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.

Today’s CIMSS Satellite Blog entry will take a step back and focus on the larger scale. We’ll begin with an animation of GOES-12 visible images from 25 June 2008 (above). Several items of interest are apparent in addition to the normal diurnal development of thunderstorms across parts of North and Central America: (1) the very large area of haziness that occupies the lower right quarter of the images is due to airborne dust from Africa; (2) thick smoke from wildfires is evident over much of northern California and Nevada; (3) a good deal of Hudson Bay in Canada is still frozen. Such is the diversity of meteorological phenomena that can be seen on satellite imagery in late June!

(1) To confirm that the haziness seen on the visible imagery above is due to African dust, we examine the Meteosat-9 Saharan Air Layer (SAL) tracking product (above). The westward progression of a large Saharan dust outbreak (orange to red color enhancement) can be clearly seen during the 21-25 June period. The areal coverage of the airborne dust over the Atlantic Ocean could also be seen on a composite of AVHRR false-color imagery from 23 June (below, displayed using Google Earth).

(2) To explore the impacts of the thick smoke over northern California, it is interesting to view an AWIPS image comparison of the MODIS Land Surface Temperature (LST) product and the MODIS visible channel (below). Note that the LST values seemed to be about 20º F cooler under the areas of thickest smoke (in the 100-110º F range, orange to light red colors) compared to areas farther to the north and to the south of the smoke (where LST values were in the 120-130º F range, darker red colors). The actual air temperatures were not affected by such a drastic amount, although air temperatures in the Sacramento Valley were in the mid-upper 80s F under the thickest smoke (compared to low to mid 90s F farther to the south in the San Joaquin Valley. At Red Bluff in northern California (KRBL) the maximum temperature on 25 June was 87º F (the surface visibility was 2.5 miles or less the entire day due to smoke), several degrees below the daytime maximum temperatures of 98º F, 95º F, and 96º F on the previous 3 days (KRBL 96-hour meteorogram).

(3) Finally, to confirm that ice still remained in much of Hudson Bay, we next turn to MODIS true color and false color images from the SSEC MODIS Direct Broadcast site (below). The majority of the bright features seen over Hudson Bay on the true color image are indeed ice, which appears as a darker red color on the false color image (in contrast to supercooled water droplet clouds, which appear as varying shades of white).