Satellite Liaison Blog

An upper level storm system brought winter weather and severe weather to a large portion of the US on 18-19 Jan 2019. GOES-16 water vapor and IR imagery show the extent and evolution of the mid latitude cyclone on the 19th (Fig 1).

Snow left by the system the previous day and evening was obvious in Day Cloud Phase Distinction RGB imagery (green) as low clouds (aqua/light blue) dissipated (Fig 2). Rapid melting of the snow was apparent during the day, especially in Texas.

Further east, thunderstorms along and near the cold front produced damaging winds and a few tornados. GOES-16 1-min imagery was available over the region to support forecast operations. The GOES-16 GLM revealed an overall lack of lightning activity along the cold front. However, it did highlight the location of (particularly strong) updraft cores and strengthening/weakening updraft trends (Fig 3).

Finally, an undular bore was captured in GOES-16 imagery traveling south along the east coast of Mexico (Fig 4).

Bill Line, NWS

A recent snowfall and relatively moist airmass in place made for a complicated scene over southern Colorado on 13 Jan 2019. As has been shown in previous blog posts, the Day Cloud Phase Distinction RGB is a great tool for differentiating low clouds (light blue) from snow (green) from high clouds (reds) during the day. In this example, the RGB imagery was utilized to monitor low cloud and fog evolution across much of the CWA in order to keep the grids/forecast accurate, but especially near KPUB, KCOS, and KALS in order to provide accurate TAFs. 1-min imagery was available from one of the GOES-17 default mesoscale sectors to provide even more detailed cloud analysis. Compare the RGB (Fig 1) with VIS (Fig 2). Cloud analysis is much easier in the RGB, particularly in areas where low clouds are present over/next to snow cover.

Note the sharp gradient in snow cover south of the Arkansas River (KPUB to KLHX to KLAA), where northerly flow becomes upslope, aiding the development of snowfall. Near and north of the river to the Palmer Divide, northerly flow is downslope, which limits snow potential in such a case. Snow is apparent again further north along the Palmer Divide from KCOS through KDEN, where northerly flow is upslope again.

Notice the stationary area of relatively high reflectance just south of KPUB? It turns out that is steam emitted from a local Power Plant (Figure 3)!

Bill Line, NWS

GOES-17 data are preliminary and non-operational.

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.

A GOES-16 1-min sector captured strong thunderstorms developing within a corridor of relatively high atmospheric moisture on Dec 14. The GOES-16 Total Precipitable Water product showed values up to 2.5″ within a large corridor of 2″+. 

The field study continued into 2019. Widespread thunderstorm activity was captured by the GOES-16 1-min sector over Argentina during the morning of 6 Jan. Figures 6 and 7 show ongoing convection after sunrise, including several storms exhibiting overshooting tops and above anvil cirrus plumes, which are indicative of particularly strong and potentially severe updrafts. Figure 8 shows the rapid development of a thunderstorm over a 95 minute period.

On 15 Jan 2019, 1-min visible imagery over Argentina provided an intriguing visualization of atmospheric shear. Loading Derived Motion Winds (DMWs) as well (updating every 5-min in mesoscale sectors), the shear can be better quantified. Sampling some winds in the area, we see 10-15 knot winds from 160 degrees at around 950 mb and 30 knot winds from 310 degrees at 550 mb yields a deep layer shear magnitude of around 40 knots.

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