MODIS visible and near-IR "snow/ice channel" images

A  wildfire (named the “Station Fire”) had been burning for several days in the Angeles National Forest in central Los Angeles county — and this fire then grew quickly in size from 5500 acres to 35,200 acres during the 29 August – 30 August 2009 period. AWIPS images of the  MODIS visible and the MODIS 2.1 µm near-IR “snow/ice” channels (above) showed the smoke plume which was drifting northward during the afternoon on 29 August, in addition to the cluster of very hot pixels (appearing brightest white on the near-IR snow/ice channel image) around the periphery of the fire complex.

The smoke plume — as well as the patches of cirrus clouds farther to the southeast — did not stand out very well against the surrounding bright, sandy desert soils of the southern California region. However, the 1-km resolution MODIS images below demonstrate that the edges of the smoke feature were easier to identify  using either the MODIS 11.0 µm IR window channel image (where the IR brightness temperature values of the smoke plume were about 25º C colder, making it appear as a lighter gray feature) or the MODIS 1.3 µm near-IR cirrus detection channel image (where the smoke appeared as a brighter feature, due to the efficient scattering properties of the smoke particles).

MODIS visible, IR window, and near-IR "cirrus detection channel" images

As the fire burned into the night and on into the morning of 30 August, 4-km resolution GOES-11 3.9 µm shortwave IR images (below) displayed a  large cluster of very hot pixels. A number of the hottest pixels had IR brightness temperatures exceeding the maximum allowable AWIPS IR value of 54.5º C, so AWIPS displayed those hottest pixels as dark black.

GOES-11 shortwave IR images

This fire was rather anomalous, in that it was not driven by strong winds (as is usually the case with  most fires in southern California). However, dry vegetation and a hot, dry air mass helped to create a situation that was favorable for rapid fire growth.

===== 31 AUGUST UPDATE =====

The fire continued to grow quickly on 31 August, reportedly reaching a size of 105,000 acres. 250-meter resolution MODIS true color and false color images from the SSEC MODIS Today site  (below) showed a large pyrocumulus cloud and smoke plume associated with the hottest fire that was actively burning in the eastern portion of the fire complex, along with several other active fires (appearing pink to white on the false color image) along the periphery of the burn area.

MODIS true color and false color images

===== 01 SEPTEMBER UPDATE =====

The thick plume of smoke could be seen moving northeastward from the Station Fire on 30 August, drifting over the Las Vegas valley region of southern Nevada during the late afternoon and early evening (below). Observe the appearance of a wave structure to the top of the smoke cloud as it moved downwind of the Spring Mountains and the 11,918 foot (3633 meter) peak of Mt. Charleston.

GOES-12 visible images

The following morning of 31 August, the thick smoke pall had drifted as far to the northeast as Nevada and Utah,  and was now beginning to move eastward  over southern Wyoming and western Colorado (below). Several pilot reports in the region placed the top of the smoke at 17,000-24,000 feet. Note that the smoke was very apparent on GOES-11 (GOES-West) imagery during the later hours of the day (above), and very apparent on GOES-12 (GOES-East) imagery during the earlier hours of the day (below) — this is due to a favorable forward scattering angle (when the sun-smoke-satellite viewing angle approaches 180 degrees), which helps to highlight the airborne smoke layer on the GOES visible imagery.

GOES-11 visible images

On the morning of 01 September, a portion of the thick smoke cloud had subsided over the Front Range of Colorado, restricting surface visibilities to 3-5 miles at places like Denver, Boulder, and Fort Collins in Colorado and Cheyenne in Wyoming. Late-morning MODIS true color imagery (below, viewed using Google Earth) showed that the smoke  had then begun to drift eastward over parts of Nebraska, Kansas, and the panhandle regions of Oklahoma and Texas .

MODIS true color image

===== 03 SEPTEMBER UPDATE =====

MODIS 3.7 µm shortwave IR image

By 03 September, the Station Fire had burned 148,000 acres, making it the largest fire in Los Angeles County history. Firefighters had made significant progress on controlling the western portions of the blaze, but active fires remained along the eastern periphery of the fire complex. An AWIPS image of the MODIS  3.7 µm shortwave IR channel (above) displayed an arc of very hot fire pixels (darkest black pixels) in the far eastern portion of the fire, located to the north/northeast of El Monte (station identifier KEMT). However, interrogating the 3.7 µm shortwave IR image using AWIPS cursor sampling reported “NO DATA” for the hottest fire pixels, since the brightness temperatures there were above the maximum AWIPS IR threshold. So where were the hottest fire pixels located at that time?

A comparison of the MODIS visible, 3.7 µm shortwave IR, and 2.1 µm near-IR “snow/ice” channels (below) demonstrated that the near-IR “snow/ice” channel imagery could be used in such a case to pinpoint the location of the hottest fires (which show up as the brightest white pixels on the snow/ice channel image).

MODIS visible, 3.7 µm shortwave IR, and 2.1 µm near-IR "snow/ice" channel images

250-meter resolution MODIS true color and false color images from the SSEC MODIS Today site (below) show the size of the Station Fire burn scar (the large red-colored patch located to the west of the smoke feature), in addition to the brighter pink active fire hot spots on the southeastern periphery of the burn area.

250-meter resolution MODIS true color and false color images

Other AWIPS examples of MODIS imagery of the Station Fire can be found on the SPoRT blog from 01 September and from 03 September.

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Vertical wind shear is the change of wind speed and/or direction with height. For tropical cyclones, wind shear is something that must be overcome if strengthening is to occur. For (minimal) tropical storm Danny, strong wind shear has persisted in keeping convection far from the storm center. When the storm center is exposed, as in the visible loop above from 28 August, the atmospheric changes that occur within the convection cannot serve to strengthen the storm center.

Wind shear can be diagnosed using satellite cloud information. It is plainly evident in the satellite loop — note how the upper level clouds are moving from the southwest — especially over the western half of the satellite loop — whereas the lower level clouds are circulating around the storm center. A diagnosis of shear from the CIMSS Tropical Weather Website shows very large shear values (>40) associated with a mid-tropospheric short wave just starting to move off the southeast coast of the United States. Values are somewhat less over Danny, and large again off to the east of Danny. As the short wave approaches Danny, shear values over Danny will increase, and chances for intensification will drop.

Other factors continue to support intensification, however, such as warm sea surface temperatures. The analysis shows ocean water temperatures in the low- to mid-80s in the vicinity of Danny. Two other features are evident in the sea surface temperature plot. The Gulf Stream shows plainly as a ribbon of warm water extending eastward from Cape Hatteras. In addition, there is a striking path of cooler temperatures off to Danny’s north and east; this is a result of the mixing and upwelling associated with the passage of Hurricane Bill last week. The energy that was lost from the warm ocean waters helped to sustain Bill’s strong winds.

Satellite-derived winds can be used to approximate Danny’s future direction. Cloud levels in visible imagery can be determined from the temperature of the cloud. Low-level steering currents (1.5-3 km) over Danny this morning were from the east-southeast whereas upper level steering currents (3-10 km) were more southerly. If Danny is a shallow feature, it might move from the east-southeast towards the southeast coast of the US, following the low-level steering. If Danny is a deeper feature, its motion should be more northerly. The forecast path from the National Hurricane Center shows northward motion, suggesting that Danny is a deep system.

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An area of disturbed weather north of the Bahamas developed into Tropical Storm Danny early on 26 August. Satellite loops of the visible imagery, above, clearly show a low-level circulation that is displaced from the area of main convection to the north and east. Until the convection and the low-level centered are more co-located, development of the system should be slow.

Observations of wind shear near the Bahamas (analysis available at the CIMSS Tropical Weather Website) show modest values near the storm, consistent with the forecast of slow strengthening.

Water vapor imagery over North America shows the environment surrounding the storm, and several features that will determine where Danny will eventually move. The environment surrounding Danny is moist — dry air intrusions into the core of the storm, a weakening influence — should be small in the next 24-48 hours. Features in the upper troposphere that may influence the path of Danny include the closed circulation over southern Mississippi and the trough dropping into the Midwestern United States. As these features approach the east coast of the United States, increasing shear may limit the strength of Danny. However, sea surface temperatures are near the seasonal peak, which favors an increase in strength.

The eventual path of Danny is difficult to predict, in part because of the disorganized nature of the storm. It should be clear, however, that interested (and/or vulnerable) parties up and down the East Coast of the United States should keep an eye on the forecast.

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The launch and post-launch checkout of GOES-14 allows a comparison of the sounder instruments on GOES-14 (see the image above) and GOES-12, the operational GOES-East (the image below). The Sounder is used to detect emitted radiation at 19 different wavelengths, in contrast to the 5 wavelengths sensed by the Imager. There is better spectral resolution on the Sounder, but spatial resolution at the sub-satellite point is limited to 8-km field-of-view separated by 10 km. At mid-latitudes, such as over the United States, resolution is about 10 km.

The GOES-14 spacecraft has a different configuration than the GOES-12 spacecraft, and as a result, the Sounder detectors are colder on GOES-14. This allows for cleaner imagery. In particular, the Sounder bands 1, 2, 12 and 15 show much less noise in GOES-14 imagery compared to GOES-12. More information on the GOES-14 instruments can be found here.

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