Long-lived low cloud edge over the Eastern Pacific Ocean

August 25th, 2010 |

Visible and infrared GOES-11 imagery over the eastern Pacific Ocean have indicated a persistent southwestward-moving cloud edge during the past several days. The visible image, above, from 1830 UTC on 25 August, shows a distinct cloud edge arcing from northwest to southeast. What is the history of this feature? The visible imagery loop below, showing 1800 UTC images over the course of 4 days, show that the feature likely has its roots in dry air exiting the North American continent. A loop of 11-micron imagery from GOES-11 every two hours (here) shows the steady progression of the feature, and the persistent sharpness of the edge. Brightness temperatures of the clouds are steady near 287-288 K; brightness temperatures in the clear region are 290-291 K.

Unusually long and thin cloud band over the Arctic Ocean

February 23rd, 2010 |
AVHRR false color RGB image + map of land boundaries (green)

AVHRR false color RGB image + map of land boundaries (green)

Andy Heidinger (NOAA/NESDIS/Advanced Satellite Products Branch) pointed out a very long and thin cloud feature, which can be seen near the center of the false color Red/Green/Blue (RGB) image created using AVHRR imagery (above). In this particular RGB image, low clouds appear dark blue, while cirrus clouds are white. The cloud feature of interest (which stretched from the North Slope region of Alaska westward across the Arctic Ocean to the north of Siberia on 23 February 2010) appeared to be over 1000 km long and less than 10 km wide — a perfect candidate for the “What the heck is this?” blog category!

With a strong high pressure cell in place over the North Pole, it is possible that this thin cloud arc marked the leading edge of a relatively weak cold frontal boundary. The southward progress of this cloud feature could be followed on a sequence of AWIPS images of AVHRR 12.0 µm IR channel data (below) — the cloud arc was highlighted with a yellow to cyan color enhancement, representing IR brightness temperatures of -10º to -20º C. In addition, well offshore of the northeastern coast of Alaska you could also see the warmer thermal signature (denoted by the yellow color enhancement) of large thin spots and cracks forming in the the sea ice covering the Arctic Ocean.

It may be pure coincidence, but when this thin cloud arc passed southward across northern Alaska coastal station PAWI (Wainwright), they briefly reported freezing fog and a drop in visibility to 0.5 mile.

AVHRR 12.0 µm IR images

AVHRR 12.0 µm IR images

The progression of this cloud band could also be seen on a sequence of grayscale AVHRR composite IR images (below, courtesy of Matthew Lazzara, AMRC). The darker appearance of the cloud arc on the grayscale images supported the idea that this was indeed a relatively warm low cloud feature.

AVHRR composite IR images

AVHRR composite IR images


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AVHRR Cloud Type product

AVHRR Cloud Type product

An AWIPS image of the AVHRR Cloud Type product (above) indicated that the cloud arc feature was composed primarily of supercooled water droplets (green color enhancement). The time of the AVHRR Cloud Type image corresponds to the time when Wainwright (station identifier PAWI) reported a brief period of freezing fog as the cloud arc passed southward through the area.

The corresponding AVHRR Cloud Top Height product (below) indicated that the top of the thin cloud band was in the 2-3 km range (darker purple color enhancement).

AVHRR Cloud Top Height product

AVHRR Cloud Top Height product

Instability vortices along a jet stream axis

September 28th, 2009 |

GOES-12 6.5 µm water vapor imagery

GOES-12 6.5 µm water vapor imagery

AWIPS images of the GOES-12 6.5 µm “water vapor channel” (above) revealed a pair of vortices immediately poleward of a well-defined jet stream  axis that was moving over the southeastern US  on 28 September 2009. It was initially thought that these vortices may have represented either a type of  Kelvin-Helmholtz instability (which can occur when there is sufficient velocity difference across the interface between two fluids) or a type of Rayleigh-Taylor instability (which can occur along an interface of two fluids of different densities) — however, a more likely explanation might be that these vortices were a result of barotropic instability, where the waves grew by extracting kinetic energy from the shear flow from which they were embedded.

If a horizontal circulation  developed due to barotropic instability being forced by the horizontal wind shear, this could result in the formation of the vortex structures seen on the water vapor imagery. The warm/dry spot on the images (exhibiting brightness temperature values as warm as -11º C, darker orange color enhancement) was probably a pocket of warm/dry air that originated from the poleward edge of the moisture gradient — once the vortex formed, the warm/dry air in the center could not escape, and its properties would be preserved.  (Thanks to Jordan Gerth, Justin Sieglaff and Chris Rozoff at CIMSS…and Michael Morgan at UW-AOS for providing valuable inputs and helping to provide an explanation)

Overlays of parameters from the 45-km resolution CRAS model at 12:00 UTC  (below) showed the presence of a 50-60 knot jet axis just south of the primary dry-to-moist gradient on the water vapor image, along with a ribbon of 500 hPa vorticity and a 500 hPa wind shear axis over the region where the water vapor vortices were forming.

CRAS45 maximum wind speed, 500 hPa vorticity, and 500 hPa shear vectors

CRAS45 maximum wind speed, 500 hPa vorticity, and 500 hPa shear vectors

A comparison of the 1-km resolution MODIS 6.7 µm water vapor image and the 4-km resolution GOES-12 6.5 µm water vapor image (below) show the advantage of improved spatial resolution for displaying the structure and gradients associated with the leading vortex around 18:15 UTC.

1-km MODIS vs 4-km GOES-12 water vapor images

1-km MODIS vs 4-km GOES-12 water vapor images

Examining the GOES-12 imager water vapor weighting function profiles at 00:00 UTC for Charleston SC (located in the “dry” portion of the sharp water vapor image gradient) and Jacksonville FL (located in the “moist” portion of the sharp water vapor image gradient) shows that there would be a pronounced downward shift in the altitude of features displayed on the water vapor image in the region of dry air located poleward of the jet stream axis.

GOES-12 water vapor weighting function profile for Charleston SC and Jacksonville FL

GOES-12 water vapor weighting function profile for Charleston SC and Jacksonville FL

A northwest-to-southeast oriented vertical cross section using GFS40 model fields (below) displayed a minor intrusion of potential vorticity (the colored image portion of the cross section) downward into the upper troposphere immediately poleward of the jet stream core (which was located between the 200 and 250 hPa pressure levels). The wind speed shear axis was located at a much lower altitude (between the 400 and 500 hPa pressure levels), closer to the altitude peak of the water vapor channel weighting function in the region of drier air.

GFS40 cross section

GFS40 model cross section

MTSAT-1R Band 5 Anomaly

September 18th, 2009 |

MTSAT-1R shortwave IR image

Night-time MTSAT-1R 3.75 µm shortwave IR image (14:30 UTC)

A curious image artifact was noted on MTSAT-1R 3.75 µm shortwave IR imagery, in the form of a very cold “false eye” appearing  just to the right of the actual eye of Typhoon Choi-Wan (17W) in the western North Pacific Ocean . The “ghost” of the eye exhibited a satellite radiance of zero (very cold, appearing bright white on the image), and was offset from the true eye by 16 pixels in the horizontal and 1-2 pixels in the vertical. This false eye was most apparent on MTSAT-1R imagery during the local night-time hours, as was seen on 16 September 2009 at 14:30 UTC (above) and 15:30 UTC (below).

MTSAT-1R shortwave IR image (15:30 UTC)

Night-time MTSAT-1R 3.75 µm shortwave IR image (15:30 UTC)

This false eye artifact was also evident during local daytime hours, but the “ghost” did not exhibit zero radiance — the feature could be seen better on the shortwave IR imagery once a contrast stretch  enhancement was applied (below). The offsets of the false eye were the same as seen during local night-time hours.

MTSAT-1R shortwave IR (original and enhanced) and visible images

Daytime MTSAT-1R shortwave IR (original and enhanced) + MTSAT-1R visible images

A similar (but less obvious) image artifact could be seen on a night-time shortwave IR image over China (below) — there was a bright white “ghost”  to the  right of the warm area that was between the two colder cloud features (again, the ghost feature was offset to the east by 16 pixels with a vertical displacement of 1-2 pixels).

Night-time MTSAT-1R 3.75 µm shortwave IR image (over China)

Night-time MTSAT-1R 3.75 µm shortwave IR image (over China)

The exact cause of these image artifacts is not known; however, since the MTSAT-1R satellite scans from left to right using a Charge-Coupled Device (CCD) array,  the satellite sensor may be overcompensating for the CCD “quantum wells” losing more charge faster than expected, subtracting more of a bias than it should (this could also be a side-effect of sensor aging). Instrument cross-talk could be another source of this type of image anomaly.

Kudos to Chris Schmidt at CIMSS for processing and analyzing these MTSAT-1R images, and supplying the explanations of possible causes of such an image artifact.