Standing waves to the lee of the Sierra Nevada

November 29th, 2009 |
GOES-11 6.7 µm water vapor images

GOES-11 6.7 µm water vapor images

Under normal conditions of westerly flow aloft, one might expect to see occasional standing waves to the east of the Sierra Nevada mountain range; however, due to the presence of a strong cut-off low over the southwestern US, the winds aloft over Nevada and California were from the northeast on 29 November 2009 — and AWIPS images of the 8-km resolution GOES-11 6.7 µm water vapor channel (above) showed a signature of mountain waves to the west of the crest of the Sierra Nevada. This type of lee wave signature on water vapor imagery indicates the potential for clear air turbulence in the proximity of the waves — however, there were no pilot reports of turbulence noted in the immediate area of the lee wave signature (possibly due to the time of day, when air traffic is generally at a minimum).

A pair of 1-km resolution MODIS 6.7 µm water vapor images at 06:20 and 10:38 UTC (below) showed the advantage of higher spatial resolution for detecting such mesoscale signatures.

MODIS 6.7 µm water vapor images

MODIS 6.7 µm water vapor images

A comparison of the 8-km resolution GOES-11 6.7 µm water vapor, 4-km resolution GOES-14 6.5 µm water vapor, and 4-km resolution GOES-12 6.5 µm water vapor images (below) further demonstrated the effects of varying spatial resolution as well as varying satellite viewing angle in resolving the lee wave signatures to the west of the Sierra Nevada. GOES-14 (positioned at 105º West longitude) had the best viewing angle of the region, and its 4-km resolution water vapor channel did a better job of depicting both the areal coverage and the temporal duration of the lee wave structure — especially compared to GOES-11 (positioned at 135º West longitude) with its 8-km resolution water vapor channel. Using GOES-14 imagery, the onset of the lee wave structure was easier to see, and the duration of the lee wave event was also longer. In addition, even though the viewing angle from GOES-12 (positioned at 75º West longitude) was very large — about 65 degrees — the 4-km resolution water vapor channel still managed to show a fairly good signature of the lee waves.

GOES-11, GOES-14, and GOES-12 water vapor images

GOES-11, GOES-14, and GOES-12 water vapor images

Note that the water vapor images also suggested the formation of a downwind “cloud banner” or “cloud crest” after about 06 UTC. The 4-km resolution MODIS Cloud Phase product (below) showed a growing ice phase cloud feature (salmon color enhancement) over central California between 06:20 and 10:38 UTC.

MODIS Cloud Phase product

MODIS Cloud Phase product

However, the 4-km resolution MODIS Cloud Top Temperature product (below) only indicated cloud top temperature values as cold as -20 to -22º C (green color enhancement) within the glaciated cloud banner feature over central California.

MODIS Cloud Top Temperature product

MODIS Cloud Top Temperature product

Another view of the central California cloud banner feature using the 1-km resolution AVHRR Cloud Type product at 09:39 UTC (below) indicated that it was composed of cirrus clouds (yellow enhancement), with supercooled water droplet clouds (cyan color enhancement) immediately upwind over the Sierra Nevada.

AVHRR Cloud Type product

AVHRR Cloud Type product

Furthermore, the corresponding 1-km resolution AVHRR Cloud Top Temperature product (below) indicated significantly colder cloud top temperature values of -60º to -70º C (blue to white colors) within the central California cloud banner feature.

AVHRR Cloud Top Temperature product

AVHRR Cloud Top Temperature product

Finally, it is interesting to note that the GOES-11 10.7 µm IR image (below, upper left panel) showed absolutely no signature of the lee cloud banner — due to the thin nature of this glaciated cloud feature, a great deal of radiation from the warmer land surface below was “bleeding up” though the ice cloud and was masking its presence on IR imagery. The GOES-11 6.7 µm water vapor image, however, did show a better signature of the presence of the cloud banner feature (lower left panel).

GOES IR, GOES water vapor, AVHRR Cloud Top Temperature, and AVHRR Cloud Top Height

GOES IR, GOES water vapor, AVHRR Cloud Top Temperature, and AVHRR Cloud Top Height

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