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.

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Wildfire activity began to increase across the northern portions of Saskatchewan and Manitoba in Canada on 30 June 2008. GOES-11 visible and 3.9 µm “shortwave IR” images (above) showed a number of thick smoke plumes (lighter gray features on the visible images) drifting southeastward from a large cluster of active fire “hot spots” (darker black pixels on the IR images). GOES-11 was placed into Rapid Scan Operations (RSO) during the afternoon hours, so images near the end of the animation were available at 5-7 minute intervals.

The largest fire was located between Pelican Narrows and Sandy Bay in Saskatchewan, as seen in an AVHRR false color image (below, viewed using Google Earth). A close-up view reveals that the fire had actually jumped the only road that was in that area (the seasonal road which connects Sandy Bay and Pelican Narrows). The GOES-11 shortwave IR brightness temperatures associated with this particular fire were as high as 341º K (68º C, 158º F), which is the saturation temperature of the 3.9 µm detectors on the GOES-11 satellite. Note that some small pyrocumulus clouds could be seen developing over this large and very hot fire on the GOES-11 visible imagery (above) as well as on the AVHRR false color image (below).

A closer view of the largest fire using AWIPS images of the 1-km resolution MODIS 3.7 µm and the 4-km resolution GOES-12 3.9 µm IR channels (below) shows the advantage of higher spatial resolution for displaying the shape and coverage of not only the largest fire cluster (located near the center of the image), but also the smaller fires in outlying areas. Many of the pixels were so hot that the IR brightness temperatures exceeded the 54.5º C upper threshold for AWIPS display, and showed up as black pixels (registered as “NO DATA“) on the imagery. The smoke from this fire was restricting surface visibility to 1 mile at Flin Flon (CYFO) and 3 miles at The Pas (CYQD) in Manitoba, even though those 2 sites were not in the direct path of the thickest portion of the smoke plume.

Some clues as to the locations of the hottest portion of the fire — which happened to be located within the eastern half of the active fire area, where the black “NO DATA” pixels were seen on the MODIS shortwave IR image — could be found by examining other MODIS images and products: note the darker black pixels on the 11.0 µm “IR Window” channel, the brighter white pixels on the 2.1 µm near-IR “Snow/Ice” channel, and the darker red pixels on the Land Surface Temperature (LST) product (below). AWIPS cursor sampling indicated that the hottest pixel on the IR Window image was 52º C (126º F), while the hottest pixel on the LST image was significantly warmer at 145º F (63º C).

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While the Atlantic Ocean tropical cyclone season has been relatively quiet thus far, the Eastern Pacific Ocean was showing some signs of activity in late June 2008. Products from the CIMSS Tropical Cyclones site such as AMSU brightness temperature (above) and Satellite Consensus (SATCON) tropical cyclone intensity estimates (below) were useful to forecasters at the National Hurricane Center, as noted in their Tropical Storm Boris discussion from 29 June 2008:

TROPICAL STORM BORIS DISCUSSION NUMBER 13
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL

800 PM PDT SUN JUN 29 2008

THE RAGGED EYE VISIBLE EARLIER HAS BEEN OBSCURED BY NEW CONVECTION NEAR THE CENTER…ALTHOUGH EXCELLENT BANDING ALOFT IS APPARENT IN AN AMSU PASS AT 0045 UTC. THE ADVISORY INTENSITY OF 60 KT IS A BLEND OF SUBJECTIVE DVORAK ESTIMATES OF 55 KT FROM TAFB AND SAB…A CIMSS ADT OF 65 KT…AND A CIMSS AMSU ESTIMATE OF 66 KT.

Not far to the west of Boris, Tropical Storm Cristina was also present over the Eastern Pacific Ocean. ASCAT satellite wind vectors were helpful in verifying the intensity of Cristina, as seen in on GOES-11 visible and IR imagery with ASCAT data superimposed (below).

A 30 June 2008 National Hurricane Center discussion for Tropical Storm Cristina also noted

TROPICAL STORM CRISTINA DISCUSSION NUMBER 11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL

200 AM PDT MON JUN 30 2008

THE CLOUD PATTERN ASSOCIATED WITH CRISTINA HAS BECOME LESS ORGANIZED OVER THE PAST SEVERAL HOURS. THERE IS A RATHER SHARP EDGE TO THE HIGH CLOUD MASS OVER THE EASTERN SIDE OF THE SYSTEM…INDICATIVE OF EASTERLY VERTICAL SHEAR. THIS SHEAR IS AT LEAST PARTIALLY DUE TO THE UPPER-LEVEL OUTFLOW FROM TROPICAL STORM BORIS LOCATED NOT FAR TO CRISTINA’S EAST.

This sharp eastern cloud edge was quite evident on GOES-11 IR imagery (below), which also displays the CIMSS wind shear product.

One factor influencing the lack of organized tropical storm activity in the Atlantic so far this season may have been the persistent Saharan Air Layer and airborne African dust that was frequently observed over the tropical Atlantic basin during the month of June 2008 — note the presence of significantly lower MIMIC Total Precipitable Water (TPW) values during the 27-30 June 2008 period (below) within the 10-20º N latitude band over the Atlantic Ocean, compared to the much higher TPW values over tropical East Pacific where Tropical Storms Boris and Cristina were seen. One impact of such a high amount of African dust may be the cooler than normal Sea Surface Temperatures across the tropical Atlantic, which would be a negative factor for tropical cyclone formation.

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Tony Cristaldi at the National Weather Service forecast office at Melbourne, Florida pointed out an interesting feature to us: a clockwise-rotating vortex over the Atlantic Ocean, located just north of the Equator off the northeast coast of Brazil. An animation of GOES-12 visible and 3.9 µm shortwave IR images (above; QuickTime animation) shows the feature as it moved westward on 26 June and 27 June 2008 (producing brief pulses of convection on both days). So, the Question of the Day is: if this was a Mesoscale Convective Vortex (MCV) that was spawned by convection over the tropical Atlantic Ocean, and it was found in the Northern Hemisphere, then wouldn’t such a feature be expected to exhibit a counterclockwise (or “cyclonic” in the Northern Hemisphere) rotation?

The answer to that question (provided via email from the bright minds of Brian Etherton at the University of North Carolina at Charlotte and Brad Barrett at the University of Oklahoma): since this mesoscale circulation possessed a small radius of curvature (implying a large Centrifugal Force) and was located near the Equator (implying a small Coriolis Force), then the flow (as governed by the Gradient Wind Balance equation) would be cyclostrophic (a balance between only the Pressure Gradient Force and the Centrifugal Force) — so the direction of flow into such a circulation could be either cyclonic (counterclockwise in the Northern Hemisphere) or, as in this particular case, anticyclonic (clockwise in the Northern Hemisphere).

In an attempt to identify the source of this curious vortex, we examined GOES-12 visible and shortwave IR imagery at 3-hour intervals during the 23-27 June period (below; QuickTime animation). The imagery seems to suggest that the source of the vortex may have been an area of convection over the tropical Atlantic Ocean, which developed just south of the Equator (centered around 1.5º S / 28.0º W) on 24 June. This convection produced a well-defined outflow boundary, which could be seen propagating northwestward on the 11:45 and 14:45 UTC visible images on 24 June. The vortex first becomes apparent on the 17:45 UTC visible image on 24 June, located some distance behind (south of) the aforementioned outflow boundary. From that point, the vortex is then difficult to follow due to other cloudiness in the region, until it is again obvious on the 11:45 UTC visible image on 26 June (located near 0.5º N / 33.5º W). After 26 June, the feature is more easily tracked using the 30-minute interval images shown above.

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