GOES-16 (GOES-East) Air Mass RGB images (above) include 3-hourly surface analyses of pressure and fronts — which showed the progression of a late-season Nor’easter during the 13 March – 15 March 2023 period. This storm produced heavy snowfall and high winds across much of the Northeast US (including a gust to 81 knots or 93 mph at Mt. Washington NH).

As the system was beginning to intensify off the coast of North Carolina on 13 March, 1-minute Mesoscale Domain Sector GOES-16 True Color RGB images from the CSPP GeoSphere site (below) revealed the hazy signature of enhanced solar reflection off an agitated sea surface (where high waves and abundant sea spray were present) — the likely result of a burst of strong middle-tropospheric winds that had descended to the surface (just south of the surface low pressure center). A similar signature of enhanced solar reflection off a highly-agitated sea surface was observed with strong West Atlantic storms in December 2022 and April 2019.

On 14 March, as the storm was slowly pivoting around Buoy 44005 in the Gulf of Maine during the time period shown in GOES-16  “Clean” Infrared Window (10.3 µm) images (above), the buoy wind speed decreased from 37 knots gusting to 45 knots (with a peak hourly gust of 51 knots) at 2150 UTC to just 5 knots gusting to 12 knots (but with a peak hourly gust of 35 knots) at 2350 UTC (below).

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If you see something, like a blog post for example (link, this shows an airmass high in Total Precipitable Water crossing the North Pole) and you’re curious what JPSS satellite imagery might look like with that something, how can you proceed? If the data are in the recent past, one could, for example, refer to the CIRA Polar slider and download an image, like this one that is time-stamped 0452 UTC on 7 March. That image, however, will not be full-resolution, and there are limits to what can be created. This blog post steps you through ways to figure out which JPSS data to download, and how to access it via NOAA CLASS, and how to display it with Polar2Grid (version 3.0, discussed in this previous blog post).

The first step is to find the times that Suomi NPP, or NOAA-20, or (eventually) NOAA-21 overflew the region. Satellite orbits are available here. The date in question is 7 March, and there are archived images for NPP, NOAA-20 and NOAA-21. Paths over the Arctic for NPP are shown below (link, here’s the similar image for NOAA-20), and the path from 1425 through 1445 UTC from Suomi NPP looks appropriate.

The next step is to access data at NOAA CLASS. We will request ‘JPSS VIIRS Sensor Data Record Operational (VIIRS_SDR)’ data. That takes you to a page that looks like this, where I have selected NPP data from 14:25 to 14:45 on 7 March 2023, from either ascending or descending passes. The Data I selected were Day Night Band (SVDNB) and I05 (SVI05) SDRs, as well as the navigation files that are needed: GDNBO, GIMGO and GITGO files. After some time, an email announces that the files are ready for download. Here is the listing after they’d been placed on my machine with the Polar2Grid software. There are eight files each of GDNBO, GIMGO, GITCO, SVDNB and SVI05 files, i.e., eight granules of data and navigation information within one directory. Note: As part of my NOAA CLASS preferences/profile, I have ‘No’ clicked for ‘Package Geolocation with JPSS Data Products’ and ‘Yes’ clicked for ‘De-aggregate JPSS Data Products’. Now it’s time to create imagery using Polar2Grid.

As always, I first interrogate to see what kind of imagery can be created with the data/navigation files that I have, i.e., I execute this command: ./polar2grid.sh -r viirs_sdr -w geotiff --list-products-all -f ../../data/Arctic/*.h5 and this command returns a list (under ### Standard Available Polar2Grid Products) of what I can request using the product flag (-p), including I05 and different flavors of Day Night Band imagery: adaptive_dnb, dyanmic_dnb, histogram_dnb, hncc_dnb (hncc: high and near-constant contrast). I then invoked the Polar2Grid command (1. and 2.) to create this imagery, (3.) to color-enhance the Infrared image, and then (4. and 5.) add coastlines and lat/lon lines. The flags in the add_coastlines.sh shell script are explained in the Polar2Grid online documentation under Section 7: Utility Scripts.

1:  ./polar2grid.sh -r viirs_sdr -w geotiff -p I05 -g polar_canada -f ../../data/Arctic/*.h5
2:  ./polar2grid.sh -r viirs_sdr -w geotiff -p adaptive_dnb dynamic_dnb histogram_dnb hncc_dnb -g polar_canada -f ../../data/Arctic/*.h5
3:  ./add_colormap.sh /home/scottl/CSPPGeo/enhancements/IR13_AWIPSAPPROXnew.txt npp_viirs_I05_20230307_142958_polar_canada.tif
4:  ./add_coastlines.sh --add-coastlines  --coastlines-outline black --coastlines-width 2 --add-grid --grid-D 10.0 10.0 --grid-d 10.0 10.0 --add-colorbar --colorbar-align bottom --colorbar-text-size 1 --colorbar-tick-marks 255 --colorbar-height 50 npp_viirs_I05_20230307_142958_polar_canada.tif
5:  ./add_coastlines.sh --add-coastlines  --coastlines-outline black --coastlines-width 2 --add-grid --grid-D 10.0 10.0 --grid-d 10.0 10.0 *dnb*20230307_142958_polar_canada.tif

Note in the polar2grid call above I have referenced one of the ‘built-in’ grids with -g, i.e., the ‘polar_canada’ grid. In this way I did not have to use the p2g_grid_helper.sh script to create a yaml file that holds gridding information. Imagery created are shown below. The toggle includes the ‘hncc’ Day Night Band image — on this day it had the best look — and the 11.45 µm I05 image both reprojected onto the ‘polar_canada’ map. The cloud feature that you might expect given the cross-polar motion of the relatively high Total Precipitable Water airmass is apparent.

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GOES-18 infrared imagery (Band 13, 10.3 µm) and derived Total Precipitable Water, above, shows tropical Invest 91P several hundred miles south of American Samoa within a corridor of moisture associated with the South Pacific Convergence Zone (SPCZ). (Note that the default color scale for Total Precipitable Water (TPW) has been rescaled so the “dryest” value is 1.0 inches). The Samoan islands are just at the northern edge of the deepest moisture — observed TPW at the Pago Pago sounding has decreased from 55 mm at 1200 UTC on 12 March to 44 mm at 1200 UTC on 14 March, the start of the animation above. MIMIC Total Precipitable water for the 24 hours ending 1300 UTC on 14 March, below, (source) shows dryer air very slowly moving over the Samoan islands from the north and northeast. (SAR and Scatterometry surface winds are all northerly around Samoa, as shown in this blog post).

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A Suomi-NPP VIIRS Day/Night Band (0.7 µm) image valid at 1252 UTC is shown below — ample illumination from the Moon (in the Waning Gibbous phase, at 56% of Full) provided a useful “visible image at night”. No bright lightning streaks were seen within the convective cluster associated with Invest 91P.

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What does the future hold for Invest 91P? The toggle below shows imagery from the SSEC/CIMSS tropical weather site (link). Shear values are low and Sea Surface Temperatures are warm. However, winds in the atmosphere (shown below, from here) are moving the invest area to the south, towards a less favorable environment.

For more information on Invest 91P (or 90P to its west!), refer to the SSEC/CIMSS Tropical Weather website (link) or the webpages of the Joint Typhoon Warning Center (link).

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The special SAR observation period over American Samoa, allowing extra observations from RADARSAT and RCM satellites (see CIMSS Blog posts here, here, here, here, here, here, here and here!) has ended, but Sentinel-1A will still routinely send back SAR observations over the Samoan Island chain. The pass above show GOES-18 Clean window imagery (10.3 µm) with SAR winds overlain.

A zoomed-in view of the SAR winds to the west of Samoa shows how parallax must be considered. GOES-18 navigation assumes clear skies, and if a tall (GOES-18 cloud-top heights — not shown — diagnose heights exceeding 44000 feet) cumulonimbus cloud is present, it will be navigated such that it is displayed farther from the sub-satellite point (0^oN, 137.2^oW) than its true position. Ice within that cumulonimbus cloud is likely altering the SAR return so that stronger winds are diagnosed. As noted in previous SAR blog posts, the presence of ice can be inferred by feathery features within the Normalized Radar Cross Section (NRCS).

The image below toggles between the NRCS and Wind Speed (from this page) and highlights the feathery structures (suggestive of thick ice within the cumulonimbus cloud) in the regions of strongest diagnosed winds. Note that the wind direction is northerly, and a wind shadow is obvious to the south of Savai’i (Salafai).

Part of the earlier of the SAR observations on this ascending Sentinel-1A pass is shown below. The coldest cloud top, in the northeastern part of the domain (orange/red enhancement) also shows a parallax shift away from the GOES-18 sub-satellite point (at about 137.2^oW) such that the cold cloud top is shifted west of the strong SAR winds (40-50 knots in the white enhancement) that are likely an artifact of the ice within that cloud. The large area of strong winds (20-25 knots, yellow/green in the color enhancement) over the center of the domain do not arise from cloud ice.

A toggle between NRCS and wind speed at 06:00:18 is shown below.

MetopC ascended over the Samoan islands after 0900 UTC on 13 March 2023, and Advanced Scatterometer (ASCAT) winds from that pass are shown below. ASCAT also diagnoses relatively weak winds just south of Western Samoa, 20-25 knot winds south of the lighter winds.

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Dora Meredith from the NWS Forecast Office in Pago Pago asked the excellent question: Would the Parallax shift be different from Himawari-9? GOES-18 is over the equator at 137^oW, about 35^o east of the Samoan Islands, and Himawari-9 is over the equator at 140.7^oE, about 45^o west of the Samoan Islands. The toggles below shows two examples of SAR winds south of Samoa in three panels; the right panel also showing GOES-18 Band 13 infrared imagery — and a parallax shift in the imagery to the west, away from GOES-18’s sub-satellite point is apparent; the left panel shows Himawari-9 Band 13 infrared imagery — and a parallax shift in the imagery to the east, away from Himawari-9’s sub-satellite point is likewise apparent.

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