Wildfires in Texas

February 27th, 2008

GOES-12 3.9µm IR images (Animated GIF)

Several large wildfires began to burn across parts of western Texas (and also southeastern New Mexico) late in the day on 25 February 2008, as seen by the large number of fire “hot spots” (black to red pixels, with red being the hottest temperatures) on GOES-12 3.9 µm “shortwave IR” images (above). Strong northerly winds — gusting as high as 67 mph (30 m s-1) at Guadalupe Pass, 51 mph (23 m s-1) at Midland, and 45 mph (20 m s-1) at San Angelo — helped the grass fires quickly grow out of control and spread rapidly southward; this extreme fire growth was evident by the fast rate of increase in areal coverage of “hot” black-to-red pixels on the shortwave IR imagery. Very warm surface air temperatures were also noted across Texas on that day, with a high of 93º F (34º C) at San Angelo and a high of 99ºF (37º C) at Del Rio (which tied the all-time record high for the month of February at that location).

MODIS + GOES-12 shortwave IR images

A comparison of the 1-km resolution MODIS 3.7 µm IR image with the corresponding 4-km resolution GOES-12 3.9 µm IR image (above) as the fire intensity and coverage was beginning to diminish around 05:00 UTC or Midnight local time (near the end of the GOES-12 shortwave IR animation shown at the beginning of this post) demonstrates the superior fire detection capability offered by the improvement in spatial resolution on the MODIS instrument. The largest of the active fires was located to the west/northwest of San Angelo (station identifier KSJT), and hot pixels associated with this large fire (black to red enhancement) could be seen across parts of several different counties.

MODIS true color image (Google Earth)

An extensive burn scar resulting from the largest fire (the “Glass Fire”, which was located between San Angelo and Midland) was apparent on MODIS true color imagery (viewed using Google Earth, above) two days later (on 27 February 2008). In fact, one small fire was still burning along the southeastern periphery of the burn scar, with a subtle smoke plume seen drifting northeastward toward Highway 87. The Glass Fire burned over 219,000 acres in Sterling county alone (InciWeb).

Cloud features related to sharp sea surface temperature (SST) gradients

February 26th, 2008

GOES-12 visible images (Animated GIF)

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.

GOES-12 10.7µm IR images (Animated GIF)

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.

MODIS sea surface temperature (SST) product

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.

“Return flow” of moisture from the Gulf of Mexico

February 25th, 2008

GOES IR images (Animated GIF)

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.

GOES fog/stratus product (Animated GIF)

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).

GOES Low Cloud Base product (Animated GIF)

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).

GOES + MODIS images (Animated GIF)

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…

The Great Lakes: a diversity of lake-effect snow mechanisms

February 20th, 2008

GOES-12 visible images (Animated GIF)

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.

MODIS true color image (Google Earth)