Tropical Storm Grace

October 5th, 2009 |

GOES-12 visible image

Late on October 4th, the weather system in the far northeast Atlantic acquired sufficient tropical characteristics to be classified as a Tropical Storm, and Grace was named. The visible image from GOES-12 above shows the counterclockwise swirl of clouds. GOES-12 is over the Equator at 75 degrees W Longitude, and Tropical Storm Grace at the time of the image above was at 45 degrees North latitude and 16 degrees W Longitude; consequently, the view angle is very oblique. Indeed, the visible image shows a convective spiral band that lies beneath the cirrus shield that covers the system. Note that no overshooting tops penetrate the cirrus overcast over the tropical system. The system sits over sea surface temperatures near 70 degrees Fahrenheit (see the Sea Surface Temperature analysis here, and those temperatures are yielding insufficient CAPEs to produce overshooting tops.

Grace developed underneath a decaying upper-level low. The low was able to draw north modestly high values of precipitable water, as shown in the MIMIC analysis here. Grace is associated with the very small region of enhanced precipitable water that is at 40 N, 20 W at the start of the loop, then moving northeastward towards Ireland.

A comparison of Terra MODIS visible and 11.0 µm IR images (below) showed that Grace exhibited a fairly well-defined banded structure and some semblance of an eye at 11:40 UTC.

MODIS visible and IR images

MODIS visible and IR images

(Added: Jesse Ferrell at AccuWeather notes that Grace was almost the farthest-east forming tropical system on record! Link).

(Added, 6 October: Grace merged with/was absorbed by a front southwest of Ireland late in the day on the 5th.) AMSU microwave data from early on the day on the 5th clearly show a warm core to the system, one of the hallmarks of a tropical Storm. For example, data from the AMSU-A instrument in NOAA-18 at 0413 UTC on 5 October show a region of warmth at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7); the 89-GHz channel on AMSU-B also shows warmth at the center of the storm. These warm signals were critical in determining that system was tropical in nature. The warmth persisted; AMSU-A data from NOAA-19 at 1406 UTC on the 5th also showed a warm core at 550 hPa (Channel 5), at 350 hPa (Channel 6) and at 200 hPa (Channel 7), as well as in the 89-GHz channel on AMSU-B. (More imagery is available here).

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