Using satellite imagery to help diagnose areas of aircraft icing potential

October 17th, 2008 |
GOES-12 10.7 µm IR images

GOES-12 10.7 µm IR images

AWIPS images of the GOES-12 10.7 µm “IR window” channel (above) showed extensive cloudiness associated with a shortwave trough that was moving eastward across the Upper Midwest region during the morning hours on 17 October 2008. IR cloud top brightness temperatures of -20º to -40º C (blue to green colors) corresponded to areas of light precipitation (radar mosaic). Also shown on the IR imagery were  aircraft Icing AIRMET advisories (outlined in red) that were issued at 09:00 UTC and 14:00 UTC –  the 14:00 UTC AIRMET was forecasting moderate icing between the freezing level (which was between 5,000 and 10,000 feet) and 20,000 feet. In fact, there were a number of aircraft pilot reports of icing (plotted in yellow) within the boundaries of these Icing AIRMETs.

A closer view using AWIPS images of the MODIS visible channel, 11.0 µm “IR window” channel, Cloud Top Temperature (CTT) product, and Cloud Phase product at 17:29 UTC (below) indicated that much of the cloud shield along the trailing (western) edge of the shortwave over Minnesota and Iowa exhibited cloud top temperatures that were below freezing (generally in the -5 to -12º C range), but the MODIS Cloud Phase product designated those trailing edge clouds as “Water droplet” clouds (blue enhancement). Within this area of supercooled water droplet clouds were several pilot reports of icing at the 8000-foot altitude  across southern Minnesota and western/central Iowa.

MODIS visible + IR window + cloud top temperature + cloud phase

MODIS visible + IR window + Cloud Top Temperature + Cloud Phase

A comparison of AWIPS images of the MODIS Cloud Phase product and the GOES-12 sounder Cloud Top Height product (below) showed that these pilot reports of icing at the 8000 foot level were well below the tops of the clouds, which were generally in the 13,000-15,000 foot range (green colors on the Cloud Top Height product).

MODIS Cloud Phase product + GOES Sounder Cloud Top Height product

MODIS Cloud Phase product + GOES-12 sounder Cloud Top Height product

Record October snowstorm across Wyoming and Montana

October 13th, 2008 |
AWIPS images of GOES-11 + GOES-12 water vapor channel

GOES-11 + GOES-12 water vapor channel images

AWIPS images of the GOES-11 + GOES-12 water vapor channels (above) showed the large size of a major snowstorm that impacted much of the western US during the 10 October12 October 2008 time frame. This storm dropped as much as 48.8 inches of snow at Cole Creek, Montana and 29.7 inches at Lander, Wyoming (their greatest October snowfall on record), and Glasgow, Montana received 12.8 inches of snow on 12 October (their heaviest snowfall for any calendar day during the month of October). At Billings, Montana the snow depth of 9 inches on 13 October was the earliest date on record for a 9-inch snow depth.

AWIPS images of the MODIS visible channel and the 1.6 µm near-IR “snow/ice” channel (below) displayed the areal coverage of snow on the ground across much of Wyoming, eastern Montana, far northwestern South Dakota (including the higher elevations of the Black Hills) and western North Dakota, stretching northeastward into portions of Saskatchewan and Manitoba on 13 October. Snow cover exhibits a darker signal on snow/ice channel imagery (in contrast to supercooled water droplet clouds, which appear as lighter shades of gray).

AWIPS images of MODIS visible and snow/ice channels

MODIS visible and snow/ice channel images

An AWIPS image of the MODIS Land Surface Temperature (LST) product (below) showed the effect of the deep snow cover on surface temperatures across much of Wyoming and Montana on 13 October — LST values within the snow swath were only in the upper 20s to low 30s F (green colors), compared to the 60s and 70s F (orange colors) north of the deep snow cover. The daily maximum temperature on 13 October at Glasgow, Montana was only 37º F (which tied the record for coldest maximum temperature for that date), while the daily high temperature of 28º F at Wisdom, Montana was the earliest date for such a cold maximum temperature.

AWIPS image of MODIS Land Surface Temperature product

MODIS Land Surface Temperature product

A 250-meter resolution MODIS true color image (below, viewed using Google Earth) displayed the extent of the snow cover across parts of Wyoming and Montana. Note that there were a few snow-free patches located within the broad swath of snow cover (a result of the complex terrain across the region).

MODIS true color image (viewed using Google Earth)

MODIS true color image (viewed using Google Earth)

Mountain waves in Colorado and New Mexico

October 10th, 2008 |
GOES-11, GOES-12, GOES-13 water vapor images

GOES-11, GOES-12, GOES-13 water vapor images

Strong southwesterly winds over the southern and central Rocky Mountains led to the development of mountain waves downwind of the higher terrain in Colorado and New Mexico on 10 October 2008. A distinct mountain wave signature was seen on 6.7 µm water vapor channel imagery from GOES-11 and 6.5 µm water vapor channel imagery from GOES-13 and GOES-12 (above). Note the improved appearance of the mountain waves on the 4-km resolution GOES-13 and GOES-12 imagery (compared to the 8-km resolution GOES-11 imagery). These mountain waves were also easier to identify and track using GOES-13, due to the more direct viewing angle of GOES-13 (positioned at 105º W longitude) and also the fact that GOES-13 was able to operate and supply imagery through the eclipse period (when the satellite was in the Earth’s shadow, and the solar panels could not supply power to the spacecraft).

AWIPS images of GOES-12 and MODIS water vapor channel data

AWIPS images of GOES-12 and MODIS water vapor channel data

AWIPS images of the GOES-12 and MODIS water vapor channel data (above) showed further improvement in mountain wave identification using 1-km resolution data from MODIS. Mountain waves on water vapor imagery have long been recognized as a signature of potential clear air turbulence, and there were indeed a few pilot reports of light to moderate turbulence over eastern Colorado and New Mexico (below).

AWIPS image of GOES-12 water vapor channel data + pilot reports

AWIPS image of GOES-12 water vapor channel data + pilot reports

Of large eyes and midget tropical cyclones

October 8th, 2008 |
GOES-13 visible images (Hurricane Norbert)

GOES-13 visible images (Hurricane Norbert)

GOES-13 visible imagery (above) revealed the rather large eye of Hurricane Norbert on 08 October 2008 — Norbert was a Category 4 storm at that time, and the eye appeared to be about 30-35 nautical miles in diameter.  Hurricane Norbert underwent a period of rapid intensification during the pre-dawn hours on 08 October, which was clearly seen on a plot of the Advanced Dvorak Technique intensity estimate (below) from the CIMSS Tropical Cyclones site.

CIMSS Advanced Dvorak Technique

CIMSS Advanced Dvorak Technique

To illustrate the diversity of tropical cyclone spatial scales, it is interesting to compare the large eye of Hurricane Norbert (in the eastern Pacific Ocean) with the small cluster of deep convection around the core of Tropical Storm Marco (2 days earlier in the Gulf of Mexico) — Marco is not much larger than the eye of Norbert! The visible and IR images shown below are from the same satellite (GOES-13) at the same time of day (20:00 UTC), displayed with the same magnification (zoomed in to an effective 0.5 km resolution). According to the National Hurricane Center discussions, Tropical Storm Marco may have been one of the smallest tropical cyclones on record, with tropical storm force winds only extending about 10 nautical miles away from the center.

GOES-13 visible images (Hurricane Norbert and Tropical Storm Marco)

GOES-13 visible images (Hurricane Norbert and Tropical Storm Marco)

GOES-13 IR images (Hurricane Norbert and Tropical Storm Marco)

GOES-13 IR images (Hurricane Norbert and Tropical Storm Marco)