Center-fixing a storm with SAR and SSMIS data

September 23rd, 2022 |
Sentinel SAR Wind Speeds, 0757 UTC on 23 September 2022 (Click to enlarge)

Sentinel overflew Tropical Storm Gaston, northwest of the Azores Islands, shortly before 0800 UTC on 23 September 2022, as shown above. The Beaufort Scale enhancement suggest peak winds derived from SAR observations to be very close to 50 knots in curved bands to the east/southeast of the center, inferred to be just off the edge of observation swath, which is at 40.76o N Latitude, 29.3o W Longitude. GOES-16 satellite imagery spanning this time around sunrise shows an exposed low-level circulation center with stronger convection building along the northern perimeter of the storm (mp4 animation shown here, created with CSPP Geosphere — direct link is here). Because GOES-16 can view the low-level circulation, the parallax shift of the center in the animation is small (smaller than the parallax shift in the SAR/GOES-16 imagery shown here Fiona) even though Gaston is near the satellite limb.

DMSP-17 carries the SSMIS (Special Sensor Microwave Imager/Sensor) and overflew Gaston just after 0900 UTC, as shown below in an image from the SSEC Tropical Website. The estimated wind speed from this image is 52 knots, close to the SAR values shown above. The 0900 UTC update from the National Hurricane Center showed a center at 40.5oN, 29.6oW and maximum sustained windspeeds at 50 knots.

85 GHz observations from SSMIS-17, 0911 UTC on 23 September 2022 (Click to enlarge)

Fiona intensifies to a Category 4 hurricane

September 21st, 2022 |

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animated GIF | MP4]

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed Fiona as it intensified to a Category 4 Hurricane just east of The Bahamas at 0900 UTC on 21 September 2022. The coldest cloud-top 10.35 µm infrared brightness temperatures were around -81ºC.

GOES-16 Infrared images with and without a overlay of GLM Flash Extent Density (below) did reveal isolated brief periods of lightning in the eyewall region of Fiona — but most of the lightning activity was associated with convection well east of the eye.

GOES-16 “Clean” Infrared Window (10.35 µm) images, with and without a overlay of GLM Flash Extent Density [click to play animated GIF | MP4]

The corresponding 1-minute GOES-16 Cloud Top Temperature and Cloud Top Height derived products are shown below — the coldest Cloud Top Temperature values were around -84ºC, while maximum Cloud Top Height values were around 58,000 feet.

GOES-16 Cloud Top Temperature and Cloud Top Height derived products [click to play animated GIF | MP4]

In a time-matched comparison of Infrared Window images from Suomi-NPP and GOES-16 at 0700 UTC (below), the coldest cloud-top infrared brightness temperatures were -86ºC and -81ºC, respectively. A slight northwestward parallax displacement was evident with the GOES-16 image.

Infrared Window images from Suomi-NPP (11.45 µm) and GOES-16 (10.35 µm) [click to enlarge]

===== 23 September Update =====

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animated GIF | MP4]

As Fiona passed just to the northwest of Bermuda during the nighttime hours on 22-23 September, it was briefly downgraded to a Category 3 storm at 0600 UTC (before being again upgraded to Category 4 at 1200 UTC on the following morning) — but a wind gust to 81 knots (93 mph) was recorded at Bermuda Naval Air Station (station identifier TXKF) shortly before 10 UTC while that airport was temporarily closed to air traffic (Bermuda discussion). 1-minute GOES-16 Infrared images (above) showed Fiona during the 0000-1000 UTC period on 23 September.

A DMSP-17 SSMIS Microwave (85 GHz) image at 1053 UTC from the CIMSS Tropical Cyclones site (below) exhibited the eye and eyewall structure about an hour after the peak wind gust at Bermuda.

DMSP-17 SSMIS Microwave (85 GHz) image at 1053 UTC [click to enlarge]

Atmospheric Bore structure over Wisconsin

September 20th, 2022 |
GOES-16 Visible imagery (0.64 ), 1056 – 1911 UTC on 20 September 2022

GOES-16 Visible Imagery, in the mp4 animation above (click here for an animated gif), shows convection initially over central Wisconsin at sunrise moving eastward over Lake Michigan into lower Michigan. In its wake, cloud lines extending east-west move southward into southward Wisconsin. Parallel lines such as these are typically associated with atmospheric bores, previously discussed many times on this blog (link). A bore is usually associated with stable air; note how the convective line over southwestern WI at around 1500 UTC dissipates after 1600 UTC as it encounters the stable air associated with the bore.

LightningCast Probabilities are consistent with the southern convective line encountering air that is more stable, as shown above in an animation that pauses at 1506 UTC; lightning probabilities decrease with the southern line as they increase with the northern line that eventually sweeps southward through southern Wisconsin, producing hail.

GOES-16 Visible imagery overlain with ProbSevere LightningCast probabilities and GLM Flash Extent Density, 1331 -1801 UTC on 20 September 2022 (Click to enlarge)

NOAA-20 overflew this region shortly after 1900 UTC on 20 September. The gridded 850-700 mb lapse rate, below, from this site, shows a region of more stable air over/around Chicago and southern Lake Michigan that is perhaps residual stability related to the bore feature.

Diagnosed 850-700 mb stability from NOAA-20 NUCAPS profiles, ca. 1915 UTC on 20 September 2022 (click to enlarge)

Shout-out to Rebecca, a forecaster at WFO GRB, for also noticing these lines!

The thunderstorms were followed by mammatus clouds over Madison, as shown in the image below, courtesy Bill Bellon, UW-Madison SSEC/CIMSS.

Mammatus clouds over Madison WI, 19 September 2022 (Click t0 enlarge). Photo Credit: Bill Bellon

TL;DR: Departing convection put down stable layer defined by atmospheric bore. Convection encountering this stable layer dissipated. Stronger convection moved in later, depositing hail.

SAR data over Hurricane Fiona

September 19th, 2022 |
Sentinel SAR wind speeds over Hurricane Fiona, 2245 UTC on 19 September 2022 (Click to enlarge)

NOAA/NESDIS STAR is producing occasional SAR imagery over Hurricane Fiona (link). NetCDF files can be imported into AWIPS and displayed, as shown above (color-enhanced with Beaufort Scaling), in an image from late on 19 September when Fiona was just north of Hispaniola. The strongest winds were associated with the northern eyewall of the storm, and an asymmetry in the storm is apparent (The NHC discussion from 2100 UTC on 19 September (here), and the discussion at 0300 UTC on 20 September (here) will give more context). The Radial wind analysis at the SAR link (here) shows the strongest winds in the NW and NE quadrants of the storm, with a peak near 100 knots. The toggle below, from late on 19 September, compares GOES-16 ABI Clean window (10.3 µm) imagery and the Sentinel SAR wind estimates (with a different color enhancement). A parallax shift in the GOES-16 imagery means the two eyes do not overlay.

GOES-16 Band 13 infrared (10.3 µm) imagery and Sentinel SAR windspeeds, 2245 UTC on 19 September 2022 (Click to enlarge)

Update on 23 September: RADARSAT-2 overflew Fiona at 2236 UTC on 22 September. The toggle below shows SAR winds estimated at that time, with peak values in the eyewall near 100 knots (for more information on this SAR pass, including a more complete image over the eye, click here). There is a parallax shift in the high clouds of the storm; Fiona at this time was near 30.5oN, 69.3oW, far from the sub-satellite point at 0oN, 75.2oW: the parallax shift for the high clouds will be away from the sub-satellite point. The time difference in the observations — 5 minutes — should not cause a big shift for a storm moving at 17 knots (i.e., 20 mph) to the north-northeast.

RADARSAT-2 SAR observations over Fiona, 2236 UTC on 22 September 2022 along with GOES-16 Clean Window Band 13 (10.3 µm) infrared imagery at 2230 UTC (Click to enlarge)