GOES Water Vapor Animations for 2015

January 1st, 2016 |

The GOES-13 and GOES-15 Imager provides routine observations at five wavelengths, including 6.5 µm, a wavelength that is sensitive to water vapor absorption (SHyMet lesson). The YouTube animations below show full-disk GOES-13 (GOES-East) and GOES-15 (GOES-West) water vapor images at 3-hour intervals for every day during 2015. GOES-15 shows the remarkable tropical cyclone activity that occurred as a result of warmer-than-normal sea surface temperatures over the central Pacific. Much less hurricane activity occurred in the Atlantic.

Once GOES-R is launched in late 2016, the ABI instrument will provide full-disk images at 5-minute intervals, rather than the 3-hour intervals shown here. The animations from GOES-R will contain 288 images per day rather than 8.



Hawai’i demonstrates that the Water Vapor channel is an Infrared channel

April 6th, 2015 |
GOES-15 6.5 µm water vapor channel images (click to play animation)

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

GOES-15 (GOES-West) 6.5 µm “water vapor channel” images (above; click image to play animation; also available as an MP4 movie file) revealed an interesting transition in the signal displayed by the 2 summits (Mauna Kea and Mauna Loa) on the Big Island of Hawai’i on 06 April 2015 — beginning as a pair of colder (darker blue color enhancement) areas during the nighttime hours, becoming a pair of warmer (brighter yellow color enhancement) areas as daytime heating warmed the land surfaces.

As was discussed in a previous blog post, the water vapor channel is essentially an Infrared (IR) channel that senses the mean temperature of a layer of moisture — usually a layer which is located in the middle troposphere. However, if the middle troposphere is dry, the water vapor detectors are able to “see” lower into the atmosphere and detect radiation from the lower atmosphere (or even high-elevation terrain features, such as Mauna Kea and Mauna Loa). A comparison of the 00 UTC and 12 UTC rawinsonde profiles from Hilo (below) showed that the middle troposphere was indeed quite dry, with the typical tropical moisture residing below the 700 hPa pressure level.

Hilo, Hawai'i rawinsonde data profiles (00, 12 UTC)

Hilo, Hawai’i rawinsonde data profiles (00, 12 UTC)

The altitude (and depth) of the layer being sensed by a water vapor channel is defined by its weighting function, which depends on (1) the temperature and moisture profile of the atmosphere, and (2) the satellite viewing angle or “zenith angle”. This site allows you to select a rawinsonde site of interest, and the GOES Imager (and Sounder) water vapor channel weighting functions are calculated and plotted. The GOES-15 Imager water vapor channel weighting functions for the 2 Hilo soundings are shown below (along with the weighting function for the US Standard Atmosphere). It can be seen that the peak of the weighting function response is at a lower altitude for both Hilo soundings than it would be for the US Standard Atmosphere, which in part allows the strong cold/warm thermal signatures of the two Big Island summits to be seen on the GOES-15 water vapor imagery.

Hilo, Hawai'i GOES-15 imager water vapor weighting functions, compared with the US Standard Atmosphere

Hilo, Hawai’i GOES-15 imager water vapor weighting functions, compared with the US Standard Atmosphere

Sensing high-altitude Sierra Nevada snow cover on water vapor imagery

February 25th, 2014 |
MODIS 6.7 µm water vapor channel image

MODIS 6.7 µm water vapor channel image

A number of previous blog posts have demonstrated the ability of the water vapor channel to sense surface features when the atmospheric column is cold and/or dry; in this example, the signal of a thin ribbon of high-altitude Sierra Nevada snow cover can be seen on an AWIPS image of 1-km resolution MODIS 6.7 µm water vapor channel data at 10:05 UTC on 25 February 2014 (above). At that time the middle to upper troposphere over much of southern California was relatively dry, as indicated by the shades of lighter blue to yellow on the water vapor image. The Blended Total Precipitable Water product indicated that TPW values were generally in the 8-10 mm range over the central Sierra Nevada region, which was actually about 130-150% of normal — however, higher resolution TPW values over the Sierra Nevada were as low as 0.7 mm and 1.3 mm according to the GOES-15 sounder and MODIS, respectively.

A similar high-altitude snow signature was seen on 4-km resolution GOES-15 6.5 µm water vapor channel images (below; click image to play animation).

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

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

The thin ribbon of high-altitude snow cover showed up as darker blue features on both the MODIS and GOES-15 water vapor images — not because there was more water vapor in that location, but because the temperature of the air above the snow pack was colder than the adjacent lower-elevation bare ground areas. This example helps to underscore the fact that the water vapor channel is essentially an InfraRed (IR) channel, which is sensing the temperature of a layer of moisture (or in this case, the temperature of a colder surface feature).

The altitude (and vertical thickness) of the layer being sensed by water vapor imagery depends on the temperature and moisture profile over that particular region, as well as the satellite viewing angle. The GOES Weighting Functions site allows you to select the rawinsonde profile closest to your area of interest, and a radiative transfer model is then used to calculate the weighting functions for the various GOES imager channels (as well as the 3 GOES sounder water vapor channels). In this case, the rawinsonde profile for Vandenberg Air Force Base (KVBG) in California (below) was the closest sounding site to the pocket of dry air over the Sierra Nevada mountains — and due to the deep layer of dry air aloft, the peak altitude of the GOES-15 6.5 µm water vapor channel weighting function was shifted downward to just below 500 hPa.

GOES-15 6.5 µm water vapor channel weighting function plot (calculated using Vandenberg CA rawinsonde data)

GOES-15 6.5 µm water vapor channel weighting function plot (calculated using Vandenberg CA rawinsonde data)

In a comparison of 3 regional rawinsonde sites (below), note how the altitude of the GOES-15 6.5 µm water vapor channel weighting function peak (as well as the vertical thickness of the weighting function plot) increases over Elko, Nevada (KLKN) and Tucson, Arizona (KTUS) where more middle to upper tropospheric moisture was present.

GOES-15 6.5 µm water vapor channel weighting function plots for Vandenberg CA, Elko NV, and Tucson AZ

GOES-15 6.5 µm water vapor channel weighting function plots for Vandenberg CA, Elko NV, and Tucson AZ

Sensing warming terrain with water vapor imagery

February 14th, 2012 |
GOES-15 6.5 µm water vapor channel images (click image to play animation)

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

GOES water vapor imagery tells you how moist or how dry the middle to upper troposphere is, right? Well, in general, yes — but it’s important to remember that the water vapor imagery actually displays the temperature of a layer of moisture that is emitting radiation. This layer of moisture emitting the radiation that is sensed by the water vapor detectors is usually located within the middle to upper troposphere, but if the atmosphere is quite dry (and/or quite cold), the water vapor channel can actually “see” further down through the troposphere and sense thermal radiation that is emitted from the surface.

On 14 February 2012, GOES-15 6.5 µm water vapor channel images (above; click image to play animation) centered on the Big Island of Hawaii showed how the two highest topographical features  (Mauna Kea and Mauna Loa) initially appeared cooler (darker blue color enhancement) than the rest of the island prior to sunrise, but then quickly warmed (exhibiting brighter yellow colors) during the morning hours as sunlight warmed the surface.

GOES-15 0.63 µm visible channel images (below; click image to play animation) showed that the peaks of Mauna Kea and Mauna Loa were cloud-free during this period.

GOES-15 0.63 µm visible channel images (click image to play animation)

GOES-15 0.63 µm visible channel images (click image to play animation)

While there was a band of higher moisture associated with the ITCZ well south of Hawaii, and another band of moisture along a cold frontal boundary well northwest of Hawaii, the atmosphere over Hawaii itself was fairly dry — MIMIC Total Precipitable Water values (below) generally in the 20-25 mm range over the islands (animation).

MIMIC Total Precipitable Water product + surface analysis

MIMIC Total Precipitable Water product + surface analysis

In this case, the middle to upper troposphere over the Hawaii region was quite dry, which had the effect of shifting the altitude of the water vapor channel weighting function (below) to an altitude low enough to enable some thermal radiation emitted from the higher terrain on Hawaii to “bleed up” through what little water vapor was present aloft and be detected on the GOES-15 water vapor channel imagery.

GOES-15 water vapor channel weighting function (using Hilo HI rawinsonde data)

GOES-15 water vapor channel weighting function (using Hilo HI rawinsonde data)