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

GOES-14 sounder visible image

GOES-14 (launched on 27 June 2009) is currently undergoing its Post Launch Test during the Summer and Fall of 2009. A GOES-14 sounder visible image (above) displayed  a variety of cloud systems across  the eastern US at 17:46 UTC on 10 September 2009. A preliminary version of the GOES-14 sounder Cloud Top Pressure derived product (below) indicated a wide range of cloud tops, from clusters of high (cold) clouds over the Gulf Coast region (blue to white color enhancement), to patches of low (warm) clouds over areas such as eastern Virginia/North Carolina and central Nebraska (orange to peach color enhancement).

GOES-14 sounder Cloud Top Pressure product

Qualitatively, the GOES-14 sounder Cloud Top Pressure values (above) agreed well with those derived using GOES-12 sounder data (below). Note that both images are displayed using their respective native satellite projections.

GOES-12 sounder Cloud Top Pressure product

Focusing on the aforementioned low cloud top features seen over eastern Virginia/North Carolina and over central Nebraska, both the GOES-12 and the GOES-14 Cloud Top Pressure (CTP) derived products indicated that those cloud tops were generally in the 700-800 mb range (orange color enhancement). Using the AWIPS cursor “Skew-T cloud height sampling” functionality, there was good agreement with the sounder CTP values: both the Moorhead City NC rawinsonde and the RUC model sounding over central Nebraska suggested that the GOES sounder IR brightness temperatures over those locations existed near the 700 mb pressure level (below).

GOES sounder IR image + Skew-T cloud height sampling (rawinsonde)

GOES sounder IR image + Skew-T cloud height sampling (RUC model)