GOES-12 visible image

GOES-14 visible image

A comparison of enhanced visible channel images from GOES-12 and GOES-14 at 13:15 UTC on 01 September 2009 is shown above — both images have been remapped to a Mercator projection over the state of Wisconsin. The obvious “meteorological” phenomenon is the early morning fog in the Mississippi, Wisconsin, and Kickapoo River basins, in addition to numerous other valleys and river basins feeding into the Mississippi River.
There are a couple of significant differences to note between the 2 visible images. First of all, the fog is a bit brighter and a little more extensive in the GOES-14 image compared to the GOES-12 image. This is primarily due to the relative age of the visible sensors (which noticeably degrades with time). The second major difference is the relative contrast of lakes, rivers, vegetation, and land usage. GOES-12 has slightly  more  contrast between  land and lakes (and/or other bodies of water) than GOES-14.

On the other hand, GOES-14 is able to discern urban centers more readily than GOES-12, as well as variations in vegetation type. Examples of this are around the large metropolitan region of southeastern Wisconsin and northeastern Illinois (i.e. Milwaukee to Chicago). Also, both the Baraboo Range (located just to the northwest of Madison) and the “Military Ridge” (which runs east to west from Madison to Prairie du Chien) stand out more boldly in the GOES-14 image compared to the GOES-12 image (AWIPS topography image). This difference is primarily due to the slight variation in the spectral width of the two visible bands on the GOES-12 and GOES-14 Imager instruments. A comparison of the visible channel spectral response function for GOES-12 and GOES-14 (below) shows that the sharper cutoff for wavelengths beyond 0.7µm on the GOES-14 visible channel makes it less sensitive to the signal from the mature corn crops, allowing greater contrast between the thick vegetation of the agricultural fields and the more sparsely vegetated cities, towns, and highway corridors.

Spectral response functions for GOES-12 and GOES-14

A time series of GOES-12 visible images (below) illustrates how the cumulus cloud field developed during the morning hours as solar heating increased — convective clouds were seen to develop right over the Baraboo Range and the Military Ridge (AWIPS topography image). At 15:15 UTC and 15:32 UTC, effect of the terrain on cumulus initiation was quite evident, while more “ridge-line cumulus” developed and filled the region by 16:00 UTC and later.

GOES-12 visible image time series

Re-mapped GOES-12 and GOES-14 visible images at 15:45 UTC are shown below, after cumulus development had progressed.

GOES-12 visible image

GOES-14 visible image

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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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