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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1-minute Mesoscale Domain Sector GOES-18 (GOES-West) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images (above) included an overlay of GLM Flash Extent Density — which showed thunderstorms that produced flash flooding, hail and an EF-1 tornado (SPC Storm Reports) in the Central Valley of California on 11 March 2023. At 2236 UTC (1 minute prior to the Tornado Warning being issued), an overshooting top along the Calaveras/Tuolumne County line exhibited an infrared brightness temperature of -53.3ºC — which roughly corresponded to the altitude of a Most Unstable Maximum Parcel Level (MU MPL) as analyzed from the 0000 UTC Oakland rawinsonde (source).  Note that for the relatively low-topped convection over California on this day, the coldest value of the default infrared enhancement was modified to -70ºC, to aid in the identification of colder overshooting tops (shades of white embedded within dark black regions).

The corresponding GOES-18 Cloud Top Height derived product at 2236 UTC — just before the issuance of the Tornado Warning — was around 32,482 feet (below). For this storm, the maximum value of the default Cloud Top Height enhancement was set to 35,000 feet.

During the period leading up to convective initiation of the hail/tornado-producing thunderstorm, 1-minute GOES-18 Visible and Infrared images (below) include contours of LightningCast Probability — a >50% LightningCast Probability (green contour) over western San Joaquin County at 2009 UTC provided a 17-minute lead time to the start of a prolonged period of GLM-indicated lightning activity for that storm, which commenced at 2026 UTC (2000 – 0100 UTC animation).

These thunderstorms developed in an environment of modest moisture and instability, as shown by 1-minute GOES-18 Visible and Infrared images combined with Total Preciptitable Water (TPW) and Convective Available Potential Energy (CAPE) derived products in cloud-free areas (below). Satellite-derived TPW values up to 0.90 inch and CAPE values as high as 400 J/kg were observed in the general vicinity of the strongest convection. Note that the default enhancements for TPW and CAPE were also modified for use with these particular storms — the maximum TPW value was set at 1.1 inches, while the maximum CAPE value was set at 500 J/kg.

We have determined a brief EF-1 tornado touched down 6SW of Tuttletown in Tuolumne County during yesterday's severe thunderstorms that also produced 1" hail and significant flash flooding. See map and press release for more info. #CAwx pic.twitter.com/bEHYHzolI4

— NWS Sacramento (@NWSSacramento) March 12, 2023

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The MIMIC Total Precipitable Water product during the period 0400 UTC on 06 March to 1200 UTC on 08 March 2023 (above) displayed the poleward transport of an atmospheric river (AR) that moved across Alaska and then the North Pole — eventually passing over Svalbard (located between Norway and Greenland). As this AR was emerging from the north coast of Alaska and approaching the North Pole, Total Precipitable Water (TPW) value anomalies were as high as 5-6 standard deviations above normal (source).

As the AR moisture plume was moving over Utqiagvik (formerly “Barrow”; station identifier PABR) Alaska at 0000 UTC on 07 March, the rawinsonde-derived TPW value was 0.62 inch (which might have been a monthly record high TPW for PABR, according to this tweet). At that particular time there was good agreement with the MIMIC TPW product, whose value in the vicinity of PABR was 0.65 inch (below).

The NESDIS Snowfall Rate product from the CICS site (below) showed the AR plume of clouds that was likely producing snowfall at the surface as it moved northward from the coast of Alaska to the Arctic Ocean.

Here’s a special image! This shows a snowfall rate product, giving liquid equivalent of snow in inches/hr. What’s special is that the remote north pole typically doesn't have observations of snowfall, but now we have a satellite product that can provide this information! #akwx pic.twitter.com/wPn4wVLpvX

— GINA (@uafgina) March 7, 2023

As the AR was passing across the North Pole, the maximum MIMIC TPW value at that location was 0.25 inch at 0900 UTC on 07 March (below).

As the leading edge of the AR eventually began moving over Svalbard (station identifier ENSB) — a distance of approximately 2000 miles from the northern coast of Alaska — light snow was observed at 2000 UTC (below) and 2100 UTC on 07 March, and then again at 0900-1000 UTC on 08 March.

Note the gradual increase in temperature and dew point as the AR continued to traverse Svalbard (below).

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Note: NOAA-21 data used in this blog post are preliminary and non-operational. Imagery was created using Polar2Grid v3.0.

The CIMSS Community Satellite Processing Package (CSPP) team has released Polar2Grid version 3.0, a software package that can be used (among many other things!) to create high-quality imagery from JPSS Sensor Data Record (SDR) files as might be produced at a Direct Broadcast site (or ordered/downloaded from NOAA CLASS). Polar2Grid v3.0 represents a significant change from Polar2Grid v2.3, and some of those changes are documented here. The latest Polar2Grid also has significantly faster processing. The three true-color images above were created using Suomi NPP, NOAA-20 and NOAA-21 VIIRS imagery. (Note that NOAA-21 data are still preliminary and non-operational, although Beta maturity has been reached. They are included here to show that Polar2Grid can produce NOAA-21 imagery every bit as stunning as imagery from Suomi NPP and NOAA-20!). An especial advantage of 3 Polar Orbiters is obvious in the imagery above: the capability to animate high-resolution JPSS data.

Polar2Grid linux-based software can be downloaded from this website in the form of a gzipped tar file (Documentation on Polar2Grid is also available.) After expanding the downloaded file, and setting an environment file, i.e., export POLAR2GRID_HOME = /directory/where/software/sits, the various packages within $POLAR2GRID_HOME/bin can be run. To create the imagery above, I first created a map on which to reproject the data. This required the use of the p2g_grid_helper.sh scripts that creates the required navigation information in a format Polar2Grid recognizes. I wanted to create something that covered the Great Lakes because all 3 JPSS satellites — Suomi-NPP, NOAA-20 and NOAA-21 — scanned the entire Great Lakes in one swath on a relatively clear day. How will the three scenes compare? After some trial and error, I used this command

./p2g_grid_helper.sh GreatLakes -84.0 45.0 500.0 -500.0 2880 1920 > GreatLakes.yaml

that created a grid (called ‘GreatLakes’) centered at 84^oW, 45^oN, with 500 m resolution in the N-S and E-W directions, with a grid size of 2880×1920. The information created was redirected into the file ‘GreatLakes.yaml’; Polar2Grid software by default now expects grid configuration files created by the p2g_grid_helper shell script to reside in files with a suffix of ‘yaml’.

To acquire data for Polar2Grid to process, I accessed the CIMSS real-time system holding files created from the Direct Broadcast site. Data are separated out by satellite, and by day/time. Multiple granules are within each day/time file, especially now that CIMSS is producing expanded files by using data from other Direct Broadcast sites! Suomi NPP data for this case, for example, are at this link from 1734 UTC on 7 March; data from NOAA-20 are from 1821 UTC; data from NOAA-21 are from 1846 UTC. I could also have requested data from NOAA CLASS by selecting ‘JPSS VIIRS Sensor Data Record Operational (VIIRS_SDR) — although this site only distributes Operational data: NOAA-21 data are not available (yet). Usually when I access JPSS data from NOAA CLASS, I first view the orbits (from this website) for the day in question (7 March in this instance) for NPP, NOAA-20 and (eventually) NOAA-21, so I can access specific times that cover the region of interest.

Once the data were on the machine that holds the Polar2Grid software, it was straightforward to create imagery, using these commands for Suomi-NPP, NOAA-20 and NOAA-21 data, respectively; all the data were in the same directory:

./polar2grid.sh -r viirs_sdr -w geotiff -g GreatLakes --grid-configs GreatLakes.yaml -p true_color -f ../../daydata/*npp*.h5
./polar2grid.sh -r viirs_sdr -w geotiff -g GreatLakes --grid-configs GreatLakes.yaml -p true_color -f ../../daydata/*j01*.h5
./polar2grid.sh -r viirs_sdr -w geotiff -g GreatLakes --grid-configs GreatLakes.yaml -p true_color -f ../../daydata/*j02*.h5

Polar2Grid’s viirs_sdr reader (the -r flag) is being used, as is the geotiff writer (the -w flag). This syntax is different from earlier versions of Polar2Grid, but more in line with that in Geo2Grid. The data are being regridded (the -g flag) to a ‘GreatLakes’ grid, and grid configurations are stored within the ‘GreatLakes.yaml’ file. The product (-p flag) being created is ‘true_color’ and the data files (the -f flag) are found in the relative directory specified. If a user is unsure of what products can be created given the data in the data directory, the –list-products-all instruction can be used in place of -p to find that out.

The three commands above create (spectacular!!) True-Color imagery from Suomi-NPP, NOAA-20 and NOAA-21, seen in the animation above. Polar2Grid can also be used to add maps to the imagery, but this blogger thought those weren’t necessary.

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