Leap Day 2012 Severe Weather

February 29th, 2012 |

Severe thunderstorms during the early morning hours of February 29th, 2012 produced several tornadoes over Missouri, including one near Branson, and one near Lebanon. (Annotated SPC storm report map here, originally from here) The GOES-13 enhanced infrared images from 0615 UTC and from 0632 UTC show a cold cloud top (brightness temperatures near 210 K or -63º C) passing just north of Lebanon (indicated by the blue square in the imagery). Similarly, GOES-13 enhanced infrared images from 0702 UTC and 0715 UTC show cold cloud tops (brightness temperatures around 212 K or -61º C) passing just north of Branson (whose position is also indicated by a blue square). (Click Here for a loop from 0545 UTC to 0845 UTC).

GOES-13 Sounder DPI LI

GOES-13 Sounder DPI LI

Satellite data gave numerous indications that severe weather was possible at these locations. For example, the Sounder-derived Derived Product Imagery (DPI) Lifted Index at 0400 UTC and at 0700 UTC (See above for the toggle between the images) shows a tongue of instability — LIs close to -4 at 0400 UTC and dropping by 0700 UTC — progressing eastward across Missouri.

UW CIMSS NearCast Product Valid at 0700 UTC 29 February 2012 (click image to play animation)

UW CIMSS NearCast Product Valid at 0700 UTC 29 February 2012

In addition, the UW CIMSS NearCasting product, above, which product uses a Lagrangian Model to move three-dimensional sounder information into the future (thereby showing where convective instability will develop) suggested that strong instability would help sustain the development of any thunderstorms over Southwestern Missouri. Each of the forecasts in the linked-to loop above focus the instability over southwest Missouri near Branson. The NearCast product indicates where the greatest instability will be in the near future and therefore serves to enhance situational awareness in a region.

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As the well-organized squall line ahead of the advancing cold front continued eastward, a tornado which produced the first documented EF4 damage of 2012 moved through the southern part of Harrisburg, Illinois around 10:56 UTC (4:56 am local time). This tornado was also responsible for 6 deaths (NWS Paducah KY Public Information Statement). See the WeatherMatrix Blog for a detailed radar-based discussion of this event.

An AWIPS image of 1-km resolution POES AVHRR 10.8 µm IR data at 11:03 UTC (shortly after the tornado moved through Harrisburg IL) with overlays of severe storm reports (below) shows that the Harrisburg supercell produced hail up to 2.50 inches in diameter and a number of damaging wind reports between 09:41 UTC and 10:56 UTC as it moved northeastward from far eastern Missouri across southern Illinois.

POES AVHRR 10.8 µm IR image + Hail, Severe Wind Gust, and Tornado reports

POES AVHRR 10.8 µm IR image + Hail, Severe Wind Gust, and Tornado reports

As seen in the 1-km resolution POES AVHRR image comparison below, along the pre-frontal squall line the 10.8 µm cloud top IR brightness temperatures were as cold as -73º C (darker black color enhancement in the IR image), cloud top heights were as high as 12 km (darker green on the Cloud Top Height product), and a large area of cloud tops was designated as “Overshooting” the tropopause (violet on the Cloud Type product).

POES AVHRR 10.8 µm IR image + Cloud Top Height, and Cloud Type products

POES AVHRR 10.8 µm IR image + Cloud Top Height, and Cloud Type products

A sequence of 4-km resolution GOES-13 10.7 µm IR images with an overlay of Automated Overshooting Tops Detection (below) showed an overshooting top associated with the supercell at 10.15 UTC over southern Illinois — nearly 45 minutes before the tornado moved through Harrisburg (station identifier KHSB).

GOES-13 10.7 µm IR images + Overshooting Top Detection

GOES-13 10.7 µm IR images + Overshooting Top Detection

Stray Light Corrections in GVAR Signal for GOES-East

February 29th, 2012 |
Four IR Channel from imager with stray light contamination

Four IR Channel from imager with stray light contamination

There are periodic, and predictable, errors within the raw signal on the GOES satellites that arise when sunlight hits the Satellite so that it emits radiation that is detected by the sensor, or when satellite structures reflect energy towards the sensors. There errors usually arise when the Sun is close to being viewed directly by the sensor near “Satellite Midnight”. NOAA/NESDIS has recently (22 February 2012) implemented a series of corrections to mitigate these errors on the GOES-13 Imager. Not only does this increase the number of useable images, but it makes derived products – cloud top pressure, for example – more accurate. Parameters pertinent to the correction are included within Block 0 of the GVAR signal. In McIDAS, these bits relating to the stray light status are included as part of the AREA line prefix.

An example of the error in the raw (or un-corrected) signal is shown at top, with data from the four infrared channels (3.9 6.5, 10.7 and 13.3 micrometers) shown. Note the comparative magnitude of the extra radiation: it is far stronger and more widespread in the 3.9 micrometer image because the sun emits so much more radiation at that wavelength. (The Imager band most affected is the visible band (click here to see two contaminated — and uncorrected — and one clean image), the images above are at night). Options to deal with the stray light errors included: (1) Send all imagery , regardless of solar position/contamination, and let users decide; (2) Cancel images if the sun is within 6 degrees (currently) or 10 degrees of the frame boundary; (3) Scan away from the sun – for example, scan only the Northern Hemisphere if the solar contamination is in the Southern Hemisphere during the Spring eclipse season; and (4) Apply an L1B algorithmic correction to minimize stray light in the images prior to GVAR broadcast. Option (4) has been implemented for GOES-13. Currently option (3) is being implemented for GOES-15.

3.9 micrometer images showing stray light contamination (left) and corrected version (right)

3.9 micrometer images showing stray light contamination (left) and corrected version (right)

The figure above shows a 3.9 micrometer image with a significant amount of stray light contamination in the southwest part of the image. The corrected version is also shown. Note that the contamination extends throughout the picture – brightness temperatures are too warm even in regions away from the large contamination (over the central United States, for example; compare the brightness temperatures of the cloud tops in the scene). The contaminated 3.9 micrometer data are corrected using two sources of information. For regions outside 6 degrees, the known amount of additional stray light is subtracted from the signal. If the sun is within 6 degrees of the pixel and the stray light signal is overwhelming, signals from the longer wavelength channels are used in combination with the 3.9 micrometer signal to estimate the true 3.9 micrometer signal. Linear relationships between the IR channels will vary with geographical location. Other thermal channel data that contain much less stray light are used in each of 256 geographic bins as input into multiple linear regressions relating 3.9 micrometer data (or 6.5 micrometer data) to 10.7 and 13.3 micrometer data. The hybrid image that results is uniformly cooler with a clear signal in a region formerly overwhelmed by stray light. The algorithm was developed by ITT and implemented by NOAA/NESDIS.

Current plans call for correcting the GOES-15 Imager during the fall 2012 eclipse season.

This ftp site contains more information. The GOES Eclipse schedule is here. This is the ‘White Paper’ on Stray Light. Finally, click here for more information on GVAR.

Finally, here is the notification from SSD that the Stray Light Correction was implemented.