MTSAT-1R visible images

One of the worst dust storms in the past 70 years swept across a large part of eastern Australia on 22 September – 23 September 2009 (Daily Mail Online photos). A sequence of MTSAT-1R visible images (above) showed the progression of the large dust cloud as it moved eastward during the daylight hours. Note the appearance of “lee waves” along the top of the dust cloud, as the strong winds interacted with the high terrain of the Great Dividing Range. An undular bore could also be seen forming out ahead of the cold front, over the offshore waters of the South Pacific Ocean.

The surface meteorogram for Brisbane, Australia (station identifier YBBN) is shown below; note that the surface visibility dropped to 0.2 km (0.1 mile) as the cold front passed, and following the frontal passage the dew point dropped from +16º C (61º F) to -16º C (+3º F).

Brisbane, Australia surface meteorogram

A larger-scale view of the dust cloud feature could be seen using MODIS true color imagery from the NASA MODIS Rapid Response site (below, viewed using Google Maps). See also the NASA  MODIS Image of the Day.

MODIS true color image

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GOES-12 10.7 µm IR images

Madison, Wisconsin received 3.63 inches (92 mm) of rainfall on 22 September 2009 — much of it within a 4 hour period — which set a record for the highest rainfall amount for any day during the month of September (additional details are available from the NWS Milwaukee/Sullivan). AWIPS images of the GOES-12 10.7 µm IR Window channel (above) showed that the cloud top IR brightness temperatures were rather unremarkable, with only a few areas exhibiting temperatures colder than -50º C (yellow color enhancement) during the heavy rainfall event. The 1-km resolution AVHRR Cloud Top Temperature values were only slightly colder, at -54º C — this corresponded to a AVHRR Cloud Top Height of 11-12 km.

GOES-12 visible images (below) did not show any pronounced cloud top texture or shadowing that would indicate strong convective elements over southern Wisconsin — in fact, there were no cloud-to-ground lightning strikes seen during the day, and no thunder was reported at any of the first order observing stations.

GOES-12 visible images

Blended Total Precipitable Water product

The Blended Total Precipitable Water (TPW) product (above) did reveal that an area of TPW values of 40 mm and greater (red colors) developed across northern Illinois and far eastern Iowa after about 08 UTC — this seemed to be associated with a disturbance that developed and moved northward along and ahead of a frontal boundary that was moving slowly eastward across the region. It should be noted that these TPW values were about 175-200% above normal.

However, about 5 hours earlier the MODIS TPW product displayed TPW values of 40 mm and higher (red colors) over parts of northwestern Illinois, at a time when the Blended TPW product was still only indicating TPW values of 30-35 mm (below). This area of higher TPW values over northern Illinois was then advected northward during the day, and lifted along and ahead of a developing warm frontal boundary. A cross section oriented north to south showed a deep column of upward vertical motion (omega values of 10 µbars per second, red colors) over southern Wisconsin.

Blended TPW + MODIS TPW products

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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).

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.

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)

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.

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MTSAT-1R IR images

MTSAT-1R 10.7 µm IR images (above) showed Super Typhoon Choi-Wan (17W) as it reached Category 5 intensity over the western North Pacific Ocean during the 15 September – 16 September 2009 period. The eye of the typhoon was quite large (which moved just to the south of Pagan in the Northern Mariana Islands), with a very cold rings of convection seen during the various eyewall replacement cycles (as also seen on this MIMIC animation). A time series of the Advanced Dvorak Technique (ADT) intensity estimate from the CIMSS Tropical Cyclones site is shown below.

Advanced Dvorak Technique intensity estimates for Choi-Wan

Terra MODIS IR image (14 September)

A pair of 1-km resolution MODIS images show the dramatic change in satellite appearance of Choi-Wan, from one with a ragged eye surrounded by cloud tops as cold as -87º C (darker purple color enhancement) on 14 September (above) to more of a classic  annular structure with a large and well-defined eye on 15 September (below).

Aqua MODIS IR image (15 September)

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