Mesoscale Convective System over Alabama

July 31st, 2012
POES AVHRR 10.8 µm IR image + SPC storm reports of hail and damaging winds

POES AVHRR 10.8 µm IR image + SPC storm reports of hail and damaging winds

A large mesoscale convective system (MCS) developed in western Tennessee, which then grew in size and intensity as it propagated southward across Alabama on 31 July 2012. An AWIPS image of 1-km resolution POES AVHRR 10.8 µm IR channel data (above) showed cloud top IR brightness temperatures as cold as -87º C (darker purple color enhancement). Overlaid on the image are the cumulative SPC storm reports of hail (green) and damaging winds (cyan).

GOES-13 10.7 µm IR channel images (below; click image to play animation) showed the increase in size of cold cloud tops as the MCS moved southward, with cloud top IR brightness temperatures as cold as -82º C (purple color enhancement).

GOES-13 10.7 µm IR channel images (click image to play animation)

GOES-13 10.7 µm IR channel images (click image to play animation)

A comparison of a 375-meter resolution (projected onto a 1-km AWIPS grid) VIIRS 11.45 µm IR image with the corresponding 4-km resolution GOES-13 10.7 µm IR image (below) showed the advantage of higher spatial resolution in the identification of locations of the coldest cloud tops and their magnitude (-88º C on the VIIRS image, vs -80º C on the GOES-13 image). Also note the slight northwestward shift in the location of features on the GOES-13 image, a result of parallax due to the larger viewing angle from the GOES-East satellite located at 75º West longtude.

VIIRS 11.45 µm IR channel image + GOES-13 10.7 µm IR channel image

VIIRS 11.45 µm IR channel image + GOES-13 10.7 µm IR channel image

With illumination from moonlight, the Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) can serve as a “visible channel” at night — and a comparison with the corresponding 11.45 µm IR image (below) aided in the identification of features such as overshooting tops and thunderstorm top gravity waves. Note how some city lights can be seen through the thinner edges of the MCS cirrus canopy.

Suomi NPP VIIRS 0.7 µm Day/Night Band image + 11.45 µm IR channel image

Suomi NPP VIIRS 0.7 µm Day/Night Band image + 11.45 µm IR channel image

This MCS produced a great deal of lightning, with over 2500 cloud-to-ground strikes in the 15-minute period preceeding the VIIRS DNB image (below). Also note the appearance of several brighter white “smeared” pixels, which indicated portions of the thunderstorm cloud top which were illuminated by lightning as the VIIRS instrument was scanning the area.

Suomi NPP VIIRS 0.7 µm Day/Night Band image + METAR surface reports + cloud-to-ground lightning strikes

Suomi NPP VIIRS 0.7 µm Day/Night Band image + METAR surface reports + cloud-to-ground lightning strikes

The Blended Total Precipitable Water (TPW) product (below; click image to play animation) indicated that the MCS was moving southward toward an axis of higher TPW values (50-60 mm or 2.0-2.4 inches, red to purple color enhancement).

Blended Total Precipitable Water product

Blended Total Precipitable Water product

A closer view using 10-km resolution GOES-13 sounder derived product images of TPW, Lifted Index, and Convective Available Potential Energy (below) showed that there were pockets of higher moisture and instability ahead of the advancing MCS.

GOES-13 sounder Total Precipitable Water, Lifted Index, and Convective Available Potential Energy derived product images

GOES-13 sounder Total Precipitable Water, Lifted Index, and Convective Available Potential Energy derived product images

Tornado near Mt. Evans, Colorado

July 28th, 2012
GOES-15 (left) and GOES-13 (right) 10.7 µm IR channel images

GOES-15 (left) and GOES-13 (right) 10.7 µm IR channel images

A tornado was observed and photographed near Mt. Evans in north-central Colorado around 2:51 PM local time or 20:51 UTC (Local Storm Report) on 28 July 2012. A comparison of McIDAS images of 4-km resolution GOES-15 (GOES-West) and GOES-13 (GOES-East) 10.7 µm IR channel data (above) showed that the thunderstorm which produced the tornado developed fairly rapidly just to the east of Copper Mountain (station identifier KCCU), but was relatively small in size and did not exhibit particularly cold cloud top IR brightness temperatures (-40º C at 20:45 UTC, and -52º C at 21:15 UTC).

AWIPS images of 1-km resolution MODIS 0.6 µm visible channel and 11.0 µm IR channel data at 20:28 UTC (below) showed that the storm was beginnng to produce a few negative cloud-to-ground lightning strikes, with a minimum IR brightness temperature around -50º C at that time.

MODIS 0.65 µm visible and 11.0 µm IR channel images (with cloud-to-ground lightning strikes)

MODIS 0.65 µm visible and 11.0 µm IR channel images (with cloud-to-ground lightning strikes)

The CIMSS Cloud Top Cooling Rate product (below) did flag the storm at 20:32 UTC as it was beginning to develop in north-central Colorado.

GOES-13 instantaneous Cloud Top Cooling Rate product

GOES-13 instantaneous Cloud Top Cooling Rate product

Saharan dust over South Florida and the adjacent offshore waters

July 25th, 2012
GOES-13 0.63 µm visible channel images

GOES-13 0.63 µm visible channel images

McIDAS images of GOES-13 0.63 µm visible channel data (above) showed the hazy signature of a dense veil of airborne Saharan dust over South Florida and the adjacent offshore waters of the Gulf of Mexico and the Atlantic Ocean on 25 July 2012. This particular major pulse of Saharan dust began to move westward across the Atlantic Ocean during mid-July.

A closer view using an AWIPS image of Suomi NPP VIIRS 0.64 µm visible channel data (below) also showed the hazy signature of the dust over the southeastern portion of the satellite scene. A pilot reported flight visibility of 4 to 5 statute miles within the dust layer over the Florida Keys.

Suomi NPP VIIRS 0.64 µm visible channel image + Pilot reports

Suomi NPP VIIRS 0.64 µm visible channel image + Pilot reports

A comparison of the VIIRS 0.64 µm visible image with the corresponding VIIRS 11.45 µm IR image (below) demonstrated that there was no dust signal at that particular IR wavelength, due to the fact tht an elevated dust layer is generally transparent to thermal radiation from the land and ocean surfaces below.

Suomi NPP VIIRS 0.64 µm visible channel + 11.45 µm IR channel images

Suomi NPP VIIRS 0.64 µm visible channel + 11.45 µm IR channel images

The VIIRS Aerosol Optical Thickness product (below) showed a strong signal (yellow to red color enhancement) over that region due to the presence of the airborne dust. Yet another strong pulse of Saharan dust could be seen exiting  the coast of Africa and moving westward across the Atlantic Ocean,

Suomi NPP VIIRS Aerosol Optical Thickness product

Suomi NPP VIIRS Aerosol Optical Thickness product

Ice floes in Hudson Bay

July 24th, 2012
Suomi NPP VIIRS 0.64 µm visible channel images

Suomi NPP VIIRS 0.64 µm visible channel images

Even though most of Hudson Bay in Canada was essentially ice-free on 24 July 2012, there were still some ice floes consisting of thick first year ice that remained in the far southwestern part of the Bay. AWIPS images of Suomi NPP VIIRS 0.64 µm visible channel data (above) showed the motion of these ice floes (along with the other cloud features in the region) between 17:50 and 19:29 UTC.

A comparison of a VIIRS 0.64 µm visible channel image with the corresponding 3-channel Red/Green/Blue (RGB) image created by using 0.64 µm, 1.61 µm, and 11.45 µm data (below) demonstrated the utility of using RGB imagery to help discriminate between ice features (darker purple color enhancemnt) and clouds (varying shades of white).

Suomi NPP VIIRS 0.64 µm visible channel + false-color Red/Green/Blue (RGB) image

Suomi NPP VIIRS 0.64 µm visible channel + false-color Red/Green/Blue (RGB) image

Similarly, a comparison of a MODIS 0.65 µm visible channel image with the corresponding RGB image created using 0.64 µm and 2.1 µm data (below) showed the ice features as darker red, compared to supercooled water droplet clouds (lighter shades of white) and glaciated clouds (lighter pink color enhancement).

MODIS 0.65 µm visible channel + false-color Red/Green/Blue (RGB) image

MODIS 0.65 µm visible channel + false-color Red/Green/Blue (RGB) image

The MODIS Sea Surface Temperature (SST) product (below) showed SST values in the middle 30s F (darker blue color enhancement) over the ice floe features, compared to SST values in the low-middle 40s F over the adjacent open waters of Hudson Bay.

MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product