until September 22, 2018.”

Examples below are from GOES-16 (East) ABI and GLM. Figures 1, 2, and 3 are early in the eclipse season (20 August), and reveal the vertical beam of stray light contamination in ABI imagery. Notice the beam is apparent in the visible and shortwave IR channel, but is not apparent in the IR window channel.

Figure 1: 20 August 2018 GOES-16 5-min 0.64 um VIS. Notice vertical beam of light passing across CONUS. Full res

Figure 2: 20 August 2018 GOES-16 5-min 3.9 um SWIR. Notice warming of features, especially cold cloud tops, as beam of light moves through region. Full res

Figure 3: 20 August 2018 GOES-16 5-min 10.3 um IRW. Full res

Figures 4, 5, and 6 are later in the eclipse season. Stray light is still apparent, as well a canceled frames in all of the channels.

Figure 4: 4 September 2018 GOES-16 5-min 0.64 um VIS. Notice canceled frames and vertical light beam extending south. Full res

Figure 5: 4 September 2018 GOES-16 5-min 3.9 um SWIR. Notice canceled frames and warming of cold cloud tops. Full res

Figure 6: 4 September 2018 GOES-16 5-min 10.3 um IRW. Notice canceled frames. Full res

GLM data are also impacted during the same period, with the solar intrusion causing false events (Fig 7).

Figure 4: 5 September 2018 GOES-16 5-min IRW and 1-min GLM FED. False detections from GLM are apparent over the western half of the US between 0430 UTC and 0445 UTC. Full res

Bill Line, NWS

The default scan mode (as of the writing of this blog post) for GOES-16 (GOES-East), mode 3, provides a full disk image every 15-min, a CONUS image every 5-min, and two movable 1-min sectors. The GOES-East mesoscale sectors have default locations over the eastern half of the United states, and can be positioned anywhere within the full disk upon request. NWS entities have the highest priority for making requests, and any NWS WFO can make a request through their regional request focal point office.

Figure 1: GOES-East Mesoscale Domain Sector coverage for the afternoon of 3 September 2018. The sector over the northern plains covers a SPC slight risk, while the sector over Florida covers TS Gordon. Full res

When making a meso request, individuals should provide: their office, desired begin and end date/time of sector coverage, the center lat/lon of the sector in decimal degrees, and the rationale for the request. Try to make the request early in the day before the event begins, if possible.

In the event of more than two requests, a priority list is consulted. The priority list can be found in the document or slides at this NOAA VLAB page. When offices make a request, they should refer to this list and make sure they use the highest possible reasoning that applies to their CWA.

When deciding on the center lat/lon of the sector, offices should consider the threat not only within their CWA, but also around them. Instead of simply using a lat/lon in the center of the CWA, requestors should choose a lat/lon that covers their area plus as much of the rest of the threat area as possible. For example, choose a lat/lon that will cover most of the SPC convective outlook area, or SPC fire weather outlook area, or WPC excessive rainfall outlook area, etc. Also, try to keep most of the sector in CONUS in order to benefit as many NWS offices as possible. One can visualize the coverage of a sector  given a center lat/lon by consulting this CIRA webpage (Fig 2).

Figure 2: Screenshot from CIRA MDS preview webpage. Full res

Finally, if you think the 1-min imagery will be even remotely helpful in your area, make the request! If there are no requests, the sectors simply scan from their default locations, possibly going unused. National Centers also make requests, but offices should not assume this will happen. For severe threats, SPC will likely make requests for ENH or higher, and often will for SLGT as well. But it is recommended the WFO make the request to ensure coverage, especially for MRGL and SLGT.  And remember, once GOES-17 goes operational as GOES-West, there will be four total 1-min mesoscale sectors to work with!

Bill Line, NWS

You may have noticed a change to the GOES-R RGBs in AWIPS since the May/June 2018 RPM update. A new and improved method for calculating the RGBs was implemented. One of the most notable benefits of this change is that users can now control the ranges of the three RGB components based on the physical units (reflectance, brightness temperature), instead of the 0-255 color range as was the case before (Fig 1). Similarly, when sampling the RGB in AWIPS, users now see the physical units for each of the three RGB components (Fig 1). The new method for computing RGBs may also lead to improved performance on AWIPS workstations. Finally, minor projection issues are resolved with the new method.

Figure 2: 5 August 2018 GOES-16 Day Cloud Phase Distinction RGB, point sampling readout, and Composite Options. Full res

Users are reminded that the default RGB ranges were chosen for a reason, and altering them will change the interpretation of the RGB, potentially negatively.

Below is an example of how a user may alter an RGB (Fig 2 and 3). The “simple” Day Cloud Phase Distinction RGB is great for differentiating various cloud types and clouds from snow from bare land, during the day. In this RGB, however, the green component (0.64 um) becomes saturated fairly quickly (reflectance >78%). So, for example, forecasters cannot interrogate  features atop deep moist convection using this RGB that are otherwise apparent in visible imagery (texture, overshooting tops, etc) . This isn’t necessarily one of the purposes of this RGB, but a forecaster may want to expand it’s capabilities regardless. With the new design, a user can simply load the composite options for the RGB, and adjust the max for the green component up to, say, around 100%.

Figure 2: 5 August 2018 GOES-16 Day Cloud Phase Distinction RGB and point sampling readout. Full res

Figure 3: 5 August 2018 GOES-16 modified Day Cloud Phase Distinction RGB and point sampling readout. Full res

This change allows for details in the visible channel to be apparent in this RGB. However, users must keep in mind the green influence will be less for all features in the RGB. Low water clouds will appear closer to blue than blue/green, and snow will be a darker shade of green. Highly reflective cold cloud tops (deep convection) may appear more red than yellow.

Bill Line, NWS

Severe storms produced large hail (including significant) and tornados across Wyoming and Colorado on 29 July 2018. Strong northwest flow over a moist airmass led to very favorable shear and instability across the region, and a SPC Enhanced Risk for severe was in place. GOES-16 1-min imagery was available to support NWS forecasters during the event. Storms early on produced overshooting tops and above anvil cirrus plumes, a signature indicative of severe and significant severe potential. The plumes appear as smooth cirrus cloud features emanating from the overshooting top and downstream over the storm anvil. Cumulus clouds are also analyzed being drawn into the southern regions of the storms, indicative of strong inflow winds.

Figure 1: 29 July 2018 GOES-16 1-min VIS. Full res

A little later over east-central Colorado, a severe storm developed that produced a measured wind gust of 83 mph, along with hail at least 2.75″ in diameter (baseball). GOES-16 1-min imagery of this storm showed a large and persistent overshooting top and an above anvil cirrus plume (VIS, Fig 2). The corresponding 1-min IRW imagery (Fig 3) shows a large overshooting top (round region of coldest pixels) along with a relatively cooler area immediately downstream of the overshooting top (completing the thermal couplet) and a relatively cool region extending downstream above the anvil (above anvil cirrus plume). Overlaying a semi-transparent IRW on the VIS, we get the sandwich image combination (Fig 4). Radar imagery of the storm showed reflectivity over 80 dBz along with significant beam attenuation, indicating a lot of hail (Fig 5).

Figure 2: 29 July 2018 GOES-16 1-min VIS. Full res

Figure 3: 29 July 2018 GOES-16 1-min IRW. Full res

Figure 4: 29 July 2018 GOES-16 1-min IRW. Full res

Figure 5: 29 July 2018 KPUX 0.5 degree radar reflectivity. Full res

Bill Line, NWS

A cluster of thunderstorms made a long trek south through the southeast Colorado plains Thursday night into early Friday morning. The storms produced heavy rainfall and gusty winds, including a 45 knot measured gust at La Junta. Additionally, the storm produced frequent lightning as evidenced by GOES-16 GLM and NLDN CG data. To the west of the storm, low clouds developed along the I-25 corridor in the anomalously moist atmosphere. Low ceilings brought IFR conditions to KCOS and MVFR to KPUB.

Cloud top brightness temperature trends from ABI IR and GLM total lightning trends were analyzed through the night in order to monitor the health of the storm cluster. Given the 45 knot wind report and constant cold IR temperatures and total lightning, SPS’s were issued for the duration of the storm. The 4-panel animation shows several long flashes extending into the anvil stratiform region of the system, from which several CGs were measured by NLDN.

Figure 1: 27 July 2018 GOES-16 FED (TL), AFA (TR), TOE (BR), and KPUX base reflectivity and NLDN CGs (BL). Full res

As the storm advanced south, low clouds spread west to the I-25 corridor. The Nighttime Microphyics RGB was utilized to track the progression of the low clouds and amend/update TAFs. The image combination below shows the nighttime microphysics RGB with IR brightness temperatures overlaid (gray-scale) for the coldest temperatures. GLM Flash Extent Density is also overlaid.  In the RGB, recall that during the warm season, the ~aqua colors represent warm liquid water clouds, the ~tan colors are cooler water clouds, and the red colors are cold ice clouds (overlaid with IR in the animation below).

Figure 2: 27 July 2018 GOES-16 5-min Nighttime Microphysics RGB (color underlay) with IR brightness temperatures overlaid (grayscale) and GLM Flash Extent Density overlaid. Full res

Bill Line, NWS

A long-lived, prolific above anvil cirrus plume (AACP) generating storm produced large hail and heavy rainfall as it tracked southeast along the Sangre de Cristo mountains in south-central Colorado on Tuesday. The largest hail report from the storm was 1.75″, 12 miles south of Westcliffe. The storm tracked over two burn scars, leading to flash flooding. The AACP signature indicates a particularly strong updraft and increased likelihood of a severe storm.

Figure 1: 25 July 2018 GOES-16 5-min visible satellite imagery. Severe thunderstorm (yellow) and flash flood (green) warnings and storm reports also included. Burn scars outlined in red. Full res

Bill Line, NWS

Severe thunderstorms developed across central Iowa during the afternoon of 19 July 2018, producing multiple tornados. Fortunately, 1-minute imagery from GOES-16 was available over the region to aid forecasters during the event.

GOES-16 derived motion winds in the area of tornadic development indicated enhanced low-level and deep layer shear just prior to initiation. With surface winds of 12 knots from 160 degrees and GOES-16 DMWs indicating ~1 km winds of 16-20 knots from around 220 degrees, 0-1 km shear was around 16 knots, which is favorable for supercell tornados (Fig 1). DMWs around 6 km were 40-50 knots from 275 degrees, which yields a 0-6 km shear vector of around 49 knots, favorable for supercell development. Figure 2 combines the mesoscale and CONUS winds to show the abundance of winds vertically and horizontally that can be available from GOES-16. Numbers from the nearby DMX VAD wind profiles are similar.

Figure 1: 19 July 2018 GOES-16 VIS, Derived Motion Winds, and surface METARs. Full res

Figure 2: 19 July 2018 GOES-16 5-min VIS and Derived Motion Winds (CONUS and Mesoscale), and METARs and surface obs. Full res

GOES-16 CAPE showed convection develop on the western edge of a CAPE max. As has been found in previous experiments, although CAPE derived from GOES may often be lower than other sources, the gradients in the moisture/instability fields are captured well.

Figure 3: 19 July 2018 GOES-16 5-min VIS and CAPE, LSR’s. Full res

NUCAPS profiles from Suomi-NPP were available ahead of the line of the line of storms around the time of initiation (Figure 4). Considering central Iowa does not have a balloon launch, temperature and moisture profiles from polar orbiting satellites have exceptional value. The surface conditions of the profiles need to be modified to match nearby obs. After doing so, the profiles near Des Moines indicated around 2,000 j/kg of MLCAPE and up to 4,000 j/kg of SBCAPE, supportive of strong updrafts (Figure 5). Further, the mid/upper levels of the profiles were dry above moist low levels, indicating favorable convective instability.

Figure 4: 1927 UTC 19 July 2018 GOES-16 VIS, Suomi-NPP NUCAPS sounding availability, METARs. Full res

Figure 5: 1927 UTC 19 July 2018 Suomi NPP NUCAPS sounding immediately ESE of Des Moines, IA. Surface modified to match nearby METARs. Full res

GOES-16 1-min imagery revealed progressively enhanced cumulus development and clumping northwest of Des Moines between 1800 UTC and 1900 UTC. Initiation took place shortly thereafter, confirmed by the first GLM total lightning detections at 1926 UTC (Figure 6). The first tornado was reported around 1950 UTC, just as GLM FED values began to rapidly increase. A max of 3 flashes/5-min was detected at 1940 UTC, and a max of 23 flashes/5-min was detected at 1950 UTC. At 2000 UTC, a max of 100 flashes/5-min was detected with this storm as it continued to produce a tornado. The updraft weakened briefly thereafter, before strengthening again from 64 flashes/5-min at 2037 UTC to 142 flashes/5-min at 2112 UTC when 1.5″ hail was reported.

Figure 6: 19 July 2018 GOES-16 1-min VIS, GLM FED 5-min accumulation, LSR’s. Full res

Further south, severe thunderstorms produced widespread damaging winds, including a reportedly fatal storm that capsized boats on Table Rock Lake. GOES-16 1-min visible imagery showed rapid updraft growth with this storm through old anvil clouds up to the tropopause, quickly producing overshooting tops and an above anvil cirrus plume, the latter of which is a sign of a particularity strong storm.

Figure 7: 19 July 2018 GOES-16 1-min VIS over southern MO. Full res

Bill Line, NWS