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 13.6^oS and around 170.5^oW, 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.

View only this post Read Less

JMA Himawari-9 Infrared Window (10.4 µm) images (above) showed Cyclone Freddy in the South Indian Ocean as it was rapidly intensifying from a Category 1 to a Category 3 storm by 12 UTC on 11 February 2023 (JTWC discussion), eventually presenting a ragged eye for several hours — then briefly reaching Category 4 intensity at 00 UTC on 12 February (SATCON | JTWC discussion). Overshooting tops within the eyewall of Freddy occasionally exhibited cloud-top infrared brightness temperatures of -90ºC and colder (yellow pixels embedded within darker purple regions).

A NOAA-20 VIIRS Day/Night Band (0.7 µm) image valid at 1825 UTC on 11 February (below) showed the eye of Freddy.

Himawari-9 Infrared Window (11.2 µm) images from the CIMSS Tropical Cyclones site (below) include contours and streamlines of deep-layer (850-200 hPa) wind shear. The intensification of Freddy occurred in spite of this unfavorable environment of moderate to strong eaterly shear — however, the storm was moving over relatively warm water (with Sea Surface Temperature values around 29ºC).

RCM/Radarsat-2 SAR winds at 1136 UTC on 11 February (source) are shown below — a maximum wind speed of 106 knots was indicated within the northeast quadrant.

View only this post Read Less

A modest snow storm affected southern/central Wisconsin on 9 February (WFO ARX story), depositing 6-12″ of snow in a band from northeast Iowa to central Lake Michigan, as shown above: the snowfall caused an obvious change in the Suomi NPP imagery between 08 February and 10 February 2023, shown above using imagery taken from the VIIRS Today website. (A similar before/after pair of images using GOES-16 data is below).

A variety of NOAA/STAR snowfall products gave information about this snowstorm during and after its passage through the midwest. The image below shows the Snowfall rate (data source) over Wisconsin and Michigan at 1816 UTC on 9 February (derived from ATMS data on NOAA-20), and a NEXRAD radar image (from the ‘radar’ tab here) over Wisconsin at about the same time. There is very good agreement between the two representations of falling precipitation over Wisconsin!

The NOAA/STAR cryosphere team developed AMSR-2 snow products using data from JAXA’s GCOM-W polar orbiter. The snowdepth product, below, shows the change in snowdepth over Wisconsin between 9 February (left) and 11 February (right). The deepest diagnosed snow on 11 February in the new snow band is about 35 cm (around 13″).

The GOES-16 Fractional Snow Cover product, below, from 1900 UTC on 11 February 2023, shows the snow band as well.

---

Thanks to Yinghui Liu, NOAA/STAR at CIMSS, and Huan Meng, NOAA/STAR in College Park, for imagery for this blog post. Thanks also to Peter Romanov, CUNY/CREST, for contributions.

View only this post Read Less

RADARSAT-2 passed over Samoan waters again on 9 February 2023, just after sunset (above) and near sunrise (at bottom). The imagery above compares the SAR winds to GOES-18 visible (with a changed enhancement to account for low light conditions, with reflectance values ranging from 0 to 6.3 only vs. the maximum default of 130) and infrared imagery at the same time. Several features of note deserve comment: both the strong convection just west/southwest of Savai’i, Samoa, and the cold cloud tops at the southern edge of the domain, have very little signal in the surface wind field.

A feature of interest to me is the slight change in windspeeds highlighted in the toggle below, extending north from the Samoan Islands. Is this slight change in SAR-derived winds, from 6 knots (bright purple, to the west) to 8 knots (darker purple, to the east) related to any obvious linear feature in the visible and infrared imagery? At first blush, a disappointing no is the answer!

An image below shows RADARSAT-2 SAR winds at 0552 and 1655 UTC (Note: the color enhancement of the winds has been changed to have a maximum of 25 kts, vs. 40 knots in the imagery above). When SAR observations occurred over Guam in November 2022, the effect of the Marianas island chain on winds extended a significant distance downwind of the island, as shown in this image from 0840 UTC on 10 November from this blog post. Why might the effects of the Samoan Islands not extend as far downstream; that is, why is the effect of the island not trapped? The toggle below shows the Guam SkewT from 1200 UTC on 10 November 2022 and the SkewT from Pago Pago at 1200 UTC on 9 February 2023; (both SkewTs are from the University Of Wyoming sounding site). One thing the Guam sounding features that is missing from the Pago Pago sounding is a stable layer from 850 to 750 hPa; that stable layer might affect how the Island Effect persists downwind.

The toggle below compares SAR winds with GOES-18 clean window imagery. At 1655 UTC, the coldest cloud tops are related to perturbations in the wind field, especially in the southern part of the domain.

View only this post Read Less