CIMSS Satellite Blog

As noted in the previous Blog entry, Tropical Storm Bertha became the Atlantic Basin’s second tropical storm on 03 July 2008. Apparently, Bertha set a new record for the furthest east named storm formation in the tropics (classified as south of 23.5N) prior to 1 August since 1950. It became classified as a tropical storm at 24.7W which easily beat the old record set by Anna in 1969 (36.0 W).
Meteosat-9 IR images with overlays of QuikSCAT and ASCAT satellite winds (above) from the CIMSS Tropical Cyclones site on following day (04 July 2008) showed some bursts of convection around the tropical cyclone, and verified the presence of tropical storm force winds. The CIMSS wind shear product (below) indicated that Bertha existed in an environment of low wind shear, which was favorable for continued intensification.

Meanwhile, on the previous day (03 July 2008), the afternoon MODIS visible image and Land Surface Temperature product (below) showed mostly cloud-free conditions and very hot surface temperatures across much of the Desert Southwest region of the United States. Death Valley in California reached a high temperature of 122º F, the hottest day so far this summer season (121º F had been reached at that location a few times in June 2008); other high temperatures in the region that day included 118º F at Bullhead City, Arizona and 115º F at Laughlin, Nevada. While the MODIS LST values were generally about 20-30º F higher than the actual air temperatures that were measured in instrument shelters about 5 feet off the ground — the highest LST values seen on this day were near 150º F (darker red colors) in parts of California, Nevada, and Arizona — the LST product is still useful for depicting where the hottest areas might be (since the coverage of stations that report air temperature over any given region might be somewhat sparse).

Jesse Ferrell highlighted an undular bore emerging off the coast of Africa this morning, as seen in the annotated 1200 UTC Meteosat-9 image above (or click here). Bores occur when impulses move underneath stable layers, exciting a series of closely-spaced lines of clouds, as shown here, for example. Bores are associated with stable air, and you can note the lack of cumuliform clouds in the vicinity of this bore as it emerges off the coast of Africa just to the north of developing Tropical Depression #2 (update: Tropical Storm Bertha). A meteorogram showing data from station GQNN (Nouakchott, Mauritania, at 18 N, 16 W) — located halfway between the mouth of the Senegal River to the south and Cape Timiris to the north, or just to the south of the east-west band of cirriform clouds that straddles the coast — shows the characteristic pressure perturbation and wind shift at 0800 UTC as the bore passed.

A loop of visible imagery shows the two bore structures moving over the waters of the Atlantic Ocean. The genesis of these bores appears to be in dying convection over eastern Africa. Note also the presence of von Karman vortex streets that occur in the marine clouds downwind of the Canary Islands, which Islands are at the extreme northern boundary of the image.

An interesting aspect of this case is that the bore is along the southern edge of a Saharan Air Layer (SAL) that has emerged from northern Africa and is moving east southeastward into the central Atlantic ahead of the developing Bertha. The dry air associated with this layer of air can be a significant deterrent to tropical cyclone intensification. Thus, if the storm moves to the northwest, or moves close enough to the SAL to entrain dry air, do not expect rapid strengthening. The dry air associated with the SAL is also apparent in the MIMIC TPW loop available here.

For the latest information on tropical storm Bertha, refer to the CIMSS tropical weather page or to the National Hurricane Center.