General interpretation « CIMSS Satellite Blog

Changes to the routine GOES-13 Scanning Schedule

March 4th, 2014

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

Tests are underway this week to determine the impact of augmented GOES-13 (GOES-East) imager coverage. The animation above shows the coverage for routine scanning on 3 March 2014 between 1645 UTC and 1945 UTC. CONUS, Extended Northern Hemisphere and Full Disk images are included. The Optimized GOES-East schedule is available at this link. Note the presence of solar RFI (radio frequency interference) in the 1645 UTC image; solar contamination resulted in no 1702 UTC image at all, as expected (link).

The difference in CONUS coverage is shown below in the toggle of the 1732 UTC image from 3 March and the 1730 UTC image from 4 March. The Optimized Image scan allows for more routine scanning of the Caribbean Sea, for example.

GOES-13 10.7 µm images at ~1730 UTC on 3 and 4 March (click to enlarge)

Side-by-side views of GOES-13 10.7 µm images. CONUS from 3 March, 1732 UTC (left) and Optimized CONUS from 4 March, 1730 UTC (right) (click to enlarge)

A side-by-side image of the regular and optimized CONUS scans is shown above. Note that the optimized scan has a slightly different time (Nominal times for each image are in the panel labels). Thus, batch jobs that access imagery by time must be altered. Side-by-side imagery for the entire test period is below. The 1645 UTC imagery should cover the same domain, but RFI interference is different on the two days. The test period ends before the 1902 UTC image. In the animation below, the CONUS images at half-past the hour show the increase in domain size.

Side-by-side views of GOES-13 10.7 µm images, 1645 UTC through 1902 UTC on March 3 2014 (Left, default schedule) and March 4, 2014 (right, optimized schedule). (click to animate)

Four-hour animation of Puerto Rico Regional Sector, 17-20 UTC on 4 March 2014 (click to enlarge)

As noted above, the optimized scan strategy significantly improves coverage in the Caribbean. In fact, the Puerto Rico Regional Sector is now almost completely covered. The animation above shows that sector for 2 hours with the expanded coverage during the test, and the subsequent two hours. Compare, for example, the 1830 UTC image, during the test, to the 1931 UTC image after the test.

Posted in Calibration/Anomalies, General interpretation, GOES-13 | No Comments »

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

February 25th, 2014

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)

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)

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

Posted in General interpretation, GOES-15, MODIS | No Comments »

Suomi NPP views of a strong midwest cyclone

February 21st, 2014

Suomi NPP VIIRS 1.38 µm near-infrared imagery (M09), 1736 UTC 21 February 2014 (click image to enlarge)

A strong late-winter cyclone brought significant snows and blizzard conditions to the upper Great Lakes/northern Plains on 21 February 2014 (NWS storm summaries: MPX | DLH | ARX). In the warm sector of the storm, there were numerous reports of tornadoes, large hail, and damaging winds in the eastern US. Suomi NPP viewed the storm multiple times, including just before 1800 UTC on 21 February.

The Suomi NPP VIIRS 1.38 µm imagery, above, was created using CSPP and highlights cirrus-level clouds, documenting just how widespread the canopy of this extratropical cyclone was (more imagery is available via ftp, and a description of the various bands is available here).

Suomi NPP VIIRS I1, Day/Night, I3, I4, I5 bands at 1736 UTC 21 February 2014 (click image to enlarge)

VIIRS imagery (375-meter resolution I-bands 1, 3, 4, and 5, along with the 750-meter resolution Day/Night Band) is available in AWIPS via an LDM subscription. The animation above cycles through these different bands as displayed using AWIPS: Visible (0.64 µm), Day/Night Band (0.70 µm), Snow/Ice Channel (1.61 µm), Shortwave IR (3.74 µm) and IR Window (11.45 µm) channels.

VIIRS 750-meter resolution M-bands can be used to create true-color imagery: the example from 1736 UTC is shown below.

Suomi NPP VIIRS True-color imagery, 1736 UTC 21 February 2014 (click image to enlarge)

Posted in General interpretation, Suomi NPP, VIIRS | No Comments »

Sensing the surface on GOES-13 water vapor imagery

January 7th, 2014

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

Most users of water vapor satellite imagery interpret the patterns they see as variations in moisture within the middle to upper troposphere — and for the most part, this is often a good first-order assumption. However, one must keep in mind that the water vapor channel is essentially an InfraRed channel, which is sensing the average temperature of a layer of moisture — and the altitude and depth of the layer of moisture being detected can change significantly, based upon such factors as the temperature and/or moisture profile of the atmospheric column, and the viewing angle of the satellite.

During an unusually cold arctic outbreak over the north-central US during the 06 January – 07 January 2014 period, the outline of various portions of the Great Lakes (in particular, Lake Superior, Lake Michigan, and Lake Erie) could actually be seen on GOES-13 6.5 µm water vapor channel imagery (above; click image to play animation). So, how is it possible to see surface features on water vapor channel satellite imagery?

In helping to understand the vertical location and vertical extent of features seen on water vapor imagery, plots of the water vapor “weighting function” (or “contribution function”) can be generated by taking into account the temperature and moisture profile of that location, along with the satellite viewing angle (or “zenith angle”). For this example, plots of GOES-13 Imager 6.5 µm water vapor weighting functions for Green Bay, Wisconsin (below) showed how the altitude and depth of the moisture layer being sensed by the water vapor channel decreased from 12 UTC on 05 January to 12 UTC on 06 January as the core of the cold arctic air moved over the western Great Lakes region. After that time, both the altitude and depth of the moisture layer being detected (as seen on the water vapor channel weighting function plots) began to increase to approximately their pervious values as somewhat warmer and more moist air began to replace the arctic air mass.

GOES-13 Imager 6.5 µm water vapor channel weighting function plots for Green Bay, Wisconsin

Getting back to seeing the outlines of portions of northern Lake Superior, western Lake Michigan, and western Lake Erie: what was being seen on the water vapor imagery was not necessarily the actual surface per se, but the signal of the strong temperature gradient between the cold snow-covered land surfaces and the still-unfrozen waters — and the signal of this strong surface temperature gradient was “bleeding upward” through what little moisture was present in the atmospheric column, and reaching the GOES-13 Imager water vapor detectors.

Posted in General interpretation, GOES-13 | No Comments »