Ice storm over the south-central US

January 28th, 2009
MODIS visible + snow/ice imagesMODIS visible + snow/ice images

AWIPS images of the MODIS visible channel and the 2.1 µm near-IR “snow/ice discrimination” channel (above) showed the areal coverage of surface snowfall and ice accrual across much of the southern Plains on 28 January 2009. Many locations across that region received a significant accumulation of ice from freezing rain (1-2 inches of ice were reported in parts of Oklahoma and Arkansas) or sleet (3-4 inches of sleet were reported in parts of Oklahoma), downing trees and powerlines and causing power outages for several hundred thousand people (for more details, see Jesse Ferrell’s WeatherMatrix blog). As the storm departed, it left a light accumulation of snow on top of some of the ice and sleet that was already on the ground.

Snow and ice on the ground are strong absorbers at the 2.1 µm wavelength, so those surface features appear darker on the MODIS Snow/Ice image — and since ice is an even stronger absorber, a surface accrual of ice appears even darker than a layer of snow on the ground. In contrast, supercooled water droplet clouds appear as brighter white features on the Snow/Ice image. Also, note that Oklahoma (where there was only 1-2 inches of snow on the ground) appeared significantly darker on the MODIS Snow/Ice image than areas in the far northern portion of the image (where there was 5-10 inches of snow cover). One rather curious feature on the imagery: the thin mesoscale streaks of snow on the ground across northern Missouri, oriented almost exactly west-to-east.

A comparison of the 1-km resolution MODIS Snow/Ice image with the 1-km resolution MODIS Land Surface Temperature product and the 10-km resolution GOES-12 sounder Skin Temperature product (below) revealed the effect that the snow and ice were having on keeping the ground (surface) temperatures down. While the MODIS LST product indicated that the vast majority of Oklahoma was at or below freezing at that time (darker green color enhancements), the corresponding GOES-12 sounder Skin Temperature product suggested that much of the western half of Oklahoma had surface temperatures that were a few degrees above freezing.

MODIS Snow/Ice + MODIS Land Surface Temperature  + GOES-12 sounder Skin Temperature

MODIS Snow/Ice + MODIS Land Surface Temperature + GOES-12 sounder Skin Temperature

A closer view of the MODIS Snow/Ice image and the MODIS Land Surface Temperature product (below) showed a swath of colder LST values (darker green colors) across southwestern, central, and northeastern Oklahoma, roughly corresponding to the areas with a more significant accrual of ice (darker enhancement on the Snow/Ice image). Note from the surface METAR reports that the air temperatures appeared to be a few degrees F colder within the swath of colder MODIS LST values — presumably due to the fact that there was a thicker ice accrual (or ice covered with snow) on the ground in that region.

MODIS Snow/Ice image + Land Surface Temperature product (with surface reports)

MODIS Snow/Ice image + Land Surface Temperature product (with surface reports)

A false color RGB composite image using MODIS channel 01 (visible), channel 06 (near-IR), and channel 31 (IR window) is shown below, which gives another depiction of the coverage of the snow and ice on the ground. The brightest pink areas are those which had the thickest accrual of ice (with a light layer of snow on top of the ice).

MODIS RGB false color image (using channels 01, 06, and 31)

McIDAS MODIS "false color RGB image" (using channels 01, 06, and 31)

False color red/green/blue (RGB) composite images using MODIS channel 01 (visible), channel 06/07 (near-IR), and channel 31 (IR window) generated using McIDAS imagery (above) and AWIPS imagery (below) gives another depiction of the coverage of the snow and ice on the ground. The brightest pink areas in Texas and Oklahoma are those which had the thickest accrual of ice (with a light layer of snow on top of the ice).

MODIS RGB false color image (using channels 01, 07, and 31)

AWIPS MODIS "false color RGB image" (using channels 01, 07, and 31)

With the enhanced graphics capabilities of the next generation of AWIPS (“AWIPS-2″ or “AWIPS Migration”), these kinds of RGB composite images will hopefully be easy to create on a routine basis.

– 30 JANUARY UPDATE –

AWIPS MODIS false color RGB image

AWIPS MODIS "false color RGB image"

Following the storm, sunshine and warmer temperatures began to melt some of the snow cover and ice cover on the ground. An AWIPS MODIS “false color RGB image (above) shows the extent of the ice and snow (pink-colored features) that remained on the ground on 30 January 2009.

Ducted internal gravity waves: another satellite signature of potential turbulence

January 27th, 2009
GOES-13 visible images

GOES-13 visible images

GOES-13 visible channel images (above) displayed a beautiful example of ducted internal gravity wave clouds over parts of Iowa, Wisconsin, Illinois, and Michigan during the daylight hours on 27 January 2009. The main linear “wave train” feature became obscured by a veil of high cirrus clouds later in the day, but other smaller/shorter wave features were seen to the north (over far northeastern Iowa and southern/central Wisconsin).

AWIPS images of the 1-km resolution MODIS visible, 3.7 µm “shortwave IR”, 6.7 µm “water vapor”, and 11.0 µm “IR window” channels (below) revealed the following points: (1) a strong component of solar reflection on the shortwave IR image (brightness temperature values were as warm as +25º to +35º C, darker gray color enhancement) suggested that the cloud billows were composed of supercooled water droplets, (2) the upward/downward gravity wave motions were also evident on the water vapor imagery, and (3) the IR window brightness temperature values were generally in the -20º to -29º C range (cyan to dark blue color enhancement).

MODIS visible + shortwave IR + water vapor + IR window images

MODIS visible + shortwave IR + water vapor + IR window images

The MODIS Cloud Phase and Cloud Top Temperature products (below) supported  the idea of predominantly supercooled water droplet clouds (blue color enhancement), with minimum Cloud Top Temperature values of -22º C along the Wisconsin/Illinois border region.

MODIS visible + cloud phase + cloud top temperature images

MODIS visible + cloud phase + cloud top temperature images

A MODIS visible image with an overlay of CIMSS Mesoscale Winds and pilot reports of turbulence (below) showed that the winds in the middle to upper troposphere were fairly strong from the southwest (several wind speeds of 160-200 knots were indicated between the pressure levels of 250 and 337 hPa), and there were a handful of pilot reports of light to moderate turbulence (with one report at an altitude of 37,000 feet over extreme northern Illinois, near the gravity wave feature).

CIMSS GOES mesoscale winds

CIMSS GOES mesoscale winds

The rawinsonde data from Davenport, Iowa (below) a few hours after the gravity wave features were seen on the satellite imagery showed a pronounced temperature inversion between the 450-500 hPa pressure levels — the air temperatures in that layer were in the -21 to -26º C range,  in agreement with the MODIS IR brightness temperature and Cloud Top Temperature values associated with the main gravity wave feature. According to the GOES-12 sounder Cloud Top Height product, the tops of these cloud features were within the 12,000-15,000 feet range (which seemed a bit on the low side, judging from the rawinsonde data).

Davenport, Iowa rawinsonde data

Davenport, Iowa rawinsonde data

Freeze in Florida

January 22nd, 2009
MODIS 11.0 µm IR + GOES-12 10.7 µm IR + GOES-12 sounder Skin Temperature

MODIS 11.0 µm IR + GOES-12 10.7 µm IR + GOES-12 sounder Skin Temperature

Much of the central and southern Florida peninsula experienced temperatures at or below freezing on the morning of 22 January 2009. Surface air temperatures were as cold as 15º F at Archbold in the Tampa NWS County Warning Area (CWA), 16º F at Plymouth in the Melbourne CWA, and 23º F at Palmdale in the Miami CWA. A comparison of AWIPS images of the 1-km resolution MODIS 11.0 µm IR window channel, the 4-km resolution GOES-12 10.7 µm IR window channel, and the 10-km resolution GOES-12 sounder Skin Temperature product (above) showed that IR brightness temperatures and sounder skin temperature values of 32º F and colder (cyan to blue to purple colors) extended well south of Lake Okeechobee at 07:15 UTC (3:15 AM local time). The coldest MODIS IR / GOES-12 IR / GOES-12 Skin Temperature values at that time were -11.5º C (11.3º F) / -9.0º C (15.8º F) / -5.5º C (22.1º F), respectively.

An AWIPS image of the 1-km resolution MODIS Land Surface Temperature (LST) product over central Florida  (below) revealed that the coldest LST values of 17.3 º F (purple color enhancement) were found just southwest of Leesburg (station identifier KLEE), along the border of Lake county and Sumter county.

MODIS Land Surface Temperature product (northern Florida)

MODIS Land Surface Temperature product (central Florida)

The MODIS Land Surface Temperature product over southern Florida (below) showed that LST values of 32º F and colder (darker blue color enhancement) were found as far south as the area east of  Naples (station identifier KAPF). Note the relative warmth of Lake Okeechobee, were LST values of 55-56º F (red color enhancement) were seen.

MODIS Land Surface Temperature product (southern Florida)

MODIS Land Surface Temperature product (southern Florida)

An animation of the GOES-12 10.7 µm IR window imagery (below) showed the southward progression of below-freezing surface IR brightness temperatures during the night-time hours, as well as the warming of the Florida peninsula after sunrise on 22 January.

GOES-12 10.7 µm IR images

GOES-12 10.7 µm IR images

GOES-12 water vapor and IR images

GOES-12 water vapor and IR images

The air mass over Florida was dry enough to allow the 7.4 µm water vapor channel on the GOES-12 sounder to actually sense the surface (above; animation) — in this case, the weighting function (below) for the 7.4 µm water vapor channel (red plot) peaked at a very low altitude, allowing the thermal signal of the contrast between cold land and warm ocean to reach the satellite.

GOES-12 weighting functions (calculated using Miami rawinsonde data)

GOES-12 weighting functions (calculated using Miami rawinsonde data)

Lake Michigan: lake-effect snow band, and lake ice

January 20th, 2009
GOES-12 and GOES-13 visible images

GOES-12 and GOES-13 visible images

A comparison of GOES-12 and GOES-13 visible images (above) showed a long lake-effect cloud band that was oriented generally north-to-south across Lake Michigan on 20 January 2009. Convergence of surface winds was helping to sustain the cloud band over the lake, which was producing localized areas of heavy snowfall where it moved onshore in Illinois (5.0 inches reported in Chicago and Burnham) and Indiana (8.0 inches reported at Whiting). One thing that becomes immediately obvious in the above image comparison is the improved navigation on the new GOES-13 satellite: there is much less image-to-image wobble compared to GOES-12. As a result of this improved navigation, one is able to get a better sense of the actual drift of the ice that is floating in parts of the lake — ice along the eastern portion of the lake was drifting slowly westward away from the Michigan shore, while ice in the northwestern part of the lake was drifting slowly southward.

AWIPS images of the GOES-12 visible, 3.9 µm shortwave IR, 10.7 µm IR window, and Sounder Cloud Top Height product (below) suggested that the lake-effect cloud band was likely composed primarily of supercooled water droplets at 17:00 UTC — there was a strong signal of solar reflection on the shortwave IR image (darker gray enhancement), and IR window cloud top brightness temperature values were only as cold as -17 to -20º C  (cyan color enhancement). The GOES-12 sounder Cloud Top Height values were generally in the 10,000-12,000 foot range (green color enhancement). Note that there were only a few GOES-12 satellite-derived wind vectors associated with the cloud band; the number and accuracy of these atmospheric motion vectors would no doubt be improved using the better navigation of the GOES-13 visible imagery.

GOES-12 visible, shortwave IR, IR window, and sounder cloud top height

GOES-12 visible, shortwave IR, IR window, and sounder cloud top height

AWIPS images of the MODIS visible, 3.7 µm shortwave IR, 11.0 µm IR window, and Cloud Phase product (below) were similar to the GOES-12 images shown above — a strong “solar reflection signal” on the shortwave IR image (darker gray enhancement), with IR window cloud top brightness temperature values in the -17 to -20º C range (cyan color enhancement) — and the MODIS Cloud Phase product indicated that most of the cloud band was of the “Mixed Phase” or “Uncertain” category (gray colors).

MODIS visible, shortwave IR, IRT window, and cloud phase

MODIS visible, shortwave IR, IRT window, and cloud phase

It is interesting to view a series of 250-meter resolution MODIS true color images (below), using the CIMSS MODIS True Color Imagery Viewer on AWIPS (available to  NWS forecast offices in Wisconsin that have installed the CIMSS MODIS imagery scripts) — the ice increased in areal coverage on 15-16 January (when inland surface temperatures were as cold as -35º F or -37º C over southern Wisconsin), but then decreased somewhat on 18 January as temperatures slowly moderated.

250-meter resolution MODIS true color images (January 13-18)

250-meter resolution MODIS true color images (January 13-18)