IFR conditions under high clouds in the Northeast

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GOES-R IFR Probabilities computed using GOES-East (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm- 3.9 µm, The ‘traditional’ fog detection product) (Upper Right), GOES-R Cloud Thickness computed using GOES-East (Lower Left), GOES-East Window Channel (10.7 µm) Brightness Temperature (Lower Right)

When high clouds overspread an area, the traditional brightness temperature difference product cannot be used to highlight areas of fog and low stratus because radiation emissions are originating from high clouds, not from the water-based low clouds.  In this example from Monday morning, 17 December 2012, IFR conditions, causing airport flight delays, are commons from Washington DC to New York, and the GOES-R IFR probability product highlights the area where IFR (and near-IFR) conditions prevail.  Modest values (around 50%) occur where the satellite predictors do not provide a fog/low stratus signal;  however, the Rapid Refresh model data does show high probability of fog and low stratus.  Where the Satellite does contribute to the product (that is, in Pennsylvania north and east of the high cloud deck), IFR probabilities are very high.

Note also that the GOES-R Cloud Thickness product (bottom left), is computed only for the highest water-based cloud (in non-twilight conditions).  It is therefore not shown under the cirrus canopy, over southern New Jersey, Delaware, and Chesapeake Bay.

Florida Radiation Fog

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GOES-R IFR Probabilities (Upper Left) computed from GOES-East, GOES-East Brightness Temperature Differences (10.7 µm – 3.9 µm) (Upper right), GOES-R Cloud Thickness product (Lower Left), GOES-East Visible Imagery (0.63 µm) (Lower Right), hourly from 0615 UTC through 1215 UTC on 14 December 2012

Light north winds over Florida over night were accompanied by the development of low stratus and fog that were captured well by the traditional fog detection product, the brightness temperature difference between 10.7 µm and 3.9 µm that occurs because of emissivity differences in water clouds at the two wavelengths.  Because of the strong satellite signal (upper right), the GOES-R IFR Probabilities over Florida were quite high.  When mid-level and high clouds are absent, IFR probabilities in regions of fog development can easily exceed 90%, as shown above.

Note, however, what happens at sunrise.  The increase in reflected 3.9 µm solar radiation causes the brightness temperature signal to vanish.  (At later times, the sign of the brightness temperature difference will flip).  The GOES-R IFR probability product maintains a coherent signal through sunrise, although values shift somewhat, as can be seen by the southwest to northeast boundary in the final image of the loop over Florida and extreme southeast Georgia.

Cloud thickness, bottom left, can be used to estimate when radiation-induced stratus or fog will ‘burn off’ — thicker clouds will take longer to dissipate.

Note in this loop that the eastern coastline of Mobile Bay shows a signal in the Brightness Temperature Difference.  This signal is likely an artifact of poor co-registration between Bands 2 and 4 (the 3.9 µm and 10.7 µm channels, respectively) on GOES-13.  NESDIS scientists and engineers are working to mitigate this time-dependent error.

The Fog/Low Stratus products can be compared to data from Suomi/NPP.  The VIIRS instrument includes a day/night band that uses reflected moonlight as a light source (below).  Unfortunately, because the moon was new on December 13th, 2012, very little reflected light is available.  Nevertheless, smearing of city lights over Florida does suggest the presence of fog.

As above, but with Day/Night band from VIIRS on Suomi/NPP instead of GOES-13 0.63 µm visible imagery.  Images from ~0630 UTC on 14 December.

Fog Detection under multiple cloud layers

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GOES-R IFR Probabilities computed from GOES-East data (Upper left), GOES-East traditional brightness temperature difference product (Upper right), GOES-East 10.7 µm imagery (Lower left), Surface observations of ceiling/visibility (Lower right)

Weather over Florida on 12 December 2012 gives another example of the importance of fused data products in computing IFR probabilities/detecting low clouds and fog.  When mulitple cloud layers are present, as today over Florida, the traditional brightness temperature difference product (10.7  µm – 3.9 µm) can struggle to produce a useful signal.  Adding information about the lowest part of the atmosphere from the Rapid Refresh model, however, allows a coherent prediction of IFR probabilities to be made.  Note that the stations with the most significant reductions in ceilings/visibility do align with the stripe of higher IFR probability over the northern part of the Florida peninsula.

Gulf of Mexico Fog

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GOES-R IFR Probabilities computed from GOES-East (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9µm ) (Upper Right), GOES-East Visible imagery (0.63 µm) (Lower Left), GOES-R Cloud Thickness (Lower Right), all from ~2030 UTC on 9 December 2012

Fog was present in the northeast Gulf of Mexico at sunset on December 9th, and the GOES-R Products was able to track its evolution as it slowly moved onshore.  Late-in-the-day visible imagery from 2030 UTC 9 December (above) shows the fog bank tucked into the northeast corner of the Gulf of Mexico.  By 0100 UTC on 10 December (below), some of the fog as moved inland, and visibilities/ceilings at Crystal River have decreased to IFR conditions.  Note in the 0100 UTC image the presence of mid-level clouds is creating a hole in the Brightness Temperature Difference Signal over the eastern Gulf of Mexico just northwest of Tampa, and in the Cloud Thickness product.  IFR Probabilities in this region are considerably reduced, as well.

As at top, but for 0100 UTC on 10 December
As above but for 0402 UTC on 10 December

By 0400 UTC on 10 December, the fog/low stratus continue to press inland, as ceilings and visibilities decrease.  Note also the presence of higher clouds in the brightness temperature difference product.  In these regions, satellite predictors are not use to compute the IFR probability product;  consequently, IFR Probabilities over and around Apalachee Bay in the extreme northeast Gulf of Mexico are somewhat reduced; in addition, the field is considerably flatter (compared to the more pixelated character close to Tampa).  At 0802 UTC (below), the presence of mid-level clouds peaks.  Note that the brightness temperature difference product shows very little strong signal over the northern part of the Florida peninsula, despite significant visibility obscurations there (consider Gainesville, for example, with a 1/4-mile visibility).

As above but for 0802 UTC on 10 December
As above, but for 1102 UTC on 10 December

By 1102 UTC, the mid-level cloudiness has dissipated, allowing the brightness temperature difference to have a distinct signal.  Consequently, the IFR probability field increases.  The combination of both satellite predictors and model (Rapid Refresh) predictors being used allows for a consistent signal throughout the night over this evolving fog/stratus deck.  In addition, the signal over the ocean, at the end of the day on 9 December, in a region where surface observations are not routine, could serve as an alert to the early development of nocturnal fog.

Note that the Tampa Bay office of the National Weather Service noted the IFR probability field in its forecast discussion:

 
000
FXUS62 KTBW 100034
AFDTBW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TAMPA BAY RUSKIN FL
734 PM EST SUN DEC 9 2012

.FOR THE EVENING UPDATE...
SURFACE HIGH PRESSURE WILL REMAIN OVER THE REGION WITH A STRONG
MID LEVEL SUBSIDENCE INVERSION AND STABLE AIR MASS OVER THE
FORECAST AREA. AREA OF FOG...LOCALLY DENSE...HAS PERSISTED THROUGH
THE DAY OVER THE COASTAL WATERS NORTH OF VENICE AND LOCALLY
ONSHORE FROM ABOUT ANNA MARIA ISLAND NORTH TO CEDAR KEY. AREA OF
LOCALLY DENSE FOG WILL GRADUALLY PUSH ONSHORE THE COASTAL
COUNTIES THIS EVENING...WITH FOG DEVELOPING OVER INTERIOR BECOMING
LOCALLY DENSE AFTER MIDNIGHT. CURRENT ZONES ARE ON TRACK WITH NO
UPDATE PLANNED.

&&

.AVIATION...
GOES-R IFR AND LIFR PROBABILITY PRODUCTS NICELY OUTLINE AN AREA OF
SEA FOG THAT HAS LINGERED ACROSS NEAR SHORE WATERS FROM BIG BEND
SOUTH TO AROUND VENICE. THIS SEA FOG IS RIGHT ALONG THE COASTLINE
AND IS VISIBLE ON SOME BEACH WEB CAMS IN THE AREA AND THEREFORE IS
POISED TO MOVE INLAND ONCE TEMPS FALL SOME. THE BIG QUESTION IN ALL
THIS IS WHETHER THE WEAK WRLY WINDS ASSOC W/ THE SEA BREEZE
CIRCULATION WILL CONTINUE LATE ENOUGH TO PUSH THE SEA FOG BANK
INLAND IMPACTING TAMPA BAY AREA SITES. PREVIOUS TAFS WERE COUNTING
ON THIS OCCURRING AND WILL CONTINUE CLOSE TO THE FORECAST ALTHOUGH
HAVE NOTED 18Z MAV AND LATEST LAMP HAVE STRONGLY BACKED OFF ON THE
FOG FOG FORECAST AND NOW CONFINED FOG TO MUCH LATER IN THE NIGHT
CLOSER TO SUNRISE. WILL HAVE TO WATCH CLOSE AND AMEND AS NEEDED IN
NEXT FEW HOURS AS ONSET OF LIFR CONDITIONS IS BIGGEST QUESTION MARK
TONIGHT. LIFR CONDITIONS ARE PRETTY CERTAIN AROUND 12Z AND WILL SEE
SLOW IMPROVEMENT TO MVFR LEVELS THROUGH 18Z. MVFR LEVELS MAY LINGER
AND CONTINUE BEYOND 18Z AS MOISTURE SURGES INTO THE AREA LEADING TO
SCT SHRA ACROSS THE ENTIRE AREA AND POSSIBLY A FEW TSTMS MAINLY
AROUND OR EAST OF SRN SITES (FMY, RSW, PGD). MODELS SUGGEST
POTENTIAL DEVELOPMENT OF MORE WIDESPREAD SEAFOG EVENT LATE TOMORROW
AS MOISTURE CONTINUES TO INCREASE AND A MORE PERSISTENT SW FLOW
DEVELOPS. 
 
 
MODIS-based GOES-R IFR Probabilities, 0334 UTC 10 December (above) and 0745 UTC 10 December (below)

MODIS-based GOES-R IFR Probabilities, above, show a similar evolution of the field, from fairly constrained over northern Florida at 0334 UTC, and determined by both satellite and model fields, to more widespread but deteremined more by model fields only at 0745 UTC.

Fog in California’s San Joaquin Valley

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GOES-R IFR Probabilities, computed from GOES-West (Upper Left), GOES-West Visible Imagery (0.62 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES-West Brightness Temperature Difference (10.7 µm – 3.9 µm) (Lower Right)

Fog is persisting in California’s San Joaquin Valley today, and the GOES-R IFR Probability and Cloud Thickness products both are describing its spatial extent.  The lowest clouds are banked against the western side of the valley, with visibilities lowest, and IFR probabilities highest, from Bakersfield to Fresno to points north.  The Sacramento Valley (not shown) is not showing visibilities near IFR conditions, and the IFR probabilities in that part of California are lower.

The impact of higher clouds

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GOES-R IFR Probabilities computed from GOES-East (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES-East 10.7 µm Imagery (Lower Right) at 0700 UTC on 6 December 2012.

Upper-level clouds, such as those apparent in both the 10.7 µm imagery and the brightness temperature difference imagery, above, lower right and upper right, respectively, have an impact on the GOES-R IFR Probability and GOES-R Cloud Thickness products.  The most obvious impact is in the Cloud Thickness product (lower left), which product is not computed in regions where high clouds are present.  The GOES-R IFR Probabilities are computed underneath high clouds, using mostly Rapid Refresh model data to determine the probability of fog and low stratus.  However, because cloud predictors are not used, IFR probabilities are somewhat lower.  In addition, the character of the field is flatter, reflecting the smoother fields that are present in the model.  This is especially obvious over northeast Louisiana and extreme east Texas in the IFR Probability image above.  Thus, the heritage product, the brightness temperature difference, gives no information from southwest Louisiana northward into southern Arkansas, but the fused GOES-R IFR Probabilities do suggest enhanced possibilities of IFR conditions in regions where reduced visibilities are reported:  central and northeast Louisiana and east Texas.  IFR Probabilities are much lower over southwest Louisiana where IFR conditions are not reported.

As above, but for 1245 UTC on 6 December 2012.

By 1245 UTC, the high clouds have lifted northeast and dissipated somewhat, so the heritage brightness temperature difference product gives information over the entire lower Mississippi River valley and over east Texas, where IFR conditions were widespread underneath very high GOES-R IFR Probabilities.  The GOES-R Cloud Thickness product indicates cloud thicknesses up to near 1200 feet, suggesting a burn-off time for radiation fog of around 5 hours.

Day/Night Band (from Suomi/NPP) imagery over Louisiana and East Texas, 0733 UTC on 6 December 2012. 

 The Day/Night band image derived from data from Suomi/NPP from 0733 UTC on 6 December, above, shows the higher and lower clouds over Texas and Louisiana.  The clouds between Houston and Dallas/Ft. Worth are low clouds, but the high clouds over Louisiana inhibit the detection of low clouds.  In addition, although the Day/Night band can give a good outline of where the clouds are, it does not show where the visibility restrictions consistent with IFR conditions are.

Visible Imagery from 2000 UTC

Visible imagery from 2000 UTC, above, shows that, although the fog has lifted, it has not burned off in 5 hours as predicted by the thickness/burn-off time relationship (here).  This may be related to the very low sun angle in December.

(Added:  This image loop shows how thin cirrus can show up in the brightness temperature difference product at night (this example is from Suomi/NPP even when fog-bound valleys are plainly evident in the Day/Night band at the same time!)

Dense fog at LAX

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GOES-R IFR Probabilities from GOES-West (Upper Left), Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) field from GOES-West (Upper Right), GOES-R Cloud Thickness from GOES-West (Lower Left), GOES-West visible imagery (Lower Right)

Dense fog developed over portions of the Los Angeles basin in the morning on 5 December 2012, and regions affected included Los Angeles International Airport.  The GOES-R IFR Probability product, above, shows the fog was most prevalent from Santa Monica to points south, mostly within a couple miles of the coast.   In particular, the GOES-R IFR probability field depicted the sharp edge to the fog field.  Surface visibilities at LAX improved to above 1 mile shortly after 1800 UTC.

From the National Weather Service in Los Angeles:

CAZ041-051700-
 /O.CON.KLOX.FG.Y.0020.000000T0000Z-121205T1700Z/
 LOS ANGELES COUNTY COAST INCLUDING DOWNTOWN LOS ANGELES-
 INCLUDING THE CITIES OF...MALIBU...SANTA MONICA...BEVERLY HILLS...
 HOLLYWOOD...LONG BEACH
 628 AM PST WED DEC 5 2012

 ...DENSE FOG ADVISORY REMAINS IN EFFECT UNTIL 9 AM PST THIS
 MORNING...

 * VISIBILITIES...ONE QUARTER MILE OR LESS AT TIMES.

 * TIMING...THROUGH EARLY THIS MORNING...WITH IMPROVING 
   VISIBILITIES BY MID MORNING.

 * IMPACTS...TRAVEL WILL BE HAMPERED BY THE DENSE FOG IN AND 
   AROUND THE LONG BEACH AREA...TO GARDENA...AND WEST LOS ANGELES 
   INCLUDING LOS ANGELES AIRPORT.

 PRECAUTIONARY/PREPAREDNESS ACTIONS...

 A DENSE FOG ADVISORY MEANS VISIBILITY WILL FREQUENTLY BE REDUCED
 TO ONE QUARTER MILE OR LESS. IF DRIVING...SLOW DOWN...USE YOUR
 HEADLIGHTS...AND LEAVE PLENTY OF DISTANCE AHEAD OF YOU. FOG CAN
 ALSO MAKE ROAD SURFACES SLICK SO AVOID USING EXCESSIVE SPEED.

Stratus vs Fog

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GOES-R IFR Probabilties over Wisconsin, computed from GOES-East (Upper left), GOES-East brightness temperature difference (10.7 µm – 3.9 µm) (Upper right), GOES-East Visible imagery (0.62 µm) (Lower left), GOES-R Cloud thickness (Lower right)

Stratus and fog look very similar from the satellite’s perspective, both during the day and at night.  That is why it is important to include surface information in a product that detects fog and low stratus.  The IFR Probability product over stratus-bound Wisconsin at mid-day on November 30 2012 shows a diagonal stripe of higher probabilities from southwestern Wisconsin to north-central Wisconsin.  Airports that are reporting IFR or near-IFR conditions are located within this stripe.  Over the rest of the state, where IFR probabilities are lower, the large majority of airports are reporting visibilities and ceilings exceeding IFR limitations.  The character of the visibile satellite data, and of the brightness temperature difference product, gives very little indication that surface visibilities are reduced primarily from northeast Iowa/southwest Wisconsin to north-central Wisconsin.

IFR Probabilities during a Big Storm

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GOES-R IFR Probabilities computed using GOES-West data (upper left), GOES-West traditional brightness temperature difference (10.7 µm – 3.9 µm ) (upper right), surface observations of visibility and ceiling (AGL) (lower left), Blended Total Precipitable Water Product (lower right)

Large extratropical storms are often accompanied by regions of IFR conditions, but the multiple layers of clouds that are produced by the storms make fog/low stratus detection by traditional means — the brightness temperature difference between 10.7µm and 3.9µm microns (at night) — a difficult prospect.  The GOES-R Fog/Low Stratus (FLS) product that fuses satellite data with model (Rapid Refresh) data allows for estimates of IFR probabilities.  The imagery above also includes precipitable water estimates from Sounder, GPS and microwave imagery (bottom right), highlighting the tropospheric river of moisture that is impinging on the West Coast.

Highest IFR probabilities are occurring in several regions in the animation above.  They occur over the Los Angeles basin, for example, where IFR conditions are reported at several airports (San Nicolas and Los Angeles airports, for example).  Reduced visibilities are also occurring in the Sierras — Blue Canyon (at 1500 m above sea level) reports IFR conditions — and IFR probabilities are higher along the spine of the mountains.  IFR probabilities are also higher along the northern California coast, and stations like Ukiah are reporting occasional IFR conditions.  Stations in the central Valley, and along the central coast, are in a region of lower IFR probabilities, and IFR conditions are comparatively rarer there.

Continuity through sunrise

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GOES-R IFR Probabilities (Upper Left), computed from GOES-East, Brightness Temperature Difference (10.7 µm – 3.9 µm) computed from GOES-East, GOES-R Cloud Thickness (Lower left), Ceiling and Visibility observations (Lower right)

The animation of the GOES-R IFR Probability product, above, shows one of its strengths:  it has a similar look during night and day.  The traditional fog product created by the brightness temperature difference between 10.7 µm and 3.9 µm data from GOES-East, switches sign as the sun rises and the amount of reflected 3.9 µm radiation increases.  The IFR Probability maintains a steady signal that matches observed IFR conditions.

Note that the Brightness temperature difference product shows a signal along the Louisiana/Texas border (over the Toledo Bend Reservoir on the Sabine River), and also over Lake Sam Rayburn, Lake Livingston and Lake Conroe in east Texas.  It is possible that there is shallow fog over these bodies of water (post-sunrise imagery shows no signal);  the signal might also arise from the approximately 1-pixel co-registration error between the 3.9 µm and 10.7 µm channels on GOES-13. This possibly erroneous signal in the brightness temperature difference does propagate into the GOES-R IFR probability field.