Saint Patrick’s Day over the Midwest was unusually clear, affording great images of the Great Lakes from the MODIS instrument aboard the Terra and Aqua satellites. The loop above includes the Visible imagery, the cirrus detection channel (1.38 micrometers), the snow-ice detection channel (2.1 micrometers), the derived lake-surface temperatures and the normalized difference vegetation index (NDVI).

Close-up views of the individual lake basins: Ontario, Erie, Michigan-Huron and Superior show mostly uniform surface temperatures in the upper 30s (Fahrenheit) as is normal in early Spring. There are warm thermal plumes in Lake Erie, however, emerging from Sandusky Bay and from the Maumee River at Toledo. MODIS-derived Lake surface temperatures in those regions are in the upper 40s. (True-color imagery for that time (here) show great turbidity over Lake Erie; perhaps the warm temperatures are a result of enhanced run-off from the Maumee and Sandusky Rivers) Temperatures over Lake Michigan are slightly warmer in the middle of the lake — near 40 F — than along the perimeter where lake temperatures are still in the mid-30s. This might be a signature of recently melted near-shore ice.

High-resolution imagery such as these will be routinely available when GOES-R is launched and becomes the operational GOES satellite over the United States.

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GOES-13 (top) and GOES-12 (bottom) visible images

McIDAS images comparing 1-km resolution GOES-12 and GOES-13 0.65 µm visible channel data (above) revealed several smoke plumes over parts of eastern Oklahoma and western Arkansas, while the corresponding 4-km resolution GOES-12 and GOES-13 3.9 µm shortwave IR images (below) showed numerous “hot spots” (black to yellow to red pixels) due to widespread fire activity that had flared up across that region on 18 March 2010. Even though there were a number of smoke plumes seen in eastern Oklahoma, the most obvious smoke plumes (in term of size and brightness on the visible imagery) were apparent in northwestern Arkansas.

GOES-13 (top) and GOES-12 (bottom) 3.9 µm shortwave IR images

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MODIS visible and 3.7 µm shortwave IR images

AWIPS comparisons of the 1-km resolution MODIS visible and 3.7 µm shortwave IR images at 19:31 UTC (above) and the 1-km resolution AVHRR visible and 3.7 µm shortwave IR images several hours later at 23:43 UTC (below) showed that many more fire hot spots could be detected with the use of higher spatial resolution shortwave IR images (GOES vs MODIS at 19:31 UTC | GOES vs AVHRR at 23:43 UTC). The spatial resolution of the shortwave IR channels on the ABI instrument of GOES-R satellite will be 2 km, which will be an improvement over the 4 km resolution on the current GOES satellites.

AVHRR visible and 3.7 µm shortwave IR images

A closer view over northwestern Arkansas using the AVHRR visible channel, Cloud Type Product, Cloud Top Temperature, and Cloud Particle Effective Radius products at 22:03 UTC (below) showed the following: (1) the 3 large smoke plumes over northwestern Arkansas as well as the smaller cumulus clouds off the the east were composed primarily of water droplets (cyan color enhancement); (2) the Cloud Top Temperature values of the large smoke plumes were quite warm (+10 to +12º C, gray color enhancement), while some of the smaller cumulus clouds off to the east were beginning to exhibit CTT values several degrees below freezing (red to orange color enhancement); (3) the Cloud Particle Effective Radius values in the smoke plumes were significantly larger (30-40 µm, darker blue color enhancement) than those of the cumulus clouds off to the east (15-20 µm, cyan color enhancement).

AVHRR visible, Cloud Type, Cloud Top Temperature, and Cloud Particle Effective Radius

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Early morning visible imagery shows a region of modest convection developing into an arc in the Gulf of Mexico south of Louisiana. Radar imagery at 11 UTC shows a concentrated region of strong convection in the marine environment. Which satellite data can be used to identify the regions most at risk for convection in a dry environment?

Total Precipitable Water (TPW) plots (above) derived from satellite microwave data (from SSM/I on DMSP-13 and -14 and from AMSR-E on Aqua) show that the region of convective development was deep within the dry air behind a storm system moving off the southeast coast of the United States. TPWs south of Louisiana are in the 8-16 mm range. Similarly, water vapor imagery from the GOES-12 imager (below) shows predominantly dry air in the region of convective development — until the convection is actually developing. However, no precursor impulses are apparent. Unfortunately, the key development occurs during the eclipse period in the GOES-12 imagery.

Imager water vapor data includes only one channel, sensing radiation with a wavelength around 6.5 micrometers. The GOES Sounder, however, has 3 water vapor channels sensing radiation with wavelengths at 6.5, 7.0 and 7.4 micrometers (The ABI on GOES-R will also have 3 channels that detect water vapor: 6.2, 6.95 and 7.34 micrometers). The weighting functions from Lake Charles, Louisiana for 00 UTC on 18 March 2010, used in the retrieval show that only the 7.4 micrometer data should detect any significant moisture because only the curve for 7.4 micrometer shows a response function that overlaps with appreciable moisture low in the atmosphere (compare the red line in the weighting function figure to the dotted black line that represents mixing ratio). Loops are shown from the GOES-12 sounder for observed radiance at 6.5 micrometer (here), 7.0 micrometer (here), and 7.4 micrometer (here). The 6.5-micrometer loop shows information from highest in the atmosphere (and that particular channel is the noisiest), the 7.0-micrometer is for data that are somewhat lower and the 7.4-micrometer data are lower still. Only the 7.4 micrometer channel suggests an impulse upstream of where the convection eventually fired. (Note that all three animations suffer greatly from the eclipse of the satellite, that is, when the satellite (and its energy-generating solar panels) are in the Earth’s shadow. Batteries on GOES-13 through GOES-15, and on to-be-launched GOES-R will power the satellite through the eclipse.) The loop of 7.4-micrometer sounder data from just before to just after the eclipse (below), however, does show an impulse rotating towards the Gulf.

The three levels of water vapor information in the sounder can be used to produce a three dimensional distribution of moisture in the atmosphere through a sounder data retrieval. If those distributions are then transported in a Lagrangian framework by winds from a numerical model, then later distributions of moisture can be computed through the period of eclipse. (Similarly, later distributions can be computed if cloudiness develops, as the clouds restrict sounder retrievals just as much as data outages do). This method has been described in a previous CIMSS GOES blog entry and forecasts are routinely available at this CIMSS website.

Nearcast predictions of precipitable water differences between two layers (900-700 mb and 700-300 mb) show a maximum (light green color) — that is, a maximum in relatively dry air over relatively moist air, i.e., convective instability) in the region of convective development off the south-central coast of Lousiana. Because numerical models showed a middle-tropospheric vorticity center moving over this region (Click here to see the 06-h forecast valid at 0600 UTC), the region to focus on for convective development can be limited to the region off the coast of southern Louisiana.

(Added: UW Convective Initiation showed no signal for this case, likely because the active initial cloud growth occurred during the eclipse).

Because GOES-13 data were flowing 18 March 2010, a comparison of the sounder channels is possible. Below are loops for 6.5 micrometers (top) and 7.4 micrometers (bottom). Only GOES-12 data is lost during the eclipse; the GOES-13 6.5 micrometer channel is far cleaner; the impulse that gives rise to the convection is evident in the 7.4 micrometer channel (detecting lower into the atmosphere) but not in the 6.5 micrometer channel. In a dry atmosphere, information from the longer-wavelength water vapor channel on the sounder can give important clues to the movement of vapor in the lower troposphere.

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The visible imagery loop from GOES-12, above, shows cirrus clouds around southern Wisconsin at 1315 UTC on 16 March (1st image in loop), and at 2215 UTC on 16 March (last image in loop), but relatively few at 1615 UTC (middle image). In contrast, the infrared imagery loop (here), indicates cirrus clouds increasing throughout the day; Brightness temperatures associated with the cirrus over southern Wisconsin are fairly warm — near 270 K — because energy from lower in the atmosphere was able to pass through the cirrus clouds, adding to the radiance emitted by the cold cirrus clouds.

Ground truth from Madison (the yellow dot in the visible and 11 micron loops) from a Tower Camera looking west at 1322 UTC, 1619 UTC (Note also the excellent example of a contrail shadow in this image) and 2224 UTC, and looking north at 1324 UTC, 1624 UTC and 2223 UTC all show similar amounts of cirrus cloud coverage.

Cirrus is difficult to detect because backscatter of radiation from the ice crystals can be limited. Indeed, the 1322 UTC Tower Cam image looking west (here) appears to show little cirrus because visible radiation from the sun rising in the east is not effectively scattered backwards by the cloud. The same thing is happening in the visible satellite imagery at 1615 UTC: apparent clarity in the visible occurs because backscatter from cirrus of solar radiation is small. The 11-micron image from the same time shows clouds over southern Wisconsin, but with warm brightness temperatures.

Although Cirrus clouds do not backscatter visible solar radiation effectively, they do very effectively backscatter radiation with wavelengths near 1.38 microns. Furthermore, radiation with a wavelength of 1.38 microns that is emitted by the Earth is strongly absorbed by water vapor. Thus, the largest signal is from solar radiation reflected off cirrus clouds. The MODIS instrument, aboard both Terra and Aqua satellites, detects radiation at 1.38 microns, and the image for 1622 UTC is shown below. Cirrus clouds are indicated over southern Wisconsin, and also over lower clouds over eastern Minnesota and Iowa. The ABI instrument, to be aboard GOES-R when it launches, will also detect radiation at 1.38 microns.

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