Interesting satellite signatures in the Arctic

July 16th, 2009 |
GOES-11 + GOES-12 water vapor imagery

GOES-11 + GOES-12 water vapor imagery

An intense upper-level low developed over the Canadian Arctic Archipelago on 14 July 2009, and subsequently migrated southwestward over the Northwest Territories, Yukon, and northeastern Alaska by 16 July 2009. AWIPS images of the GOES-11 + GOES-12 water vapor channel composite (above) showed a well-defined signature of the intensifying upper low during this period, with a large area of warmer/drier air (darker blue colors) within the circulation.

What was remarkable about this warm/dry signature on the water vapor imagery is the fact that such detail could be seen, in spite of the very large satellite viewing angle of the GOES satellites. Water vapor imagery is prone to the effect of “limb brightening” as the water vapor weighting function is shifted to higher, colder altitudes over the higher latitude regions — this tends to make water vapor imagery appear rather cold and “washed out” over the Arctic much of the time. However, in this case the dynamic tropopause (taken to be the pressure of the 1.5 Potential Vorticity Unit surface) was brought downward to the 850 hPa pressure level on 14 July (below) as the low intensified — and snow was even reported at Norman Wells (station identifier CYVQ) in the Northwest Territories on 16 July.

GOES water vapor imagery + GFS90 PV1.5 pressure

GOES water vapor imagery + GFS90 PV1.5 pressure

On the GOES water vapor imagery above there was a hint of some embedded vortex structure developing within the upper low circulation. These vorticies (which were likely small stratospheric intrusion vorticies) were much easier to identify on the 1-km resolution MODIS water vapor image than on the “8-km” resolution GOES-11 water vapor image (below) — this is partly due to the upward shift of the water vapor weighting function mentioned previously, and also due to the fact that the 8-km GOES-11 water vapor pixels were effectively about 30 km in size due to the very large satellite viewing angle.

GOES-11 + MODIS water vapor images

GOES-11 + MODIS water vapor images

Another interesting satellite signature was the appearance of a “thermal anomaly” over the Arctic Ocean north of Alaska on the GOES-11 10.7 µm IR window channel image at 09:00 UTC (below, bottom panels). At that particular time, there is a great deal of solar reflection off the water and ice surface, which appears very bright on the visible imagery (top panels), and very hot (dark black enhancement) on the 3.9 µm shortwave IR imagery (center panels). The intense solar reflection effectively causes the IR window channel brightness temperature to “roll over” from very warm to very cold values (black to white color enhancement). This IR window channel thermal anomaly does not appear if cloud cover masks the highly reflective nature of the water and ice in the Arctic Ocean.

GOES-11 visible, 3.9 µm IR, and 10.7 µm IR images

GOES-11 visible, 3.9 µm IR, and 10.7 µm IR images


GOES-11 visible images

GOES-11 visible images

Later in the day, GOES-11 visible imagery showed that the southern edge of the ice in the Arctic Ocean had receded a considerable distance from the northern coast of Alaska (above). The ice edge could be seen in greater detail using 1-km resolution MODIS true color imagery (below, courtesy of the GINA, University of Alaska Fairbanks SwathViewer).

MODIS true color image (courtesy of GINA, University of Alaska Fairbanks)

MODIS true color image (courtesy of GINA, University of Alaska Fairbanks)

Tropical Storm 07W (Molave): very cold cloud tops

July 16th, 2009 |
MTSAT-1R IR images

MTSAT-1R IR images

MTSAT-1R InfraRed (IR) images from the CIMSS Tropical Cyclones site (above) showed the rapid development of a cold Central Dense Overcast (CDO) as Tropical Storm 07W (Molave) intensified east of the Philippines in the West Pacific Ocean on 16 July 2009. It is interesting to note that the CDO was centered a considerable distance to the southwest of the low-level circulation of the tropical cyclone.

MTSAT-1R IR image

MTSAT-1R 10.3 µm IR image


4-km resolution MTSAT-1R 10.3 µm IR cloud top brightness temperatures (above) were a cold as -92.4º C (purple color enhancement) at 12:30 UTC. 1 hour and 15 minutes later, a 1-km resolution MODIS 11.0 µm IR image (below) indicated that cloud top IR brightness temperatures were still as cold as -92.1º C.

MODIS IR image

MODIS IR image