Reflection of sunlight from the Topaz Solar Farm in southern California

June 12th, 2019 |

GOES-17

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

1-minute Mesoscale Domain Sector GOES-17 (GOES-West) “Red” Visible (0.64 µm) images (above) revealed a bright reflection of sunlight off the large arrays of solar panels at Topaz Solar Farm in southern California (Google maps) — located between Black Mountain and California Valley — on 12 June 2019. Of particular interest are the vertical “stripes” emanating from the bright reflection signature in the 0.64 µm images, extending both northward and southward from the solar farm. These image artifacts are likely related to saturated ABI detector column amplifiers, due to an excess charge induced by intense sunlight reflection off the large solar panels.

Visible images displayed using McIDAS (below) are in the native GOES-17 satellite projection — removing the re-mapping inherent in the AWIPS images shown above — so the vertical striping artifacts are correctly oriented with respect to how the ABI swaths are scanned.

GOES-17 "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

In multi-panel GOES-17 images that showed all 16 ABI bands (below) this reflection signature was apparent in the other visible and in most of the other infrared channels. The reflected energy was so intense that the Shortwave Infrared (3.9 µm) images displayed infrared brightness temperatures of 138.7ºC (411.85 K), the saturation temperature of the 3.9 µm detectors. Another interesting artifact is the so-called “Dark Pixels Around Bright Objects” that appear in the Visible (0.47 µm and 0.64 µm) and Near-Infrared (0.86 µm, 1.61 µm and 2.24 µm) spectral bands.

Multi-panel images of all 16 ABI bands of GOES-17 [click to play animation | MP4]

Multi-panel images of all 16 ABI bands of GOES-17 on 12 June [click to play animation | MP4]

However, note the absence of a solar farm signature in the Cirrus (1.37 µm), Water Vapor (7.3 µm, 6.9 µm and 6.2 µm) and CO2 (13.3 µm) images — the presence of a layer of moisture within the mid-troposphere (centered near the 500 hPa pressure level) absorbed upwelling radiation from the surface, then re-emitted radiation at the colder temperature of that moisture aloft (thereby masking the bright/hot solar farm signature). Plots of Infrared and Water Vapor weighting functions (below) showed significant peaks at higher altitudes (due to the aforementioned layer of mid-tropospheric moisture) for Bands 8, 9, 10 and 16 — while the other Infrared spectral bands had their strongest weighting function peaks at the surface, with minimal contributions from higher altitudes.

Infrared and Water Vapor weighting functions calculated using rawinsonde data from Vandenberg CA at 00 T on 13 June [click to enlarge]

Infrared and Water Vapor weighting functions calculated using rawinsonde data from Vandenberg Air Force Base CA at 00 UTC on 13 June [click to enlarge]

It is interesting to examine GOES-17 imagery from 5 days earlier (below) — due to a drier air mass over the area on 07 June (with a Total Precpitable Water value of 0.57 inch, vs 0.70 inch on 12 June), a faint signature of the solar farm reflection could even be seen in the Band 4 Cirrus (1.37 µm) imagery.

Multi-panel images of all 16 ABI bands of GOES-17 [click to play animation | MP4]

Multi-panel images of all 16 ABI bands of GOES-17 on 07 June [click to play animation | MP4]

Plots of the Infrared and Water Vapor weighting functions for that earlier day (below) showed higher-altitude peaks for bands 8, 9, 10 and 16 (similar to what was seen in the 12 June case).

Infrared and Water Vapor weighting functions calculated using rawinsonde data from Vandenberg Air Force Base CA at 00 UTC on 08 June [click to enlarge]

NUCAPS Sounding Availability

May 16th, 2019 |

NUCAPS soundings from NOAA-20 at 0653 UTC on 16 May 2019, 34.4 N, 75.8 W (Click to enlarge)

The Cross-Track Infrared Sounder (CrIS) on Suomi NPP suffered an anomaly back in late March and the mid-wave portion of the detectors are not functioning as designed; the wavelengths affected include those sensitive to water vapor. Because of this data outage, NUCAPS soundings are not being produced from Suomi NPP. Suomi NPP was the sole data source for NUCAPS in National Weather Service offices over the contiguous United States.

As shown above, NUCAPS soundings are being produced by NOAA-20, which, like Suomi NPP, carries both the CrIS and the Advanced Technology Microwave Sounder (ATMS). NOAA-20 NUCAPS soundings are scheduled to replace the Suomi NPP NUCAPS soundings in National Weather Service Forecast Offices in late May 2019. NOAA-20 is in the same orbit as Suomi NPP, but offset by half an orbit; overpasses are offset by about 45 minutes, so the NUCAPS data should show up in forecast offices at about the same time of day. (Compare these Suomi NPP orbits over North America to these from NOAA-20; Orbital tracks for most polar orbiters are here.) Time latency for NOAA-20 soundings is improved over Suomi-NPP however; there will be less wait needed for the soundings.

NUCAPS soundings are also produced from Metop-A and Metop-B, satellites that carry the Infrared Atmospheric Sounding Interferometer (IASI) and the Advanced Microwave Sounding Unit (AMSU) and Microwave Humidity Sensor(MHS) instruments.

NUCAPS soundings from NOAA-20, Metop-A and Metop-B are available at this site. That site includes a map (shown here) To access the soundings, move the map to your desired location, and click on the small box in the upper left of the map (under the +/- that cause the map to zoom in and out).  After clicking the box, use a left click and mouse drag on the map to define a region where sounding points will appear. (Alternatively, click the ‘Thumbnail Viewer’ box above the map; as you mouse over the points, a sounding will appear in the window.) The points are color-coordinated based on how old the latest sounding is. Zoom in, and choose your point.  Three profiles are displayed: The initial regression profile (labeled MW+IR Regr), the microwave-only profile (labeled MW phys) and the final physical retrieval profile (labeled MW+IR phys).  The resultant sounding you see will be the latest, but 10 soundings near that point over the past several days can be accessed as well.

NUCAPS soundings from Suomi NPP are not gone for good, however.  The CrIS has redundant electronics, and ‘A’ side — that has partially failed — and a ‘B’ side that has not been tested since before launch (Suomi NPP was launched on 28 October 2011!  Here is one of its first images).  The ‘B’ side electronics can be activated, and if they work, NUCAPS algorithms would have to be recalibrated for an essentially new data source.  This would take several months.  Alternatively, NUCAPS for Suomi NPP could be reformulated to account for the missing data with the ‘A’ side electrontics, something that also would take several months.  A decision on the path to take is forthcoming.

Change to the GOES-R ABI Band 7 (3.9 µm) Resampler

May 1st, 2019 |

GOES-17 3.9 µm imagery around a fire at 23:30 UTC on 17 February 2019 with the former interpolation scheme (left), the updated interpolation scheme (center) and the difference field between the two (right). The yellow box shows the approximate fire location over Mexico. (Image courtesy Chris Schmidt, CIMSS)

GOES-R Advanced Baseline Imagery (ABI) detections must be interpolated from the detector grid on the satellite to a grid that is fixed and geographically referenced. This is accomplished by applying a truncated sinc function in both north-south and east-west directions to the data on the detector grid. Sinc functions include small negative tails adjacent to the large central maximum; for fifteen out of sixteen ABI bands, those subtractions are not detectable. For Band 7, however, the shortwave infrared band at 3.9 µm, the ABI band with the largest dynamic range (and 14 bits of information), the interpolation from detector space to the fixed grid pixel can introduce negative values of radiances and careful observers have seen Cold Pixels Around Fires, the so-called CPAF effect.

An improved interpolation for Band 7 only has been implemented (on 23 April for GOES-16 and on 18 April for GOES-17) in the GOES-R Ground System that reduces the negative tail in the Truncated Sinc function. In the single image above, from GOES-17 at 23:30 UTC on 17 February, the “old” truncated sinc function (denoted ‘Original’ in the image) has generated a falsely cold pixel — white in the greyscale enhancement — off the southeast corner of the warm pixels shown in black.  The cold pixels are not present when the new, improved interpolation scheme is used. Note, however, that the Data Max annotated in the image has cooled by 2K with the improved interpolation;  a fire is nevertheless obvious.

Consider the animation below, for example, (from this blog post on the Cranston fire), that used the ‘old’ interpolation scheme.  Cold pixels (in white) occasionally appear around the periphery the fire (in red) in the center of the image. The new interpolation means that such cold pixels will no longer appear in the data.

GOES-16 ABI visible imagery (0.64 µm) and shortwave infrared imagery (3.9 µm) over the Cranston fire, 1842 UTC on 25 July 2018 to 0227 UTC on 26 July 2018  (Click to enlarge)

The image below shows a fire at 1641 UTC on 29 April 2019, after the CPAF change was implemented into the GOES-R Ground System (two different enhancements are shown). No artificial cold pixels are present. The hottest pixel is 405 K, which would have produced a CPAF under the original truncated sinc kernel.

GOES-16 3.9 µm Imagery at 16:41 UTC on 29 April 2019 (Image courtesy Chris Schmidt, CIMSS)(Click to enlarge)

GOES-R Band 2 (“red visible”) Calibration Changes

May 1st, 2019 |

GOES-16 ABI Band 2 Visible Imagery (0.64 µm) at 1811 UTC on 21 April (before the calibration change) and at 1811 UTC on 25 April 2019 (after the calibration change) in and around Pima County (outlined in black) in southern Arizona (click to enlarge).

On 23 April 2019, a Ground Systems update resulted in a change to the ‘brightness’ (in the form of dimming) of the GOES-16 Band 2 (0.64 µm) visible imagery (as noted in this calibration events log).  The  calibration coefficients used for this band were determined in the lab before the launch. (Other calibration information is collected on-orbit.) GOES-R Advanced Baseline Imager (ABI) visible imagery have been compared to visible imagery from polar-orbiting satellites in the past several years (see this page), and GOES-R ABI Band 2 (0.64 µm) radiances were consistently larger than measurements from Suomi NPP and NOAA-20 using the Visible-Infrared Imaging Radiometer Suite (VIIRS).

ABI calibration coefficients for GOES-16 were modified on 23 April, using plausible values from pre-launch lab measurements. Thus, Band 2 radiances decreased by about 6.9%. This will have an impact on the computed albedo as shown above: the later date (after the calibation change) is slightly darker than the earlier (before the calibration change). This change means that Band 2 radiances and albedo are more closely aligned with values from other satellites. A similar toggle over Texas is shown below.

Note: A similar change for GOES-17 was implemented at 1540 UTC on 27 April 2019.

GOES-16 ABI Band 2 Visible Imagery (0.64 µm) at 1811 UTC on 21 April (before the calibration change) and at 1811 UTC on 25 April 2019 (after the calibration change) in and around Fayette County, TX (outlined in black), just west of metropolitan Houston (click to enlarge).