Blowing dust off the Alaskan coast

November 6th, 2006 |

GOES-11 visible images (QuickTime animation)
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.
GOES-11 IR difference images (QuickTime animation)

Hawaiian island lee cloud line, and cold frontal rope cloud

November 6th, 2006 |

GOES-11 visible image animation
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).

The Problem of Parallax

November 2nd, 2006 |

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.satelliteviewwithcloudparallax.gif

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.

Visible image from 2045 UTC 11 April 06
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.
Color-enhanced 11-micron windown channel infrared image from 2045 UTC 11 April 06

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

Composite Radar from 2048 UTC 11 April 06

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-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-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)

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)

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)


Subtropical cyclone in the North Pacific??

November 1st, 2006 |

GOES-11 IR winds
What appeared to be a rather unusual example of a subtropical cyclone was evident over the northern East Pacific Ocean on 01 November (AWIPS surface analysis + buoys) — however, this compact cyclone was likely associated with a deep cold-core cutoff low that had been over that region for a few days (HPC 500 hPa analyses). GOES-11 IR cloud drift winds (above) showed the broad cyclonic flow around the periphery of the surface low (closer view of AWIPS low-level winds | AWIPS upper-level winds). GOES-11 visible channel imagery (QuickTime animation) showed a cloud structure that almost resembled the eye of a tropical cyclone.

A similar signature was also seen on the GOES-11 10.7µm IR window channel imagery (below), with a ring-like feature of cold cloud top temperatures (-50 to -55 C, yellow to orange enhancement) that was very persistent for much of the day (QuickTime animation). Additional satellite imagery can be found by selecting the Central Pacific 91C.INVEST link at the NRL Tropical Cyclones site.
GOES-11 10.7µm IR image