Category Archives: Day/Night Band

GOES Resolution might miss valley fog (Plus: What does Stray Light look like?)

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), Suomi/NPP Brightness Temperature Difference (Lower Left), MODIS-based IFR Probability (Lower Right), all imagery at 0615 UTC on 18 September 2013 (click image to enlarge)

Nominal GOES resolution for the Brightness Temperature Difference product that is used in the GOES-R IFR Probability is 4 km at the sub-satellite point, and it worsens as you move into mid-latitudes. Rapid Refresh Model resolution is even coarser than the satellite. When fog is forming in narrow valleys, then, there can be a significant lag in the time from when it starts to form to when the satellite data, and the satellite/model fused product, detects it. In the 0615 UTC image above, for example, only a few pixels of strong GOES-detected Brightness Temperature Difference, and enhanced IFR Probabilities, exist. In the 0630 UTC image, below, there has been little change in the GOES-based imagery. However, the Suomi/NPP data at the time, at 1-km resolution, suggests fog is forming in many of the river valleys of Pennsylvania, but it is still sub-gridscale as far as GOES can detect.

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), Toggle between Suomi/NPP Day/Night band imagery and Suomi/NPP Brightness Temperature Difference (Lower Left), MODIS-based IFR Probability (Lower Right), all imagery at ~0630 UTC on 18 September 2013 (click image to enlarge)

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), Suomi/NPP Brightness Temperature Difference (Lower Left), MODIS-based IFR Probability (Lower Right), all imagery at 0645 UTC on 18 September 2013 (click image to enlarge)

Fifteen minutes later, the MODIS-based IFR probabilities (above) suggest a strong possibility of IFR conditions in many of the river valleys of Pennsylvania. However, Suomi/NPP and MODIS data come from polar orbiters so that high resolution information is infrequent. When GOES-R is launched, ABI will have nominal 2-km resolution in the infrared, which resolution is intermediate between GOES and MODIS.

The higher-resolution polar orbiters’ occasional views can give a forecaster an important heads’ up for fog formation. By 0815 UTC the GOES-based information is showing higher IFR probabilities in the river valleys of Pennsylvania, but a Suomi/NPP overpass shows that it is still underestimating the areal extent of the fog.

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), Toggle between Suomi/NPP Day/Night band imagery and Suomi/NPP Brightness Temperature Difference (Lower Left), MODIS-based IFR Probability (Lower Right), all imagery at ~0815 UTC on 18 September 2013 (click image to enlarge)

Note that this is a time of year when stray light does occasionally enter the GOES signal, causing contamination. This occurred — and was very obvious — around 0400 UTC on 18 September. As is typical, it was present for only one scan. See below.

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), GOES-R Cloud Thickness (Lower Left), MODIS-based IFR Probability (Lower Right), all imagery at ~0815 UTC on 18 September 2013 (click image to enlarge)

Fog over Kansas

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), GOES-R Cloud Thickness (Lower Left), Suomi/NPP Day/Night Band (Lower Right), all imagery at ~0830 UTC on 17 September 2013 (click image to enlarge)

Light winds with a small upslope component allowed for the formation of fog over the High Plains on the morning of 17 September 2017. The image above shows the GOES-R IFR Probability, GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm), the GOES-R Cloud Thickness and the Day/Night band from Suomi/NPP that provides for nighttime visible imagery. In the imagery above, a large region over southeastern Kansas is overlain by higher ice-based clouds (likely cirrus) such that the brightness temperature difference product does not give the signal that is common with fog and low stratus (in the enhancement used here, fog and low stratus occur where the brightness temperature difference is colored orange or yellow). The Day/Night band visible imagery also suggests high clouds over southeast Kansas. Surface observations do show reduced visibilities, at or near IFR conditions. In this region, the IFR Probability Product gives useful information by using Rapid Refresh Data to diagnose the possibility of low-level fog. The probabilities are smaller — in the 40- to 50% range — but that is because no satellite data are being used as predictors. IFR Probabilities are very high only if both predictors — satellite and Rapid Refresh — are associated with high probability of fog/low stratus.

Note also how the Cloud Thickness product yields no information where high clouds are present. The Cloud Thickness is the thickness of the highest liquid cloud layer; the presence of ice clouds or mixed phase clouds precludes the determination of how thick a water cloud is because the satellite cannot view the water-based cloud.

As above, but at 1215 UTC (click image to enlarge)

The 1215 UTC image shows the effect of twilight conditions moving westward across Kansas — GOES-R Cloud Thickness is not computed during twilight conditions that occurring over eastern Kansas, although they are still computed around Dodge City, where the computed cloud thickness is just over 1000 feet thick. The 1200 UTC Sounding from Dodge City, below, does show a nearly-saturated layer at the surface (about 927 mb, or about 2700 feet ASL) up to about 870 mb (4600 feet ASL).

Upper Air Sounding from Dodge City, KS, 1200 UTC (click image to enlarge)

Fog/Low Clouds over the Northwestern US

Brightness Temperature Difference from GOES-W and GOES-R IFR Probability computed from GOES-West and Rapid Refresh data, both images at 1100 UTC 9 September (click image to enlarge)

Fog and low clouds off the coast of Washington and in Montana are instructive in describing some strengths of the GOES-R IFR Probability algorithm. A general statement is that the IFR Probability improves — sometimes greatly — the approximation of where fog and low clouds are present. In the example above, the brightness temperature difference shows a positive signal — suggestive of low clouds — along the Washington and Oregon coasts (where IFR conditions are reported within the marine layer of stratus/fog). The brightness temperature difference also has a strong signal over Montana; there is a scattershot signal as well over Nevada, Idaho and western Oregon/Washington. These are regions where the GOES-R IFR Probability field is correctly minimizing the probability of fog/low clouds. Over Montana, the Brightness Temperature Difference signal is driven by an elevated stratus deck. The positive signal in arid Nevada, Idaho, Oregon and Washington arises from soil emissivity variability. The IFR Probability field maintains a strong signal where IFR conditions are reported, and reduces the strong signal in regions where fog/low stratus likely do not exist.

Brightness Temperature Difference from GOES-W and and from Suomi/NPP, both images at ~1100 UTC 9 September (click image to enlarge)

Suomi/NPP was passing over the Pacific NW at 1100 UTC, the times of the imagery above, and it provided a higher-resolution look. Suomi/NPP data allows for better definition of the — apparently — thickest stratus/fog layer right along the Washington and Oregon coast. The superior resolution also allows for a better approximate of the dendritic nature of river valley clouds, as demonstrated over southern British Columbia. However, the brightness temperature difference here is also unable to distinguish between fog on the ground and stratus that at mid levels.

Brightness Temperature Difference from GOES-W, Day/Night Band, Brightness Temperature Difference from Suomi/NPP, Day/Night Band, GOES-based GOES-R IFR Probabilities, all at ~1100 UTC 9 September (click image to enlarge)

The animation above loops through the GOES-Based Brightness temperature difference, the Suomi-NPP Brightness temperature difference, the Suomi-NPP Day/Night band, and the GOES-R IFR Probability field computed with GOES-W and Rapid Refresh data.

Fog over southern New England

The National Weather Service in Taunton, MA, tweeted an image of fog over coastal southern New England early on September 3rd. How well did the GOES-R IFR Probability detect this event?

GOES-R IFR Probability (click image to play animation)

The animation, above, at hourly time-steps (GOES-R IFR Probability is computed every 15 minutes, usually, but matching surface observations occur only hourly) can be used to validate the accuracy of the IFR Probability product: highest probabilities in the animation are occurring over coastal New England where visibility is lowest. The increase in probabilities from 1002 UTC to 1102 UTC over the Gulf of Maine reflects the night-time (1002 UTC) vs. daytime (1102 UTC) predictors being used in the IFR Probability algorithm. The flat nature of the IFR probability field over parts of the Gulf of Maine suggests that model data only is being used to compute the IFR probability field there. This occurs where high clouds obscure the satellite view of low clouds.

The brightness temperature difference fields in the animation above (click image to see the animation) have a characteristic appearance associated with high clouds south and east of Cape Cod. In fact, Nantucket is mostly overlain by high clouds. The presence of those high clouds makes satellite detection of fog/low stratus difficult, but a fused/blended product such as the GOES-R IFR probability field that combines satellite and model data is able to alert an observer to the presence of reduced visibilities.

The GOES-R Cloud Thickness (Lower left in the animation above) gives an estimate to the depth of the highest water-based cloud layer. This product is not computed during twilight conditions — from about 1015 to 1215 UTC in the animation above. The animation clearly shows the thickest fog/low stratus deck moving eastward through southeast New England during the early morning hours.

The Suomi/NPP Day/Night band allows for visible imagery at night, detecting visible moonlight and Earthglow that is reflected off clouds, and also emitted light from cities. The 0615 UTC image is here. Clouds are visible in the image, but there is no information on the level of the cloud.

GOES-13 Visible Imagery (click image to play animation)

Visible imagery from after sunrise shows the low clouds persisting south of New England. In addition, a bore (manifest as a series of parallel lines) propagates south of an area of convection that appears to have spawned the bore.

Northern Indiana Fog Event

Dense fog developed overnight over NW Indiana and adjacent regions. The GOES-R IFR Probability Product highlighted the regions with the densest fog, refining somewhat the regions highlighted by the brightness temperature difference, the heritage fog-detection product. The image above shows the IFR Probability (Upper Left), Brightness Temperature Difference (Upper Right), Day/Night Imagery from Suomi/NPP (Lower Left) and GOES-R Cloud Thickness (Lower Right). Forecast offices were alert to the possibility of fog, as shown in AFDs from LOT (Chicago) and IWX (Northern Indiana)

000
FXUS63 KIWX 290619
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
219 AM EDT THU AUG 29 2013

.SYNOPSIS…
ISSUED AT 137 AM EDT THU AUG 29 2013

AN UPPER LEVEL RIDGE WILL CONTINUE ACROSS THE MIDWEST…CAUSING
TEMPERATURES TO STAY MUCH ABOVE NORMAL WITH DRY CONDITIONS. HIGHS
TODAY AND FRIDAY WILL BE IN THE MID 80S TO LOWER 90S. LOWS TONIGHT
WILL BE AROUND 65 DEGREES.

.UPDATE…
ISSUED AT 137 AM EDT THU AUG 29 2013

UPDATED THE GRIDS/FCST TO MENTION AREAS OF FOG DEVELOPING.
CLEARING SKIES OVER MUCH OF THE AREA COMBINED WITH RESIDUAL
MOISTURE AND LIGHT WINDS FAVOR FOG FORMATION. CURRENT THINKING IS
THE FOG EXPECTED TO BECOME WIDESPREAD BY LATE TONIGHT…AND WILL
PROBABLY BECOME DENSE IN AREAS. HAVE HELD OFF ON A DENSE FOG
ADVISORY TO TRY TO GET A BETTER IDEA WHERE THE FOG WILL FORM…
ESPECIALLY AS SOME CLOUDS LINGER OVER NRN INDIANA.

000
FXUS63 KLOT 290906
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
406 AM CDT THU AUG 29 2013

.DISCUSSION…
324 AM CDT

THE FORECAST CHALLENGES INCLUDE FOG IN THE NEAR TERM…THEN
INCREASING TEMPS AND THUNDERSTORM CHANCES FRIDAY…FOLLOWED BY A
COOL DOWN AND RETURN TO MORE SEASONABLE CONDITIONS TOWARD END OF THE
HOLIDAY WEEKEND.

GOES IMAGERY THIS MORNING SHOWS EXPANDING AREA OF STRATUS ACROSS
MUCH OF THE FORECAST AREA WITH SURFACE OBS AND WEBCAMS SHOWING A
SMATTERING OF DENSE FOG HERE AND THERE. ALL INDICATIONS ARE THAT THE
STRATUS IS THIN AND SHOULD BURN OFF QUICKLY THIS MORNING EXPECT NEAR
THE LAKE FRONT WHERE MARINE STRATUS AND POSSIBLY EVEN SOME FOG COULD
HOLD ON INTO THE AFTERNOON. ANOTHER COMPARATIVELY COOL DAY NEAR THE
LAKE WITH 70S RANGING QUICKLY UPWARD TO 80S INLAND JUST A FEW MILES
WITH UPPER 80S TO NEAR 90 EXPECTED SOUTHERN AND WESTERN PORTIONS OF
THE CWA.

GOES-R IFR Probability (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), Suomi/NPP Day/Night Band (Lower Left), GOES-R Cloud Thickness (Lower Right) (click image to play animation)

Aspects of the loop above require comment. Note that initially there is a region of enhanced brightness temperature difference over northern Indiana, between Fort Wayne (KFWA) and Muncie (KMIE), in a region where IFR conditions are not reported. The GOES-R IFR Probabilities in this region are very small (correctly), likely reflecting the influence of the Rapid Refresh model on the IFR Probability product. In other words, this is a region for which the model is not predicting low-level saturation, at least early in the evening. The Brightness Temperature Difference signal is likely a result of elevated stratus.

Later on in the animation, IFR probabilities start to increase between NW Indiana and Saginaw MI, a region where visibilities are decreasing but where the brightness temperature difference, initially, has little signal.

In this case, the IFR Probability Fields improved on the Brightness Temperature Difference predictions — suppressing a signal in regions of stratus, and developing a signal before a signal is present in the brightness temperature difference field alone. This is the power of a fused data product.

The IFR Probability field will also provide a consistent signal through sunrise.

Fog dissipation is shown in the animation below (Click the image to start the animation). Cloud thickness is especially thick over central Illinois, and fog was slow to burn off there. Stratus also persisted over southern Lake Michigan.

Fog on Cape Cod

Fog developed over Cape Cod and the Islands overnight into the early morning on August 28th. The animation above (click image to animate) shows high IFR probabilities over land adjacent to the ocean. Observations show IFR or near-IFR conditions in these regions. IFR conditions decreased after sunrise. By 1410 UTC, the final image in the loop, IFR conditions persisted mostly only over Nantucket and Cape Cod east of Falmouth. This is where highest IFR probabilities persisted. The GOES-based IFR Probabilities suggest a sharp edge to the lowest visibilities over far eastern Massachusetts, which edge was just east of a Newport (RI) to Taunton (MA) line. MODIS-based IFR probabilities at 0218 UTC, below, also suggest a sharp western edge to the IFR conditions.

VIIRS data from Suomi/NPP includes both the Day/Night Band and a Brightness Temperature Difference. These data are toggled with the GOES-based IFR Probabilities below. Resolution limitations inherent with GOES data preclude the accurate detection of fog in small river valleys.

Valley Fog in Pennsylvania

The above animation bewteen the Suomi/NPP VIIRS Day/Night Band and the brightness temperature difference between the longwave infrared image and the shortwave infrared image (that highlights water-based clouds because of emissivity differences at the two wavelengths) shows fog/low stratus in the river valleys of Pennsylvania. The high spatial resolution of Suomi/NPP allows remarkable detail, and the near-Full Moon provides ample illumination. How well did more conventional satellite imagery depict the developing fog? The GOES-13-based IFR Probability Field, below, shows relatively high values in regions over Pennsylvania that are near the river valleys, but GOES lacks the spatial resolution to portray adequately the horizontally confined river valley fog — although someone with knowledge of Pennsylvania Geography can infer a lot.

The strength of GOES imagery is temporal consistency and 15-minute timesteps. Polar orbiter data can only give occasional looks. For example, MODIS Imagery can be used to generate brightness temperature differences and IFR Probabilities, below, but they are produced only every 90 minutes at most (although they will still give useful information, even at the edges of the MODIS swath where resolution is degraded).

The animation of IFR Probabilities from GOES-East, below, nicely depicts the slow increase in Valley Fog. This animation in concert with knowledge of the geography, augmented with the occasional high-resolution imagery from polar orbiters, as above, should allow a forecaster to describe the location of fog development overnight.

Visible imagery, below, shows the dissipation of the fog during the morning of August 21st.

Fog was also abundant over Pennsylvania the morning of 20 August. GOES-14, in SRSO-R mode, captured the dissipation. Link. (Courtesy Dan Lindsey, NOAA).

Late Summer Fog Detection over Alaska

GOES-15 0.62 µm Visible Imagery (Click to see animation)

It can be challenging to determine where fog and low stratus are problematic over Alaska. The visible imagery, above, shows an abundance of clouds (and, over the Northwest Territores, smoke) — but it’s difficult to guess which of the clouds are associated with low stratus or fog and accompanying low visibilities. Knowing where visibilities are low is vital in a state like Alaska where small aircraft provide vital transportation services.

After dark, increasingly frequent at this time of year in Alaska, the brightness temperature difference field (between brightness temperatures at 10.7 µm and 3.9 µm) can be used to identify water-based clouds because of differences at those two wavelengths in the emissivity properties of liquid water droplets that make up clouds. That animation is shown below. As with the visible imagery, the brightness temperature difference tells you about the top of the cloud. The important visibility restrictions occur at the bottom of the cloud.

GOES-15 Brightness Temperature Difference (10.7 µm – 3.9 µm) (Click to see animation)

GOES-R IFR Probabilities computed from GOES-15 and Rapid Refresh Data (Click to see animation)

Neither the visible nor brightness temperature difference loops above highlight cloudy regions with restricted visibility. The GOES-R IFR Probability product that fuses satellite information with model output, a loop of which is available above, does. Many offshore regions show high probabilities of IFR conditions. These are likely regions of advection fog where relatively moist air is flowing over cold ocean waters and being cooled to its dewpoint. (Click here to see a surface plot over northwest Alaska — dewpoints are in the 40s and 50s near the Bering Sea) An area of high IFR probabilities that occurs over land is over southwest Alaska between Dillingham and Bethel. Dillingham does report IFR and near-IFR conditions as the higher IFR probabilities rotate through. A close-up image of the IFR Probability, hourly, over southwest Alaska is shown below; note how ceilings and visibilities in Dillingham (PADL) lower as the IFR probabilities increase.

GOES-R IFR Probabilities computed from GOES-15 and Rapid Refresh Data (Click to see animation)

VIIRS data from Suomi/NPP can also be used to approximate where fog may occur. Both the Day/Night band and the Brightness Temperature Difference product can tell a forecaster where clouds exist. However, surface conditions beneath the clouds detected by Suomi/NPP may or may not be consistent with IFR conditions. In the animation below, a new algorithm has been applied to the Day/Night band to extract information in the transition zone between day and night. Some artifacts of that algorithm are evident (manifest as what appear to be cloud bands perpendicular to the satellite path), but useful information about clouds do appear elsewhere. The brightness temperature difference product is only computed where solar reflectance is essentially nil. GOES Imagery is included in the loop to show where sunrise has occurred.

Suomi/NPP Day/Night band, Brightness temperature difference, and GOES Imager visible data, all near 1245 UTC on 8 August 2013 (Click to see animation)