Ian and Charley eye comparison

September 28th, 2022 |
GOES-16 and GOES-12 visible (0.64 um) imagery at 1731 UTC on 28 September (Ian) and 1915 UTC on 13 August 2004 (Charley). (Click to enlarge)

The toggle above compares eyewall sizes from 2004’s Hurricane Charley and 2022’s Hurricane Ian near landfall in/around Fort Myer FL. Charley was a small cat-4 storm at landfall (link) with a 10-nm wide eye.. Hurricane-force winds with Charley extended only 15-25 miles out from the eye (link). In contrast, Ian has a much larger eye (30 nautical miles) and hurricane-force winds extend 30-40 miles from the eyewall (link)! Other GOES-12 Imagery of Charley is available in the CIMSS GOES Gallery here.

Current information on Hurricane Ian is available at the National Hurricane Center.

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.

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)

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.