General interpretation « CIMSS Satellite Blog

Freezing fog in the Upper Midwest region

February 6th, 2012

GOES-13 fog/stratus product (click image to play animation)

AWIPS images of the 4-km resolution GOES-13 10.7 µm – 3.9 µm “fog/stratus product” (above; click image to play animation) showed a large area of fog and/or stratus (yellow to orange color enhancement) that was increasing in areal coverage during the pre-dawn hours on 06 February 2012. Although the fog/stratus product is useful for locating the presence and temporal trends of such features, it does not offer any reliable indication of whether it is fog on the ground or stratus cloud aloft.

One product that attempts to give the forecaster some quantitative information is the GOES Low CLoud Base (LCB) prodcut (below; click image to play animation), which attempts to blend surface observations with satellite data to indicate whether the cloud base is above or below the threshold of 1000 feet.

GOES-13 Low Cloud Base product (click image o play animation)

With 1-km resolution data, the MODIS instrument aboard the polar-orbiting Terra and Aqua satellites offers a similar “fog/stratus product” (below) that provides better clarity, especially regarding the exact location of the edges of the fog and/or stratus.

MODIS fog/stratus product images

In this particular case, a number of locations beneath the western and southern edge of the fog/stratus feature were expereincing freezing fog (below) and visibilities of 1/4 mile or less, which was creating hazardous road conditions and prompting the issuance of Freezing Fog Advisories.

MODIS fog/stratus product with METAR surface reports

As part of CIMSS participation in GOES-R Proving Ground activities, products are being developed which can provide more quantitative information about such parameters as Fog Depth and the Probability of Marginal Visual Flight Rules (MVFR) or Instrument Flight Rules (IFR) conditions (below). In this case, across the southwestern part of Iowa (where widespread freezing fog was being reported), the fog depth was as high as 1400-1500 feet, with probabilities of MVFR and IFR conditions as high as 75-90% and 60-75%, respectively.

MODIS Fog Depth, MVFR Probability, and IFR Probability products

Shortly after sunrise, it is interesting to note that a comparison of 1-km resolution POES AVHRR 0.63 µm visible channel, 3.74 µm “shortwave IR” channel, and 10.8 µm channel “IR window” channel images (below) revealed that part of the swath of fresh snow cover (as deep as 4-6 inches) across western Iowa could be seen through the translucent western edge of the fog/stratus deck that was beginning to burn off during the morning hours. The fog/stratus deck appears warmer (darker gray enhancement) om the 3.74 µm image, due to the sensitivity of that channel to the reflection of solar radiation off the tops of supercooled water droplet clouds.

Farther to the south, note the presence of narrow fingers of valley fog in the Ozark Mountains and surrounding regions in Oklahoma, Arkansas, and Missouri.

POES AVHRR 0.63 µm visible, 3.74 µm "shortwave IR", and 10.8 µm "IR window" images

Posted in AVHRR, Fog detection, General interpretation, GOES-13, GOES-R, MODIS, POES, Winter weather | No Comments »

Unusually large and persistent dry region aloft over the central Pacific Ocean

January 31st, 2012

GOES-15 full disk 6.5 µm water vapor channel images (click image to play animation)

An animation of McIDAS images of GOES-15 (GOES-West) 6.5 µm water vapor channel data from 27 January to 31 January 2012 (above; click image to play animation) revealed the presence of an unusually large and persistent warm/dry region over the central Pacific Ocean (centered southwest of Hawaii). Water vapor brightness temperatures as warm as +3.5 C (darker orange color enhancement) were seen — such warm values are rarely seen on water vapor imagery.

AWIPS images of the MIMIC Total Precipitable Water (TPW) product (below; click image to play animation) showed a number of areas exhibiting TPW values as low as 15 mm (0.6 inch) within the large dry feature seen on GOES-15 water vapor images. At 00 UTC on 30 January, the Hilo, Hawaii rawinsonde data indicated a TPW value of 0.93 inch, with very dry air above the 800 hPa pressure level. The strong subsidence aloft was acting to prevent significant rainfall over the Hawaiian Islands, prompting a Red Flag Warning to be issued for elevated fire danger during the 29-30 January period. It was also very warm in Hawaii — the highest temperature recorded in the 50 United States was 84 F at Kahului on 28 January, and 85 F at Kalaeloa on 30 January.

MIMIC Total Precipitable Water product (click image to play animation)

A comparison of an AWIPS water vapor image composite with the corresponding MIMIC TPW product (below) offers a good example of the fact that not all features of high TPW are evident on water vapor imagery. In particular, note the plume of higher TPW being drawn northeastward along a frontal boundary west and northwest of Hawaii — this plume crossed through the western portion of the dry feature on water vapor imagery.

Water vapor image + MIMIC TPW product + surface analysis

Posted in General interpretation, GOES-15 | No Comments »

Rain-cooled ground across much of Texas and Oklahoma

January 27th, 2012

1-day observed precipitation for 25 and 26 January 2012

Maps of the 1-day total precipitation for 25 January and 26 January 2012 (above) showed that much of Texas and Oklahoma received several inches of rainfall during that 48 hour period.

AWIPS images of the MODIS 0.65 µm visible channel and the corresponding MODIS Land Surface Temperature (LST) product (below) revealed a large swath of rain-cooled ground across much of that region. LST values where heavy rain fell were in the 60s F (yellow to light orange color enhancement), in contrast to LST values in the 70s and 80s F (darker orange to red color enhancement) to the north and the to the south of the rain-cooled areas.