An AWIPS image of the 12 April 2007 MODIS Sea Surface Temperature (SST) product over the northwestern Atlantic Ocean (above) showed a very sharp SST gradient between the cold waters of the Labrador Current (30-40º F, blue colors) and the warm waters of the Gulf Stream (55-65º F, green colors). An interactive Java applet allows you to explore the SST values over that region (click on the SST image in the applet to activate the cursor readout). A MODIS SST image farther to the south (below) reveals SST values that were significantly warmer (75-80º F, orange to red colors) in the Gulf of Mexico and along the southern portion of the Gulf Stream. SST and other Level 2 MODIS Science Products are available daily on the CIMSS MODIS Direct Broadcast Products site.

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GOES-12 visible channel images (above; Java animation) revealed an undular bore that was propagating southeastward across the Gulf Coast region of the US on 11 April 2007. This feature was aligned along a pre-frontal trough axis that was located ahead of an advancing cold front. A closer view of the southwestern end of the bore (below; Java animation) shows the fine wave cloud structure as the feature moved over the Gulf of Mexico (the wave train contained as many as 15-20 individual, narrow “roll cloud” bands). Rawinsonde data from Corpus Christi and Brownsville Texas showed that a very pronounced temperature inversion was present in the 1200-2300 ft (360-700 m) layer above the surface, which likely acted as a ducting mechanism for the bore; GOES-12 10.7µm InfraRed (IR) imagery exhibited fairly warm (+12 to +16 C) brightness temperatures in the area of the bore clouds, suggesting that those cloud features were at or below the top of the inversion. Note how the northward-moving marine layer cloud features that were present offshore appeared to dissipate with the passage of the undular bore. A similar case was seen in that same general region on GOES-10 rapid-scan imagery on 19 March 1998.

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MODIS true color imagery (above) revealed a wide swath of fresh snow cover across much of the northcentral US on 04 April 2007. Snowfall during the 03-04 April period included 9 inches in North Dakota, 11 inches in South Dakota, 13 inches in Minnesota (the 12.1 inches that fell in Duluth was the highest daily snowfall for the month of April), 18 inches in Wisconsin, and 25 inches in Michigan. The deep snow cover helped minimum temperatures drop to -3º F at Beulah in North Dakota and +2º F at Roscoe in South Dakota. On the corresponding MODIS false color image (below), snow cover (and ice crystal clouds) appear as varying shades of red, while supercooled water droplet clouds appear as shades of white (bare ground appears as cyan colors). A Java applet allows you to interactively fade between these 2 MODIS images.

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A lee-side frontal gravity wave was seen on GOES-12 6.5µm “water vapor” imagery (above; Java animation) — this feature was associated with a cold frontal boundary that was moving southward across eastern New Mexico and western Texas on 03 April 2007. These types of vertically-propagating gravity waves are stationary with respect to the flow (they follow the cold front) and are forced by nongeostrophic and nonhydrostatic accelerations in the frontal zone (these waves above frontal surfaces are dynamically equivalent to stationary “trapped lee waves” that appear over mountains). Wave breaking induces narrow zones of subsidence that can be sensed as warm/dry bands on the water vapor imagery. Note that the “wave signature” was not readily apparent on the 8-km resolution GOES-11 water vapor channel image; the gravity wave packet eventually became less obvious on GOES-12 4-km resolution water vapor imagery after about 17 UTC, but was still evident on 1-km resolution MODIS water vapor channel imagery at that time. A comparison of GOES-12 visible and water vapor imagery indicates that no clouds were forming along the cold frontal zone in the New Mexico / Texas border region; however, the vertically-propagating gravity waves along the far western end of the front appeared to generate cirrus cloud patches in the foothills of New Mexico.
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AWIPS images of NOAA wind profiler data (below) show that the depth of the cold air was fairly shallow, with winds shifting to northeasterly within the lowest 1 km of the atmosphere at Tucumcari, New Mexico (TCUN5) and Jayon, Texas (JTNT2) as the front moved southward through those locations. Even though the GOES-12 water vapor channel weighting function (calculated using Amarillo, Texas rawinsonde data) peaked at around 450 hPa (about 6.5 km above the surface), a signal of the vertically-propagating wave breaking above the top of the cold front was still evident on the water vapor channel imagery.

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