Japan’s deadliest tornado on record struck the town of Saroma (near the northern coast of the island of Hokkaido) around 04:00 UTC (1 PM local time) on 07 November 2006 (CNN news report). QuickTime animations of MTSAT-1R visible channel (above) and 10.7µm IR channel images (below) did not show any typical severe convection signatures such as an “enhanced-v” signature, but these images were only available at 30 minute intervals. Widespread convection was developing over that region in advance of an approaching mid-latitude cyclone that was rapidly intensifying — water vapor channel imagery (QuickTime animation) indicated that the upper level flow was strongly divergent over Hokkaido island, creating an environment favorable to supporting upward vertical motions and subsequent convective development.

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A QuickTime animation of GOES-11 visible channel images (above) revealed multiple plumes of glacial sediment blowing offshore along the coast of Alaska on 06 November 2006. Strong chinook winds in the glacial valleys were lofting dust and carrying it out over the adjacent Gulf of Alaska. This phenomenon had been occurring on other days in early November (as seen on a MODIS true color image 5 days earlier).

A longer (14-hour) animation using the GOES-11 10.7µm – 12.0µm IR difference product (below) shows a subtle blowing dust signal that can be followed during the non-daylight hours as well (when visible channel imagery is not available). The airborne particulate matter associated with the largest dust plume reduced the surface visibility to 2-3 miles at Cordova, Alaska (station identifier PACV) late in the day; also, note the rapid rise in temperature farther to the east at Yakutat, Alaska (station identifier PAYA), as easterly chinook winds arrived and gusted to 18 mph at 20:00 UTC.

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Two interesting semi-linear cloud features were apparent in the vicinity of the Hawaiian Islands on 06 November: (1) a cloud line to the lee of the Big Island of Hawaii, due to easterly “trade winds” within the marine boundary layer converging after flowing around the island (also note the small cyclonic eddy that formed immediately northwest of the Big Island), and (2) a long, narrow “rope cloud” that stretched for a considerable distance across the Pacific Ocean (to the north and northwest of Hawaii), which marked the leading edge of a cold frontal boundary which had become quasi-stationary (large-scale GOES-11 visible image animation).

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Parallax can mean different things in different sciences (See, for example, this link that describes how parallax is used to compute distances in astronomy), but in satellite meteorology, parallax is the apparent shift in an object’s position (away from the sub-satellite point) as a result of viewing angle. (Here is an example.) Parallax generally increases as you move away from the sub-satellite point. It is also large for higher clouds. Consider the simplified example below.

It shows a lake (blue surface) viewed obliquely from a distant satellite. So this surface feature is far from the sub-satellite point. If a very tall cloud develops between the surface lake and the observing satellite, the satellite will still interpret the information as coming from the surface — that is, where the lake is. In reality, however, the tall cloud is displaced towards the sub-satellite point. Note also another consequence of the viewing angle: the temperature of the cloud will reflect the temperature of the side of the cloud that the satellite is viewing. The colder cloud top will be in a different pixel.

Parallax in geostationary imagery becomes obvious when cloud imagery is compared with surface-based observations. The example below is from April 2006, and shows strong convection over northern Wisconsin just south of Lake Superior. More modest convection is over southern Wisconsin near Madison.

The enhanced infrared imagery can be used to infer the height of the cloud (cold clouds are usually higher in the atmosphere). Displacement (parallax) is greater for higher clouds. The coldest clouds are associated with the convection just south of Lake Superior.

The radar for the same time shows a line of convection displaced to the south of the convection.

The displacement is difficult to see in the three individual images, but stands out in the fader that can be seen here (Link requires Java).

The bottom line: when you see a very cold cloud top on satellite imagery, and the cloud top is far from the sub-satellite point, it’s very likely that the position of the cloud feature over the surface is closer to the sub-satellite point than is indicated in the image mapping.

(Added, 2013: The examples below show how the satellite navigation can place severe weather events in a location that is not where you might expect it to be. By parallax-correcting the observation, the severe weather report location is more properly aligned with the cloud top as seen in satellite images. The images below provide GOES-13, GOES-14 and GOES-15 views of a large hail event in northwest Wisconsin; imagery courtesy of Bob Rabin and Jim Nelson, CIMSS)

GOES-13 0.63 µm visible image and original and parallax-corrected storm report (click to enlarge)

GOES-14 0.62 µm visible image and original and parallax-corrected storm report (click to enlarge)

GOES-15 0.62 µm visible image and original and parallax-corrected storm report (click to enlarge)

It is also possible to remap the satellite image, rather than the storm report. The GOES-14 satellite image below was first remapped to a Mercator projection, and that image was then parallax corrected (using infrared imagery to estimate the height of the cloud; note that low clouds show very little parallax correction).

GOES-14 0.62 µm visible image remapped to Mercator projection and then parallax-corrected (click to enlarge)

Added, 2017: The National Weather Service Operational Proving Ground produced a YouTube video, below, discussing the causes and effects of parallax.

NWS OPG “Satellite Parallax Causes and Effects” (click to play YouTube video)

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