MODIS visible images

An interesting large “dark feature” showed up on AWIPS images of MODIS visible channel data over Lake Michigan on 04 June 2009 (above). This dark feature appeared to be shifting slowly southward during the 105-minute period between the time of the Terra MODIS image (16:53 UTC, or 11:38 am local time) and the time of the Aqua MODIS image (18:38 UTC, or 1:38 pm local time).

It is revealing to examine the corresponding MODIS Sea Surface Temperature (SST) images (below), which indicate the presence of a significantly warmer area of SST values located within the region of the “dark feature” that was seen on the visible imagery. MODIS SST values were as warm as 52.4º F on the Terra image, and 57.3º F on the Aqua image (darker green colors) — a good 10-15º F higher than the surrounding cooler waters (39-42º F, darker blue colors). There are only 2 buoys in Lake Michigan — Buoy 45007 in the northern portion of the lake (reporting a water temperature of 39º F), and Buoy 45002 in the southern portion of the lake (reporting water temperatures of 41-43º F) — so the MODIS SST imagery provided potentially useful information in an otherwise data-void area.

MODIS Sea Surface Temperature images

As it turns out, this large dark feature on the visible imagery corresponded to portions of the lake water that were very smooth. Since high pressure was centered over this region — note that the NAM12 model surface winds (below) were only in the 0-5 knot range  — the wind-driven wave heights were minimal, allowing for a nearly flat water surface. The presence of very light winds also allowed the “skin temperature” of the water surface to warm very quickly, as we have seen over Lake Michigan in the past. The warmest SST values were located near the center of the “light winds region”, where the winds apparently had been calm for several hours (or longer).

MODIS visible image with NAM12 surface winds

So, why did this large area of “smooth water” appear darker in the MODIS visible images? The answer lies in the fact that with imagery from polar-orbiting satellites such as Terra and Aqua, there is often a significant amount of sun glint off the rough water surfaces below the satellite overpass — these areas of sun glint make the rougher water surfaces appear brighter in visible imagery. However, in the area of the calm winds, the water surface was very flat; this flat water surface then reflected like a mirror (with all the light being reflected back in one direction — but in this case, that one direction was not directly back toward the satellite!).

This idea is supported by the corresonding MODIS 3.7 µm shortwave IR imagery (below) — this particular channel is very sensitive to reflected solar radiation, which makes the rougher waters (which give off a lot of sun glint) appear warmer (darker) on the shortwave IR imagery. The MODIS 3.7 µm IR brightness temperature values were about 20º C colder (denoted by the lighter gray shades) in the region of the smooth water, since there was very little solar radiation being reflected directly back toward the satellite.

MODIS 3.7 µm shortwave IR images

Very fine structure in the edges of the “smooth water areas” can be seen by examining 250-meter resolution MODIS true color imagery (below). This AWIPS MODIS true color imagery viewer is available as part of the CIMSS “MODIS Imagery in D-2D” project (currently only over Wisconsin at this time).

MODIS 250-meter true color image + MODIS 1-km visible image

One final question remains: why wasn’t this “dark signature”  of the calm Lake Michigan waters seen on the “1-km” resolution GOES-12 visible imagery (below)? Once again, the answer revolves around the issue of sun glint: with a geostationary satellite such as GOES-12 (which is positioned over the Equator), the sun glint pattern is generally restricted to the Equatorial regions. The lack of sun glint over Lake Michigan did not allow the GOES-12 visible imagery to be useful for diagnosing those areas of calm winds over the water.

GOES-12 visible images

A hat-tip to Kathy Strabala and Chris Moeller at CIMSS for bringing this case to our attention, and offering a physical explanation of what was going on!

===== 08 JUNE UPDATE =====

Another good case of this type of “calm winds / smooth water” satellite signature presented itself on 08 June 2009 over the northern Gulf of Mexico. An AWIPS image of the MODIS visible channel (below) shows an elongated dark region that was oriented approximately west to east — but in this case, there was also a large bright spot located within the elongated dark region. Note the orientation of the Terra satellite overpass, as shown in the MODIS Orbit Itinerary Viewer to the left of the MODIS visible image: the satellite overpass was just to the left (west) of the bright spot on the visible image. This was due to the geometry of the sun and the satellite at that time: the sun was still to the east of the satellite position during the late morning hours, which affected the path of the rays of sunlight reflection such that the bright reflection spot seen in the visible image was located to the right (east) of the satellite overpass point.

MODIS visible image + Terra satellite overpass geometry

===== 09 JUNE UPDATE =====

The temporal evolution of the changing satellite image appearance of such an area of calm winds / smooth water is demonstrated on GOES-12 visible and 3.9 µm shortwave IR images (below). This particular region of calm winds and smooth water was located off the Pacific coast of Central America on 09 June 2009. Note how the appearance of the area on visible imagery changes from dark to bright to dark again as the sun’s location passes overhead during the rotation of the Earth — and the brightness temperature on the shortwave IR images gets much hotter (darker black in appearance) as the reflected solar radiation becomes the dominant component in the 3.9 µm IR brightness temperature.

GOES-12 visible and 3.9 µm shortwave IR images

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Meteosat-9 IR images

An Air France Airbus A330-200 — Flight #447 en route from Rio de Janeiro, Brazil to Paris, France — crashed in the tropical Atlantic Ocean on 01 June 2009 (surface analysis). Shortly after the last radio contact about 350 miles (565 km) northeast of Natal, Brazil (station identifier SBNT), the aircraft likely traversed an area of intense deep convection which had formed within a broad band of high total precipitable water along the Intertropical Convergence Zone (ITCZ). This convection could be seen on EUMETSAT Meteosat-9 10.8 µm IR images (above) in the region between 2º N to 4º N latitude and 25º W to 35º W longitude.  Some of the individual convective clusters appeared to be developing very quickly — this leads to speculation that turbulence in the vicinity of these rapidly-developing storms may have played a role in the accident.

A closer look at the ITCZ convection is shown using Meteosat-9 IR imagery with a magnification factor of 2 (below; also available as a QuickTime animation). There were a number of times when the minimum cloud top IR brightness temperature was -80º C or colder (light purple color enhancement), with the coldest cloud top temperature of -82º C occurring at 00:15 UTC. While this is certainly a cold cloud top temperature value, it cannot be considered “extreme” by any means: cloud top temperatures in tropical weather systems have been known reach -90º C or colder on occasion.

Air France Flight 447 last radioed their position at the  “INTOL” waypoint  at 01:33 UTC, and according to their flight plan they were then supposed to proceed to the “SALPU” and the “ORARO” waypoints along Airway UN873. At the INTOL waypoint, they communicated that they expected to reach the “TASIL” waypoint around 02:20 UTC (these waypoints are labeled on the IR images below). During  the 02:10-02:14 UTC timeframe, a series of automated ACARS fault messages was transmitted by the aircraft when it was approximately 54 miles from reaching the TASIL waypoint (the aircraft had possibly just cleared the northern fringes of the band of ITCZ convection around that time).

Meteosat-9 IR images (magnified by a factor of 2)

The brightness temperature difference values between the Meteosat-9 water vapor and IR window channels (6.2 µm – 10.8 µm) were calculated in an effort to try and highlight the most vigorous areas of convective development (below). The assumption is that when intense convection overshoots the tropopause into the warmer stratosphere, the water vapor that is pushed above the cloud top emits radiation at a warmer temperature than the actual cloud top below. Many pixels in the band of ITCZ convection exhibited WV-IR brightness temperature difference values in the 3-5º C range (darker red color enhancement). Of particular interest is the comparatively small cluster of convection that developed very rapidly around 02:00 UTC, near 1.75º N latitude and 31.7º West longitude (north of waypoint “SALPU”) — this cluster of convection exhibited WV-IR brightness temperature difference values as high as 4º C at 02:15 UTC. Could this rapidly-developing convective cell have generated severe turbulence that affected Air France flight 447 as it was passing nearby, en route to waypoint “TASIL”?

Meteosat-9 Water Vapor – IR brightness temperature difference

A comparison of the 3-km resolution Meteosat-9 10.8 µm IR and the 4-km resolution GOES-10 10.7 µm IR images (below; magnified to an effective resolution of 1 km) shows that the cloud top IR brightness temperatures  generally appeared to be about 3-6º C warmer on the GOES-10 imagery — the coldest GOES-10 IR brightness temperature was -77º C at 01:15 UTC. The coarser 4-km GOES-10 IR pixel resolution tended to “smooth out” the small-scale temperature structure of the cold cloud tops; therefore, a finer cloud top temperature structure was apparent on the 3-km resolution Meteosat-9 IR imagery.

Meteosat-9 and GOES-10 IR images

The 00:00 UTC rawinsonde report from Fernando de Noronha, Brazil (below) indicated that the tropical tropopause level was probably located near the 100 hPa pressure level (at a height of 16,649 meters, or 54,623 feet), where the minimum temperature was -77.7º C. The presence of cloud top IR brightness temperatures colder than -80º C on the Meteosat-9 imagery suggests that many of the strongest updrafts were likely penetrating the tropopause — and such overshooting thunderstorm updrafts have been known to initiate strong gravity waves aloft that have generated moderate to severe turbulence.

Fernando de Noronha rawinsonde report

Meteosat-9 6.25 µm “water vapor channel” images (below) showed none of the typical water vapor signatures associated with turbulence in the immediate region of the ITCZ convection — however, it did indicate the presence of a southwestward-propagating wave (located between 7-8º N latitude and 35-40º W longitude) that appeared to be responsible for initiating the formation of a patch of high clouds near 8º N 38º W. The water vapor imagery depicted a region of drier mid-tropospheric air immediately to the north of the ITCZ convection, suggesting synoptic-scale subsidence aloft in that area. Also note that within this region of drier air to the north of the ITCZ there was an interesting pattern of subtle impulses which were propagating westward.

Meteosat-9 water vapor images

Meteosat-9 water vapor winds from the CIMSS Tropical Cyclones site valid at 00:00, 03:00, and 06:00 UTC (below) showed that the upper tropospheric winds were weakly divergent over area of the ITCZ convection (150-300 hPa divergence plot), with only a minimal amount (5-10 knots) of deep layer wind shear in that particular region. The water vapor imagery also depicted  a “dry/moist gradient” signature associated with a subtropical jet stream which was moving over the northwestern coast of Africa — while the deep layer wind shear was increasing between the ITCZ convection and the subtropical jet (to a maximum value exceeding 60 knots), it is questionable whether the aircraft made it far enough to the northeast to be affected in any way by this increasing wind shear.

Meteosat-9 water vapor winds

===========================================

See also:

Air France Flight 447: A detailed meteorological analysis

NOVA: Crash of Flight 447

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GOES-12 10.7 µm IR images

The first tropical depression of the 2009 season formed off the US East Coast on 28 May 2009. An AWIPS animation of GOES-12 10.7 µm “IR window” images (above) revealed several bursts of convection as the canopy of cold cloud tops slowly increased in areal coverage.

One of the convective bursts occurred around 15:00 UTC , and a comparison of the 1-km resolution MODIS 11.0 µm IR window and the 4-km resolution GOES-12 10.7 µm IR window images around that time (below) depicted cloud top brightness temperatures several degrees colder on the MODIS image (-72º C, vs -68º C on the GOES-12 IR image).

MODIS 11.0 µm and GOES-12 10.7 µm IR images

Products from the CIMSS Tropical Cyclones site (below) showed that the coldest SSM/I microwave brightness temperatures (red colors) were found in the southeastern quadrant of the cold IR cloud shield. In addition, it could be seen that the tropical depression was situated over the warmer waters of the Gulf Stream (SST values greater than 24º C, green colors), which was likely aiding in the intensification process. The deep layer wind shear was also light, which was another factor that favored further intensification.

GOES-12 IR + DMSP Microwave + Sea Surface Temperature

The Blended Total Precipitable Water product (below) showed that TPW values were as high as 50-57 mm (2.0 to 2.2 inches, red colors) in the vicinity of the tropical depression. The POES AMSU Rainfall Rate product depicted rainfall intensities as great as 29 mm per hour (1.14 inch per hour) around 13:30 UTC.

Blended Total Precipitable Water + GOES-12 IR images

===== 29 MAY UPDATE =====

On the following day (29 May 2009), GOES-12 visible images from the CIMSS Tropical Cyclones site (below) indicated that the low-level circulation  had become separated from the cluster of deep convection which was located in the southeast  quadrant of the tropical depression — this was due to increasing amounts of deep layer wind shear across the region.

GOES-12 visible images

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MODIS 11.0 µm IR + GOES-12 10.7 µm IR images

AWIPS images of the 1-km resolution MODIS 11.0 µm  and the 4-km resolution GOES-12 10.7 µm “IR window” channels (above) showed a cluster of very cold cloud top temperatures (-88º C on MODIS, and -82º C on GOES, violet colors) associated with deep convection over the northwestern Gulf of Mexico on 28 May 2009. Also of interest is the appearance of  both transverse banding and an orthogonal gravity wave structure in the northwestern portion of the anvil edge (near the Mexico border).

A comparison of the MODIS 0.6 µm “visible channel”, 1.3 µm “cirrus detection channel”, 6.7 µm “water vapor channel”, and the 11.0 µm “IR window channel” (below) showed that the various satellite  channels differed in their  ability to detect the true western and northwestern extent of the cirrus anvil edge.

MODIS visible, cirrus detection, water vapor, and IR window channel images

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