Caughlin Wildfire near Reno, Nevada

November 18th, 2011 |
GOES-15 3.9 µm shortwave IR images (click image to play animation)

GOES-15 3.9 µm shortwave IR images (click image to play animation)

The 2000-acre “Caughlin Fire” started burning around 08:45 UTC (1:45 am local time) in the hilly terrain near Reno, Nevada, and soon grew out of control due to strong winds gusting as high as 74 mph. McIDAS images of GOES-15 3.9 µm shortwave IR data (above) showed the “hot spot” (black to yellow to red enhanced pixels) associated with the fire. At least 30 homes were destroyed, with many more damaged by the fire. Thousands of residents were evacuated.

Evidence of the strong winds across the region could be seen on an AWIPS image of MODIS 6.7 µm water vapor channel data (below), with a number of very pronounced mountain waves showing up on the image. These mountain waves persisted for several hours, and were responsible for pilot reports of severe turbulence, wind shear, and 50-knot crosswinds during descent to final approach into the Reno airport. The highest wind gust reported at the Reno airport was 44 mph, and surface visibility was also reduced to 6 miles at the airport due to smoke.

MODIS 6.7 µm water vapor channel image

MODIS 6.7 µm water vapor channel image

Mysterious Gravity Wave Over the Eastern Pacific Ocean

November 14th, 2011 |

We received the following email from Ken Waters of the National Weather Service forecast office in Phoenix, Arizona:

I noticed something interesting in this morning’s visible imagery off the Baja California coast.

Here’s a link: https://docs.google.com/open?id=0B2ktDMIN5qWfODI2OTYzYjgtZjJkMS00MTU5LTk2ODctYzdhNzY5M2Y2MWIx

I’m looking at the apparent wave pattern that’s going “upstream” towards the northeast whereas the low level flow is mostly towards the south. Is that a gravity wave? If so, what causes it? The vis can only go so far back so I looked at the IR and couldn’t find anything obvious.

Great question Ken — it certainly appears to be some sort of internal gravity wave, but what caused it and why was it propagating against the ambient flow shall remain a bit of a mystery until we can dig into this case a bit further. One more event for the “What the heck is this?” blog category.

GOES-15 6.5 µm water vapor channel images + GOES-15 0.63 µm visible channel images

GOES-15 6.5 µm water vapor channel images + GOES-15 0.63 µm visible channel images

McIDAS images of GOES-15 6.5 µm water vapor channel data (prior to daylight) and then GOES-15 0.63 µm visible channel data after sunrise (above) on 14 November 2011 tell us one thing: this gravity wave was apparently fairly deep in the vertical, since it exibited a signal on both the water vapor channel imagery (which generally senses radiation from the middle troposphere: San Diego 12:00 UTC rawinsonde water vapor chanel weighting function profile) as well as on the lower-tropospheric cloud features seen on the visible channel imagery.

Note that there was a second packet of shorter-wavelength gravity waves that could be seen in the far southwestern portion of the GOES-15 visible image satellite scene toward the end of the animation. This second packet of gravity waves was very evident on a 500-meter resolution Aqua MODIS Red/Green/Blue (RGB) true color image at 21:21 UTC (below).

Aqua MODIS true color image

Aqua MODIS true color image

Gravity waves are usually ducted within a well-defined temperature inversion. A look at the 12:00 UTC rawinsonde profile from San Diego, California (below) did indicate the presence of a few inversions that might have been capable of ducting such a gravity wave — but the inversions existed at multiple levels.

San Deigo, California 12:00 UTC rawinsonde profile

San Deigo, California 12:00 UTC rawinsonde profile

An AWIPS image of 18:00 UTC MODIS 0.65 µm visible channel data with overlays of 1-hour interval MADIS satellite winds (below) did not reveal any atmospheric motion vectors with a southwesterly component – but these would likely have been rejected by the winds quality control algorithms, since such a motion would have differed too greatly from the model first guess wind fields at 850 hPa, 500 hPa, 300 hPa, and 250 hPa.

MODIS 0.65 µm visible channel image + MADIS 1-hour interval satellite winds

MODIS 0.65 µm visible channel image + MADIS 1-hour interval satellite winds

Regarding the effect of the gravity wave seen on the lower-tropospheric clouds bands, a MODIS 11.0 µm IR image detected cloud top IR brightness temperatures around +4ºC, which on a RUC model sounding at that location apparently corresponded to a cloud top height around 12,550 feet (below) — however, this value seemed to be a bit high judging from the appearance of the cloud band features on the GOES and MODIS visible and IR imagery.

MODIS 11.0 µm IR image + RUC model sounding

MODIS 11.0 µm IR image + RUC model sounding

On the other hand, POES AVHRR Cloud Type and Cloud Top Height products indicated that these low-level cloud bands were water droplet clouds, with cloud top heights of around 1 km (below) — much more typical for marine boundary layer cloud features over this region.

POES AVHRR Cloud Type product

POES AVHRR Cloud Type product

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

Mountain waves over Colorado and New Mexico

November 12th, 2011 |
GOES-11, GOES-13, GOES-15, and MODIS water vapor channel images

GOES-11, GOES-13, GOES-15, and MODIS water vapor channel images

A comparison of 8-km resolution GOES-11 6.7 µm water vapor channel, 4-km resolution GOES-13 and GOES-15 6.5 µm water vapor channel, and 1-km resolution Aqua MODIS 6.7 µm water vapor channel images (above) demonstrated how differences in satellite viewing angle as well as differences in satellite sensor spatial resolution have an impact in being able to resolve the structure and areal coverage of small-scale features such as the mountain waves that existed across much of southeastern Colorado and northeastern New Mexico around 19:45 UTC on 12 November 2011.

There were a number of pilot reports of moderate to severe turbulence aloft across the region – and at the surface, wind gusts as high as 115 mph were reported. As can be seen in a comparison of 1-km resolution MODIS 0.65 µm visible channel and MODIS 6.7 µm water vapor channel images (below), many of the mountain waves were located in cloud-free areas — this highlights the value of water vapor channel imagery for identifying such regions of potential aircraft turbulence.

MODIS 0.65 µm visible channel + MODIS 6.7 µm water vapor channel images

MODIS 0.65 µm visible channel + MODIS 6.7 µm water vapor channel images

Intense Bering Sea Extratropical Cyclone

November 9th, 2011 |
MTSAT-1R 6.7 µm water vapor channel images

MTSAT-1R 6.7 µm water vapor channel images

McIDAS images of MTSAT-1R 6.7 µm water vapor channel data (above) showed an intense extratropical cyclone that was moving toward the Bering Sea region during the 07 November – 08 November 2011 time frame. Of particular interest was the presence of a very warm/dry (dark black) circular region within the dry slot sector of the developing cyclone, which could have been associated with a strong potential vorticity anomaly.

A color-enhanced comparison of MTSAT-1R and GOES-11 6.7 µm water vapor channel data (below; click image to play animation) demonstrated the difference that satellite viewing angle (MTSAT looking from the west; GOES-11 looking from the east) and satellite sensor spatial resolution (the MTSAT-1R water vapor channel is “4 km” at nadir, while the GOES-11 water vapor channel is “8 km” at nadir) play in the ability to resolve such potentially important dynamical features. The core of the aforementioned MTSAT-1R dry feature moved directly over Shemya Island around 12:00 UTC on 08 November (MODIS IR image with surface analysis), where a surface wind gust of 83 mph was recorded at Shemya Air Force Base. Then, once the storm began to move northward over the Bering Sea, a more “curved banding” structure was seen on water vapor imagery as the cyclone began to wrap filaments of dry air around the deepening storm center. Although the sun angle was low, some of the “banding structure” could be seen in GOES-11 0.65 µm visible channel images.

MTSAT-1R (left) and GOES-11 (right) 6.7 µm water vapor channel images (click image to play animation)

MTSAT-1R (left) and GOES-11 (right) 6.7 µm water vapor channel images (click image to play animation)

While the dry slot features began to lose their definition in the geostationary MTSAT-1R and GOES-11 water vapor images (in part due to the upward shift in the peak of the water vapor channel weighting function with increasing satellite viewing angle), a direct overpass of the Aqua satellite around 23:45 UTC on 08 November provided a nice view using the 6.7 µm water vapor channel on the MODIS instrument (below). Using the MODIS imagery, good dry slot structure could be seen, even after the storm had moved northward over the Bering Sea.

Aqua MODIS 6.7 µm water vapor channel image

Aqua MODIS 6.7 µm water vapor channel image

A sequence of AWIPS images of 1-km resolution MODIS 11.0 µm and POES AVHRR 12.0 µm InfraRed data (below; click image to play animation) showed the storm at various phases as it was rapidly deeping during its northward trek over the Bering Sea.

MODIS 11.0 µm and POES AVHRR 12.0 µm InfraRed images (click image to play animation)

MODIS 11.0 µm and POES AVHRR 12.0 µm InfraRed images (click image to play animation)

This ended up being one of the strongest Bering Sea storms on record — the winds exceeded hurricane force across a very expansive area, producing high seas and major coastal flooding and beach erosion along parts of western Alaska. At the Tin City Airways Facility Sector (located near the western tip of the Seward Peninsula), they reported sustained winds of 72 mph with gusts to 85 mph — and the minimum altimeter air pressure was 28.46 inches. A peak gust of 89 mph was reported nearby at Wales. As the storm moved over St, Lawrence Island, minimum altimeter air pressure readings were 28.21 inches and 28.28 inches at Gambell and Savoonga, respectively.

The entire evolution of the storm during the 08-09 November time period can be seen on an animation of 15-minute interval GOES-11 10.7 µm IR images (below; click image to play animation).

15-minute interval GOES-11 10.7 µm IR images (click image to play animation)

15-minute interval GOES-11 10.7 µm IR images (click image to play animation)