The AWIPS image of MODIS 11.0µm-3.7µm “fog/stratus product” (above) reveals several long plumes embedded within the extensive stratus cloud deck (yellow to red enhancement) that covered much of northern and central Minnesota during the pre-dawn hours on 19 October. These plumes likely originated at large coal-fired power plants (or paper mills?) located across that region — emissions from these industrial sources may have acted as cloud condensation nuclei, causing a higher concentration of smaller supercooled cloud droplets downwind of the plants.

The MODIS Cloud Phase product (below, lower left panel) showed that this stratus deck was primarily a water-phase cloud (blue enhancement); MODIS 11.0µm IR window channel brightness temperatures were generally around -14 C across that region, indicating that the stratus cloud was composed of supercooled water droplets. Note that these power plant plumes were not evident on the 4 km resolution GOES fog/stratus product (below, upper right panel). The GOES sounder Cloud Top Height values were around 11-12 kft over the area.

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A large Mesoscale Convective System formed over the Gulf of Mexico (just off the Texas coast) on 15 October, and this convection exhibited some very cold cloud top temperatures on AWIPS images of IR window channel data (above) — IR brightness temperatures were as cold as -91 C (-132 F) on the 1-km resolution 11.0 µm MODIS IR channel, and as cold as -87 C (-125 F) on the 4-km resolution GOES IR channel. Such cold cloud top temperatures are common in a tropical atmosphere, where the tropopause pressures are usually lower and the tropopause temperatures colder than atmospheres at mid-latitudes; the rawinsonde reports from Brownsville, Texas indicated a tropopause near the 100 mb level, with a nearly moist adiabatic lapse rate that is also typical of tropical air masses.

GOES sounder total precipitable water values were also quite high across the Caribbean and Gulf of Mexico, with PW exceeding 60 mm (2.4 inches) at some locations (below). This feed of tropical moisture helped to fuel severe convection on the following day which produced a few tornadoes along the Gulf Coast, along with record daily rainfall amounts at Houston, Texas (5.17 inches) and Galveston, Texas (3.91 inches) which caused fatal flash flooding to occur.

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GOES-12 6.5 µm water vapor channel imagery (QuickTime animation, above) revealed a series of vortices migrating southward along the western periphery of the large cold-core polar vortex that was centered over  southern Ontario on 13 September. Instability-like fragmentation of a potential vorticity (PV) strip along a stratospheric intrusion can lead to the development of this type of isolated subvortex structure (see Wirth et al, 1997). Within these mesoscale vortices, the tropopause was likely displaced downward several kilometers as vertical winds induced by local stratospheric intrusions brought drier air into the upper troposphere. AWIPS imagery of GOES sounder total column ozone (QuickTime animation, below) showed elevated ozone values (350-375 Dobson Units, green enhancement) co-located with the dry vortex signatures — elevated ozone is another signature of stratospheric air. Note that these vortices were forming in cloud-free air (GOES-12 4 panel image).

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A deep cold core upper-level low rapidly intensified over the northcentral US and southcentral Canada on 11 October — cold air in the wake of a strong southward-moving cold frontal boundary allowed many sites across the Upper Midwest and the western Great Lakes region to see their first snow of the season (13-22 inches fell in northern Wisconsin and the UP of Michigan, snow pellets were seen here in Madison area, and Detroit, Michigan experienced their earliest measurable snowfall on record); this system later gave Buffalo, New York it’s snowiest October day ever (14.0 inches on 13 October). The AWIPS image of GOES sounder total column ozone (above) shows a lobe of elevated ozone (350 Dobson Units or higher, green to red enhancement) which was moving southward across the Dakotas, Minnesota, and Iowa (QuickTime animation). This upper-tropospheric ozone feature corresponded with the lowering tropopause heights (denoted by the potential vorticity contours greater than ~2.0 PVU within the 310-320 K isentropic layer) associated with the core of the deepening 500 hPa vortex. Ozone and potential vorticity are both tracers of stratospheric air — high values in the upper troposphere are seen when tropopause heights drop (due to tropopause folding around jet streaks or upper-level frontogenesis, or deepening of upper-level cyclones).

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