Tropical Storm Dolly in the southern Gulf of Mexico

September 2nd, 2014
Suomi NPP VIIRS Day Night Band (0.70 µm) image (click to enlarge)

Suomi NPP VIIRS Day Night Band (0.70 µm) image (click to enlarge)

Tropical Storm Dolly has formed in the western Gulf of Mexico. The Suomi NPP VIIRS Day Night Band imagery, above, shows the exposed low-level swirl of the storm (then still a tropical depression). North-northwesterly shear (shown here, from this site) means the deep convection (shown below) is displaced to the east of south of the the low-level circulation (click here for a toggle between the Day Night Band and the 11.45µm imagery). Cloud-top IR brightness temperatures from VIIRS were as cold as -87º C. ASCAT winds from 0230 UTC show a region of tropical storm-force winds associated with convection east and north of the circulation center.

Suomi NPP VIIRS Infrared (11.45 µm) image (click to enlarge)

Suomi NPP VIIRS Infrared (11.45 µm) image (click to enlarge)

Early-morning visible imagery from GOES-13, below, shows the large area of convection over the southern Gulf. The NHC-reported positions of the storm at 0900 UTC and 1500 UTC (22.6º N, 94.8º W and 23.4º N, 96.5º, respectively) are indicated by the red boxes on the images. Strong convection just south of the surface circulation developed at sunrise, obscuring the low-level swirl. That strong convection is especially apparent in the GOES-13 10.7 imagery, at bottom. GOES IR Brightness Temperatures were as cold as -84º C in the animation. (For more on Dolly from the National Hurricane Center, see this link).

GOES-13 Visible Imagery (0.63 µm) (click to animate)

GOES-13 Visible Imagery (0.63 µm) (click to animate)

GOES-13 Infrared Imagery (10.7 µm) (click to animate)

GOES-13 Infrared Imagery (10.7 µm) (click to animate)

01 September, and no named tropical cyclones!

September 1st, 2014
Global composite of IR images, with tropical cyclone center points (click to play Animated GIF)

Global composite of IR images, with tropical cyclone center points (click to play Animated GIF)

It is rather rare to have no named tropical cyclones anywhere on the globe on the first day of September, but that was the case on 01 September 2014. A sequence of hourly global composite IR images from the SSEC RealEarth web map server (Animated GIF | MP4 movie file | YouTube video) showed the void of tropical activity — until Tropical Depression 5 formed late in the day in the Gulf of Mexico. This feature later became Tropical Storm Dolly early on 02 September.

Hurricane Cristobal and the Saharan Air Layer

August 28th, 2014
Meteosat-10 Saharan Air Layer product (click to play animation)

Meteosat-10 Saharan Air Layer product (click to play animation)

The CIMSS Saharan Air Layer (SAL) product (above; click image to play animation) showed a large pocket of SAL (yellow to red color enhancement) drifting westward over the far western Atlantic Ocean and toward the Gulf of Mexico on 28 August 2014.

On GOES-13 0.63 µm visible channel images (below; click image to play animation), the hazy signature of the SAL dust could be seen surging westward, not far to the south of Category 1 intensity Hurricane Cristobal.

GOES-13 0.63 µm visible channel images (click to play animation)

GOES-13 0.63 µm visible channel images (click to play animation)

The SAL also exhibited a warm/dry signature (yellow to orange color enhancement) on the corresponding GOES-13 6.5 µm water vapor channel images (below; click image to play animation).

GOES-13 6.5 µm water vapor channel images (click to play animation)

GOES-13 6.5 µm water vapor channel images (click to play animation)

At 17:16 UTC, a Suomi NPP VIIRS true-color Red/Green/Blue (RGB) image from the SSEC RealEarth web map server (below) showed that Hurricane Cristobal had developed an eye formation.

Suomi NPP VIIRS true-color image

Suomi NPP VIIRS true-color image

A comparison of AWIPS-2 images of Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel data (below) revealed that the coldest cloud-top IR brightness temperatures (-77º C, lighter gray color enhancement) were located within convection just southwest and southeast of the eye.

Suomi NPP VIIRS 0.64 µm visible image and 11.45 µm IR channel image

Suomi NPP VIIRS 0.64 µm visible image and 11.45 µm IR channel image

Stray Light in GOES-13 Imagery

August 27th, 2014
GOES-13 3.9 µm infrared channel images (click to play animation)

GOES-13 3.9 µm infrared channel images (click to play animation)

GOES-13 is currently in Autumn Eclipse Season, when the Earth-Satellite-Sun geometry means that solar energy can reach the satellite sensors directly. NOAA NESDIS has software to mitigate the effects of Stray Light in the Sensor Processing System (SPS) that transforms the raw GOES Imager data to navigated and calibrated (GVAR) data. However, earlier this month, the SPS at Wallops inadvertently omitted the Stray Light Correction. The animation above, from 16-27 August, shows how Stray Light intruded into the 3.9 µm imagery on the GOES-13 Imager; on 25 August the Stray Light Correction was turned back on, and the final two images show no major Stray Light effects over the satellite view (Stray Light is still recorded in outer space). The animation above is for 5:15 UTC, when Stray Light affected the eastern part of the full disk scan. At 4:45 UTC, Stray Light affected the western part of the disk, and at 05:00 UTC, the central part of the disk.

Click here for more about the Stray Light Correction.