Sentinel 1A SAR winds near American Samoa on 11 February 2023
Sentinel-1A overflew the Samoan Islands shortly after sunset on 11 February and captured a packet of very strong winds just to the west of Tutuila Island of American Samoa. The toggle above compares Band 13 imagery with two different enhancements of the SAR Winds: one (here) highlighting the different Beaufort Scales, and one (here) showing just gridded fields (from 0-65 knots). Is there a good relationship between the wind structures and the Band 13 brightness temperatures?
The first thing to do is to examine the strong winds to the west of Tutuila for any possible artifacts that might arise from ice within the clouds. The toggle below (between this wind image and this Normalized Radar Cross Section — NRCS — image from this website) shows that features north of Tutuila, between 13.4 and 13.6oS and around 170.5oW, for example, have bright white, diffuse NRCS features that suggest ice; such features are absent in the clouds around western Tutuila, however, suggesting they contain little ice to affect the SAR return (with the exception of features at the very southern edge of the domain). The SAR data suggest modest northerly winds (around 10 knots) over most of the domain. That is in agreement with ASCAT data from MetopB (0800 and 2100 UTC on 12 February) and MetopC (2030 UTC on 12 February), taken from this website.
The animation below shows persistent cooler cloud top features moving northward over western Tutuila. The feature over western Tutuila at 0550 UTC spawns the very strong winds that show up in the SAR data. Immediately in the lee of the island (if one assumes southwesterly outflow!), relatively calm winds are diagnosed.
GOES-18 Band 3 Near-infrared (“Veggie Band”, 0.86 µm) imagery, below, also shows the northward motion of cloud features over western Tutuila. The absence of widespread strong winds in other features within this line of enhanced cloudiness suggests that interaction with the topography of Tutuila was a possible contributing factor to the outflow generation.
What was the weather like at Pago Pago International Airport around sunset? The screen capture below (from this site) shows a shower near 7 PM on the 11th (Samoa Standard Time), which is 0600 UTC on 12th.
