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!

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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.

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.

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.

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GOES-10 IR imagery with QuikSCAT winds (above) sourced from the CIMSS Tropical Cyclones site showed that the maximum surface winds associated with Subtropical Storm Olga were located well to the north of the center of the circulation early in the day on 11 December 2007. However, ASCAT wind data later in the day (below) indicated that the radius of the maximum surface winds had decreased somewhat, suggesting a transition from subtropical storm to tropical storm status. Reconnaissance aircraft data confirmed this trend, and Olga was named a Tropical Storm late in the day. Olga produced nearly 10 inches of rain across the island of Puerto Rico.

A NOAA-17 AVHRR 3-channel red/green/blue (RGB) false-color image (below) revealed that the center of Olga was partially exposed as the storm began to interact with the rugged terrain on the island of Hispaniola, with some convection around the core of the storm (primarily within the northern quadrant).

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Just as the 2007 Atlantic Tropical Cyclone season started off a bit early (with Subtropical Storm Andrea in early May), it also is ending a bit late with the formation of Subtropical Storm Olga on 10 December 2007. An animation of GOES-10 IR images (above) sourced from the CIMSS Tropical Cyclones site shows the cluster of cold cloud top temperatures (red to white enhancement) associated with Olga, moving just north of Puerto Rico.

An analysis of the Deep Layer Mean wind field (above) indicated that an upper level low existed just to the south of Olga. The majority of the 00 UTC 11 December 2007 model forecast tracks (below) moved Olga westward toward the Dominican Republic and Jamaica.

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