MODIS 6.7 µm water vapor image (with and without map overlay)

Under normal atmospheric conditions, the weighting function of most water vapor channels tends to peak at altitudes within the 500-300 hPa pressure range, allowing features within the middle to upper troposphere to be viewed on the water vapor imagery. However, under special conditions — for example, either a very dry or a very cold air mass — the altitude of the water vapor weighting function is shifted downward such that we are able to “see the surface” on water vapor imagery. Such was the case with the MODIS 6.7 µm water vapor image over the Baja California region on 08 July 2010 (above), where the outline of the coast was very obvious on the image.

Even though the water vapor channel was not “seeing the surface” per se, a signal of the strong surface thermal contrast (between the very warm land and the much cooler water) was able to override the weak signal from what little middle-tropospheric water vapor was present. Other cases of strong land/water temperature contrasts have been seen on water vapor imagery, such as with very cold and very dry arctic air masses back in February 2007, December 2006, and January 2004.

However, in this case, the signal of the land/water thermal contrast was not evident on the corresponding GOES-11 6.7 µm / GOES-13 6.5 µm water vapor composite image. Because of the large viewing angle of the geostationary satellites (around 40 degrees for GOES-11 and around 55 degrees for GOES-13 for the Baja California region), the water vapor weighting function was apparently shifted upward to a high enough altitude to preclude detection of the surface land/water thermal signal.

GOES-11 6.7 µm + GOES-13 6.5 µm water vapor composite (with and without map overlay)

Surprisingly, not even the GOES-11 sounder 7.4 µm water vapor channel image (below) was able to detect the strong surface thermal signal — the weighting function of this channel often peaks much lower in the troposphere (usually around 850-700 hPa). Again, perhaps the large geostationary satellite viewing angle was a factor. With the MODIS instrument flying directly overhead, there was no corresponding upward shift in the water vapor channel weighting function.

GOES-11 sounder 7.4 µm water vapor image (with and without map overlay)

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GOES-13 0.63 µm visible images

A stagnant area of high pressure situated over the Northeast and Mid-Atlantic states contributed to a prolonged heatwave over that part of the US, with widespread temperatures over 100º F (38º C) for several days — including 106º F (41º C) at Williamsburg, Virginia and Frederick, Maryland on 06 July 2010. McIDAS images of GOES-13 0.63 µm visible channel data (above) showed a large area of very hazy sky conditions covering much of the Northeast and Mid-Atlantic regions on that particular day — and corresponding MODIS Aerosol Optical Depth values were also quite high within the hazy areas seen on visible imagery, with surface Air Quality Index values deteriorating into the “Moderate” to “Unhealthy for Sensitive Groups” categories.

An AWIPS image of the MODIS Land Surface Temperature (LST) product (below) revealed that while many air temperatures (measured in an instrument shelter about 5 feet above the surface) were only as warm as about 100º F at 18:05 UTC (2:05 pm local time), the “skin temperature” of the ground surface was much warmer — with many areas exhibiting LST values in the 120-130º F range (darker orange to red color enhancement). The warmest LST value seen at that time was 136º F in southeastern Pennsylvania. As an aside, MODIS Sea Surface Temperature values were as warm as 87º F in the southern portion of Chesapeake Bay.

MODIS Land Surface Temperature product + surface METAR reports

Other notable features seen on the GOES-13 visible imagery above included (1) the development of a line of severe thunderstorms along a frontal boundary from Nebraska and South Dakota, which produced a number of tornadoes and large hail up to 2.75 inches in diameter, (2) a extensive area of cloudiness over the Yucatan Peninsula of Mexico, which eventually developed into Tropical Depression #2 over the Gulf of Mexico on the following day, and (3) the dissipation of fog and stratus along the California coast during the late morning and afternoon hours (with convection developing further inland over the Sierra Nevada mountain range).

In terms of the coastal fog and stratus in southern California, GOES-15 0.63 µm visible channel images (below) showed how slow these features were to burn off in some areas. In fact, a number of locations in the San Diego, California area experienced record low maximum temperatures for the date — including a daily high temperature of only 65º F at San Diego International Airport (labeled SAN on the images), which was 10 degrees below the normal high temperature (75º F) for San Diego on 06 July. It is also interesting to note that heating of the higher terrain of some of the offshore islands appeared to help initiate the earlier clearing of the marine layer stratus cloud deck.

GOES-15 0.63 µm visible channel images

A 250-meter resolution MODIS true color Red/Green/Blue (RGB) image (created using Bands 1/4/3) from the SSEC MODIS Today site (below) shows even greater detail in the structure of the coastal fog/stratus features at 21:23 UTC (2:23 pm local time).

MODIS true color RGB image (created using bands 1/4/3)

===== 10 JULY UPDATE =====

The large pocket of haze seen on the GOES-13 visible imagery over the Northeast US on 06 July slowly migrated southward along the East Coast during the next several days. An animation of hourly GOES-13 visible images (below) showed that the haze was concentrated over the Mid-Atlantic states on 07 July, but then moved further south to settle along and off the coast of the North and South Carolina, Georgia, and Florida during the 08/09/10 July period. A large cyclonic circulation over the far western Atlantic Ocean was largely responsible for helping to draw the hazy air mass southward.

GOES-13 0.63 µm visible images (06 July - 10 July)

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GOES-13 10.7 µm IR imagery

GOES-13 10.7 µm IR imagery from the CIMSS Tropical Cyclones site (above) showed the development of large convective bursts around the center of Hurricane Alex during the 29 June – 30 June 2010 period. Alex became the first June hurricane in the Atlantic Basin since the 1995 tropical cyclone season (which produced Hurricane Allison).

An AWIPS image of POES AVHRR 11.0 µm IR channel data (below) showed very cold IR brightness temperatures of -80 to -90º C (violet color enhancement) associated with the convective bursts as well as the distant bands of intense convection surrounding the hurricane.

POES AVHRR 10.8 µm IR image

Deep layer wind shear (below) over the western Gulf of Mexico remained very light, which was favorable factor for further intensification prior to making landfall.

GOES-13 IR image + deep layer wind shear

DMSP SSMI/S 85 GHz microwave imagery (below) showed the possible development of some inner banding structure, as well as the larger and more intense bands of convection far from the center of Alex.

SSMI/S microwave image

AWIPS images of the MIMIC Total Precipitable Water product (below) indicated that a rich source of moisture remained in place across the entire Gulf of Mexico region (with TPW values in excess of 60 mm).

MIMIC Total Precipitable Water product

UPDATE: AWIPS images of the MODIS 0.65 µm visible and 11.0 µm IR channel data with an overlay of ASCAT scatterometer surface winds (below) depicted the eye of Hurricane Alex at 16:58 UTC on 30 June.

MODIS 0.655 µm visible and 11.0 µm IR images + ASCAT scatterometer winds

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POES AVHRR Cloud Top Temperature, Cloud Top Height, and Cloud Type products

Later in the day, AWIPS images of the POES AVHRR Cloud Top Temperature (CTT), Cloud Top Height (CTH), and Cloud Type products at 22:05 UTC (above) displayed a large area of CTT values in the -80 to -83º C range (violet color enhancement), with CTH values as high as 17 km (darker blue color enhancement). The Cloud Type product categorized a large portion of the coldest/highest cloud tops surrounding the eye as “overshooting” (lighter violet color enhancement), in general agreement the the GOES InfraRed/Water Vapor difference overshooting top detection technique of Olander and Velden (2009).

GOES-15 and GOES-13 0.63 µm visible channel images

The eye of Hurricane Alex became more well-defined on GOES visible imagery as it approached the coast of Mexico, as seen on a comparison of GOES-15 and GOES-13 visible images at 15 minute intervals (above) and also on GOES-13 Rapid Scan Operations (RSO) images at 5-10 minute intervals (below). An impressive convective burst was evident just as the eye was nearing the coastline — in fact, Alex rapidly intensified into a 90 knot Category 2 hurricane just prior to making landfall, as can be seen on this plot of the CIMSS Automated Dvorak Technique. This made Alex the first Category 2 or stronger hurricane to occur in the month of June since Hurricane Alma back in 1966.

GOES-13 0.63 µm visible channel images (Rapid Scan Operations)