Satellite Liaison Blog

GOES-16 has periodically been scanning 1-min mesoscale sectors in South America for the RELAMPAGO Research Mission since this past October. Strong thunderstorms in Argentina were captured in the mesoscale sector on 11 December 2018. 

Focusing in on one of the storms for a 1.5 hour period (FIg 1-3), a persistent and large overshooting top is clear, along with a downstream above anvil cirrus plume, both indicating a particularly strong updraft and possible severe weather at the surface.  Shown are VIS, IR, and a VIS/IR sandwich combo.

Also shown are 15-min loops and wider views of the storm cluster moving across Argentina.

Bill Line, NWS

Both the GOES-16 (GOES-East) and GOES-17 (future GOES-West) ABI’s operated in Mode 4 today (Dec 4, 2018) for close to four hours in order to “test storage updates” (according to NESDIS). Recall Mode 4, also known as Continuous Full Disk Mode, scans a full disk (entire hemisphere) image every five minutes, and that is all. The default Mode 3 “Flex Mode” only scans a full disk image every 15-minutes, but also scans a CONUS image every 5-minutes, and two mesoscale sectors every 1-minute. See Table 1 for a comparison between scan modes. Mode 6 is under evaluation.

Figure 1 shows a world perspective of the two ABI’s operating in Mode 4 (1825 UTC to 2200 UTC) and then switching back to Mode 3 (2215 UTC to 0200 UTC). Notice the imagery appears to “speed up” when switching from 5-min to 15-min resolution. Note the 2015 UTC GOES-17 and 2020 UTC GOES-16 images are incomplete.

On a related note, there is not yet a nicely stitched East/West image composite available in AWIPS. Users can easily load both the East and West imagery together (as is done in this post), but there will be a large region of poor quality imagery overlapping better quality imagery along the edge of the top image. This is due to degraded spatial resolution far from the satellite sub-point. Further, the limb cooling effect will lead to differences in brightness temperature for IR channels (especially water vapor) along the transition.

Bill Line, NWS

GOES-17 data are preliminary and non-operational at the time of this post.

The GOES-17 ABI will be operating in Mode 6 as a test starting today, Nov 27 at 1400 UTC through 1700 UTC on Nov 30. Recall in Mode 3, the current default operating mode for GOES-16 and GOES-17 ABI, we get a Full Disk sector every 15-min, a CONUS/PACUS sector every 5-min, and two mesoscale sectors every 1 minute each. In Mode 6, the ABI scans a full disk sector every 10-min, a CONUS/PACUS sector every 5-min, and two mesoscale sectors every 1-minute each. As you can see, the only difference between the two modes is that Mode 6 provides a full disk image more often (six per hour) than mode 3 (four per hour). See table 1 for a comparison between the two scan modes. Also shown is Mode 4, which is currently an option on the ABI’s. 

Figure 1 is a 60 minute snapshot of all four sectors (Full Disk, PACUS, Meso-1, Meso-2) from the GOES-17 ABI in Mode 6 on Nov 27. As a comparison, Figure 2 provides the same snapshop for GOES-16 ABI operating in Mode 3. 

Figures 3 and 4 compare full disk scanning for GOES-17 in Mode 6 and GOES-16 in Mode 3 for a three hour period. During this period, the full disk is scanned by GOES-17 18 times, and by GOES-16 12 times.

Bill Line, NWS

GOES-17 data are preliminary and non-operational at the time of this post.

A Winter Storm delivered snow across a large swath of the US during Thanksgiving weekend. Clearing behind the system revealed the fresh snowfall. As has been discussed in previous blog posts, the increased spectral resolution of the ABI compared to previous GOES Imagers makes it easier to identify/differentiate surface and atmospheric features. By combining multiple channels into channel differences and RGB’s, it makes the job of the forecaster even easier to analyze these complicated scenes.

The scene across the central US behind the system included bare ground, lakes, fresh snow, water clouds and ice clouds. The latter three features are all highly reflective and difficult to differentiate in visible imagery (Fig 1).

Figure 1: 26 November 2018 GOES-16 0.64 um VIS. Full res

By combining the visible channel with the 1.61 um snow/ice near IR channel and the 10.3 um IR window channel, we have one image that contrasts those features while maintaining the 0.5 km high resolution of the VIS (Fig 2). This combination is known as the Day Cloud Phase Distinction RGB. In this example, the max bounds of the green and blue components were lowered to 50 and 40, respectively, while the min red component was decreased to 0. These changes were made in order to best highlight the features I was interested in for this situation (snow vs low clouds vs high clouds in a cold airmass). Bare ground is blue, water bodies are very dark blue to black, snow is green, water clouds are light blue/green, ice clouds are red.

26 November 2018 GOES-16 Day Cloud Phase Distinction RGB. Full res

Bill Line, NWS

With GOES-17 now at the 137.2W GOES-West position (not yet operational, though), Alaska has significantly improved satellite coverage. Imagery from GOES-17 is available every 15-min over Alaska from the full disk sector, and every 1-min (or 30 seconds) in a mesoscale sector upon request (previous generation GOES provided 15-min coverage over Alaska with a 30-min gap every 3 hours). However, spatial resolution is degraded so far from the satellite subpoint over Alaska. A 1 km pixel will have an approximate pixel area of around 4 km over southern Alaska and the Aleutian Islands, degrading to 8+ km over northern Alaska. Figures 1, 2, and 3 show 15-min full disk VIS, IR and WV imagery during the day on 19 Nov. An occluded low pressure system is diagnosed along the southern Alaska coast.

Figure 1: 19 Nov 2018 GOES-17 0.64 um VIS. Full res

Figure 2: 19 Nov 2018 GOES-17 10.3 um IRW. Full res

Figure 3: 19 Nov 2018 GOES-17 6.2 um WV. Full res

Since GOES-17 arrived at GOES-West and dataflow resumed, NWS offices in Alaska have taken advantage of the 1-minute mesoscale sectors.  On the evening of 16 Nov, WFO Juneau requested and was granted a sector through the next evening  for “Winter Storm Warning in area with no RADAR coverage.” During the morning of 17 Nov, WFO Anchorage requested a sector to run through the evening for “Winter Wx Advisory, no radar coverage, models under performing.” These sectors were positioned next to each other, providing widespread 1-min imagery over Alaska. The mesoscale sector coverage from the AWIPS GOES-West perspective is shown in Figure 4. For a more direct view of Alaska, AWIPS users can take advantage of the Alaska perspective, shown in Figure 5. The meridional stretching of the sectors (and embedded pixels) is apparent in the Alaska perspective. The southern extent of the sectors are cut off in the Alaska perspective in this example. Alaska offices have requested mesoscale sectors numerous additional times since the 17th.

Figure 4: 17 Nov 2018 GOES-17 1-min meso-1 and meso-2 sector 0.64 um VIS from GOES-West perspective. Full res

Figure 5: 17 Nov 2018 GOES-17 1-min meso-1 and meso-2 sector 0.64 um VIS from Alaska perspective. Full res

Most derived products are not yet available in AWIPS from GOES-17, but all imagery channels, channel differences, and RGB’s are. The Day Cloud Phase Distinction (DCPD) RGB, which provides great contrast of snow (green) vs low clouds (light blue) vs high clouds (red), benefits from some slight modifications to the recipe. These changes are necessary due to lower reflectance and cooler temperatures over Alaska compared to over CONUS, especially this time of year. Figure 6 shows the default DCPD RGB over Alaska on 17 Nov, while figure 7 shows the same RGB and time period with some adjustments to the RGB component ranges. Similar tweaks can be made to other RGBs in order to make them more usable in cold airmasses.

Figure 6: 17 Nov 2018 GOES-17 1-min Day Cloud Phase Distinction RGB over Alaska. Full res

Figure 6: 17 Nov 2018 GOES-17 1-min *modified for AK* Day Cloud Phase Distinction RGB over Alaska. Full res

As of the writing of this post, GOES-17 imagery is preliminary and non-operational.

Bill Line, NWS

GOES-17 arrived to the 137.2W, GOES-West position, on Nov 13. Dataflow from GOES-17 resumed on Nov 15. The below examples exemplify the scope of ABI sector coverage from the GOES-West position.

GOES-17 full disk sector imagery (updates every 15-minutes when in mode 3, every 5-minutes when in mode 4) from 137.2W includes much of North America (with best coverage over western parts) and much of the Pacific Ocean (with best coverage over eastern parts). The sector includes coverage of Hawaii and Alaska. Below are examples of GOES-17 Full Disk VIS, IR, and WV from 137.2W.

Figure 1: 17 November 2018 GOES-17 15-min Full Disk 0.64 um VIS Imagery from 137.2W. Higher res

Figure 2: 17 November 2018 GOES-17 15-min Full Disk 10.3 um IRW Imagery from 137.2W. Higher res

Figure 3: 17 November 2018 GOES-17 15-min Full Disk 6.2 um WV Imagery from 137.2W. Higher res

GOES-17 CONUS sector imagery (updates every 5-minutes) from 137.2W includes the western 1/3 of the US and Hawaii. Below are examples of GOES-17 CONUS VIS and IR imagery from 137.2W.

Figure 4: 17 November 2018 GOES-17 5-min CONUS 0.64 um VIS Imagery from 137.2W. Higher res

Figure 5: 17 November 2018 GOES-17 10.3 um IRW 5-min CONUS Imagery from 137.2W. Higher res

Like it’s predecessor, GOES-17 has two movable 1-min mesoscale sectors. These sectors can image anywhere within the full disk. Below is an example of the two GOES-17 mesoscale sectors with IR imagery from the default “GOES-West” mesoscale sectors positions.

Figure 6: 17 November 2018 GOES-17 10.3 um IRW 5-min CONUS Imagery from 137.2W. Higher res

Finally, full disk imagery of all 16 GOES-17 ABI channels from the 137.2 GOES-West position.

Figure 7: 17 November 2018 GOES-17 15-min Full Disk All 16 Channel Imagery from 137.2W. Higher res

A reminder that GOES-17 is expected to become the operational “GOES-West” on Dec 10. As of the writing of this post, imagery is preliminary and non-operational.

Bill Line, NWS

A winter storm brought snowfall to much of southern Colorado on 11 November 2018. Behind the storm system on the 12th, a variety of low and high clouds lingered during the day after the snowfall had ended. Interrogating visible satellite imagery alone, it is difficult to differentiate the clouds from surface snow cover given both are highly reflective. The additional bands on the GOES-16 ABI compared to previous GOES imagers allows for the creation of RGB combinations that help differentiate various clouds from each other and from snow. This especially relevant to aviation forecasting when monitoring cloud cover near TAF sites.

The Day Cloud Phase Distinction has been described for this purpose in previous blog posts. The scene on the 12th was another great example of the advantages of using the Day Cloud Phase Distinction RGB over visible imagery alone. In the visible imagery (Fig 1), while it is apparent that there is widespread snow cover with clouds moving overhead, it is difficult to quickly diagnose clouds vs snow. The RGB (Fig 2) makes it clear where there is bare ground (dark blue), snow (green), liquid clouds (light blue), and ice clouds (red).

Figure 1: 12 November 2018 GOES-16 0.64 um VIS over southeast Colorado. Full res

Figure 2: 12 November 2018 GOES-16 Day Cloud Phase Distinction RGB over southeast Colorado. Full res

The Day Snow-Fog RGB can also be utilized for differentiating snow from low clouds and high clouds (Fig 3), but does not provide useful information over the Day Cloud Phase Distinction RGB. Further, it does not utilize the 0.64 um 500 m band, so it lacks the detail that is inherent in the Day Cloud Phase Distinction RGB, which does utilize the high resolution visible band.

Figure 3: 12 November 2018 GOES-16 Day Snow Fog RGB over southeast Colorado. Full res

Figure 4 provides a comparison of the visible channel with the two RGBs discussed along with labels for the various cloud and surface types.

Figure 4: 1732 UTC 12 November 2018 GOES-16 0.64 um VIS, Day Cloud Phase Distinction RGB, and Day Snow Fog RGB over southeast Colorado. Full res

Bill Line, NWS