Sentinel-1A overflew the Hawai’ian islands shortly before sunrise on 15 March and the derived winds from this descending pass are shown above (they are also available here, through this website). Southwesterly flow and the topography of Oahu has excited gravity waves that are observable downstream from the island in the SAR analysis. GOES-18 Band 13 (“Clean window”, 10.3 µm) imagery, below (that includes the Sentinel-1A SAR winds), also shows suggestions of mostly stationary clouds oriented parallel to the wind features. (This toggle showing 1624 UTC SAR winds and brightened visible imagery and 1646 UTC shows clouds aligned with the wind features as well).

Advanced Scatterometer (ASCAT) winds from the manati site, below, show the southwesterly winds to the northeast of Oahu from both ascending passes between 0700 and 0800 UTC, and from descending passes between 1900 an 2100 UTC.

Trapping the energy in the lower part of the troposphere requires the presence of an inversion. SkewT/LogP charts from Lihue to the west of Oahu, and from Hilo to the east (from the University of Wyoming Sounding Site) both show low-level stable air.

SAR data shows winds of 30 knots just north of Maui above (red in the enhancement). Should you believe those wind speeds? Sometimes (not today, but sometimes) reflection off ice within the clouds results in computed wind speeds that are too high. This typically occurs when feathery structures appear in the Normalized Radar Cross Section fields, shown below. The absence of such structures north of Maui lends credence to the computed wind speed.

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The waves also appeared in Water Vapor imagery, most prominently in the low-level (Band 10, 7.34 µm) imagery shown below.

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The conclusion from these images: the perturbation induced by the topography on Oahu affects the atmosphere from the sea surface all the way up into the upper troposphere!

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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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