Low-Earth Orbit satellite views of Ian as it formed, and comparisons to Geostationary imagery

September 26th, 2022 |

Polar-orbiting satellites have microwave detectors that give important information about the low-level structure of an evolving tropical cyclone. If high clouds are omnipresent, it can be difficult for an analyst to diagnose storm strength with accuracy. Microwave energy penetrates clouds, however, and low-earth orbit (LEO) observations of microwave frequencies can reveal much about a storm’s structure.

24 September: South of Haiti

Consider the imagery below, showing the cluster of thunderstorms associated with then-Tropical Storm Ian south of Haiti. Based on just the still infrared image (admittedly, this would be easier with an animating image!), where would you place the center? Microwave data — 36.5 GHz and 89 GHz data from GCOM-W1 (from the AOML Direct Broadcast site here) suggest a center in between the top large regions of cold cloud tops in the infrared imagery (the 0900 UTC discussion has a center near 14.7oN, 73.5oW). MIMIC Tropical Cyclone imagery (from this link) for Ian on 24 September (here) can help a user determine where the center is as well.

GOES-16 ABI Band 13 Infrared (10.3 µm) imagery, and GCOM-W1 AMSR-2 Microwave imagery (36.5 and 89.0 GHz), 0620 UTC on 24 September 2022 (Click to enlarge)

25 September: Southwest of Jamaica

One day later, imagery from ABI and GCOM-W1 show a better-defined tropical system at 0700 UTC (Here’s the NHC discussion from 0900 UTC, at which time the center was at 14.9oN, 78.8oW). Even from the still ABI image, one could infer a center based on the spiral bands. Microwave information (36.5 and 89.0 GHz) certainly will increase confidence. Indeed, the low-level microwave signal (i.e., from 36.5 GHz) suggests a center very near the 0900 UTC location. The MIMIC TC animation from 0000 UTC 25 September – 0000 UTC 26 September (link) is showing a stronger signal for a center as well.

GOES-16 ABI Band 13 Infrared (10.3 µm) imagery, and GCOM-W1 AMSR-2 Microwave imagery (36.5 and 89.0 GHz), 0700 UTC on 25 September 2022 (Click to enlarge)

26 September: south of Western Cuba

NOAA-20 ATMS imagery (88 GHz) over Ian, 0606 and 0746 UTC on 26 September 2022 (Click to enlarge)

The LEO coverage on 26 September is a great example of why multiple LEO satellites are vital. The early-morning coverage from NOAA-20 is shown above; the gap between the two satellite passes is in an unfortunate spot for monitoring this tropical cyclone! However, Suomi NPP orbits overlap NOAA-20, and on this day Suomi NPP overflew the center of the storm, as shown below. The cadence was NOAA-20 to the east, 45 minutes later Suomi-NPP over the center, 45 minutes later NOAA-20 to the west. Here is an animation of the three passes. Polar monitoring capabilities will receive a big boost when JPSS-2 (slated to become NOAA-21) is launched (tentatively scheduled for 1 November 2022).

Suomi-NPP ATMS Microwave Imagery, 88.0 GHz, 0656 UTC on 26 September 2022 (Click to enlarge)

Ian at 0700 UTC on 26 September, below, is on the cusp of being upgraded to a hurricane (0600 UTC intermediate advisory), and an animation of the Band 13 imagery (a still image is shown below for comparison to the ATMS imagery) shows the center of rotation even though an eye is not present in the infrared (although one in the microwave).

GOES-16 ABI Band 13 Infrared (10.3 µm) imagery, and Suomi-NPP ATMS Microwave imagery (88.0 GHz), ca. 0700 UTC on 26 September 2022 (Click to enlarge)

ATMS and AMSR2 imagery as shown above are created from passive microwave sensors; that is, the sensors are detecting the microwave imagery emitted by the ocean, land, clouds and atmosphere. Other LEO satellites emit energy (“ping”) in the microwave and listen for a return signal. This leads to both scatterometry (not shown, as from the Advanced Scatterometer — ASCAT — instrument on Metop-B and Metop-C — available here) and Synthetic Aperture Radar imagery (available here for tropical cyclones), and shown below. The image below shows infrared and GLM imagery for then-newly upgraded Hurricane Ian (link). Although a distinct eye is still not present in the infrared imagery, SAR wind data defines an obvious region of reduced winds. Maximum SAR winds in this image are just above 70 knots.

GOES-East ABI Band 13 Infrared imagery (10.3 µm), GLM 1-minute aggregate Total Optical Energy (TOE) and RSAT-2 SAR Winds over Ian, 1110 UTC on 26 September 2022 (Click to enlarge)

VIIRS and ATMS imagery of Hurricane Ian on 27 September is here. For the latest information on Hurricane Ian, please refer to the National Hurricane Center. People in southern (and especially southwestern) Florida should be paying very close attention to this storm.

Fiona makes landfall in Nova Scotia

September 24th, 2022 |

GOES-16 Mid-level Water Vapor (6.9 µm) images [click to play animated GIF | MP4]

Hurricane Fiona transitioned to a strong post-tropical cyclone just prior to making landfall in Nova Scotia, Canada around 0700 UTC on 24 September 2022 — 1-minute Mesoscale Domain Sector GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) showed the storm during the 0200-1800 UTC period. This anomalously-deep cyclone may have set a new Canadian record (931 hPa) for minimum mean sea level pressure (surface analyses). The highest surface wind gust was 97 knots / 112 mph / 179 km/h at mesonet site Arisaig (along the northeastern coast of mainland Nova Scotia), with a peak wind gust of 90 knots / 104 mph / 167 km/h at METAR site Grand Etang CWGQ (just off the west coast of Cape Breton Island) shortly before 0800 UTC. A preliminary storm summary issued by Environment Canada is available here.

One feature of interest was a region of lee waves generated by strong easterly boundary layer winds interacting with the Highlands terrain in the northern portion of Cape Breton Island — these waves spread westward across parts of the Gulf of Saint Lawrence (below).

Comparison of GOES-16 Water Vapor image at 0700 UTC and topography [click to enlarge]

Sentinel-1A Synthetic Aperture Radar (SAR) water surface winds at 1015 UTC (below) captured lighter winds of 20 knots or less (brighter shades of violet) within the post-tropical cyclone’s eye as it was emerging northwestward from Cape Breton Island — along with an arc of strong winds within the storm’s eastern semicircle, where speeds of 100 knots or higher (shades of red) were seen. Off the north coast of Cape Breton Island, the peak wind gust at St. Paul Island (CWEF) just prior to 1000 UTC was 85 knots — and SAR winds at 1015 UTC in the vicinity of CWEF were in the 75-80 knot range.

Sentinel-1A Synthetic Aperture Radar (SAR) winds at 1015 UTC [click to enlarge]

The animation of GOES-16 airmass RGB, below, shows the evolution of Fiona and its interaction with a Potential Vorticity Maximum (orange/red in the RGB).

GOES-16 airmass RGB, 0000 UTC 21 September – 0000 UTC 25 September

Perhaps you are skeptical that the Orange/Red signature in the Airmass RGB is a Potential Vorticity Signature. Consider the animation below, downloaded from the TropicalTidbits website and showing 00 – 30 hour GFS model output from the run initialized at 0000 UTC on 23 September. A rich source of cyclonic potential vorticity air (in orange) wraps around Fiona as it moves north. Similar behavior is apparent in the Airmass RGB animation above. This cross section (also from the excellent Tropical Tidbits site) of the model data (24 h into the forecast run) also shows a classic stratospheric intrusion structure.

330 K Potential Vorticity color-shaded in PV Units at 6-h time-steps, 0000 UTC on 23 September 2022 through 0600 UTC on 24 September 2022 (Click to enlarge)

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)

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)