Severe weather over northeast Illinois

August 9th, 2021 |
GOES-16 “Red”Visible (0.64 µm) Imagery, 1900 UTC 9 August – 0040 UTC 10 August 2021

Severe thunderstorms developed over northeast Illinois late in the afternoon on 9 August, and a series of tornadoes resulted. Storm Reports (from the Storm Prediction Center) are shown below. The mp4 animation above (click here for a large animated gif) shows 1-minute GOES-16 Mesoscale Sector 2 visible imagery (0.64 µm) from 1900 UTC on 9 August through 0040 UTC on 10 August. The active convection is apparent.

SPC Storm Reports from 9 August 2021 (Click to view site at SPC)

Part of the region hit by severe weather is in a persistent drought, as shown below (an image from this website). Rains that accompanied the severe weather provided some drought relief. (Click here for hourly CMORPH2 precipitation estimates from RealEarth)

Drought Intensity over the Midwestern United States, 3 August 2021 (Click to enlarge). Portions of southeast Wisconsin and northeast Illinois are under Severe Drought (Orange Enhancement)

RCM3 (RADARSAT Constellation Mission 3) Synthetic Aperture Radar (SAR) winds from a 2340 UTC overpass on 9 August (from this site) show a wind feature over Lake Michigan associated with the convection. These wind estimates might be affected by ice within the glaciated clouds

RCM3 SAR esimates of wind speed, 2340 UTC on 9 August 2021 (Click to enlarge)

SAR winds over Tropical Storm Mirinae

August 8th, 2021 |
Himawari-8 “Red visible” (0.64 µm) on 2010 UTC on 8 August 2021, data courtesy JMA (Click to enlarge)

Tropical Storm Mirinae, shown above, to the east of Japan (click here to see the same image with latitude/longitude lines), was overflown on 8 August by Radar Constellation Mission Satellite one (RCM-1) at 2013 UTC. The Synthetic Aperture Radar (SAR) imagery (from this site) is shown below, with both polarities. The two polarities help mitigate wind speed errors that might arise from ice. A peak wind of nearly 60 knots is indicated.

RCM1 Synthetic Aperture Radar winds, 2013 UTC on 8 August 2021, HH Polarization at 5.4 GHz (click to enlarge)
RCM1 Synthetic Aperture Radar winds, 2013 UTC on 8 August 2021, HV Polarization at 5.4 GHz (click to enlarge)

The storm profile winds from the Mirinae at this time are shown below (courtesy Christopher Jackson, GST/NOAA). Such plots are useful for determining max wind radii.

Mirinae SAR winds as a function of distance from the center, 2013 UTC o 8 August 2021 (Click to enlarge)

Comparing SAR data over Lake Superior to radar

August 5th, 2021 |
SAR winds from RCM-2, 2340 UTC on 5 August 2021 (Click to enlarge)

Synthetic Aperture Radar (SAR) winds are derived from a microwave signal pinged from a satellite; backscatter is converted into winds (given a background field that is typically from a numerical model). The wind structure here is suggestive of a bowing feature to a convective downdraft. How does it compare to radar or satellite imagery? GOES-16 Satellite imagery for the hour bracketing the imagery above is shown below. Convection (weak) is apparent moving east from the tip of Keewenaw peninsula.

GOES-16 “Red” Visible (0.64mm) imagery, 2301-2356 UTC on 5 August 2021 (click to enlarge)

What did radar imagery look like at this time? The imagery below, courtesy Nick Langlieb, the SOO at WFO Marquette, shows radar echoes (base reflectivity) at a different level than the Lake-surface values sampled by SAR, so a direct comparison is a challenge (click here to see Correlation Coefficient at the same time). It’s peculiar that no radar signal is apparent to match the strong SAR winds just offshore from the tip of the Keewenaw peninsula (between 88º and 87.5º W) — although there is a feature oriented north-south a small distance to the west of the peninsular tip; there seems to be a better (but not exact) match with the SAR winds near 87º W.

1.3º – Base Reflectivity, 2340 UTC on 5 August 2021 (click to enlarge)

SAR winds near Tropical Storm Nepartak

July 27th, 2021 |
RCM1 SAR Winds over Tropical Storm Nepartak just east of Honshu, 0828 UTC on 27 July 2021 (Click to enlarge)

RADARSAT Constellation Mission One (RCM1) passed over Nepartak at 0828 UTC on 27 July 2021, and the image above shows the Synthetic Aperture Radar (SAR) winds derived at that time. There is a widespread region of ~40-knot winds (cyan to green in the color enhancement), a bit stronger than the 30-knot winds viewed by MetopA at 1040 UTC (shown here, in this blog post). How certain can a forecaster be of the even-stronger winds that exist in an area near the coast near 37.4ºN, 141.4ºE, and in an arc from 37.5ºN, 142.5ºE to 37.2ºN, 144ºE? There are isolated SAR estimates in that band that are near 60 knots! Are there other data sources to confirm that kind of wind? (See information at the bottom for clarification!)

The image below shows Himawari-8 Clean window imagery at 0829 UTC, just after the image above. Three points with cold cloud tops, suggestive of more vigorous convection, are indicated: (37.6ºN, 141.18ºE); (37.57ºN, 143.01ºE); (37.35ºN, 144.01ºE). The structures in the infrared imagery do match the structures in the SAR winds, but offset a bit to the north in the Himawari-8 imagery, as expected because of the parallax shift: features will be displaced away from the sub-satellite point, with the displacement increasing for higher clouds, and for greater distance from the sub-satellite point (on the Equator at 140.2ºE for Himawari-8). Convective downdrafts could be responsible for the highest winds shown in the SAR analysis.

Himawari-8 Clean Window Infrared (10.41 µm) imagery, 0829 UTC on 27 July 2021 (Click to enlarge)

The strong winds in the image are not observations of strong winds by SAR. Rather, these are most likely the result of reflection off of ice in the atmosphere — ice that is likely to be present around convective towers that have glaciated. (Thanks to Christopher Jackson, GST/NOAA, for this information!)