MODIS visible and near-IR snow/ice channel images

CIMSS has been distributing MODIS imagery and products in AWIPS (via LDM subscription) since 2006 — and we are now in the process of testing and evaluating AVHRR imagery and products for distribution to NWS forecast offices via a similar process. Let’s now utilize a few of these MODIS and AVHRR products to interrogate snow cover and cloud features across parts of Idaho and western Montana on 13 November 2009. A comparison of the 1-km resolution MODIS visible channel and 2.1 µm near-IR “snow/ice channel” images (above) showed a broad area of snow cover across the region, which fell during the previous 2 days. Snow cover (in addition to dense tree cover, and water) are strong absorbers at the 2.1 µm wavelength, so these features appear much darker on the snow/ice channel image — therefore, bright features on the visible image that are also dark on the snow/ice image are indeed snow. The maximum snow depth at the time was 17 inches (43 cm) at Bozeman (station identifier KBZN), located near the center of the images.

A comparison of the 1-km resolution MODIS snow/ice channel and the MODIS Land Surface Temperature (LST) product (below) revealed that this area of snow cover was having an obvious effect on Land Surface Temperatures across the state of Montana: LST values in the bare ground portions of the east were in the 40º to 50º F range (green to yellow colors), while the areas with deep snow on the ground exhibited LST values in the +5º to +15º F range (cyan to blue colors). The daily high temperatures across the state of Montana on 13 November ranged from 20º F at Three Forks in the southwest to 44º F at Glendive in the far east — and the coldest morning low was -14º F at Wisdom in the far west.

MODIS snow/ice channel and Land Surface Temperature product

Now let’s focus our attention on the cloud features that were over parts of Idaho and Montana at that time. A comparison of the 1-km resolution MODIS 11.0 µm “IR window” and 3.7 µm “shortwave IR” images (below) showed that there were some very cold cloud features that were likely cirrus (brightness temperatures of -30º to -40º C, blue to green colors) over northern Idaho and far northwestern Montana on the IR window image — however, there was a large area of clouds located just to the east of those cirrus clouds that exhibited significantly warmer (+15º to +25º C, darker gray) appearance on the 3.7 µm shortwave IR image. The shortwave IR channel is very sensitive to the reflection of solar radiation of the tops of supercooled water droplet clouds — so a quick comparison of the IR window and the shortwave IR channels offers some cursory information on the character and composition of various cloud features. Note that there also appeared to be a few other darker patches of supercooled water droplet clouds located over parts of southwestern Montana and southern Idaho.

MODIS 11.0 µm IR window and 3.7 µm shortwave IR images

The 4-km resolution MODIS Cloud Phase product (below) offered confirmation about the presence of ice crystal cirrus clouds (salmon color enhancement) over northern Idaho and far northwestern Montana, with supercooled water droplet clouds (blue color enhancement) located farther to the east.

MODIS Cloud Phase product

The 1-km resolution AVHRR Cloud Type product (below) supported the MODIS Cloud Phase product, indicating supercooled water droplet cloud (cyan color enhancement) to the east of the various classifications of ice crystal cloud (yellow, orange, and red color enhancements) over northern Idaho and far northwestern Montana.

AVHRR Cloud Type product

The 1-km resolution AVHRR Cloud Top Temperature (CTT) product (below) showed that the area of supercooled water droplet cloud exhibited CTT values of -18º to -20º C (cyan colors), with the cirrus cloud features farther to the west exhibiting CTT values as cold as -40º to -50º C (darker blue colors).

AVHRR Cloud Top Temperature product

The 1-km resolution AVHRR Cloud Top Height product (below) indicated that the tops of the supercooled water droplet clouds over northwestern Montana were around 4 km or 13,000 feet (light yellow color enhancement), with the tops of the cirrus clouds farther to the west at a much higher 8 km or 26,000 feet (darker orange color enhancement).

AVHRR Cloud Top Height product

The 1-km resolution AVHRR Cloud Particle Effective Radius product (below) indicated that the supercooled water droplet cloud particles in northwestern Montana were generally in the 20-25 micrometer range (cyan colors), with the cirrus cloud ice crystals farther west at a much larger 40-50 micrometers (darker blue colors).

AVHRR Cloud Particle Effective Radius product

For the sake of comparison, let’s also examine the corresponding “10 km” resolution GOES Sounder Cloud Top Height (CTH) derived product image (below), which actually has an effective field of view closer to 20 km for large satellite viewing angles over the northern Lower 48 states — Sounder CTH values ranged from 7,000-12,000 feet (orange to yellow to green colors) for the supercooled water droplet clouds in northwestern Montana up to 35,000 feet (lighter cyan colors) for the cirrus clouds located just to the west.

Note that the GOES Sounder Cloud Top Height product (as well as some of the AVHRR cloud products shown above) indicated a number of “false cloud features” in the area of the deep snow cover over southwestern Montana — the large temperature gradients associated with the edges of such areas of snow cover can sometimes fool the cloud product algorithms into portraying cloud top height or cloud top temperature data where no clouds actually exist.

GOES Sounder Cloud Top Height derived product image

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A strong storm is bringing high winds and rain to the east coast of the United States from North Carolina northward to New Jersey. This dangerous weather will persist through tomorrow. The weather results from the combination of the extratropically transitioned remnants of Ida — over southern North Carolina — and a strong high pressure system over New England. (See a surface analysis here). Various satellite-derived products can be used to explore this system.

Consider the water vapor loop above. Towards the end of the loop, features in the vapor are developing and moving westward over Virginia and North Carolina. That observation combined with the continued eastward motion in the water vapor signal over the southeast part of the US suggests the formation of a closed circulation. Such a development will slow the eastward progression of the system, prolonging the period of stormy weather on the coast.

Satellite observations of total precipitable water (a blended product from AMSU and SSM/I on the NOAA series of Polar Orbiters) show large values — greater than 200% of normal — over the eastern United States. Superimposed near-surface winds from the QuikScat scatterometer show a broad region of gale-force winds over the Ocean. The long fetch of the wind over open ocean will allow large waves to develop. (A zoomed-in version of the QuikScat winds, here, includes a 57-knot wind with a rain flag of only 1% — meaning it’s a “good” wind. Peak surface wind gusts from reporting stations on land at this time included 44 knots at Norfolk, Virginia, 43 knots at Wallops Island, Virginia, and 42 knots at Elizabeth City, NC). The long duration of the storm event and the winds will exacerbate matters. A loop of precipitable water derived from SSMI and AMSRE (here) shows the tropical origins of the moisture over the eastern part of the United States, and also the movement of more moisture in from the east.

Abundant moisture is leading to large rainfalls. Rainfall rates are estimated using data from the AMSU instrument on the NOAA series of POES spacecraft. There are numerous pixels in the short loop above, including suggesting rains exceeding 20 mm per hour. There is also a westward drift suggested in the loop.

Visible image loops (rocking loops) from GOES-12 and GOES-14 show the westward drift of clouds into western Virginia and the Carolinas as the system starts to close off. A near-surface circulation center can also be inferred over southern North Carolina.

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MODIS true color images (30 September, 01 November, 10 November)

A sequence of three MODIS true color images from the SSEC MODIS Today site (above) showed an increasing level of turbidity of the water along the Gulf Coast — the 3 images are from 30 September, 01 November, and 10 November 2009. This increase in turbidity can be directly attributed to the runoff of sediment-rich water due to heavy precipitation across much of the Gulf Coast states from late October into early November, as shown in the 14-day observed precipitation map (below). Special thanks to Steve Davis and Jeff Craven at the National Weather Service forecast office at Milwaukee/Sullivan for creating/capturing these images and bringing this case to our attention!

14-day observed precipitation

AWIPS images of the MODIS Sea Surface Temperature (SST) product (below) showed that the Gulf of Mexico immediately offshore was significantly colder due to this discharge of sediment-rich water from rivers draining from the Gulf Coast states — SST values were in the low to mid 60s F (darker green colors) right along the coast, compared to the mid 70s to near 80º F (darker red colors) farther offshore.

MODIS Sea Surface Temperature product (November 10 - 12)

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A morphed microwave imagery loop (from the MIMIC website at CIMSS) shows 48 hours of structural changes as observed from microwave imagery. Note the strong eyewall-like structures at the beginning of the loop, with persistent strong convection noted west of the storm center. As the storm moves over colder water, and as southwesterly shear increases, the structure deteriorates and strong convection becomes concentrated in regions some distance east and north of the storm center.

The color-enhanced loop, above, of 11-micron brightness temperature also shows Hurricane Ida (dowgraded to a Tropical Storm at 9 AM EDT on Monday) moving northward from the Straits of Yucatan into the central Gulf of Mexico. As it moves, the appearance of the storm deteriorates markedly. At the start of the loop, a relatively warm region in the center of the central dense overcast (CDO) that might be the eye of the storm migrates to the southwest edge of the CDO and then vanishes. This change shows the effects of strong southwesterly winds that are moving the tops of the thunderstorm away from the center of the storm.

Maps of shear (wind vector differences between the upper and lower troposphere) show a more hostile environment for the storm between 1200 UTC Sunday — when Ida was a strengthening category 1 hurricane in the Straits of Yucatan — and 1200 UTC Monday, when Ida was a weakening category 1 hurricane over the central Gulf of Mexico. Maps of mid-level shear, that is changes in wind between the lower and middle troposphere, for 1200 UTC Sunday and Monday tell a similar tale.

Ida’s path is forecast over progressively cooler sea-surface temperatures. That in combination with the hostile shear environment suggest that re-strengthening to hurricane status is unlikely. Strong high pressure off the East Coast of the United States, however, as shown in an analysis here suggest a large pressure gradient that will support strong winds over the entire southeast part of the United States as Ida approaches the central Gulf Coast.

Visible imagery, above (from GOES-14), show the results of shear on the cloud patterns. Deep convection is offset to the northeast of the circulation center (shown as the yellow dot in the imagery, from the 1500 UTC National Hurricane Center discussion). Latent heat in the storms cannot affect the southern/western semicircles of the storm if the storms are all displaced by shear to the north and east, as in this rocking loop. A similar loop from GOES-12 is shown here.

Precipitable water estimates from microwave imagery, above, show deep tropical moisture over the Gulf of Mexico. As this tropical moisture moves over the cooler air at the surface over the southeast part of the United States, it will cool and water will condense out. Heavy rains are predicted in the next two days as that happens.

For more information on Ida, please visit the CIMSS Tropical Weather Website or the National Hurricane Center website.

Added: Late afternoon infrared satellite imagery from GOES 14 and visible imagery from GOES 14 show convection nearly wrapping around the center of the storm, just south of the mouth of the Mississippi River. However, persistent shear appears to be over-riding that tendency.

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