Blowing dust in Texas

January 29th, 2008

GOES-12 6.5µm water vapor images (Animated GIF)

A very cold arctic air mass was in place across parts of the Canadian Prairies, with many sites in Alberta reporting surface temperatures colder than -40º F (-40º C) on the morning of 29 January 2008; the leading edge of this arctic air was surging rapidly southward across the Great Plains of the US as a strong cold frontal boundary. AWIPS images of the GOES-12 6.5µm “water vapor channel” (above) actually showed a signature of a gravity wave along the leading edge of the cold front as it moved southward across Nebraska and Kansas into Texas and Oklahoma. This lee-side cold frontal gravity wave feature could be detected on the water vapor channel imagery due to the fact that the airmass ahead of the front was quite dry, which shifted the altitude of the GOES-12 water vapor channel weighting function to much lower altitudes compared to what would be seen in a more typical airmass.

Strong pre-frontal and post-frontal winds were responsible for creating a large area of blowing dust across Texas during the afternoon hours, as seen on consecutive Terra (18:15 UTC, or 12:15 pm local time) and Aqua (19:50 UTC, or 1:50 pm local time) MODIS true color images from the SSEC MODIS Today site (below). This blowing dust reduced surface visibility to as low as 2 miles at San Angelo, Texas.

MODIS true color images (Animated GIF)

The windy conditions (several wind gusts in Texas were in excess of 60 mph) and very dry air were also creating an environment favorable for small wildfires; note that a number of fire “hot spots” (black pixels) could be seen on an AWIPS image of the MODIS 3.7µm shortwave IR channel (below).

MODIS 3.7µm IR image

The SSEC MODIS Today true color image from the Aqua satellite displayed using Google Earth (below) showed exactly which counties and highways in Texas were being impacted by the large cloud of blowing dust. In addition, a smoke plume from small wildfire that was burning just southeast of Abilene, Texas could be seen streaming south/southeastward.

MODIS true color image (Google Earth)

Cold night-time temperatures in the Upper Midwest

January 24th, 2008

GOES-12 10.7µm IR images (Animated GIF)

AWIPS images of the 4-km resolution GOES-12 10.7µm IR channel (above) showed that the land surfaces across the Upper Midwest region exhibited very cold brightness temperature values (darker blue enhancement) during the pre-dawn hours on 24 January 2008 — GOES IR brightness temperatures were as cold as -36ºC in northern Wisconsin, and -38ºC in northern Minnesota. Most of the area seen in the images above was cloud-free (except for the lake-effect cloud bands downwind of Lake Superior and Lake Michigan); the cloud-free surfaces could then be seen warming very quickly after sunrise. Much of the northcentral US had a deep snow cover of at least 5-10 inches, with snow depths of 15-30 inches common in northern Minnesota, Wisconsin, and Michigan; this deep snow cover (along with cloud-free skies and light winds) allowed for very strong radiational cooling of the air near the surface.

A higher-resolution (1-km) view using the MODIS 11.0µm IR channel and the MODIS fog/stratus product (below) revealed an amazing amount of structure in the surface brightness temperature field over the region (much of which was driven by terrain, with cold air drainage into low-lying areas such as river valleys); the coldest MODIS IR brightness temperatures sampled on the AWIPS images were -38ºC in northern Wisconsin and -42ºC in northern Minnesota. The MODIS fog/stratus product (created by computing the 11.0µm – 3.7µm brightness temperature difference) confirmed that there were no areas of fog or stratus cloud contributing to the interesting IR temperature structure seen across much of the Upper Midwest region around 08:20 UTC (2:20 AM local time). Note the appearance of “urban heat islands” (warmer IR temperatures, cyan enhancement) around cities such as Minneapolis, Minnesota and Sioux Falls, South Dakota.

MODIS IR + fog/stratus product (Animated GIF)

NWS cooperative observer overnight minimum temperatures were as cold as -31ºF (-35ºC) at Sparta, Wisconsin and -39ºF (-39ºC) at Embarrass, Minnesota. There was also a reported minimum temperature of -51ºF (-46ºC) at a Minnesota Department of Transportation site northwest of Duluth (below), but the temperature data from that particular site appeared to be suspect.

Minnesota mesonet minimum temperatures

Such cold temperatures aided in the formation of ice along the near-shore waters of western Lake Superior, as seen the following afternoon in a 250m-resolution MODIS true color image from the MODIS Today site (below). Lake Superior is the largest and deepest of the Great Lakes, and is usually the last to experience significant ice formation during the winter months.

MODIS true color image

Swath of snow cover in the Deep South

January 20th, 2008

MODIS Today true color image

A Terra MODIS true color image from the SSEC MODIS Today site (above) revealed a swath of fresh snow cover across parts of Mississippi, Alabama, and Georgia on 20 January 2008. Snowfall amounts in the state of Alabama (which fell on the previous day) were as high as 5.8 inches near Verbena, 5.0 inches at Orrville, and 4.0 inches at Toxley (located on the MODIS imagery using Google Earth, below); snowfall up to 3.0 inches was reported in Mississippi, with 1.5 inches falling in Georgia.

MODIS imagery in Google Earth (Animated GIF)

AWIPS images of the MODIS visible channel, the 1.6µm “snow/ice channel”, and the Land Surface Temperature product (below) confirmed that this feature was indeed snow cover (snow is a strong absorber at the 1.6µm wavelength, and appears darker on the “snow/ice channel” image); in addition, the land surface temperatures within the area of snow cover were generally several degrees F colder (upper 20s to low 30s F, darker green enhancement) compared to the surrounding areas with bare ground (where land surface temperatures were generally in the mid 30s to around 40 F).

MODIS visible + snow/ice + land surface temperature images (Animated GIF)

Ice forming in Lake Michigan

January 20th, 2008

MODIS true color image

The coldest arctic air of the 2007/2008 winter season (so far) settled in over the Great Lakes region on 19-20 January 2008. Most reporting stations in Wisconsin experienced a daytime maximum temperature below 0ºF on 19 January, with the coldest overnight minimum temperature on 20 January of -34ºF at Nekoosa in central Wisconsin. As this cold air streamed eastward across Lake Michigan, ice began to form along parts of the western and southern nearshore waters as seen on the MODIS true color image (above) from the SSEC MODIS Today site. Also note that the four larger lakes in the Madison area (located toward the upper left corner of the image) had all frozen solid again — they had all frozen completely by late December, but then the largest of Madison’s lakes (Lake Mendota) began to partially open during a brief warm period in early January 2008.

In a comparison of AWIPS images of the MODIS visible and 1.6µm “snow/ice channel” (below), the lake ice (and adjacent snow-covered land surfaces) exhibited a darker signal on the snow/ice image, in contrast to the brighter signal exhibited by the supercooled water droplet lake-effect snow cloud bands that covered much of the central and eastern portion of Lake Michigan.

MODIS visible + snow/ice images (Animated GIF)