Using MODIS to detect ice-covered ground

November 30th, 2006

MODIS visible channel
A fast-moving winter storm was responsible for widespread snow, sleet, and freezing rain across much of central US on 29 November 2006; several counties in southeastern Iowa reported 1/8 to 1/4 inch of ice accumulation from the storm (MODIS false-color composite image). AWIPS MODIS images from the following day (30 November 2006) show the extent of the ice-covered ground: the area affected by the ice storm in southeastern Iowa appears to be bare ground on the visible channel image (above), but note the dark signal over that same region on the Band 7 (2.1µm) near-IR channel (below) — this dark signal confirms the presence of a coating of ice. This ice glaze was not apparent on the visible channel image since it had very little impact on the albedo of the ground surface, but the ice coating is strongly absorbent in the 2.1µm portion of the electromagnetic radiation spectrum (which yields a dark signal on that particular channel). Just to the north of the large patch of ice-covered ground is a narrower feature over east-central Iowa which exhibits a somewhat darker signal on the 2.1µm image; this is due to cloud shadows being cast onto the surface from the high cloud layer that was present (note that those shadows were also apparent on the visible channel image). An examination of the corresponding 11.0µm IR window and the 3.7µm shortwave IR images (Java applet) shows that there was very little thermal contrast between this ice-covered ground and the adjacent bare ground over the remainder of Iowa (due to the presence of a cold arctic air mass over the entire region on that day).

The remainder of the ice-covered ground (across southcentral Iowa into northern Missouri) was revealed by MODIS true color and false color image composites on the following day (01 December 2006), as clouds from a departing winter storm cleared from that area.
MODIS 2.1µm near-IR channel

Contrails over the southeastern US

November 29th, 2006

MODIS 1.3µm near-IR image
AWIPS MODIS imagery revealed widespread condensation trails (or “contrails”) from commercial aircraft flying across the southeastern US on 29 November 2006. These contrails were best seen using the Band 26 (1.3µm) near-IR channel (above) and the Band 27 (6.7µm) water vapor channel (below). Rawinsonde data from Peachtree City GA (KFFC) and Greensboro NC (KGSO) indicated increasing moisture aloft within the range of altitudes common for commercial jets (around 35,000 feet) — the contrails were relatively long-lived within this relatively moist environment (GOES-12 water vapor channel animation).
MODIS 6.7µm water vapor image

Cold air in northwestern Canada

November 27th, 2006

NOAA-15 10.8µm IR image
A very cold arctic air mass had been building over central Alaska and northwestern Canada during the latter half of November 2006 (surface air temperatures colder than -40 F/-40 C have been reported daily over that region since 21 November). A NOAA-15 AVHRR 10.8µm “IR window channel” image centered over the southern Yukon Territory (above) on 27 November (surface analysis) revealed that the coldest air (-40 to -50 C, darker blue enhancement) was settling into the lower elevations of the river valleys. Narrow lakes along and south of the Yukon Territory / British Columbia border exhibited significantly warmer IR brightness temperatures (-10 to -20 C, orange to yellow enhancement), due to heat radiating upward through the snow and ice covered lake surfaces.

A similar IR image centered a bit farther east over the Northwest Territories (below) showed warmer brightness temperatures over the higher terrain of the Mackenzie and Selwyn Mountains (-20 to -30 C, yellow to cyan enhancement) — those higher terrain features rose above the level of the strong temperature inversion which was trapping the coldest air near the surface at lower elevations. This IR image also revealed a comparatively warm signature (0 to -20 C, red to yellow enhancement) from the snow and ice covered surface of Great Bear Lake in the northern portion of the image. The 1-km resolution of the NOAA-15 AVHRR instrument showed the small-scale structure of these temperature features much better than the “4-km” resolution of GOES-11 (which had degraded to an effective resolution of about 12 km, due to the ~65 degree satellite viewing angle) — this is quite apparent looking at a NOAA-15 / GOES-11 IR image fader (Java applet).
NOAA-15 10.8µm IR image

Mountain waves: GOES-11 versus GOES-13 water vapor channel

November 22nd, 2006

GOES-11/GOES-13 water vapor image
An extensive area of mountain waves was apparent on the “water vapor channel” images from GOES-11 and GOES-13 on 22 November 2006 (above). Animation of these images (QuickTime | Java) shows that the mountain waves were present for several hours across a good deal of the Northwest US, becoming well-defined over Wyoming after about 10:00 UTC. Due to the relatively dry air mass that was present over that region, the GOES-11 water vapor channel weighing function was peaking at a fairly low altitude (around 600 hPa, or about 3 km above ground level). The improvement in spatial resolution of the water vapor channel (from 8km on GOES-11 to 4 km on GOES-13) allows such mountain wave features to be detected with better clarity; in addition, the 6.5µm water vapor channel on GOES-13 is spectrally wider than the 6.7µm water vapor channel on GOES-11, which accounts for some of the improved mountain wave detection capability. This improved 4 km resolution water vapor channel is also available on GOES-12.
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Wind speed isotachs at the 500 hPa level (below) indicated that strong winds associated with a passing jet stream axis were responsible for generating these mountain waves (also note the corresponding gradient in GOES Sounder total column ozone, poleward of the axis of strongest winds). This mountain wave signature on water vapor channel imagery is an indicator of turbulence potential; while there were no pilot reports of turbulence during that 05-14 UTC time period, the Graphical Turbulence Guidance product did indicate a Moderate to Severe potential for turbulence across the region (over Wyoming in particular).
AWIPS GOES sounder ozone, GOES imager water vapor