As noted in the previous Blog entry, Tropical Storm Bertha became the Atlantic Basin’s second tropical storm on 03 July 2008. GOES-12 IR images and QuikSCAT and ASCAT satellite winds (above) from the CIMSS Tropical Cyclones site on following day (04 July 2008) showed some bursts of convection, 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 (to be released at 10 AM on 6 July in accordance with data restrictions) 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.

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).

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.

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.

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).

A large mesoscale convective system (MCS) developed over Nebraska during the pre-dawn hours on 24 June 2008, and AWIPS images of the GOES-12 10.7 µm IR channel (above) showed the extensive coverage of very cold cloud top temperatures (-60 to -70º C, red to black color enhancement) associated with this convective activity. This MCS was responsible for some weak tornadoes and small hail across parts of Nebraska (SPC storm reports), but the main impact was heavy rains that produced flooding — in Nebraska, 3.70 inches was reported at Gibbons, and 2.10 inches fell at Waterloo in a 1-hour period.

As the MCS moved eastward and began to decay during the daytime hours over Iowa and Missouri, the cyclonic circulation of a mesoscale convective vortex (MCV) became apparent over southeastern Nebraska on both the GOES-12 IR imagery above, and also on GOES-12 visible channel imagery from the UW-Madison AOS site (below). This MCV apparently played a role in the development of new convection later in the day over southeastern Nebraska, which produced hail up to 1.0 inch in diameter.

Numerous wildfires were started by lightning activity across Northern California on 21 June 2008, and GOES-11 visible images from 22 June (above) showed a large number of smoke plumes drifting across that region. A MODIS true color image from the SSEC MODIS Today site (below) showed that areas of thick smoke remained across much of northern California on 23 June, with some of the smoke still trapped in the valleys of the North Coast Range. The thick smoke drifting eastward on 22 June reduced surface visibility to 2 miles at Redding (station identifier KRDD), and was causing major air quality problems over a good deal of that area.

An AWIPS image comparison of the 1-km resolution MODIS 3.7 µm and the 4-km resolution GOES-11 3.9 µm shortwave IR channels from around 06 UTC on 23 June (11 PM local time on 22 June) demonstrated the improved ability to detect many of the smaller fires using higher spatial resolution data (below) . In addition, the actual locations of the larger fires were more correctly depicted on the 1-km MODIS image; the comparatively large 4-km GOES IR field of view (and to a lesser extent, the large geostationary satellite viewing angle) tends to diminish the accuracy of such small-scale image details. Improved fire monitoring will eventually be possible using the Advanced Baseline Imager (ABI) instrument on the upcoming GOES-R satellite (scheduled to be launched in 2014), which will offer similar IR imagery at a 2-km spatial resolution (at 5-minute intervals on a routine basis).

UPDATE: A MODIS true color image from 24 June (below, displayed using Google Earth) showed that the thick smoke had increased in areal coverage across much of northern California. MODIS Aerosol Optical Depth values as high as 1.0 were seen across much of the Sacramento Valley region of northern California; surface visibilities on that day at Redding and Red Bluff were as low as 1 mile due to smoke.

Numerous severe thunderstorms developed along the foothills of the Rockies and the High Plains during the afternoon hours on 19 June 2008. AWIPS images of the MODIS 11.0 µm “IR window” channel, Cloud Top Temperature product, and 3.7 µm “shortwave IR” channel (above) showed some of these areas of convection around 20:00 UTC (2 PM local time). Shortly after the time of these MODIS images, hail up to 2.75 inches in diameter was reported near Pueblo, Colorado (SPC storm reports). About 90 minutes after the time of the MODIS images, hail of 4.50 inches in diameter and a wind gust to 81 mph was produced by the storm that was seen developing in the Texas panhandle region. Although these storms exhibited similar cold cloud top temperatures on the IR window image (-60 to -77º C, red to black to gray color enhancement) and the Cloud Top Temperature product (-60 to -69º C, darker blue to light violet color enhancement), note the darker gray appearance of many of the westernmost storms on the 3.7 µm shortwave IR image — this darker signal is due to a dominance of smaller anvil top ice particles (in contrast with the brighter white appearance due to larger anvil top ice particles associated with the storms farther to the east). Anvils composed of smaller ice particles are more reflective of incident solar radiation, which results in significantly warmer shortwave IR brightness temperatures (in this case, +5 to +20º C, compared to -10 to -15º C in the brighter white anvil features composed of larger ice particles). According to Lindsey et al. (2006), regions such as the high plains and mountains (having environments with relatively dry boundary layers, steep lapse rates, and large vertical shear values) tend to favor thunderstorms with enhanced 3.9-μm reflectivity.

With cloud top temperatures in the -60 to -70º C range, no supercooled water droplets could have been present at the cirrus anvil top — this was confirmed by the MODIS “cirrus detection channel” image in tandem with the MODIS Cloud Phase product (below), which indicated ice phase (salmon color enhancement) for all the convective storm cirrus anvil features.

Additional reference:

  Cloud-top Structure of Northeast Colorado Thunderstorms May 24, 2005

Both GOES-11 (GOES-West) and GOES-12 (GOES-East) were placed into Rapid Scan Operations (RSO) during the afternoon and early evening on 18 June 2008, to monitor the development of severe thunderstorms across the central US (SPC storm reports). In RSO mode, images are available at 5-7 minute intervals (instead of the usual 15 minute intervals for standard operations) — however, the RSO image times are not exactly the same for GOES-11 and GOES-12. A side-by-side comparison of GOES-11 and GOES-12 visible channel images centered on Bismarck, North Dakota (above) showed the formation and intensification of thunderstorms that produced hail as large as 2.75 inches in diameter at 22:48 UTC and 4.25 inches in diameter at 00:22 UTC (both times at a location between Bismarck KBIS and Garrison KN60), and a tornado around 00:25 UTC (just west of Bismarck KBIS). Both the GOES-11 and GOES-12 images are displayed in their native satellite projections, so the cloud features (and the area shown) appear a bit different due to the differing satellite viewing angles.