An SPC Day 1 Moderate Risk for severe thunderstorms was issued for March 28, 2017 as yet another potent shortwave trough moved through the southern Plains. SPC requested GOES-East Rapid scan mode, and both GOES-16 mesoscale sectors were moved over the moderate risk area, meaning 30-sec imagery was available.
Below is a side-by-side comparison of GOES-16 0.5 km visible 30-sec imagery and 1-min imagery. The scene is of rapidly developing convection among mature storms. Overshooting tops and above-anvil cirrus plumes are apparent, indicators of strong-to-severe convection. When compared side-by-side, the 30-sec imagery does indeed have a slightly smoother appearance.
-Bill Line, NWS
“The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing. Users bear all responsibility for inspecting the data prior to use and for the manner in which the data are utilized.”
On the morning of 03/08/17, Eric Stevens and I had the opportunity to visit the Anchorage NWS Weather Forecast Office as part of a coordinated trip to discuss a collaboration between the Satellite Proving Ground for Marine, Precipitation, and Satellite Analysis and the Alaska Region Satellite Proving Ground. Although not the purpose of our visit, forecaster Mike Lawson (Satellite Focal Point) showed some multispectral imagery of that morning’s Bogoslof eruption that highlighted an area of Sulfur Dioxide (SO2) that was drifting from the volcano, southeast into the Gulf of Alaska. What surprised us was how well the SO2 cloud showed up on Himawari using the Air Mass RGB, which is not typically used for this type of feature identification. The SO2 signal (pink feature moving southeast from Bogoslof) is due to the 7.3 𝞵m water vapor channel having some sensitivity to SO2 absorption. We took a closer look at other imagery (difference, single channel, and multispectral) to see if the feature was still evident.
Himawari Air Mass RGB animation of the Bogoslof eruption on the early morning of 03/08/17. The pink area as the ash dissipates is a sulfur dioxide cloud that is picked up by the 7.3 um water vapor band.
Some information on the eruption and observations:
1) Alaska Volcano Observatory (AVO) says the eruption began at 10:36pm Tue Mar 7, continued for three hours, and sent ash up to 35k ft. Article on-line at…
2) GINA’s polar orbiter imagery from the hours after the eruption, using the classic old channel difference between 11um and 12um, showed minimal signature. While this differencing approach is doable with data gathered by imagers on polar orbiters, the GOES N-O-P satellites lack the required 12um channel. One of the many benefits of the GOES-R series and the ABI instrument is that the 12um channel is now returning to geostationary orbit, so the differencing approach seen in polar data over Alaska today gives a taste of what the new GOES-West will deliver in the future, not to mention the numerous more sophisticated multispectral products that can be generated from ABI data.
VIIRS 11um – 12um volcanic ash product valid on 03/08/17.
As shown above, in this specific case the longwave IR differencing method doesn’t appear to work very well. The differencing technique is tuned to detect volcanic ash, but not to detect SO2. Per the figure below (from the web site http://all-geo.org/volcan01010/2015/04/how-do-satellites-map-volcanic-ash-clouds/ ), this lack of signal can be explained by an absence of ash in the cloud. If the ash precipitated out of the cloud before this S-NPP pass, or if there was not much ash in the cloud to begin with, this product will not offer much insight. But as shown in the following screen captures, the multispectral Airmass, Ash, and Dust imagery derived from the Himawari 8’s Advanced Himawari Imager (AHI) can detect ash as well as SO2.
The satellite proving ground was designed to show new products to assist in these cases and in this case, we are delivering products to the NWS that are an improvement over what was available previously. Below are a couple VIIRS Red-Green-Blue products (Dust RGB and Volcanic Ash RGB) that can sense SO2 in the atmosphere due to the sensitivity of the 8.6 𝞵m channel to SO2 absorption.
VIIRS Dust RGB with the SO2 signal identified by the light green color south of the Aleutian Islands valid on 03/08/17.
VIIRS Volcanic Ash RGB with the SO2 cloud identified as the light green coloring south of the Aleutian Islands valid on 03/08/17.
Similar to the Himawari Air Mass animation above, but from the MODIS instrument on the Aqua satellite. The SO2 cloud is identified in this imagery as the pink coloring south of the Aleutian Islands, valid on 03/08/17.
These new multispectral capabilities in concert with some of the more traditional methods of identifying and tracking ash plumes and SO2 will provide forecasters and analysts more information on the hazards associated with volcanic eruptions. It certainly shed some light on the risks that Bogoslof presented on this particular day as SO2 can be quite deadly.