## 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.

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)

## Mountain wave clouds over southern California

December 21st, 2014 |

GOES-15 6.5 µm water vapor channel images click to play animation)

AWIPS images of 4-km resolution resolution GOES-15 (GOES-West) 6.5 µm water vapor channel data (above; click image to play animation) showed the development of a patch of mountain wave or “lee wave” clouds immediately downwind of the higher elevations of the western Transverse Ranges in southern California on 21 December 2014.  These clouds developed in response to strong northerly winds interacting with the west-to-east oriented topography (12 UTC NAM 700 hPa wind and height). As seen on the plotted surface reports, at Sandberg (station identifier KSDB) the highest wind gust was 52 knots or 59 mph  at 17:42 UTC — and later in the day there also a peak wind gust of 87 mph at Whitaker Peak and 86 mph at Montcito Hills. In addition, there were isolated pilot reports of moderate turbulence in the vicinity of the mountain wave cloud at 20:21 UTC and 23:06 UTC;  farther to the east there was a pilot report of moderate to severe turbulence at 01:27 UTC.

A comparison of 1-km resolution MODIS 6.7 µm and 4-km resolution GOES-15 6.5 µm water vapor channel images around 21:00 UTC (below) demonstrated the advantage of higher spatial resolution (and the minimal parallax offset) of the polar-orbiter MODIS imagery for more accurate location of the mountain wave cloud.

MODIS 6.7 µm and GOES-15 6.5 µm water vapor channel images

At 20:42 UTC (below), the coldest 1-km resolution POES AVHRR Cloud Top Temperature value associated with the mountain wave cloud feature was -69º C (darker red color enhancement), with the highest Cloud Top Height value being 14 km or 45,900 ft (cyan color enhancement)., which is actually colder and higher than the tropopause on  the 12 UTC rawinsonde report at Vandenberg AFB. The highest elevation in the western portion of the Transverse Ranges where the mountain wave cloud formed is Mount Pinos at 8847 feet or 2697 meters, so it appears that a vertically-propagating wave developed which helped the cloud reach such a high altitude.

POES AVHRR Cloud Top Temperature and Cloud Top Height products

At 21;20 UTC, a comparison of 375-meter resolution (projected onto a 1-km resolution AWIPS grid) Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR channel, and 11.45 µm IR channel images (below) showed that while the coldest cloud-top 11.45 µm IR brightness temperatures were around -60º C, the 3.74 µm shortwave IR temperatures were in the +5 to +10º C range — this indicates that the mountain wave cloud was composed of very small ice particles, which were efficient reflectors of solar radiation contributing to much the warmer shortwave IR brightness temperatures.

Suomi NPP VIIRS 0.64 µm visible, 3.74 µm shortwave IR, and 11 45 µm IR channel images

A 375-meter resolution Suomi NPP VIIRS true-color Red/Green/Blue (RGB) image from the SSEC RealEarth web map server is shown below.

Suomi NPP VIIRS true-color image

## Hurricane Arthur transitions to an extratropical cyclone

July 6th, 2014 |

GOES-13 6.5 µm water vapor channel images with surface pressure and frontal analyses

GOES-13 6.5 µm water vapor channel images with overlays of surface pressure and frontal analyses (above) showed Category 2 Hurricane Arthur (NHC discusions | blog post) transitioning to a powerful extratropical (or “post-tropical”) storm as it moved northward over the Maritime Provinces of Canada on 05 July 2014. Impacts of Hurricane Arthur along the East Coast of the US included a peak wind gust of 101 mph at Cape Lookout, North Carolina, and over 6 inches of rainfall in eastern Maine.

A long animation of 4-km resolution GOES-13 6.5 µm water vapor channel images covering the period 00:15 UTC on 05 July to 12:15 UTC on 06 July (below; click image to play animation; also available as an MP4 movie file) showed a very pronounced area of dry air (bright yellow to red color enhancement) wrapping into the circulation of the storm. Also evident on the water vapor imagery was the subsequent development of a “sting jet” signature along the southwestern and southern flank of the storm — this feature was associated with very strong winds (peak gust of 138 km/h or 86 mph) being transported down to the surface over parts of Nova Scotia and New Brunswick (Canadian Hurricane Centre statement). The sting jet signature resembles a “scorpion tail” (22:45 UTC image); note that there is a significant parallax offset with the >50 degree satellite viewing angle of GOES-13 imagery over this region, so the sting jet signature was actually located farther to the south over Nova Scotia (where the strongest surface winds were observed). Other notable sting jet cases appear here, here and here.

GOES-13 6.5 µm water vapor channel images (click to play animation)

As an aside, it is interesting to examine the effect that the northeastward passage of Hurricane Arthur had on the pattern of sea surface temperatures in the far western Atlantic Ocean off the East Coast of the US. The Suomi NPP VIIRS Sea Surface Temperature (SST) product at 17:27 UTC on 05 July (below) revealed a number of filaments and eddies along the path of the tropical cyclone. A comparison with the 00 UTC 05 July Real-Time Global Sea Surface Temperature High-Resolution (RTG_SST_HR) analysis showed that even a 1/12 degree resolution model had difficulty resolving many of these subtle SST features — this helps to underscore the value of high-spatial resolution satellite imagery for making highly-accurate assessments of such fields as SST.

Suomi NPP VIIRS Sea Surface Temperature product, with a comparison to the RTG_SST_HR analysis

## Severe thunderstorm over the Black Hills of South Dakota

May 27th, 2014 |

GOES-13 10.7 µm IR channel images (click to play animation)

An isolated severe thunderstorm developed over the northern portion of the Black Hills of South Dakota around 18 UTC (Noon local time) on 27 May 2014, and moved southeastward producing hail as large as 2.75 inches in diameter and a tornado (SPC storm reports), as well as up to 3 inches of heavy rainfall. 4-km resolution GOES-13 10.7 µm IR channel images (above; click image play animation) showed the cold cloud-top IR brightness temperatures associated with the storm (which reached a minimum of -59º C at 21:40 UTC). Convective initiation was aided by convergence of a surface cold frontal boundary with the topography of the Black Hills, as seen here.

A comparison of 375-meter resolution Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images at 19:38 UTC (below) revealed a minimum cloud-top IR brightness temperature of -68º C. Subsequent cumulus cloud development is seen to be suppressed in the stable outflow region in the wake of the storm.

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images

A comparison of the VIIRS IR image with the closest available GOES-13 IR image (below) demonstrated the advantage of higher spatial resolution of polar-orbiting satellite imagery, as well as the lack of parallax error associated with the geostationary-orbit GOES imagery. The coldest cloud-top IR brightness temperature on the GOES-13 image was -57º C, compared to -68º C on the VIIRS image.

GOES-13 10.7 µm IR channel and Suomi NPP VIIRS 11.45 µm IR channel images

The NOAA/CIMSS ProbSevere model identified this storm as a potential producer of severe weather. The animation below shows the evolution of the MRMS radar signal with ProbSevere overlain from 1812 UTC through 2000 UTC. The Radar object that is highlighted has strong satellite growth rate (observed at 1725 UTC), MUCAPE of 1750 J/kg and Effective Shear of ~19 knots. At 1810 UTC, when MESH values are 0.53″, ProbSevere is 18%; four minutes later at 1814 UTC MESH increased to 0.94″ and ProbSevere increased to 69%. The National Weather Service issued the first warning at 1905 UTC and severe hail (1.5″ in diameter) occurred at 1910 UTC (when MESH was 1.64″ and ProbSevere was 94% and less than an hour after ProbSevere increased above 50%).

NOAA/CIMSS ProbSevere over South Dakota, times as indicated (Click to enlarge)

ProbSevere later in the day, after 2200 UTC, continues to track the hail-producing storm in far southwestern South Dakota. Although ProbSevere is designed to show when the first severe reports might occur, it does continue to provide useful information. In time, as below at 2222 UTC, the satellite growth parameters are replaced by ‘Mature Storm’ as a reminder that this tracked feature is not new.

Annotated NOAA/CIMSS ProbSevere over South Dakota, times as indicated (Click to animate)