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.

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.

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Clear skies over the Great Lakes (except for Lake Superior) allowed for VIIRS data to record temperatures over the Great Lakes in the early morning of 5 August 2021. In situ data from moored buoys closely match ACSPO (Advanced Clear-Sky Processor for Oceans) values. Southwestern Lake Erie is warmest, with temperatures as warm as 78ºF (pink/white enhancement). Green Bay and Saginaw Bay are as warm as 75ºF. The analysis below (from this link at the Great Lakes Environmental Research Lab) also shows values largely in agreement with the above analysis.

AWIPS-ready files of these fields are available from the CIMSS LDM feed.

There plots of basin-wide average temperatures suggest that the Great Lakes (except for Lake Superior) are a bit cooler in early August of this year than they have been at this time in the recent past.

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The animation above shows NUCAPS sounding availability plots from sequential NOAA-20 overpasses on 5 August 2021. Stations that are north of about 39 N will occasionally see profiles from two sequential overpasses, as was the case above at Buffalo NY, and also Pittsburgh PA. A benefit of NUCAPS soundings is that they show up halfway between 0000 and 1200 UTC upper-air soundings. The animations below show 0000 UTC and 1200 UTC soundings from Buffalo (top) and Pittsburgh (bottom) with two soundings from NUCAPS from the closest point to the upper air launch.

When sequential overpasses happen, a forecaster can use information to tell how things are changing on 90-minute timescales. Even with only one profile, however, one can view changes between the synoptic upper-air soundings. It is worth mentioning a couple obvious differences between the two data sources: Radiosondes are sampling a line that moves up in the atmosphere over a period of time (up to two hours!); that line can extend over a significant horizontal distance; in contrast, NUCAPS observations are volumetric and instantaneous. NUCAPS profiles are very smooth compared to radiosondes, and extreme values, or very thin layers are unusual in NUCAPS profiles.

NUCAPS profiles are available at this OSPO site. Gridded NUCAPS fields are here.

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True-color imagery from the VIIRS Today website, below (click here for a direct link to the image below at the VIIRS Today site), shows an unusual sunglint pattern over the eastern Gulf of Mexico, to the southwest of the Florida peninsula. Typically, sunglint features are fairly wide in VIIRS imagery, as evidenced from this Suomi-NPP image, also from 2 August, but to the east of Florida. However, the winds over the eastern Gulf were very light on 2 August, so surface wave action was reduced. When the ocean approaches glassy calm, solar reflection becomes more unidirectional, and a brighter spot becomes visible in True-Color imagery. When seas are choppier (as was the case to the east of Florida), solar reflection off the ocean is diffuse, and a less concentrated region of brightness results. An good analogy might be reflection off flat aluminum foil (representing a flat sea state) or very crinkled aluminum foil (representing an agitated sea state).

What evidence is present of light winds? Consider the Metop-A Scatterometry image, below, from this site. Both the ascending pass (about 0145 UTC) and descending pass, below (about 1410 UTC; orbit imagery available here, from this site) show very weak winds over the Gulf to the southwest of Florida.

The effect of the sun glint occurs in both visible and shortwave infrared channels. The imagery below, downloaded from the CIMSS Direct Broadcast ftp site (https://ftp.ssec.wisc.edu/pub/eosdb/j01/viirs/, from this ephemeral site — https://ftp.ssec.wisc.edu/pub/eosdb/j01/viirs/2021_08_02_214_1837/images/ — in particular), shows the five Image bands from VIIRS, a prominent signal is apparent in all but the longest wavelength.

Does the sunglint affect products? The Cloud Mask product is tricked by the large reflectance into believing a cloud is present, as highlighted by the box below. The image also includes Cloud Type and Cloud Phase. Because liquid water clouds are believed present, the ACSPO Sea-surface temperature algorithm (at bottom) produces no values in that region. Careful inspection of the image, however, shows marginally warmer waters (a lighter pink color is apparent) adjacent to the sunglint where the lack of wind means surface mixing of waters is suppressed and a thin layer of very warm water can develop on the top of the ocean.

This animation steps through all the VIIRS M-bands. The sunglint is apparent in nearly all of them, except for wavelengths longer than 4 µm. The sunglint is also missing from the 1.38 µm (M09) imagery because of strong absorption of energy at that wavelength by water vapor.

Other examples of diagnosing areas of light winds over water (via a lack of sunglint or moonglint) can be found here, here, here and here.

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