Interesting SAR Wind structures over the Great Lakes
The RCM-1 satellite carrying SAR instruments overflew Lakes Superior and Erie/Ontario at 2350 30 April 2023 and 1135 UTC 1 May 2023, respectively. What kind of wind structures were observed, and how easily can one explain them? The imagery above shows a GOES-16 Visible animation from 2301 UTC on 30 April through 0001 UTC on 1 May 2023. Higher-level clouds are overspreading central Lake Superior from the east (part of a large occluded system ove the Great Lakes), and northwesterly winds might be inferred over the lake — especially given the 0000 UTC surface analysis shown below. Note in particular the relative closeness of the two isobars extending northward from the western Upper Peninsula of Michigan!
SAR winds for just before 0000 UTC are shown below. Rather than one region of stronger winds associated with those more closely-packed isobars, 3 or 4 separate bands of slightly-stronger northwesterly winds (a still image is here; winds are upwards of 30 knots — yellow/green in the enhancement — vs. about 20 knots — cyan in the enhancement — in between the strongest winds) are apparent both upwind and downwind of Isle Royale! Isle Royale does have topographic features (here’s an image) — but it’s hard to relate those features to the downstream wind bands. AWIPS presentations of the wind speed does not include direction; those directions are available online however, at this website. This toggle shows both the derived wind speed and the Normalized Radar Cross Section field.
Twelve hours later, RCM-1 had a descending pass over the eastern Great Lakes between 1100 and 1200 UTC. GOES-16 Visible imagery from 1116 to 1136 UTC, bracketing the RCM-1 overpass, is shown below. The easterly flow on the Poleward side of the occluded low is readily apparent in the higher clouds over northern Lake Huron. Similarly, southwesterly winds are apparent over Lake Erie. Here is the 1200 UTC surface analysis.
Strong southwesterly winds (around 30 knots — yellow/green in the enhancement used) are apparent over northeast Lake Erie, except for the nearshore waters off Erie County (NY) at the extreme eastern edge of Lake Erie, where winds are closer to 15 knots (dark blue/purple in the enhancement used). There is a linear low cloud feature extending west-northwestward from Chautauqua County — the westernmost county in New York). It is close to the transition zone between lighter winds upstream and stronger winds downstream. The sector with the southernmost winds is here; here‘s the view with winds primarily over Lakes Ontario and Huron.