Category Archives: Pacific Northwest

The Challenge of Satellite Fog Detection at Very Small Scales


Hazards as depicted by front page at 1400 UTC on 10 November 2016 (Click to enlarge)

Isolated regions of dense fog developed over eastern Oregon early in the morning on 10 November 2016, and one county — around Baker City — was placed in a Dense Fog advisory (Counties in the Willamette Valley of western Oregon, and near Glacier Park in Montana were placed under Dense Fog Advisories a bit later in the morning on 10 November). Click here to see a 1400 UTC mapping of IFR/LIFR conditions from the Aviation Weather Center.

600 AM MST THU NOV 10 2016

500 AM PST THU NOV 10 2016









What kind of Fog-detection products are available to assist a forecaster in seeing at a glance that fog is developing? How useful are they for small-scale features such as that in Baker County Oregon?

Brightness Temperature Difference Fields (3.9 µm – 10.7 µm) have historically been used to detect fog; the difference field keys on the Emissivity Differences that exist in water-based cloud droplets: they do not emit 3.9 µm radiation as a blackbody, but do emit 10.7 µm radiation more nearly as a blackbody, so computed brightness temperatures are different: cooler at 3.9 µm than at 10.7 µm. The Brightness Temperature Difference fields for 0900 and 1200 UTC are shown below (Note the seam — GOES-13 data are used east of the seam, GOES-15 data are used to the west). There is no distinct signal over Baker County, nor any pattern that can really help identify regions of fog. Cirrus is present over western Oregon (depicted as dark grey or black in this enhancement); satellite-only detection of low fog is not possible if cirrus prevents a view of the surface.


GOES Brightness Temperature Difference Fields (3.9 µm – 10.7 µm) at 0900 and 1200 UTC on 10 November 2016 (Click to enlarge)

For a small-scale event, the nominal 4-km pixel size on GOES-15 and GOES-13 (a size that is closer to 6-7 km over Oregon because of the distance from the sub-satellite point) may prevent satellite detection of developing fog. The toggle below shows Brightness Temperature Difference fields at 0928 UTC from MODIS on Aqua, as well as the GOES-R IFR Probability fields computed using the MODIS data.  As with GOES data, the presence of cirrus in the Brightness Temperature Difference field is obvious and shown by a black enhancement.  Little signal is present over Baker County.  (There is a strong signal, however, in the valleys of northwest Montana and northern Idaho — compare this to the GOES-based brightness temperature difference above).

Note:  MODIS resolution is 1-km;  data from the Advanced Baseline Imager (ABI) on GOES-R will have nominal 2-km resolution at the sub-satellite point.


MODIS Brightness Temperature Difference fields (3.7 µm – 11 µm) and MODIS-based GOES-R IFR Probability, 0928 UTC on 10 November 2016 (Click to enlarge)

What does the GOES-based GOES-R IFR Probability field show during the early morning hours of 10 November? The animation below, from 0800-1200 UTC, shows some returns in/around Baker County. It would have been difficult to use this product alone to diagnose this fog feature however. (It did do a better job of diagnosing the presence of fog over northwest Montana and western Oregon where Advisories were later issued).


GOES-R IFR Probability fields, hourly from 0800 – 1200 UTC on 10 November 2016 (Click to enlarge)

Small-scale Fog Event near Puget Sound


GOES-R IFR Probabilities computed with GOES-15 and Rapid Refresh Data, 1100-2200 UTC on 9 February 2016 (Click to enlarge)

GOES-R IFR probabilities on Tuesday 9 February captured the development of a small-scale fog event in/around the southern part of Puget Sound in Washington State. The animation above shows high IFR Probabilities developing shortly before sunrise and persisting through most of the day in a region including Shelton, Olympia, Tacoma and Seattle.

MODIS visible data, below, from the Terra Satellite overpass shortly after 1800 UTC, below, shows the fogbank over the most of Puget Sound, extending inland only over the southern part of the sound.


MODIS Visible Imagery (0.65 µm), 1822 UTC on 9 February (Click to enlarge)

Suomi NPP viewed Puget Sound on two consecutive overpasses on 9 February, and visible imagery from those passes, just before 2000 UTC and near 2130 UTC, are shown below.  Fog Dissipation is apparent in the later image, which is consistent with the animation of IFR Probability at the top of this post, which animation shows IFR Probabilities declining in value after 2100 UTC.


Suomi NPP Visible Imagery (0.64 µm), 1953 and 2122 UTC on 9 February (Click to enlarge)

Fog and Low Stratus in Idaho’s Snake River Valley


GOES-R IFR Probability, hourly from 0500 through 1500 on 8 February 2016 (Click to enlarge)

River Valleys will be prone to fog when they are capped by strong High Pressure, as occurred early in the morning on 8 February 2016 over Idaho. The animations, above and below, show GOES-R IFR Probability fields and GOES Brightness Temperature Difference Fields, respectively. GOES-R IFR Probability fields include information from the Rapid Refresh about near-surface saturation. Compare IFR Probabilities at Burley Municipal Airport (KBYI) in Cassia County with those at Jerome County Airport (KJER) between 0800 and 1100 UTC, when visibilities and ceilings at the two airports vary. IFR Probabilities in general are higher when reported ceilings and visibilities are consistent with IFR conditions, and they are lower when IFR conditions are not reported.

Extensive mid-level stratus over the eastern portions of northern Idaho and western Montana have reduced values of IFR Probabilities (compared to the Snake River Valley where IFR Probabilities are large). This is a benefit of a fused product: Information from two sources combined is more powerful than either of the two sources individually. IFR Probability fields also have superior results when mid-level or higher clouds overspread an area. This occurs around 1200-1300 UTC over the northeastern portion of the Snake River Canyon around Rexburg and Idaho Falls, locations that maintain IFR (or near-IFR) conditions although the brightness temperature difference field has little signal. IFR Probabilities remain enhanced in the region, however, with a signal — a less pixelated, flatter field — that suggests only Rapid Refresh Data are being used as predictors of IFR.


GOES Brightness Temperature Difference (10.7µm – 3.9µm), hourly from 0500 through 1500 on 8 February 2016 (Click to enlarge)

Fog vs. Stratus over the Pacific Northwest

Brightness Temperature Difference (10.7 µm – 3.9 µm) from GOES highlight regions of water-based clouds:  water-based clouds emit 10.7 µm radiation nearly as a blackbody does, but those clouds do not emit 3.9 µm radiation as a blackbody.  Thus, the brightness temperature computed from the radiation detected by the satellite (GOES-15 in this case) — a computation that assumes a blackbody emission — is relatively cooler for the 3.9 µm data compared to the 10.7 µm data.  A water-based cloud is normally stratus, and the pertinent question for aviation purposes (for example) is:  Is the ceiling of that cloud near the surface?  (That is:  Is the stratus also a fog bank, or is it “just” mid-level stratus?)


GOES-15 Brightness Temperature Difference (10.7 µm – 3.9 µm) and GOES-based GOES-R IFR Probabilities, 0500 UTC 15 December 2015 (Click to enlarge)

The toggles above (0500 UTC) and below (0900 UTC) show how the GOES-R IFR Probability fields capably screen out many regions of mid-level stratus. This is achieved by fusing the brightness temperature difference information with data from the Rapid Refresh Model. If the lowest 1000 feet of the Rapid Refresh Model is not near saturation, probabilities of IFR conditions are reduced.


As above, but at 0900 UTC 15 December 2015 (click to enlarge)


As above, but at 1200 UTC 15 December 2015 (click to enlarge)

Toggles from 1200 UTC (above) and 1400 UTC (below) continue to show IFR Conditions mostly confined to regions near the Willamette Valley in eastern Oregon — banked up against the higher terrain to the east of the Willamette, and also over the higher terrain of northeastern Oregon (Click here for a toggle between the 1400 UTC IFR Probability field and Topography). IFR Conditions are a function of ceilings above ground (not above Mean Sea Level), so it’s important to recognize the influence of topographic features on an IFR Probability field. Fog/Low stratus can bank up against a topographic feature, and/or it can shroud the top of a topographic feature.

Note also how at 1400 UTC high clouds have impinged upon extreme northwest Oregon and coastal western Washington. In these regions IFR conditions nevertheless persist under the high clouds, but satellite data alone does not indicate low cloudiness. In this region, the inclusion of Rapid Refresh data in the GOES-R IFR Probability algorithm allows the IFR Probability field to continue to provide useful information about the presence of fog/low stratus.


As above, but at 1400 UTC 15 December 2015 (click to enlarge)

MODIS and Suomi NPP afforded high-resolution images of the fog/stratus banks over the Pacific Northwest on 15 December. The brightness temperature difference fields and MODIS-based IFR Probability fields from MODIS at 0533 and 0945 UTC, below, support the observations from the coarser-resolution GOES fields above.


As above, but for MODIS data at 0533 UTC 15 December 2015 (click to enlarge)


As above, but for MODIS data at 0945 UTC 15 December 2015 (click to enlarge)

GOES-R IFR Probability fields are not yet computed using data from the Suomi NPP Satellite, but the Day Night band and the Brightness Temperature Difference field give information about the presence of cloudiness. For the case of Suomi NPP data, however, it’s more important to consider surface-based observations to confirm regions of low clouds/fog or mid-level stratus. Note also that December 15 was shortly after a New Moon, and the crescent moon that could give illumination was below the horizon (that is, it had set) at 0918 and 1059 UTC.

Note that Suomi NPP Near-Constant Contrast Day Night Band imagery was scheduled to start flowing in to AWIPS II on 14 December 2015 via the SBN. It should be available in NWS offices now.


Suomi NPP Day Night Band Visible Imagery (0.70 µm) and Brightness Temperature Difference (10.35 µm – 3.74 µm), 0918 UTC 15 December 2015 (Click to enlarge)


Suomi NPP Day Night Band Visible Imagery (0.70 µm) and Brightness Temperature Difference (10.35 µm – 3.74 µm), 1059 UTC 15 December 2015 (Click to enlarge)

Resolution Benefits from MODIS


GOES-R IFR Probability fields, every 2 hours from 0200 through 1400 UTC on 16 October 2015 (Click to enlarge)

The GOES-R IFR Probability fields over Oregon and Washington on the morning of 16 October 2015 correctly diagnose low ceilings and reduced visibilities along the coast, even when high clouds intervene at the end of the animation above (especially evident at 1300 UTC, below). In addition to marine stratus that is reducing visibility/ceilings along the coast, surface observations suggest a valley fog is forming in/around Centralia/Chehalis Washington (KCLS). However, GOES-based IFR Probability fields do not become enhanced in that area. Why not?


GOES-R IFR Probability computed from GOES-15, 1300 UTC on 16 October 2015. Note the Character of the field near Newport OR (KONP); the uniformity of the field is characteristic of regions where satellite signals cannot be used because of high clouds (Click to enlarge)

The valley of the Chehalis River, in which the fog is forming, is far too narrow to be resolved by GOES-15, which satellite has 4-km pixel sizes at the sub-satellite point. (Pixels are closer to 6 km in size over southern Washington). Higher-resolution MODIS data (with a 1-km pixel size) can be used to create GOES-R IFR Probabilities, and MODIS overpasses viewed Centralia/Chehalis at 0650, 0924 and 1105 UTC. The imagery below toggles between MODIS-based and GOES-based GOES-R IFR Probabilities at those times.  Even as early as 0645 UTC, the MODIS-based IFR Probability fields are suggesting that a fog is starting to develop.  At later times the MODIS-based IFR Probability values are much larger than the GOES-based values.  MODIS data can give an early alert to the development of small-scale fog.


MODIS-based (0650 UTC) and GOES-15-based (0645 UTC) GOES-R IFR Probability fields (Click to enlarge)


MODIS-based (0924 UTC) and GOES-15-based (0930 UTC) GOES-R IFR Probability fields (Click to enlarge)


MODIS-based (1105 UTC) and GOES-15-based (1100 UTC) GOES-R IFR Probability fields (Click to enlarge)

Fog/Low Stratus over coastal Oregon


Suomi NPP Visible Imagery from the Day Night Band, 0944 and 1125 UTC on 30 September 2015 (Click to enlarge)

The near-Full Moon provided ample illumination of fog/low stratus near the Oregon Coast early in the morning of 30 September, as shown above. Note also the clear skies near North Bend along the southern Oregon coast. What did the GOES-R IFR Probability field show at these two times?  IFR Probability fields also suggest clear skies around North Bend/Coos Bay (and offshore), as observed.  The fine fingers of fog/low stratus that are moving up river valleys in the 1125 UTC image most notably are not resolved by GOES-15 (which has a 4-km pixel size at the sub-satellite point). Both fields do capture the slow increase in cloud cover over land. Available surface observations show near-IFR conditions at some stations in Oregon.


GOES-R IFR Probabilities computed with GOES-15 and Rapid Refresh Data, 0945 and 1115 UTC on 30 September 2015 (Click to enlarge)

Occasional glitches in GOES-R IFR Probability fields from GOES-West


GOES-R IFR Probability fields computed using GOES-15 are periodically — once or twice per night — showing unusual behavior, as documented in the short animation above. The 0730 UTC shows a greatly expanded region of modest IFR Probability values compared to 0715 UTC; at 0745 UTC, fields return to ‘normal’. This aberrant behavior does not occur during the day, nor does it occur every night, nor at specific times. This intermittent type of error makes it difficult to determine and exact cause, but it appears to be related to GOES-15 3.9 µm emissivity. That field is missing when the erroneous fields are produced. This could be an issue of timing — that is, the algorithm requests the field before it is created.

CIMSS scientists are actively working to determine the underlying cause of this error.

======================== Added August 4 2015 =======================

Tweaks to the processing flow at CIMSS at the end of July appear to have fixed this problem, as it has not occurred in August.

Using MODIS and GOES IFR Probabilities to gauge fog motion in the Pacific Northwest


MODIS-based brightness temperature difference fields, ~0550 and ~1000 UTC on 6 July (Click to enlarge)

MODIS-based Brightness Temperature Difference fields, above, from 0547 (Terra) and 0958 (Aqua) detect a large area of marine stratus over the Pacific Ocean that is penetrating inland up river valleys along the coasts of Washington and Oregon.  Dark reatures that are consistent with higher clouds are also present over southern Oregon. GOES-R IFR Probability fields can be computed from MODIS data, and those fields (below) show high probabilities along the coast, in regions where IFR or near-IFR conditions are observed. Aspects of the GOES-R IFR Probability field deserve comment. Where high clouds are present in the MODIS data, GOES-R IFR Probabilities are largely controlled by model-based fields that are typically smooth. This is the case over the northwest corner of the IFR Probability field, for example, and also off the southern Oregon coast at ~0550 UTC. The blocky nature of the IFR Probability fields off the central Oregon coast at ~0550 UTC is likely related to model relative humidity fields that show saturation both near the surface and aloft.


MODIS-based GOES-R IFR Probabilities, ~0550 and ~1000 UTC on 6 July 2015 (Click to enlarge)

Suomi NPP Day Night Band visible imagery had ample illumination early on 6 July with a waning gibbous moon. Imagery below from 1000 UTC (very close to the Aqua pass) also shows thin tendrils of fog moving up river valleys. The VIIRS instrument that provides the Day Night imagery is only on one satellite, however, (compared to MODIS on both Aqua and Terra) so such views are infrequent.


Suomi NPP Day Night Band Visible imagery and Brightness Temperature Difference fields at 0959 UTC, 6 July 2015 (Click to enlarge)

MODIS and Suomi NPP imagery suggest fog is penetrating preferentially into river valleys along the west coast. This should color the interpretation of GOES-based GOES-R IFR Probabilities. GOES-15 lacks the horizontal resolution to resolve fully most river valleys (Rapid Refresh data, similarly, does not resolve small valleys); however, GOES-15 does have excellent temporal resolution, and combining that with the intermittent information from polar orbiters such as Terra, Aqua and Suomi-NPP provides a fuller picture of the evolution of near-surface IFR conditions. The animation of GOES-R IFR Probabilities from GOES-15 is shown below.


GOES-15 based GOES-R IFR Probabilities, 0445-1345 UTC, 6 July 2015 (Click to enlarge)

Fog over Western Oregon


GOES-R IFR Probabilities computed from GOES-15, hourly from 0500 through 1700 UTC, 26 March 2015 (Click to enlarge)

GOES-R IFR Probabilities during the morning of 26 March 2015 expanded eastward from the Pacific Ocean as fog and IFR conditions developed over the western quarter of Oregon. At sunrise, IFR Probabilities dropped along the coast in Northwest Oregon, and IFR conditions were not reported at either Tillamook or Newport, but IFR Probabilities remained high in the Willamette and Umpqua River Valleys, where IFR conditions persisted.

The Brightness Temperature Difference Product, below, for the same time shows a signal in regions where fog/IFR conditions were not reported. The use of model data (Rapid Refresh) in the GOES-R IFR Probability algorithm helps screen out regions where low-level saturation is not occurring — either because the clouds detected by the brightness temperature difference are mid-level, or because the brightness temperature difference field is driven by soil-based emissivity differences at 3.9µm and 10.7µm and not cloudiness at all. The brightness temperature difference signal vanishes near sunrise as solar 3.9µm radiation starts to be reflected off clouds; the sign flips after sunrise and the low clouds appear dark.


GOES-15 Brightness Temperature Difference (10.7µm – 3.9µm), hourly from 0500 through 1700 UTC, 26 March 2015 (Click to enlarge)

Both IFR Probabilities and Brightness Temperature Difference fields largely miss the isolated IFR conditions over northeast Oregon, around Baker and Meacham.

Successive Suomi NPP Scans Show Stratus/Fog movement


Suomi NPP Day/Night Band at 0926 and 1106 UTC on 9 October 2014 (Click to enlarge)

(A Blog post on Suomi NPP Imagery over the western US from 10 October is available here).

Polar orbiters typically don’t give good temporal resolution, especially near the Equator. In mid-latitudes, however, Polar Geometry can yield views over a wide area on two successive scans. This happened along the West Coast early in the morning on 9 October. Two regions show noticeable changes in stratus between the two times: stratus/fog extends farther down the Salinas Valley at the southern edge of image, and stratus/fog expands over southwestern Puget Sound in Washington. The Brightness Temperature Difference field (11.35µm – 3.74µm) from Suomi NPP for the same times shows a similar evolution over the Salinas Valley, but the view of Washington is obscured by thin cirrus. These cirrus (that show up as black enhancements below) are mostly transparent in the Day Night Band but not in the infrared bands.


Suomi NPP Brightness Temperature Difference (11.35µm – 3.74µm) at 0926 and 1106 UTC on 9 October 2014 (Click to enlarge)

MODIS instruments onboard Terra and Aqua yield spectral data that can be used to generate GOES-R IFR Probability Fields. The animation below shows high-resolution imagery of where IFR Probabilities are highest, but only at three distinct times: 2152 UTC on 8 October and 0537 and 0949 UTC on 9 October. An increase in IFR Probabilities around Monterey Bay is apparent (and consistent with the Suomi NPP Observations above); IFR Probabilities also increase along the Oregon Coast and around Puget Sound.


MODIS-based GOES-R IFR Probabilities at 2152 UTC 8 October, 0547 UTC 9 October and 0949 UTC 9 October (Click to enlarge)

How do GOES-based observations complement the Polar Orbiter data above?


GOES-15 based GOES-R IFR Probabilities, hourly from 0200 through 1400 UTC, 9 October 2014 (Click to enlarge)

The hourly animation above shows the slow increase of IFR Probability in/around Monterey Bay, and also a push of higher IFR Probability onto the Oregon Coast that occurs after the last MODIS-based IFR Image shown farther up. (Higher IFR Probabilities also spill into San Francisco Bay). Do surface observations of ceilings and visibilities agree with the IFR Probability fields? The loops below, from Oregon (below) and Monterey Bay (bottom) suggest that they do. For example, Eugene OR (and stations north and south of Eugene) show IFR conditions as the high IFR Probability field moves in after 1100 UTC. GOES-based data is valuable in monitoring the motion of IFR Probability fields; keep in mind, though, that small-scale features may be lost. For example, it is difficult for GOES to resolve the Salinas Valley.


GOES-15 based GOES-R IFR Probabilities, hourly from 0200 through 1500 UTC, 9 October 2014, with surface and ceiling observations superimposed (Click to enlarge)


GOES-15 based GOES-R IFR Probabilities from 0400 through 1400 UTC, 9 October 2014, with surface and ceiling observations superimposed (Click to enlarge)