Tropical Storm Bret

June 19th, 2017 |

GOES-16 “Veggie” Band (0.86 µm) animation of Tropical Storm Bret, 1545-2030 UTC on 19 June 2017 (Click to animate)

GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing.

The fast-moving tropical system in the southern Caribbean Sea has developed a closed circulation and has been named Bret.  Tropical Storm Bret, shown above in an animation of GOES-16 Near-Infrared (0.86 µm) imagery that highlights land/water contrasts (the Orinoco River in Venezuela and Caribbean Islands — some with cloud streamers in their lee — north of Venezuela stand out clearly), is forecast to remain very close to the South American coastline.  Such proximity to land will likely hinder development. Further, wind shear in the atmosphere over the storm is predicted to increase.

Bret is embedded within a ribbon of very moist air (associated with the ITCZ) that stretches from Africa to the northwest Caribbean, as shown in the animation below (taken from this site) that shows morphed microwave observations of total precipitable water.

Microwave estimates of Total Precipitable Water for the 24 hours ending 1900 UTC on 19 June 2017 (Click to enlarge)

For more information on Bret, refer to the National Hurricane Center and the CIMSS Tropical Cyclones sites (where you can also follow the future of the system emerging into the Gulf of Mexico).

Scattering and Shadows

June 19th, 2017 |

GOES-16 Imagery over New England from 1022 through 1117 UTC on 19 June 2017. Blue Band (0.47 µm), upper left; Red Band (0.64 µm), upper right; ‘Veggie’ Band (0.86 µm), lower left; ‘Snow/Ice’ Band (1.61 µm), lower right. Click to enlarge.

GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing

The animation of GOES-16 imagery, above, showing, clockwise from upper left, the GOES-16 0.47 µm, 0.64 µm, 1.61 µm and 0.86 µm channels, shows lows clouds over southeast New England, with a few mid-level clouds aloft. The moving mid-level clouds are casting shadows on the lower clouds beneath. Note that the shadows appear darkest at the longest wavelength. This is shown in the AWIPS cursor readout below as well — for the point selected, the 1.61 µm reflectance is 5.4%, and it increases to 18.9% at 0.47 µm. Why does it change by wavelength?

AWIPS read-out of reflectance in a shadow east of Cape Cod. Note the reflectance increases as wavelength decreases. (Click to enlarge)

Rayleigh scattering in the atmosphere is a function of wavelength: scattering is more effective at shorter wavelengths. Thus, the atmosphere is scattering the greatest amount of 0.47 µm radiation (compared to the longer wavelengths shown here). Shadows are darker (there is less detected reflectance) at longer wavelengths because less longer-wavelength radiation is scattered towards the satellite sensor.

The tweet from the National Weather Service in Melbourne, below, shows another shadow scene with 0.86 µm imagery.