Tropical Storm Arthur forms east of Florida

July 1st, 2014 |
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 first tropical depression (update: Arthur was named as a tropical storm at 1500 UTC 1 July) of the season in the tropical Atlantic has formed just to the east of Florida. The visible imagery animation, above, shows persistent strong thunderstorms with overshooting tops in the area of disturbed weather over the Gulf Stream and the Bahamas. Refer to the National Hurricane Center and the CIMSS Tropical Cyclones sites for particulars on the future track of this system. Note that current forecasts have the system strengthening to a hurricane in the next few days, and close to the North Carolina coast on July 4th.

Metop ASCAT surface scatterometer winds at 1541 UTC, below, indicated that the strongest winds (green barbs, 30-39 knots) were found within the northeastern quadrant of the tropical storm.

GOES-13 visible images with Metop ASCAT surface scatterometer winds (click to play animation)

GOES-13 visible images with Metop ASCAT surface scatterometer winds (click to play animation)

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GOES-13 10.7 µm infrared channel images (click to play animation)

GOES-13 10.7 µm infrared channel images (click to play animation)

The tropical Atlantic has lately been besieged by Saharan Air Layer (SAL) dust (see, for example, this post from last week, or this image from today); that dry air suppresses tropical cyclone formation. The animation of GOES-13 10.7 µm imagery, above, shows that this Tropical Depression formed out of an impulse that sank southward from the Carolinas over the past 6 days, so its gradual development has not been impeded by the SAL.

The VIIRS instrument on board the Suomi NPP satellite provided high-resolution imagery over this tropical system shortly after midnight on the 1st (see below). A large cirrus shield with brightness temperatures cooler than -70º C (Green in the enhancement) with a few overshooting tops that are colder than -85º C are present. An analysis of some NUCAPS Soundings from this overpass is here.

Suomi NPP VIIRS 11.35 µm infrared imagery, Day/Night Band imagery (0.70 µm) and lightning data at ~0715 UTC on 1 July 2014 (click to toggle through images)

Suomi NPP VIIRS 11.35 µm infrared imagery, Day/Night Band imagery (0.70 µm) and lightning data at ~0715 UTC on 1 July 2014 (click to toggle through images)

Arthur’s projected track moves the storm up the East Coast over very warm waters associated with the Gulf Stream. Both MODIS and VIIRS analyses of SSTs show widespread temperatures in excess of 80º F.

A comparison of Suomi NPP VIIRS 11.45 µm IR channel images at 0717 UTC and 1840 UTC, below, showed that the areal coverage of cold cloud tops was increasing during the day on 01 July, but the deep convection remained well to the southeast of Arthur’s low-level center of circulation.

Suomi NPP VIIRS 11.45 µm IR channel images

Suomi NPP VIIRS 11.45 µm IR channel images

At 1840 UTC, a comparison of the Suomi NPP VIIRS 11.45 µm IR channel image with the corresponding 0.64 µm visible channel image with an overlay lightning data, below, revealed a large number of cloud-to-ground strikes within the 1-hour period ending at 1900 UTC.

Suomi NPP VIIRS 11.45 µm IR channel image and 0.64 µm visible channel image (with lightning data)

Suomi NPP VIIRS 11.45 µm IR channel image and 0.64 µm visible channel image (with lightning data)

===== 02 July Update =====

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

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

Arthur continued to slowly intensify on 02 July, and began to show hints of an organized eye structure on GOES-13 0.63 µm visible channel images (above; also available as an MP4 movie file).

A comparison of AWIPS-2 images of Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images (below) showed that the coldest cloud tops were north of the center of Arthur at 1822 UTC. A buoy just southwest of the center reported winds gusting to 52 knots (60 mph).

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

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

Even though an eye was not evident on GOES-13 10.7 µm IR channel imagery around 2045 UTC, a DMSP SSMIS 85 GHz microwave image at 2049 UTC did display a well-organized eye signature (below).

GOES-13 0.63 µm visible channel image and DMSP SSMIS 85 GHz microwave image

GOES-13 0.63 µm visible channel image and DMSP SSMIS 85 GHz microwave image

NUCAPS soundings near a Tropical Disturbance

July 1st, 2014 |
Suomi/NPP 11.35 µm infrared channel, 0717 UTC on 1 July 2014 and NUCAPS sounding locations in green (click to enlarge)

Suomi/NPP 11.35 µm infrared channel, 0717 UTC on 1 July 2014 and NUCAPS sounding locations in green (click to enlarge)

Suomi/NPP overflew the developing tropical depression #1 (now Tropical Storm Arthur) east of Florida early in the morning on July 1st. The CrIS and ATMS instruments on board S/NPP provide data for NUCAPS soundings that are routinely distributed to AWIPS II. The image above is an overlay of the 11.35 µm infrared imagery with the sounding locations plotted as green dots. Seven sounding locations are indicated on the image above (Here is the image without the seven sounding locations) How well do NUCAPS soundings represent the tropical atmosphere that is supporting the development of Arthur?

The 7 soundings indicated in the plot above are: 1 (Just south of Pensacola, FL), 2 (Off the coast of Georgia), 3 (northeast of Arthur in the tropical Atlantic), 4 (Cape Canaveral), 5 (north of Tampa Bay), 6 (the western tip of Cuba) and 7 (northeastern Cuba).

GOES Sounder DPI Total Precipitable Water at 0700 UTC on 1 July 2014 (click to enlarge)

GOES Sounder DPI Total Precipitable Water at 0700 UTC on 1 July 2014 (click to enlarge)

How does Precipitable Water from the NUCAPS soundings compare to observations from other satellite-based systems? GOES Sounder DPI TPW from 0800 UTC shows values around 50 mm over interior the southeast United States, and over the tropical Atlantic to the northeast of the tropical system. A corridor of lower values, around 30-35 mm, extends northeast of Jacksonville, FL. Smaller values (30-40 mm) also extend southeastward from the lower Mississippi River valley into the Gulf of Mexico. A similar pattern in the precipitable water is evident in the blended product, here. Precipitable water values from the NUCAPS soundings appear, for this case, to be too low. The value at Cape Canaveral (point 4), for example, is 1.59″ (40 mm, versus close to 50 mm from the Sounder and the Blended Product); off the coast of Georgia (point 2), 1.30″ (33 mm vs. close to 41 mm from the Sounder and Blended Product); south of Pensacola (point 1), 1.25″ (31 mm vs. 35 mm from the Sounder and the Blended Product); north of Tampa Bay (point 5), 1.46″ (37 mm vs 47 mm from the Sounder and the Blended Product); northeast of the tropical system (point 3), 1.84″ (47 mm vs 49 mm from the Sounder/Blended Product); western Cuba (point 6), 1.70″ (43 mm, similar to the 44 mm from the Sounder/Blended Product); and northeast cuba (point 7), 1.22″ (31 mm vs. 39 from the Sounder and 34 from the Blended Product). The lowest 3 kilometers of the atmosphere (where most of the moisture resides) is the most difficult part for a satellite-based sounding, but there do appear to be differences between the two satellite-based sounding products (GOES and NUCAPS) in this case.