Cold temperatures in Alaska

September 30th, 2008
NOAA-16 11.0 µm IR image + surface reports

NOAA-16 11.0 µm IR image + surface reports

A NOAA-16 AVHRR 11.0 µm IR image (above) showed cold brightness temperatures of -10º to -20º C (violet colors) across much of northwestern Alaska on 30 September 2008. The coldest IR brightness temperatures of around -27º C or -17º F (darker blue colors) corresponded to the higher elevations of the Brooks Range which runs from west to east across northern Alaska. The coldest minimum temperature reported in Alaska that morning was -13º C (+9º F) at Anatuvuk Pass (station identifier PAKP, which had a temperature of +16º F at 16 UTC), but the coldest brightness temperatures on the IR image were located well to the west of that station. Two days earlier, Alaska reported their first temperature colder than -18º C  (0º F)  this season: both Chalkyitsik and Denali National Park registered a low temperature of -1º F on the morning of 28 September.

A comparison of MODIS true color images and topography using the Swath Viewer from the Geographic Information Network of Alaska (below) indicated that there was a good deal of snow cover over parts of the higher terrain in northern Alaska, especially across the northern slopes of the Brooks Range (due to a recent period of upslope flow moving inland from the Arctic Ocean, which dropeed several inches of new snow).

MODIS true color images + topography

MODIS true color images + topography

Cold front approaching Hawaii

September 30th, 2008
AWIPS satellite images + surface analysis

AWIPS satellite images + surface analysis

A cold frontal boundary was moving southward across the Pacific Ocean and approaching the Hawaiian Islands on 29 September30 September 2008, as seen on a comparison of AWIPS visible, IR, and water vapor satellite imagery and corresponding surface analysis (above). According to the boundary layer Meteorological Assimilation Data Ingest System (MADIS) atmospheric motion vectors (below), the front was moving southward at a speed of around 15-20 knots — IR cloud top temperatures were warmer than 0º C along the frontal cloud band north of Hawaii, suggesting rather shallow cloud features.

AWIPS image of GOES-11 10.7 µm IR channel and MADIS winds

AWIPS image of GOES-11 10.7 µm IR channel and MADIS winds

A closer view using GOES-11 visible imagery (below) revealed that a series of mesoscale vorticies had developed along the frontal boundary. Another interesting feature was the persistent volcanic plume downwind of the big island of Hawaii (streaming toward the southwest), due to ongoing activity at the Kilauea volcano since Spring 2008 (see the April 2008 CIMSS satellite blog entry). Also note the long, thin line of cumulus clouds below the volcanic plume, a result of lee-side convergence.

GOES-11 visible images

GOES-11 visible images

A comparison of GOES-11 and GOES-13 visible images (below) shows that the volcanic plume was even more apparent with the larger viewing angle and more favorable “forward scattering” geometry from the GOES-13 satellite (positioned at 105º W longitude, vs. 135º W longitude for GOES-11).

GOES-11 and GOES-13 visible images

GOES-11 and GOES-13 visible images

Subtropical Storm Laura

September 29th, 2008
GOES-12 IR image + QuikSCAT winds

GOES-12 IR image + QuikSCAT WindSat wind vectors

An image of the GOES-12 10.7 µm IR channel (with an overlay of QuikSCAT WindSat wind vectors) from the CIMSS Tropical Cyclones site (above) revealed that wind speeds were near 50 knots within a curved band of deep convection located just to the east of the center of Subtropical Storm Laura on 29 September 2008.

Animations of the GOES-12 10.7 µm IR channel and visible channel images (below) showed the curved band of deep convection developing further and wrapping around the northern and then the western quadrants of the storm during the hours that followed; small-scale swirls were also seen on the visible imagery, rotating around the low-level center of Laura.

GOES-12 10.7 µm IR images

GOES-12 10.7 µm IR images

GOES-12 visible images

GOES-12 visible images

Laura was eventually classified as a Tropical Storm on the following day (30 September).

Hurricane Kyle: the transition to extratropical

September 28th, 2008
GOES-12 water vapor images

GOES-12 6.5 µm water vapor images

GOES-12 6.5 micrometer “water vapor channel” imagery (above) revealed a pronounced warming/drying signature (darker orange colors) as Hurricane Kyle was beginning the transition to an extratropical system on 28 September 2008. GOES-12 water vapor brightness temperatures were as warm as 268º K (-5.15º C) at 16:45 and 17:02 UTC — and the rapid trend of warming/drying suggested that strong subsidence was occurring in that region.

A comparison of the 4-km resolution GOES-12 and the 1-km resolution MODIS water vapor channel data (below) yielded similar brightness temperature values within the core of the warm/dry region (-5.1º C on MODIS, -7.5º C on GOES-12).

GOES-12 6.5 µm and MODIS 6.7 µm water vapor images

GOES-12 6.5 µm and MODIS 6.7 µm water vapor images

Curiously, the GOES-12 sounder total column ozone product (animation) did not exhibit a high ozone feature co-located with the warm/dry pocket seen on the water vapor imagery (below) — if this dry air were due to a stratospheric intrusion or a tropopause fold, ozone values would normally increase to the 350-400 Dobson Unit range (green to red colors).

AWIPS images of GOES-12 water vapor and total column ozone

AWIPS images of GOES-12 water vapor and total column ozone