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

March 15th, 2012 |
MODIS true-color Red/Green/Blue (RGB) images

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

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

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

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

Fog and Low Cloud Detection

March 14th, 2012 |
GOES-13

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.

MODIS

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.

MODIS

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)

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

Possible Sting Jet in Upper Midwest

March 12th, 2012 |
GOES-13 6.5 µm water vapor channel images (click image to play animation)

GOES-13 6.5 µm water vapor channel images (click image to play animation)

The water vapor animation from GOES-East on March 12th shows a structure rotating through the upper-level trough, which structure looks very much like a so-called “Sting Jet”. (In the animation above, the sting jet structure crosses the Missouri/Kansas border south of Kansas City, propagates across northern Missouri and eastern Iowa before moving northward into Wisconsin). (A more obvious Sting Jet event is discussed here; A Monthly Weather Review article on Sting Jets is here).

RUC wind analyses show that the sting jet structure was associated with a wind maximum on the 315 Kelvin isentropic surface. This Loop shows the maximum moving from northeastern Missouri into Central Wisconsin between 1000 and 1400 UTC on March 12th. Stability in the lower troposphere on March 12th (as suggested by this sounding from the Quad Cities in Iowa/Illinois) was strong enough to inhibit vertical mixing of stronger upper-tropospheric air down towards the surface. The circulation around the jet was sufficient, however, to generate showers over the upper Midwest, as shown in this loop.

MODIS 6.5 µm water vapor channel image

MODIS 6.5 µm water vapor channel image

MODIS water vapor imagery, above, from 0841 UTC on 12 March shows the sting jet structure in north-central Missouri, and curving back to central Nebraska and central South Dakota.

GOES-13 0.63 µm visible image

GOES-13 0.63 µm visible image

(Added 13 March: SPC Storm Reports show a rare March tornado north of I-69 in lower Michigan. The visible imagery above, bracketing the observed time of the tornado (near the yellow box), shows a strong thunderstorm. By this time, the possible sting jet has rotated northward into western Ontario, so its influence on the environment in Michigan would be secondary. The sounding from DTX at 2300 UTC shows a favorable low-level wind profile.)

Ice in the Bering Sea

March 11th, 2012 |
POES AVHRR 0.86 µm visible channel images

POES AVHRR 0.86 µm visible channel images

A sequence of AWIPS images of POES AVHRR 0.86 µm visible channel data (above) showed the areal extent of ice in the Bering Sea during the 09 March – 11 March 2012 period. The far southern edge of the ice could be seen moving a bit further to the south during this time.

The corresponding Sea Ice Analaysis chart issued by the Ice Desk at the Anchorage, Alaska National Weather Service forecast office (below) provided a more detailed analysis of the age and thickness of various portions of this ice in the Bering Sea.

National Weather Service Sea Ice Analysis chart

National Weather Service Sea Ice Analysis chart