Subtropical storm Raoni off the coast of South America

June 28th, 2021 |

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed the circulation of Subtropical Storm Raoni (discussion issued by MARINHA) off the coast of Argentina/Uruguay on 28 June 2021.

In the corresponding GOES-16 “Clean” Infrared Window (10.35 µm) images (below), the coldest cloud tops exhibited infrared brightness temperatures around -50ºC (shades of yellow).

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

===== 29 June Update =====

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 Visible images (above) showed that Subtropical storm Raoni continued its northeastward motion, and was located off the coast of far southeastern Brazil on 29 June (12 UTC surface analysis | discussion).

GOES-16 Infrared images (below) indicated that Raoni was unable to maintain a persistent closed ring of cold clouds around its center.

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

A sequence of ASCAT surface scatterometer winds (source) from Metop-A and Metop-C (below) revealed swaths of wind speeds of 40 knots or greater within the northeastern and southwestern sectors of the system.

ASCAT winds from Metop-A and Metop-C [click to enlarge]

ASCAT winds from Metop-A and Metop-C [click to enlarge]

Tropical Depression Four off the South Carolina coast

June 28th, 2021 |

GOES-16 Red Visible (Band 2, 0.64 µm) imagery, 1056 -1431 UTC on 28 June 2021 (click to animate)

Update: Tropical Depression 4 strengthened to become Tropical Storm Danny at 1905 UTC on 28 June (link).

GOES-16 visible imagery (0.64 µm), above, from the morning of 28 June 2021, shows a compact low-level circulation east of South Carolina, steadily moving towards the coast. Deep convection with this system is offset to the east, suggestive of shear, and the shear analysis from the SSEC Tropical website, below, does show easterly shear over the system.

Wind shear analysis, 1200 UTC on 28 June 2021 (Click to enlarge)

VIIRS Day Night Band imagery, below, from Suomi-NPP at 0723 UTC on 28 June, a time with ample lunar illumination, shows convection over the center of the storm at that time.

Suomi-NPP VIIRS Day Night Band Visible (0.70 µm) imagery, 0723 UTC on 28 June 2021 (Click to enlarge)


The wind shear analysis from 1900 UTC, below, is more in line with what might be expected in a system with an exposed low-level circulation center.

Wind shear analysis, 1900 UTC on 28 June 2021 (Click to enlarge)

 

The National Hurricane Center initiated statements on this tropical system at 1500 UTC on 28 June 2021. Refer to those webpages (link) for further information. Tropical Storm warnings are in effect for parts of the South Carolina coast, from Edisto Beach to the S. Santee River.

SAR Winds over Lake Michigan compared with Radar winds

June 28th, 2021 |

RCM3 SAR winds over Lake Michigan, 1201 UTC on 28 June 2021 (Click to enlarge)

RADARSAT Constellation Mission 3 (RCM3) Synthetic Aperture Radar (SAR) wind data over southern Lake Michigan at 1201 UTC on 28 June, above (click to enlarge, taken from this website), shows a small region of strong winds just offshore of southeast Wisconsin and northeastern Illinois.  GOES-16 Band 2 visible imagery (clipped from the CSPP Geosphere website, click here for the direct link to the imagery — valid until mid-July) shows modest convection just off the Wisconsin/Illinois shorelines.

CSPP Geosphere visualization of GOES-16 Visible (Band 2, 0.64) imagery, 1200 UTC on 28 June 2021 (Click to enlarge)

Winds from the Sullivan WI (WFO MKX) radar, below, (courtesy John Gagan, SOO), show a similar structure. The challenge in comparing the SAR winds and the radar winds: The Sullivan radar beam in that location is about 5000 feet above the surface. Storm-relative velocities are about 20 kt.

1201 UTC WFO MKX Radar display showing Base Reflectivity (upper left), Storm Relative Velocities (upper right), ZDR (Lower left) and Base velocities (lower right).  Beam angle of 0.5 degrees. (Click to enlarge)

Strong winds from SAR over the South Pacific

June 28th, 2021 |

GOES-17 ABI Band 13 infrared (10.3 µm) imagery, 0500-0600 UTC on 28 June 2021 (click to animate)

The animation above shows convection over the south Pacific to the east of American Samoa (note the Manu’a Islands just east of 170 W Longitude in the animation). Extensive cloud cover will limit the ability of the GOES-17 instrument to detect low-level cloud motions underneath the deep convection, which motions can be used to infer wind speeds. Other data sources are needed. In addition to scatterometry from the MetOp series of satellites, synthetic aperture radar (SAR) that flies on the RADARSAT Constellation Mission (RCM) satellites (and on Sentinel) (available at this website) can give high-quality estimates of wind speeds. Given the animation of full-disk imagery above (at ten-minute timesteps), taken from AWIPS and therefore sub-sampled down to 6-km horizontal resolution, how fast would you guess wind speeds might be near the surface under the convection. SAR data below shows winds near 25 m/s, i.e., storm-force winds, under the convection.

RCM2 SAR surface winds, 0526 UTC on 28 June 2021 (Click to enlarge)

A full-resolution image of GOES-17 clean window infrared data (10.3 µm) (created using Geo2Grid) is shown below, at the closest time to the observed winds shown above. The deepest cloud tops are close to the strongest surface wind speeds.

GOES-17 ABI Band 13 infrared (10.3 µm) imagery, 0530 UTC on 28 June 2021 (click to enlarge)