Fog detection « CIMSS Satellite Blog

Undular bore along the leading edge of a “pneumonia front” over Lake Michigan

March 15th, 2012

MODIS true-color Red/Green/Blue (RGB) images

250-meter resolution Terra and Aqua MODIS true-color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (above) revealed the presence of an undular bore along the leading edge of a southward-moving cold front (also known regionally as a “pneumonia front“) on 15 March 2012.

AWIPS images of the MODIS Sea Surface Temperature (SST) product (below) indicated that SST values in the central portion of Lake Michigan (behind the cold frontal boundary) were generally in the upper 30s to low 40s F (darker blue color enhancement). At Racine in southeastern Wisconsin (station identifier KRAC), the surface air temperature dropped from 69 F to 47 F in 2 hours after the front passed. Just to the north, the high temperature of 72 F at Milwaukee (station identifier KMKE) set a record high for the date — then the temperature there dropped into the upper 40s F a few hours later.

MODIS Sea Surface Temperature product + Surface frontal analysis

During the previous night-time hours, the southerly to southwesterly flow of unseasonably warm air (with dew points in the 50s F) over the cold waters led to widespread lake fog across almost all of Lake Michigan, as was seen in a 03:22 UTC MODIS fog/stratus product (below).

MODIS fog/stratus product

A few hours later (at 07:33 UTC), isolated convective rain showers were moving across the southern half of Lake Michigan — these showed up at the darker gray to black enhanced features on the MODIS fog/stratus product image (below). The corresponding MODIS Instrument Flight Rules (IFR) and Low Instrument Flight Rules (LIFR) Probability products indicated a number of areas with IFR and/or LIFR probabilities in excess of 80-90% (darker red color enhancement).

MODIS fog/stratus product + IFR Probability product + LIFR Probability product

Posted in Aviation, Fog detection, General interpretation, MODIS, Red/Green/Blue (RGB) images | No Comments »

Fog and Low Cloud Detection

March 14th, 2012

Regular readers of this blog will be quick to recognize the GOES imagery above as a low cloud detection product that exploits the differences in emissivity properties for water droplets that exist between 3.9 and 10.7 µm, two radiation bands that are detected on the GOES imager. The emissivity differences mean that 10.7 µm brightness temperatures will be warmer than 3.9 µm brightness temperatures, so a difference field will highlight where low clouds exist. This can be done with GOES imagery, above, or with MODIS imagery, below. Note that the existence of the low cloud may or may not suggest fog: only the top of the cloud is detected; whether or not the cloud rests on the ground cannot be determined easily from satellite.

Both the GOES and MODIS imagery show slow expansion to the low cloud field over the 3 hours, as might be expected given slow cooling at night. However, careful inspection reveals a variety of regions that show obstructions to visibility but no indication of fog or low clouds. For example, Montgomery (KMGM), Mobile (KMOB) and Tuscaloosa (KTCL) all show fog observations in the absence of a clear signal of detected low cloud. Similar observations occur over eastern Georgia and the Carolinas.

New low-cloud detection algorithms that incorporate model information (from the RUC, or, in the near future, the Rapid Refresh) can quantitatively describe the evolution of the low cloud and fog field. These algorithms were initially developed (and trained) using GOES data and are distributed to the AWIPS environment here at CIMSS. The loop above shows probabilities of Low IFR visibilites over the deep south during the morning. (Click for the 0400 UTC and 0700 UTC examples). Because the GOES-R product synthesizes model and satellite data together, better fog/low cloud detection occurs in regions where high clouds make low cloud detection difficult. Note how the probability of a visibility obstruction increases during the course of the night, and how places that develop fog are among the first to see a LIFR signal.

The visible imagery from GOES-13, below, shows the characteristic erosion of the low clouds, from outside in, during the course of the subsequent day. As usually happens, cumulus development is suppressed in regions where low clouds persist during the morning.

GOES-13 Visible (0.63 µm) (click image to play animation)

Posted in Fog detection, General interpretation, GOES-13, GOES-R | No Comments »

Fog and low clouds over the Ohio River Valley region

February 17th, 2012

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

AWIPS images of the GOES-13 fog/stratus product (above; click image to play animation) did not do a particularly good job of depicting a well-defined signal of the fog/stratus that was in place across parts of the Ohio River Valley region on 17 February 2012. Note that a number of stations were reporting night-time fog and/or freezing fog in parts of far eastern Missouri, southern Illinois and Indiana, and western Kentucky.

4-km resolution GOES-13 and 1-km resolution MODIS fog/stratus product

Even with the improved 1-km resolution of the MODIS fog/stratus product, no good signal was displayed at 03:40 UTC (above) or at 07:52 UTC (below) over the areas that were reporting fog/freezing fog.

4-km resolution GOES-13 and 1-km resolution MODIS fog/stratus product

As part of CIMSS participation in GOES-R Proving Ground activities, new satellite products are being developed and tested — and one such product is an Instrument Flight Rules (IFR) Probability product (below; click image to play animation). This IFR Probability product blends satellite data and model fields to display regions where the cloud ceiling is likely to be between 500 and 1000 feet. The IFR Probability product did a much better job at highlighting the relatively large area where IFR cloud ceiling conditions were present at a number of reporting stations (some of which were experiencing cloud ceilings of 100 to 200 feet, along with freezing fog limiting the visibility to 0.3 miles at times) — for example: Effingham IL (K1H2) / Mount Vernon IL (KMVN) / Harrisburg IL (KHSB) / Lawrenceville IL (KLWV) / Bloomington IN (KBMG) / Cape Girardeau MO (KGCI) / Pahducah KY (KPAH).

GOES-13 IFR Probability product (click image to play animation)

GOES-13 0.63 µm visible channel images (below; click image to play animation) showed the areas of fog/stratus burning off quickly after sunrise — and the fog/stratus appeared to be relatively shallow in nature. The fact that the fog/stratus did not appear to be very thick may have been a factor that limited detection by the traditional GOES-13 (10.7-3.9 µm) and MODIS (11-3.7 µm) fog/stratus products.