Autumnal equinox as seen on GOES-14 Full Disk images

September 22nd, 2012 |
GOES-14 0.63 µm visible channel images (click image to play animation)

GOES-14 0.63 µm visible channel images (click image to play animation)

The autumnal equinox occurred at 14:49 UTC on 22 September 2012. On this day, GOES-14 was providing Full Disk images at 30-minute intervals (as opposed to the normal operational 3-hour interval) as a part of the SRSOR testing period.

Increased noise in GOES-13 Imager Channels

September 21st, 2012 |
Toggle between GOES-13 6.5 µm and MODIS 6.76 µm imagery at 0740 UTC on 21 September

Toggle between GOES-13 6.5 µm and MODIS 6.76 µm imagery at 0740 UTC on 21 September

The GOES Imager infrared channels are displaying increasing amounts of small-scale noise that is manifest in imagery as herringbone or checkerboard features, as shown in the comparison above with a MODIS water vapor imagery. The animation below, an AWIPS composite of GOES-13 and GOES-15, shows increasing amounts of noise with a peak near 0830 UTC (when the loop pauses), before the noise relaxes. There seems, then, to be a diurnal component to the noisy behavior.

GOES-13/GOES-15 6.5 µm imagery (click Image for Animation)

GOES-13/GOES-15 6.5 µm imagery (click Image for Animation)

The noise is apparent in the GOES-13 imagery in the loop above, but not in the GOES-15 imagery that is displayed on the western side of the north-south seam from eastern Washington southward to Baja California.

GOES-13 and GOES-14 6.5 µm imagery, 1045 UTC 21 September 2012

GOES-13 and GOES-14 6.5 µm imagery, 1045 UTC 21 September 2012

Similarly, a comparison of GOES-13 and GOES-14 data over the north-central United States shows a clean signal in GOES-14. Compare, for example, the checkerboard signal over northern Illinois in the GOES-13 imagery to the same region in the GOES-14 imagery.

GOES-13 3.9 µm imagery, 0815 UTC 21 September 2012

GOES-13 3.9 µm imagery, 0815 UTC 21 September 2012

The noise is also apparent, though more subtle, in other channels, especially the shortwave Infrared (3.9 µm), as evidenced above. The herringbone/checkboard pattern is present both in the very cold cloud tops of the convection over southern Missouri and in the mid- and high-level clouds over the central Mississippi Valley. The noise in the 3.9 channel is also apparent in the heritage fog product that simply takes the difference between the 10.7 µm and 3.9 µm brightness temperatures. (The noise is not apparent on this day in the GOES-R IFR Probability Product that fuses the brightness temperature difference field with model data).

GOES-13 µm imagery (click Image for Animation)

GOES-13 6.5 µm imagery (click Image for Animation)

An animation of GOES-13 Water Vapor imagery at 0815 UTC from 7 September 2012 through 21 September 2012 shows that the noise is a recent, but now persistent, issue with the Imager. It may have started around September 12.

GOES-13 6.5 µm imagery from Imager (top) and Sounder (bottom), 0845 UTC 21 September 2012

GOES-13 6.5 µm imagery from Imager (top) and Sounder (bottom), 0845 UTC 21 September 2012

The noise is present in both Imager and Sounder data. This suggests that the root cause may be with the satellite platform itself, not necessarily with the instruments on board. NOAA/NESDIS scientists are actively working to determine the cause of the noise in the signal.

Radiation Fog in River Valleys in Pennsylvania and New York

September 20th, 2012 |
Brightness Temperature Difference between Shortwave and Longwave IR Channels on GOES (10.7 µm and 3.9 µm channels), MODIS (11 µm and 3.7 µm channels), and VIIRS (11.35 µm and 3.74 µm channels)

Brightness Temperature Difference between Shortwave and Longwave IR Channels on GOES (10.7 µm and 3.9 µm channels), MODIS (11 µm and 3.7 µm channels), and VIIRS (11.35 µm and 3.74 µm channels)

Clear skies and light winds allowed for the formation of fog in river valleys of Pennsylvania and New York in the early morning of 20 September 2012. The animation above shows the traditional method of detecting fog or low stratus from satellite (GOES-East, MODIS on Aqua and VIIRS on Suomi/NPP): the brightness temperature difference between the shortwave infrared (between 3.7 and 3.9 µm) and longwave infrared (near 11 µm) channels. Liquid clouds do not emit radiation as a blackbody at shortwave IR wavelengths but they do emit radiation more nearly as a blackbody at longwave IR wavelengths. Consequently, inferred blackbody temperatures based on the radiation detected by the satellite (the temperature conversion assumes blackbody emissions) are cooler for shortwave radiation than for longwave radiation, and a difference field will highlight regions of low clouds comprised of water droplets, such as fog or stratus.

The image loop above demonstrates the importance of resolution in correctly describing the dendritic features of fog in river valleys. MODIS and VIIRS infrared channels have resolutions near 1.0 km; GOES has a nominal sub-satellite point resolution of 4 kilometers. Consequently, the GOES brightness temperature difference field cannot accurately detect the sharp edges to the valley fog features.

Day/Night Band image from VIIRS on Suomi/NPP

Day/Night Band image from VIIRS on Suomi/NPP

Fog in valleys can also be detected using the Day/Night band that is on board Suomi/NPP, as shown above. The light source for this image is emitted light from the surface (that is, city lights) and reflected lunar illumination. Lunar illumination is a function of phase, and the moon on 20 September is nearly new. Despite the lack of lunar light, clouds are observed in valleys in New York and Pennsylvania.

MODIS-based GOES-R IFR Probabilities

MODIS-based GOES-R IFR Probabilities

Fog and low stratus in a river valley is not necessarily an obstruction to aviation. The GOES-R IFR probability product can help determine how likely IFR conditions are. This product is currently produced from MODIS data and from GOES-East and GOES-West data. The product uses both the brightness temperature difference field and Rapid Refresh moisture profiles to assign probabilities of IFR conditions. MODIS IFR Probabilities are highest along the Chemung and Susquehanna Rivers in southern Upstate NY; at 0700 UTC, Elmira NY was reporting visibility of 1/4 mile in Fog. No other stations in the Susquehanna River watershed had visibilities that low. Note that the character of the IFR Probability field is different over Steuben County compared to elsewhere. High clouds that are apparent in the brightness temperature difference field (and in the longwave infrared radiation field) mean that satellite data cannot be used to compute IFR probabilities, so only model fields are used, and the model fields have lower resolution than the MODIS data.

The 0700 UTC GOES-R IFR Probability field computed using GOES-East imager data is shown below. Because of the coarser resolution of the GOES Imager pixel, the small valleys cannot be resolved. Nevertheless, higher probabilities are present near the valleys because sub-pixel scale cloud features do have an impact on the emitted shortwave and longwave radiation. GOES-R IFR Probabilities increased over the course of the night, so that by 1100 UTC, there were signals over most of north-central Pennsylvania and in most river valleys of central Pennsylvania, and airports in those valleys — Williamsport (KIPT), Selinsgrove (KSEG), Clearfield (KFIG), for example — all reported IFR conditions.

GOES Imager-based GOES-R IFR Probabilities

GOES Imager-based GOES-R IFR Probabilities

Note that these products are available in WFOs. From the 0958 UTC AFD in State College, PA:

.NEAR TERM /UNTIL 6 PM THIS EVENING/…

EARLY AM MODIS 11-3.7UM IMAGERY SHOWING DENDRITIC PATTERN OF FOG IN THE VALLEYS…THE RESULT OF A COOL…CALM MORNING AND RELATIVELY WARM RIVER/STREAM WATERS. LATEST 3KM HRRR SFC RH FIELDS SUGGEST THE MOST PERSISTENT FOG WILL BE UNDER SFC RIDGE AXIS ACROSS THE SUSQ VALLEY…WHERE PATCHES COULD LINGER UNTIL 14Z-15Z.

Noise in the Shortwave GOES-13 Sounder Channels

September 19th, 2012 |
Multipanel display of GOES-East Sounder Channels

Multipanel display of GOES-East Sounder Channels

The shortwave infrared channels on the GOES-13 sounder have displayed increasing amounts of noise over the past months. The multi-channel display, above, shows distinct noise in the channels at 4.57 µm, 4.53 µm, 4.45 µm and 4.13 µm. Animations of the GOES-13/GOES-15 imagery from AWIPS shows that the noise is far more obvious in the GOES-13 channel. It is especially noteworthy in the 4.45 µm channel (called the 4.5 µm channel in AWIPS) but also readily apparent in the 4.13 µm channel (called the 4.0 µm channel in AWIPS).

Data from these noisy channels are used in the computation of derived products. The effect of the noise is manifest as small-scale noise in the derived fields of total precipitable water, for example, and the bad data is most noticeable at night, because the shortwave data are used to identify cloudy pixels; during the day, visible imagery identifies cloudy pixels. Thus, maps of, for example, lifted index have too many pixels flagged as cloudy at night compared to during the day. Also compare the 0600 UTC Total Precipitable Water (TPW) image to the 1900 UTC image.

Sounder Cloud-Top Pressure From GOES-13 and GOES-14 (click image to toggle between images)

Sounder Cloud-Top Pressure From GOES-13 and GOES-14 (click image to toggle between images)

Because the shortwave data are used to identify cloudy pixels, night-time estimates of cloud-top pressure as also impacted, as shown above. The GOES-13 Sounder-derived cloud-top pressure includes several regions that are indicated as cloudy because of the noisy shortwave data. A similar field derived from GOES-14 sounder data does not include the cloudy pixels. Data from 1800 UTC — when visible imagery can be used to screen out clear pixels with more efficacy — shows less contamination that can be traced to the shortwave fields. That is, the fields derived from the GOES-13 and GOES-14 Sounder look more similar.

NESDIS scientists are evaluating how the shortwave bands can be excluded from the GOES-13 Sounder computation to reduce night-time noise in the output fields. Real-time GOES Sounder imagery is available at this link. Derived products, including Convective Available Potential Energy (CAPE), TPW, Lifted Index and cloud-top properties can also be viewed at that link.

(Some imagery in this blog post courtesy of Tim Schmit, NOAA/NESDIS)

This plot, courtesy of Mat Gunshor, CIMSS, shows the standard deviation of the difference between observed brightness temperature and forward-calculated brightness temperatuers. A big jump in noise occurred in late August.