Using polar-orbiter MODIS and AVHRR imagery to supplement degraded geostationary GOES imagery

March 22nd, 2012 |
MODIS 6.7 µm water vapor image + GOES-13 6.5 µm water vapor image

MODIS 6.7 µm water vapor image + GOES-13 6.5 µm water vapor imageÂ

After 20:30 UTC on 21 March 2012, GOES-15 (GOES West) experienced a bad momentum unload, and at this point the satellite went into  a sun acquisition mode (a “safe mode”) and stopped transmitting data (latest GOES-15 status messages). NOAA/NESDIS immediately began to operate GOES-13 (GOES East) in continuous Full Disk scan mode, in order to provide imagery as far west as possible every 30 minutes. However, due to the extreme viewing angle from GOES-13 (positioned over the Equator at 75 degrees West longitude), the image quality over the western US was degraded (since the effective pixel resolution was so large). In addition, Alaska, Hawaii, and the Pacific Region were effectively without useful GOES imagery (although some of these areas could make use of Japanese MTSAT imagery to help fill the GOES-15 imagery gap).

Shortly after the GOES-15 outage, AWIPS comparisons of a 1-km resolution MODIS 6.7 µm water vapor image with the corresponding GOES-13 6.5 µm water vapor image (above) as well as the MODIS 11.0 µm IR channel image and the corresponding GOES-13 10.7 µm IR image (below) demonstrate the value of using higher spatial resolution polar-orbiter (from the Terra and Aqua satellites) MODIS imagery to supplement the degraded geostationary GOES-13 imagery during the GOES-15 outage. In these 2 examples, the GOES-13 viewing angle for San Francisco, California is 65 degrees, making the effective resolution of the “4 km” IR and water vapor image pixels closer to 16 km. In addition, the large satellite viewing angles were creating a significant “parallax error“, shifting the apparent location of high cloud features to the northwest.

MODIS 11.0 µm IR image + GOES-13 10.7 µm IR image

MODIS 11.0 µm IR image + GOES-13 10.7 µm IR image

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MODIS 6.7 µm water vapor image + GOES-13 6.5 µm water vapor image

MODIS 6.7 µm water vapor image + GOES-13 6.5 µm water vapor image

On the following day,  AWIPS comparisons of a MODIS 6.7 µm water vapor image with the corresponding GOES-13 6.5 µm water vapor channel image (above) as well as a MODIS 11.0 µm IR channel image with the corresponding GOES-13 10.7 µm IR channel image (below) showed a storm moving into the far southern panhandle of Alaska. In the case, the GOES-13 satellite viewing angles for Seattle, Washington and Juneau, Alaska were 71 degrees and 82 degrees, respectively.

MODIS 11.0 µm IR image + GOES-13 10.7 µm IR image

MODIS 11.0 µm IR image + GOES-13 10.7 µm IR image

Similarly, an AWIPS comparison of a 1-km resolution POES AVHRR 12.0 µm IR image with the corresponding GOES-13 10.7 µm IR image (below) shows another source for polar-orbiter satellite imagery to supplement GOES in this type of situation.

POES AVHRR 12.0 µm IR image + GOES-13 10.7 µm IR image

POES AVHRR 12.0 µm IR image + GOES-13 10.7 µm IR image

CIMSS participation in GOES-R Proving Ground activities includes making a variety of POES AVHRR and  MODIS images and products available for National Weather Service forecast offices to add to their local AWIPS workstations. Currently there are 57 NWS offices receiving MODIS imagery and products from CIMSS.

Warm water temperatures in the Great Lakes

March 21st, 2012 |
MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product

On 20 March 2012, the Milwaukee/Sullivan NWS forecast office posted a story about possible record warm water temperatures in southern Lake Michigan. Then, on 21 March 2012, the Lake Erie water temperature measured at Buffalo, New York was 39º F, which tied for the warmest water temperature ever measured there during the month of March (Lake Erie water temperature records).

An AWIPS image of the MODIS Sea Surface Temperature (SST) product (above) showed that there were a number of pockets of warm SST values in the 50s F (green to yellow color enhancement) within Lakes Michigan, Huron, Erie, and Ontario, with only a few isolated SST values in the 30s F (darker blue color enhancement) in each of those lakes.

GOES-15 Imager Calibration Issues

March 19th, 2012 |
Color-enhanced GOES-15 Imager Water Vapor Imagery (click image to play animation)

Color-enhanced GOES-15 Imager Water Vapor Imagery (click image to play animation)

The Sensor Processing Subsystem (SPS) for GOES-15 (operational as GOES-West) was changed at 20:45 UTC on March 12, 2012. This change that should have been transparent to the users introduced an error to the calibrated data that was especially apparent in the Imager Water Vapor Channel (Channel 3). The SPS is used to generate the GVAR data stream. The error was traced to a database issue in the new SPS that included the incorrect coefficients for the conversion to brightness temperature for the on-board blackbody. The blackbody temperature is used as part of the calibration of the IR bands. As a result, water vapor brightness temperatures were about 5% too cool for approximately five days. Other Imager channels were not so dramatically affected and Sounder Channels were not affected.

For example, in the image toggle above, the ‘Before’ image, at 20:30 UTC 12 March, shows the warmest brightness temperatures over the eastern Pacific Ocean west of Baja California. The brightness temperatures in that yellow region cooled by 2 K over those 30 minutes. Similarly, cold brightness temperatures over cirrus shield off the coast of Oregon/California cooled by 1-2 K. (The cold cloud tops over southwest Oregon also cooled, but that cooling is likely influenced by synoptic forcing). In contrast, Sounder data (below), shows little cooling between 20:02 UTC and 21:02 UTC on March 12th 2012. Brightness temperatures in the warm band west of Baja peak near 244.5 K in both images; brightness temperatures in the cirrus shield off the coast of California remain near 218 K (and actually show signs of warming between 2002 and 2102 UTC).

Color-enhanced GOES-15 Sounder Water Vapor Imagery (click image to play animation)

Color-enhanced GOES-15 Sounder Water Vapor Imagery (click image to play animation)

Color-enhanced GOES-15 Imager and Sounder Water Vapor Imagery

Color-enhanced GOES-15 Imager and Sounder Water Vapor Imagery

The four-panel image above also highlights the change in Imager data, and the static nature of the Sounder data. The enhanced Imager data, in the two panels on top, shows less yellow (that is, cooler temperatures) at 2100 UTC vs. 1800 UTC. Sounder data shows very little change over the same three hours.

The change in Brightness temperatures was first brought the attention of NOAA scientists via email from ECMWF. In preparation for the planned routine use of GOES-15 Imager data in the global model at ECMWF (starting in April of 2012), that center is monitoring observed and modeled radiances, and a big change started late in the day on March 12, 2012. This website shows the monitoring over the past three weeks. A similar site that shows how NCEP monitors Satellite Data is here. In addition, NOAA/NESDIS STAR compares GOES Imager observations to observations from high-spectral resolution measurements.

The image above shows GOES Imager Water Vapor Mean brightness temperatures over water from 90 S to 90 N and 110 W to 170 W. Note the big drop in mean temperature late in the day on March 12, and the big increase last in the day on March 16 2012. Comparisons between geo-located GOES-15 and MTSAT Mean Water Vapor Brightness temperatures, below, show good agreement until 2045 UTC on March 12, when the SPS change that accessed incorrect values in a different database was implemented. Database values were checked in a test starting at 21:00 UTC on March 14th until before the 00:52 UTC image on March 15th before the correct database values were used starting again at 2045 UTC on March 16th. (Link). At that time in both graphs, GOES-15 brightness temperatures return to more appropriate values.

Interpretation of the Sounder vs. Imager water vapor imagery should be colored by the spectral response functions (SRF) of the two instruments. The GOES-15 Imager SRF for water vapor is a broad one centered near 6.5 micrometers. The three GOES-15 Sounder water vapor channel SRFs are much narrower and are centered at 6.5, 7.0 and 7.4 micrometers. In addition, nominal resolution for the GOES-15 imager is 4 kilometers at the sub-satellite point versus something closer to 10 kilometers for the GOES-15 Sounder.

Special thanks to Cristina Lupu at ECMWF for alerting us to this issue.

Near-record March temperature in South Dakota

March 18th, 2012 |
MODIS 0.65 µm visible channel image + MODIS Land Surface Temperature product

MODIS 0.65 µm visible channel image + MODIS Land Surface Temperature product

On 18 March 2012, the maximum temperature at Winner, South Dakota (station identifier KICR) reached 92º F (33º C), which was just shy of the State’s all-time record high temperature for the month of March (which was 94º F or 34º C, set at Tyndall in 1943).

An AWIPS image comparison of MODIS 0.65 µm visible channel data with the corresponding MODIS Land Surface Temperature (LST) product (above) showed pockets of very warm LST values in the 95-105º F range (darker red color enhancement) across parts of South Dakota at 19:10 UTC (1:10 pm local time). Differences in soil type and vegetaion density can contribute to higher LST values — and there is no direct 1:1 correspondence between LST values and air temperature values measured in an instrument shelter at a height of 5 feet off the surface. Daily record high temperatures were also set on 18 March at other locations in central and eastern South Dakota: Pierre (88º F), Sioux Falls and Aberdeen (85º F), Sisseton and Watertown (82º F), and Mobridge (81º F).

Land surface temperature values were not available in the immediate vicinity of Winner at the time of the MODIS images, due to patches of thin cirrus cloud over that area — the cloud mask prevents the calculation of LST products in cloudy areas. These patches of thin cirrus clouds could be seen as colder IR brightness temperatures (yellow to green color enhancement) in the MODIS 11.0 µm IR channel image (below). The warmer north-to-south oriented line located to the northwest of Winner was the burn scar from the Okreek fire which burned on 05 October 2011.

MODIS 11.0 µm IR channel image

MODIS 11.0 µm IR channel image