The Gálvez-Davison index (GDI) is a useful derived product (available online here from NCEP) for predicting tropical convection in tradewind regimes. In general, the larger the GDI, the more likely that showers and thunderstorms will be present. The toggle above compares GDI with K-Index derived from NUCAPS data (the Sounding Availability plot shows in green where the NUCAPS retrieval completed successfully; i.e., where the data are considered most appropriate). There’s good spatial correlation between the GDI and the diagnosed stability.

The toggle below compares GDI from GFS to the GOES-17 K-index (a Level 2 GOES-R derived stability product computed in clear skies only) that is displayed on top of GOES-17 Band 13 Clean Window infrared (10.3 um) imagery. A corridor of lower stability is readily apparent in the K Index, stretching across the Samoan Islands. Relatively stable air overlays Fiji, and a large region of instability lies east of American Samoa. The GDI agrees very well with the K Index derived from the satellite data, and convection as diagnosed by the Band 13 imagery is limited to regions where GDI values are larger. It is a good practice is to compare the K Index and the GDI. If the K Index starts to diverge from the predicted GDI, it’s a good idea to ask why!

Thanks to Dora Meredith and Elinor Lutu-McMoore, WGO PPG, for their assistance with this blog post!

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RADARSAT-2 observations of SAR data near/around the island of Guam in the Marianas Islands continue in November (as previously discussed here, here and here). Overpasses on 15 November (above) and 17 November (below) contain features that deserve comment. For example, there is an obvious seam (and a less-obvious second seam) in the SAR wind analysis above. The “Beam Seams” arise because the 500-km wide scan for SAR winds is created from smaller individual beam positions.   Each beam position has its own set of adjustments applied during SAR image processing and sometimes there is slight disagreement from one beam to the next.   The image above has 3 subswaths – so 2 beam seams. The seams are also apparent in the normalized radar cross section (NRCS) image here.

Although SAR winds as displayed in AWIPS do not have a wind direction, a direction can be inferred from the ‘shadow’ of weaker winds in the lee of Guam. Imagery at this website does include the constraining GFS wind barbs (as shown here). Note that MetopB Advanced Scatterometer (ASCAT) winds show winds (from this site) in the 15-20 knot range in the region to the west of Guam, in agreement with the SAR observations.

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SAR winds on 17 November at 0830 UTC, shown below with Himawari-8 infrared imagery, show winds of similar strength to those on the 15th. (Click here to view the analysis with wind barbs, or the NRCS analysis, both from this site.) On this day, the Marianas Islands all have an apparent effect on the downstream winds (Click here to see the AWIPS image of SAR winds; here’s a similar example over the Caribbean). The wake from Rota, for example, extends all the way from Rota to the western edge of the SAR scan! The wake from Guam is being affected by convection, but it too can be followed to the western edge of the SAR domain. Islands in the Marianas have an effect on winds that stretches for 100s of kilometers on some days, such as the 17th, but not on others, such as the 15th. (Note also: very faint Beam Seams are detectable on the 17th).

This SAR scan includes the islands of Tinian and Saipan, as shown below. Are there still large ships near Saipan, as noted in this blog post where 5 are present? Per the SAR data below, perhaps only one! Also, the lighter winds downwind of both Saipan and Tinian are very apparent in the image below: winds of 20 knots on the windward side of the islands (green in the enhancement used) and 10 knots or less (blue and purple) on the leeward side.

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You can find information on RADARSAT-2 at this WMO link.

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Overlapping 1-minute Mesoscale Domain Sectors provided 30-second images from GOES-16 (GOES-East) ABI spectral bands 01-16, in addition to a Rocket Plume RGB (above) — which displayed visible reflectance and/or thermal signatures of the NASA Artemis Space Launch System rocket booster (as it moved quickly northeastward) and its low-altitude launch condensation cloud (as it drifted slowly eastward away from the coast) on 16 November 2022. One or both of these signatures was seen in all 16 of the ABI spectral bands, as well as the RGB imagery.

A closer view of the area immediately offshore of the Kennedy Space Center launch site (below) showed the faint visible reflectance signature in Band 01, along with the brighter signature in Band 02.

GOES-16 Rocket Plume RGB images created using Geo2Grid are shown below.

GOES-18 (GOES-West) caught a glimpse of the rocket booster’s thermal signature (shades of green) far to the east of Florida during its Full Disk scan that began at 0650 UTC (below).

Thermal signatures of the rocket booster (brighter shades of pink) and its low-altitude condensation cloud (darker shades of blue) were also apparent in GOES-16 Nighttime Microphysics RGB images from the CSPP GeoSphere site (below).

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The animation above shows the Nighttime Microphysics RGB (saved from the CSPP Geosphere site) over the islands of American Samoa. (An animated gif is available here). The annotated animation below highlights several features that are typical of convective development detection at night. Low clouds become apparent shortly before 1450 UTC (when they are circled); subsequent color changes in those low clouds (in this case, a reddening that then detaches from the cloud) are most likely orphaned anvils (highlighted with arrows in the 1500-1530 UTC imagery — but the orphaned anvil can be tracked at later times, as it moves farther to the south) whose appearance frequently precedes successful convection; further reddening of the low clouds, as highlighted by arrows from 1610 to 1630 UTC, shows a transition of the low clouds (made up of water droplets) to higher clouds that are glaciating. At the end of the animation, note that active convection is near the Manu’a Islands.

Careful inspection of the Night Microphysics RGB can help you anticipate where convection might be developing at night.

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