Detecting ship condensation trails in the Gulf of Alaska

November 19th, 2010 |
POES AVHRR images and derived cloud products

POES AVHRR images and derived cloud products

A comparison of AWIPS images of the 1-km resolution POES AVHRR 0.86 µm visible channel, 12.0 µm IR channel, Cloud Top Temperature product, Cloud Type product, Cloud Top Height product, Cloud Optical Depth product, and Cloud Particle Effective Radius product (above) demonstrated that the Cloud Particle Effective Radius product was the best tool to use in locating the pattern of ship condensation trails (or “ship tracks”) that was located over the northern Gulf of Alaska on 19 November 2010.

The Cloud Type of the stratus cloud deck where the ship tracks were seen was primarily of the “supercooled” classification (green color enhancement), with Cloud Top Temperatures generally in the -3º to -4º C range and Cloud Top Heights of 2-3 km. According to the Cloud Particle Effective Radius product, the size of the particles within the ship track features was about 23-25 µm — somewhat smaller than the 27-31 µm in the surrounding undisturbed stratus cloud deck. This is evidence of the “Twomey effect”, where sulfate aerosols (found in the exhaust plumes of the diesel-burning ships) act as cloud condensation nuclei, which leads to greater numbers of smaller cloud droplets.

As part of the CIMSS involvement in GOES-R Proving Ground activities, POES AVHRR satellite images and products are currently being made available in an AWIPS format for interested NWS forecast offices to add to their local AWIPS workstations (via LDM subscription). For more information, see the  AVHRR Imagery and Products in D-2D site. A VISIT training lesson has also been created for these AVHRR images and products.

Cold air outbreak (and ice formation) across Hudson Bay, Canada

November 18th, 2010 |
GOES-13 0.63 µm visible channel images

GOES-13 0.63 µm visible channel images

A strong cyclone was located over the northern portion of Hudson Bay, Canada on 18 November 2010 — and McIDAS images of GOES-13 0.63 µm visible channel data (above) showed the formation of widespread cloud bands due to very cold arctic air flowing over the still-unfrozen waters of Hudson Bay.

The strong westerly component of the surface winds was apparently helping to cause some of the land-fast ice along the far western shoreline to begin to drift eastward into the open waters of Hudson Bay. This was more easily seen on a zoomed-in version of the GOES-13 visible images. According to lower-tropospheric GOES-13 atmospheric motion vectors, the speed of motion of the cloud band features was generally in the 25-35 knot range.

A closer view using AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data (below) revealed better detail in the cloud band structure — but also suggested the initiation of ice formation along the western and southwestern near-shore waters of Hudson Bay.

MODIS 0.65 µm visible channel images

MODIS 0.65 µm visible channel images

To verify that the brighter near-shore features seen on the MODIS visible images were indeed ice forming in Hudson Bay, a pair of false-color Red/Green/Blue (RGB) images were created using the MODIS 0.65 µm visible channel data as the Red component and the MODIS 2.1 µm “snow/ice channel” data as the Green and Blue components of the image. Snow cover (which was generally 3-5 inches at the first-order reporting stations in the region) and thick ice show up as darker red features on the RGB images.

MODIS false-color Red/Green/Blue (RGB) images

MODIS false-color Red/Green/Blue (RGB) images

An AWIPS image of the POES AVHRR Cloud Type product (below) indicated that many of the cloud bands likely consisted of supercooled water droplets (green color enhancement), although a number of the cloud bands were beginning to show signs of glaciating farther downstream (as indicated by the yellow and orange color enhancements).

POES AVHRR Cloud Type product

POES AVHRR Cloud Type product

The POES AVHRR Cloud Top Height product (below) showed that the tops of most of thee cloud bands were in the 3-4 km range.

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

Another indication of the change from supercooled water droplets to more of a glaciated composition could be seen on the POES AVHRR Cloud Particle Effective Radius product (below), with an increasing presence of the larger size ice crystals showing up as increasingly darker blue colors farther downstream.

POES AVHRR Cloud Particle Effective Radius product

POES AVHRR Cloud Particle Effective Radius product

Blowing glacial silt from the Alaska Panhandle region

November 17th, 2010 |
GOES-11 0.65 µm visible channel images

GOES-11 0.65 µm visible channel images

McIDAS images of GOES-11 0.65 µm visible channel data (above) showed hazy plumes of glacial silt blowing southwestward from the Alaska Panhandle region out over the adjacent offshore waters of the Gulf of Alaska on 17 November 2010. A NOAA-19 false-color Red/Green/Blue (RGB) image created using AVHRR channels 1, 2, and 4 (below) offered another view with a bit more contrast, making the plumes of airborne particulate matter somewhat easier to see.

NOAA-19 AVHRR false-color Red/Green/Blue (RGB) image

NOAA-19 AVHRR false-color Red/Green/Blue (RGB) image

Mountain waves over the Mid-Atlantic states

November 17th, 2010 |
MODIS 6.7 µm water vapor and GOES-13 6.5 µm water vapor images

MODIS 6.7 µm water vapor and GOES-13 6.5 µm water vapor images

An AWIPS image comparison of  MODIS 6.7 µm and GOES-13 6.5 µm “water vapor channel” data (above) demonstrated the advantage of improved spatial resolution for detecting the structure and areal coverage of mountain waves that were present over the Mid-Atlantic states on 17 November 2010. The spatial resolution of the MODIS water vapor image is 1 km, compared to 8 km for the GOES-13 water vapor image (note that the native resolution of the water vapor channel on GOES-13 is actually 4 km, but this image was viewed on the AWIPS “CONUS” scale, which downgrades the resolution to 8 km).

About 2 hours later, an AWIPS image of the POES AVHRR Cloud Top Height product (below) indicated that the tops of the mountain wave cloud bands at that time were generally in the 3-4 km range, which corresponded well to some of the altitudes of pilot reports of moderate turbulence (8,000-10,000 feet above ground level).

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

As part of the CIMSS involvement in GOES-R Proving Ground activities, MODIS and POES AVHRR satellite images and products are currently being made available in an AWIPS format for interested NWS forecast offices to add to their local AWIPS workstations (via LDM subscription). For more information, see the MODIS Imagery in D-2D and AVHRR Imagery and Products in D-2D sites. VISIT training lessons are also available for these MODIS and AVHRR images and products.