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!)

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