Some interesting cloud features were seen in the western North Atlantic Ocean on 26 February 2008, about 300 miles southeast of New England (and about 300 miles south of Nova Scotia, Canada): a narrow line of cumulus clouds (oriented approximately north-south), and a patch of stratus cloud (or possibly fog?) that developed just east of the cumulus line. These cloud features were apparently related to very sharp gradients in the Sea Surface Temperature (SST) over that particular area.

AWIPS images of the GOES-12 10.7µm IR channel (above) showed that there were large cold eddies (darker green enhancement) located over the western North Atlantic, along the north wall of the Gulf Stream (RTG_SST analysis). These cold water eddies remained stationary as patches of colder clouds (cyan to blue enhancement) drifted eastward across the region.

An 8-day composite of the MODIS Sea Surface Temperature product (above) confirmed the presence of a very sharp SST gradient in that area (near 40º N latitude, 65º W longitude), with a patch of water having SST values in the 40s F (cyan to blue enhancement) located just to the east of water having SST values in the 60s F (green enhancements) — black features on the SST image are clouds. Low-level baroclinicity enhanced by the sharp SST gradient was aiding the development and maintenance of the cumulus line; as warm air moving eastward encountered the colder pocket of water, stratus cloud quickly formed as the marine boundary layer air was rapidly cooled to its dew point.

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With a dome of high pressure centered over the southeastern US early in the day on 25 February 2008, the Gulf Coast of Texas began to experience a southeasterly onshore flow during the pre-dawn hours. AWIPS images of the GOES-12 10.7µm IR channel (above) revealed a subtle signature of slightly warmer cloud top temperatures (darker gray enhancements) associated with the “return flow” of fog and stratus as moisture over the Gulf of Mexico began to move inland across Texas.

The GOES-12 fog/stratus product (above) was better able to detect the inland progression of the leading edge of the fog/stratus features (darker yellow to orange enhancements), as well as the development of separate areas of radiation fog further inland (lighter yellow enhancement).

A new satellite product that has recently been added to AWIPS (beginning with Operational Build 8.2) is the GOES Low Cloud Base product (above), which provides an indicator of whether the base (or bottom) of a cloud/fog feature meets the aviation criteria of Instrument Flight Rules (IFR) with bases less than 1000 feet above ground level (red enhancement), or Marginal Visual Flight Rules (MVFR) with bases greater than 1000 feet but less than 3000 feet above ground level (green enhancement), or cirrus cloud (blue enhancement). Note that the GOES Low Cloud Base product is only valid during night-time hours; this is also true of the GOES fog/stratus product (due to the fact the the 3.9µm shortwave IR channel used for those satellite products is very sensitive to reflected solar radiation during daylight hours).

A comparison of the 4-km resolution GOES-12 IR image, fog/stratus product, and low cloud base product with the 1-km resolution MODIS fog/stratus product (above) shows the advantage of better spatial resolution for detecting the leading edge of the inland-moving fog/stratus features, and also for estimating what portions of the areas of fog/stratus might be vertically deeper (denoted by the darker orange to red enhancements). The spatial resolution of the IR channels on the Advanced Baseline Imager (ABI) instrument aboard the GOES-R satellite (planned to be launched in 2014) will be 2 km, which will provide improved detection of mesoscale features compared to the 4 km IR channels and products now available from the current generation of GOES imagers. And what about the Sounder instrument aboard GOES-R and beyond? We refer you to the VISIT Meteorological Interpretation Blog for a discussion of that particular topic…

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GOES-12 visible channel imagery (above) displayed several different lake-effect snow (LES) producing mechanisms across the Great Lakes on 20 February 2008: multiple LES bands over Lake Superior, meso-vorticies over both Lake Michigan and Lake Huron, and a single LES band over Lake Ontario. Even though the Great Lakes water temperatures were getting quite cold (generally around 32-40ºF), a very cold arctic air mass (overnight minimum temperatures on 20 February were as cold as -33ºF at Grand Forks, North Dakota and Embarrass, Minnesota; -30ºF at Upson, Wisconsin; -28ºF at Stammbaugh, Michigan) was spreading across the region (GOES-12 IR image + surface reports) creating a large water-air temperature difference.

So what about Lake Erie? Since Lake Erie is the most shallow of the five Great Lakes, it often freezes the earliest; due to a large concentration of ice over that particular lake (as seen on SSEC MODIS Today true color imagery from 4 days earlier, viewed using Google Earth, below), Lake Erie was not able to contribute the necessary heat and moisture flux needed to produce LES mechanisms on the scale that the other lakes were producing on this day.

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AWIPS images of the MODIS visible channel, the 1.6µm near-IR “snow/ice channel”, and the 11.0µm “IR window” channel (above) revealed a large patch of supercooled water droplet stratus cloud (and/or fog) over northeastern Utah on 19 February 2008. This stratus/fog cloud feature was confined to the lower elevations (AWIPS topography image | Johns Hopkins topography image) where several river valleys converge just south of Vernal, Utah (station identifier KVEL); at the time of the images, the surface visibility at Vernal was restricted to 1 mile with fog. The areal extent of this cloud feature was difficult to judge using either the 1-km resolution MODIS visible image (due to the bright white appearance of both the stratus/fog and the surrounding non-cloudy snow cover) or the 1-km resolution MODIS IR window image (due to the light gray enhancement of both the cloud feature and the cold air that had collected within the remainder of the lower elevations) — but on the snow/ice image, the contrast of the brighter gray-to-white enhancement of the supercooled water droplet stratus/fog feature really stood out against the dark appearance of the surrounding snow-covered terrain.

The corresponding 250-meter resolution MODIS true color image from the SSEC MODIS Today site (below, viewed using Google Earth) showed the “translucent” nature of the patch of stratus/fog — note that you can actually see the outlines of some of the converging rivers (the Green River, White River, and Duchesne River) through the optically-thin cloud feature.

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