Another subtropical storm in the Atlantic?

December 29th, 2007

GOES-12 10.7µm IR images (Animated GIF)

If you thought that Tropical Storm Olga was the last gasp of the 2007 North Atlantic basin’s Tropical Cyclone season, think again: GOES-12 10.7 µm IR channel imagery (above; closer view) and 6.5 µm water vapor channel imagery (below) revealed a circulation southwest of the Azores on 29 December 2007 (centered near 28º North latitude, 47º West longitude) that appeared to be acquiring subtropical characteristics as it began to produce gale-force winds and some convection (evident in this 500-m resolution MODIS visible image) within the northeastern quadrant of the disturbance.

GOES-12 6.5 µm water vapor imagery (Animated GIF)

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Meteosat-8 visible image

A Meteosat-8 visible channel image (above) showed the circulation well off the coast of Africa at 12:00 UTC on 29 December. In addition, note the “hazy” appearance of the cloud-free region over and just south of the Cap-Vert region of northwestern Africa (not the larger sun glint feature seen farther south over the subtropical South Atlantic — the Saharan Air Layer (SAL) tracking product (below) suggests that this could be an area of airborne dust (yellow to orange enhancement) moving westward from the Sahara desert across the far eastern Atlantic Ocean (a good deal of the yellow-to-orange signal across the rest of the North Atlantic is a “false positive” SAL/dust signal).

Saharan Air Layer (SAL) tracking product

Google Earth Imagery from SSEC

December 17th, 2007

AVHRR imagery in Google Earth (Animated GIF)

Two new sources of Google Earth satellite imagery are now available from SSEC: AVHRR images from the NOAA operational polar orbiting satellites, and MODIS images from the NASA Terra and Aqua satellites. An example of the AVHRR “false color” imagery (above) zooms in to show the deep snow cover that existed over much of the Upper Midwest on 17 December 2007. Note the darker appearance of the Chicago IL and Milwaukee WI metro areas; even though those cities had a significant amount of snow on the ground, the higher concentration of trees, buildings, and paved surfaces all contributed to a somewhat “darker” satellite scene in those urban areas (compared to the adjacent outlying rural areas). Also evident on the AVHRR image were well-defined lake-effect snow bands over Lake Michigan that were moving inland over portions of Michigan and Indiana — these snow bands were on the far western periphery of a large winter storm that was centered over the Northeast US.

MODIS Google Earth imagery

MODIS “true color” imagery from the MODIS Today site (above) revealed a large number of lake-effect snow (LES) bands over parts of Lake Superior, Lake Michigan, and Lake Huron as cold arctic air streamed southward across the Great Lakes on 14 December 2007; these LES bands produced 3-6 inches of snowfall at some locations in the Upper Peninsula of Michigan. On that same day, MODIS imagery farther to the east showed a variety of banded cloud features over the Mid-Atlantic states (below).

MODIS Google Earth imagery

Snow vs stratus cloud discrimination

December 17th, 2007

MODIS visible + snow/ice channel images (Animated GIF)

AWIPS images of the MODIS visible and 1.6 µm “snow/ice channel” images from 17 December 2007 (above) demonstrate the utility of the snow/ice channel for helping to identify areas of thin supercooled water droplet stratus clouds that exist over a snow-covered satellite scene (this MODIS imagery was mentioned in the NWS Milwaukee/Sullivan Area Forecast Discussion that afternoon). Note the semi-transparent nature of many of the stratus cloud patches, which allowed surface features (such as rivers, cities, and densely-forested areas) to be seen on the visible image.

The GOES-12 sounder Cloud Top Height product (below) suggested that the tops of the thicker stratus cloud areas located over northern Wisconsin and northern Illinois were 9000-9800 feet above ground level (yellow enhancement). Surface METAR data under those same cloud features indicated that the cloud bases were only 300-800 feet above ground level; so if the stratus clouds were really close to 9000 feet thick, it was remarkable to be able to see hints of surface features through such a cloud layer!

GOES-12 sounder Cloud Top Height

Ice in the Upper Midwest

December 13th, 2007

MODIS visible + snow/ice images (Animated GIF)

AWIPS images of the MODIS visible and “snow/ice” channels from 13 December 2007 (above; closer view) reveal that much of the southeastern half of Iowa received a significant glazing of ice (0.25 to 1.0 inch) during a freezing drizzle/freezing rain event on 10-11 December. Snow and ice particles are very strong absorbers at the 1.6 µm wavelength of the MODIS snow/ice channel [Baum et al., 2000], so the thick coating of ice on top of existing snow cover shows up as a very dark feature on those images (even darker than adjacent areas to the north which had more snow cover but did not receive significant amounts of freezing rain). In contrast, supercooled water droplet clouds show up as much brighter features on the MODIS snow/ice channel image.

A false-color RGB composite using the MODIS visible and 1.6 µm images (below) shows the region of significant ice glazing as the transition to deeper red colors.

MODIS false color composite image

A comparison with the MODIS snow/ice channel image from the previous day (below) shows that the significant ice accumulation (darker black enhancement) also extended to the northeast, covering parts of northern Illinois and extreme southern/southeastern Wisconsin. It is interesting to compare this event with another case of significant ice glazing that was seen on MODIS imagery in the southern Plains back in February 2002.

MODIS snow/ice images (Animated GIF)

For photos of the ice accumulation in extreme northwestern Missouri, see the website from storm chaser Mike Hollingshead.

Reference: Baum, B.A., P.F. Soulen, K.I. Strabala, M.D. King, S.A. Ackerman, W. P. Menzel, and P. Yang: Remote sensing of cloud properties using MODIS airborne simulator imagery during SUCCESS, J. Geophys. Res., 105, 11781-11792, 2000.