Using SAR Winds to center-fix Tropical Cyclone Estelle in the Eastern Pacific

July 20th, 2022 |
OGES-17 Visible (Band 2, 0.64 µm) and Infrared (Band 13, 10.3 µm) at 0150 UTC on 20 July 2022 (click to enlarge)

GOES-17 visible and infrared imagery shows Tropical Storm Estelle over the eastern Pacific to the west-southwest of Baja California. Although there are regions of strong convection, satellite presentation of the storm suggests a modest tropical storm. Center-fixing a storm such as this (with just one image vs. an animation!) is complicated by both parallax (GOES-17 is overhead at 0oN, 137.2oW) and the lack of an easily-discerned eye feature in this system.

Instruments that view surface winds, via Scatterometry or via Synthetic Aperture Radar (SAR), can offer better center fixes. Consider the toggle below of Radarsat-2 SAR winds over Estelle. SAR Winds (available here; SAR winds over Tropical Systems can be found here.) show a lopsided storm, with most of the strong winds on the poleward side of the center. The SAR winds that use 0.5-degree GFS wind data as a first guess show a characteristic hourglass feature near the center that advertises an ambiguity between the first guess winds and the observations. That feature is missing in the SAR wind field that is a product of the 0.25-degree GFS winds, suggesting that the 0.25-degree wind field is more accurate. Both fields show a rather baggy center at or just south of 20oN.

RSAT-2 SAR Winds over Estelle benchmarked by 0.5-degree and 0.25-degree model output, 0150 UTC on 20 July 2022; note the different color scales used in the two images (Click to enlarge)

Scatterometry from MetopB and HY2B between 1730 UTC 19 July and 0230 UTC 20 July, respectively show a storm moving west-northwest, passing north of 20oN just after 0230 UTC.

ASCAT (MetopB) and HY2B Scatterometry winds at 1730 (19 July) and 0230 (20 July), respectively.

SAR Winds over the tropical Pacific Ocean

July 11th, 2022 |
GOES-17 ABI Band 13 (Clean Window Infrared, 10.3 µm), 0400-0510 UTC on 11 July 2022, along with SAR Wind observations at 0513 UTC (Click to enlarge)

This NOAA/NESDIS website shows small footprints where SAR observations of ice and wind (from the RADARSAT Constellation Mission — RCM — satellites and from Sentinel) are available. AWIPS-ready data are also available from an ftp site. Consider the animation of GOES-17 Band 13 imagery above, just south to the Equator, and to the west of 160oW longitude. The slightly cooler brightness temperatures at the eastern edge of the arc of clouds moving to the west is associated with two patches of strong surface winds. The toggle below zooms in on the region of winds. Surface wind speeds are close to 15 m s-1 with this weak line of tropical convection.

GOES_17 ABI Band 13 (Clean Window infrared, 10.3 µm) at 0510 UTC and RCM1 SAR Winds at 0513 UTC, 11 July 2022 (Click to enlarge)

Bore feature in SAR winds over Lake Michigan

June 6th, 2022 |
RCM2 Wind Speeds, 1152 UTC on 6 June 2022 (Click to enlarge)

Color-enhanced wind speeds observed from Synthetic Aperture Radar (SAR) data on the second RADARSAT Constellation Mission satellite (RCM-2), shown above in AWIPS (Click here for a similar image online at this website) show parallel lines of enhanced wind speeds, a wind structure suggestive of a bore (click here for many previous blog posts on this phenomena), over southern Lake Michigan. Peak wind values (in yellow over the water) are near 14 m/s, with minima in between the lines showing winds around 5-6 m/s. (Note that SAR wind information over land is invalid). When this kind of event happens under mostly clear skies, parallel lines of clouds (that are perpendicular to the observed wind) develop. In this case, widespread clouds prevented satellite detection of cloud bands. The toggles below show Visible (Band 2, 0.64 µm) and Infrared (Band 13, 10.3 µm) at the time of the SAR observations.

GOES-16 Band 2 visible (0.64 µm) and RCM Winds, 1152 UTC on 6 June 2022 (Click to enlarge)
GOES-16 Band 13 infrared (10.3 µm) and RCM Winds, 1152 UTC on 6 June 2022 (Click to enlarge)

Modest convection (cloud tops are only around -48oC) over central Lake Michigan likely generated the bore. For a bore to propagate, a strong inversion is required. Soundings at Green Bay WI and Gaylord MI likely are unrepresentative of the environment over southern Lake Michigan. The sounding at White Lake Michigan — near Detroit — (from this website) does show a surface inversion, as does the sounding at Davenport IA. The cool late-Spring waters of Lake Michigan will serve to anchor a similar low-level inversion over the Lake; bore features travel along those inversions.

SAR winds and Lake-Effect cloud bands

December 17th, 2021 |
SAR winds from satellite RCM2 at 23:48:58 UTC on 16 December 2021 (Click to enlarge)

Synthetic Aperture Radar (SAR) winds in select small domains are routinely available (with good latency, i.e., within 2 or 2-1/2 hours) at this website. Coverage over the Great Lakes typically occurs within an hour of 0000 UTC and 1200 UTC on each day. The image above (direct link) shows derived SAR winds (from the RCM2 satellite) over Lake Superior at 2348 UTC on 16 December 2021. Winds over Lake Superior are around 40 knots; weaker wind speeds are indicated in the lee of Isle Royale, the various Apostle Islands, Upper Michigan’s Keewenaw Peninsula, near Marquette Bay, and right offshore Minnesota. Fingers of stronger winds extend east-southeastward from just off the Minnesota shoreline. This small horizontal variability in the wind speeds is too small to be detected by other microwave detectors (such as ASCAT on Metop-B and Metop-C).

RCM2 SAR Winds at 2348 UTC, 16 December (left) along with annotated GOES-16 ABI Band 7 (3.9 µm) imagery at 2346 UTC, 16 December 2021 (right); arrows suggest the same structures in both figures (Click to enlarge)

The toggle above shows the winds with a time-matched image of GOES-16 Band 7 (3.9 µm) data (created using Geo2Grid), stretched to enhance low-level clouds. The second toggle includes best-guess location-matched arrows of features in the SAR winds and the GOES-16 ABI brightness temperature. That figure is reproduced below with an AWIPS display of GOES-16 3.9 µm imagery at 2346 UTC (again with a stretched colortable to emphasize low-level temperature contrast).

RCM2 SAR Winds at 2348 UTC, 16 December (left) along with annotated GOES-16 ABI Band 7 (3.9 µm) imagery at 2346 UTC, 16 December 2021 (right); arrows point to similar structures in both images (Click to enlarge)

GOES-16 satellite imagery over Lake Superior will have a parallax shift because the location is far from satellite nadir. Parallax shift is related to cloud-top heights, and derived Cloud-Top Heights (a Level 2 product) over the region show cloud tops between 3000 and 5000 feet near Minnesota, rising to about 7000 feet over Lake Superior in between Minnesota and the Keewenaw Peninsula. This parallax shift means that features will be displayed to the north and a bit to the west of their true location, displaced away from the GOES-16 sub-satellite point at 0oN, 75.2o W.

GOES-16 Derived Cloud Height, 2346 UTC on 16 December 2021 (Click to enlarge)

These images suggest that lake-effect clouds are regions of enhanced wind speeds. The inferred convective roll vortices present in the satellite imagery are also regions of enhanced convergence and upward moisture transport.