Category Archives: Suomi/NPP

Radiation Fog over Texas, Day 2

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GOES-R IFR Probabilities computed from GOES-East and the Rapid Refresh Model (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness computed from GOES-East (Lower Left), Suomi/NPP VIIRS Day/Night Band (Lower Right), 0615 UTC on 24 January 2013

Clear skies and light winds again allowed for the development of radiation fog over southeast Texas.  How did the development, and the detection of fog, differ for this event from the event 1 night previous (as discussed here).  The 0615 UTC imagery, above, shows a separation between where the Brightness Temperature Difference (the heritage fog detection product) and where the GOES-R IFR probabilities are suggesting fog is present.  The Heritage Fog Product is focused on the Rio Grande Valley whereas the GOES-R IFR Probability is focused (correctly, as it turns out) on southeast Texas.  Note also that the Brightness Temperature Difference product has a signal representative of higher clouds (the dark region) over northeast Texas.

As above, but at ~0715 UTC

 At 0715 UTC, IFR probabilities are increasing over southeast Texas between Houston and San Antonio.  Suomi/NPP Day/Night band imagery at that time shows evidence of clouds in the region of highest IFR probabilities.  The traditional brightness temperature difference product continues to highlight a region near the Rio Grande Valley.

As above, but at ~0845 UTC

At 0845 UTC, IFR probabilities continue to increase over southeast Texas in the region surrounding Houston and a region near San Antonio.  The Day/Night band detects the cloudiness present in those regions.  High clouds persist over northeast Texas.

As above, but at 1315 UTC

Shortly before sunrise, fog is widespread over southeast Texas.  IFR probabilities are highest where both model and satellite predictors can be used.  Over northeast Texas, where cirrus clouds are present and where the brightness temperature difference product can therefore not provide guidance, the Rapid Refresh data are suggesting that fog is present (as observed), but IFR probabilities are lower because the satellite predictor is not used.

Fog/Low Stratus over the High Plains

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Surface Weather Maps, 0300, 0600 and 0900 UTC on 22 January

High Pressure with origins in the Arctic has pushed cold air into the central United States.  The western edge of the cold dome shows as a stationary front that stretches from central Kansas northwestward into Montana and beyond.  During the early morning hours of 22 January, a small region of IFR conditions developed over western Nebraska.  How did the GOES-R Fog products do in describing this region?

GOES-R IFR Probabilities and surface plots of ceilings/visibilities (Upper Left), GOES-East Brightness Temperature Difference (Upper Right), Suomi/NPP Day/Night Band (Lower Left), GOES-R Cloud Thickness (Lower Right) for various times from 0102 through 1102 UTC 22 January 2013

The animation above shows increasing IFR probabilities over southwest and west-central Nebraska over the course of the night in a region where IFR conditions are developing.  Note how the IFR probabilities are not enhanced in regions where the traditional brightness temperature difference product does have a signal — over eastern Nebraska and northeastern Kansas.  In these regions, the Rapid Refresh Model fields likely include no saturation in the lowest model layers.  Suomi/NPP Night-time Visible imagery, below, at 0752 UTC and at 0933 UTC also show the extent of the fog and low stratus.   However, it’s impossible to tell from the satellite where the visibility obstructions are most likely — that’s why the model data are important in this fused product.  Note the distinct change in illumination between 0752 UTC and 0930 UTC.  The Waxing Gibbous moon set around 0900 UTC.

As above, but for 0745 – 0800 UTC on 22 January 2013

As above, but for 0930 UTC on 22 January 2013

Two examples from 17 January 2013

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GOES-R IFR Probabilities computed from GOES-East, 1700 UTC on 17 January 2013

 The image above shows how IFR probabilities can maximize over higher terrain where mountains rise up into a somewhat uniform cloud deck.  IFR probabilties are highest over the Laurel Highlands of Pennsylvania southward along the spine of the Appalachian Mountains in West Virginia (and also in the highlands of north-central Pennsylvania).  IFR Conditions are reported at K2G4 in Maryland, which is 890 meters above sea level, and near-IFR conditions are present at Johnstown, PA (KJST) and Elkins, WV (KEKN), two stations above 600 meters above Mean Sea Level.  In contrast, KCBE and KW99, Cumberland Maryland and Petersburg, WV, are both lower than 300 m above sea level, and IFR condition are not present there.

GOES-R IFR Probabilities (Upper Left), GOES-East Visible Imagery (Upper Right), GOES-East Brightness Temperature Difference (Lower Left), Suomi-NPP 1.61 µm Reflectivity (Lower Right)

GOES-R IFR probabilities maximized near the Missouri River Valley in eastern Nebraska around mid-day on 17 January 2013.  IFR conditions were reported.  The largest visibility restrictions appear to occur over a band of snow that extended southwest to northeast, roughly parallel to the N. Platte River, and IFR probabilities are highest in that region. The snow band shows up well in the visible imagery, and as a black swath in the 1.61 reflectivity (snow absorbs radiation at 1.61µm).

Evolution of Fog/Low Stratus over Florida

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GOES-R IFR Probabilities (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES-East 6.5 µm imagery (Lower Right), from ~0000 UTC on 3 January 2013

GOES-R IFR Probabilities captured the evolution of IFR (and Low IFR) conditions over and around the Florida peninsula from late on 2 January through morning on 3 January 2013.  Advection fog over the chilly coastal waters of the eastern Gulf of Mexico stayed mainly offshore (although Sarasota at 00 UTC reports IFR conditions) and is captured well by the GOES-R product.  This is a region underneath high cirrus and as such, the traditional brightness temperature product is blind to the existence of low clouds there.

Over the course of the night, fog and low stratus developed over land, and the GOES-R IFR probability product captured that evolution as well (below, hourly imagery).  Again, there are regions where the brightness temperature difference product is not useable because of multiple cloud layers, and the Rapid Refresh Model output is controlling the IFR Probabilities — these are regions where the IFR probability field is very smooth and typically exhibits lower probability values even though IFR conditions may be observed (For example, at Gainesville and Jacksonville at 0600 UTC).  By morning, visibilities were under 1/4 mile over much of the central Florida Peninsula (For example, Orlando).

As above, but hourly imagery from 0000 UTC through 1400 UTC on 3 January 2013.

The 3/4-full moon allows for plenty of illumination for the Day/Night band on VIIRS, which is flying on Suomi/NPP.  The 0700 UTC imagery, below, demonstrates the difficulty of using the DNB at night to detect fog — city lights that shine through low clouds.  Fog is detected in rural regions, but where city lights exist, the signal is difficult to extract.

GOES-R IFR Probabilities (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), Suomi/NPP VIIRS Day/Night Band 0.7 µm imagery (Lower Right), from ~0700 UTC on 3 January 2013

The visible imagery at 1500 UTC, below, shows the horizontal extent of the stratus deck through central Florida.  The region matches well with the IFR Probabilities because visible imagery during the day is used as a cloud-clearing mechanism in the GOES-R algorithms.  Note also how the reflected 3.9 µm solar radiation during the day renders the brightness temperature difference product ineffectual.

GOES-R IFR Probabilities (Upper Left), GOES-East Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Left), GOES-East visible (0.63 µm) imagery (Lower Right), from ~1500 UTC on 3 January 2013

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.

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!)

Low Clouds over the Florida Peninsula

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GOES-R IFR Probability computed from GOES-East and Rapid Refresh data (Upper left), Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) Product from GOES (Upper right), GOES-R Cloud Thickness product (Lower Left), Brightness Temperature Difference (10.8 µm – 3.74 µm) Product from VIIRS on Suomi/NPP (Lower Right)

Tuesday morning’s low cloud/fog event over southern Florida is an excellent example of how the fused product better distinguishes between fog (that reduces visibility at the surface) and low stratus (that does not reduce visibility at the surface).  The traditional brightness temperature difference products from both GOES (Upper Right) and VIIRS (Lower Right) show a strong signal over both Florida coasts — Atlantic and Gulf — but the IFR probabilities are highest along the Atlantic Coast — where IFR and near-IFR ceilings/visibilities are observed.  Over and off the west coast of Florida, despite the very strong satellite signal, IFR probabilities are low because saturation is not occurring in the lowest part of the Rapid Refresh Model — and IFR conditions are not observed.

GOES-R Fog Products vs. ‘Traditional’ Products over Florida

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GOES-R IFR Probabilities computed from GOES-East (Upper left), Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) (Upper right), Brightness Temperature Difference (10.8 µm – 3.74 µm) from Suomi/NPP data (Lower Left), GOES-R IFR Probabilities computed using MODIS data (Lower Right)

Fog and low clouds that developed over central Florida (again) on Monday morning, 19 November, highlight some strengths and shortcomings of the different fog detection techniques.  The GOES-R IFR Probability (upper right) shows highest probabilities in the region where the traditional brightness temperature difference product has a distinct signal.  But there is also a region of IFR probabilities (albeit low) east of Tampa Bay where the traditional brightness temperature difference product has no distinct signal.  This is likely a region of developing fog/low stratus.  Note that the higher-resolution Brightness Temperature Difference product from VIIRS on Suomi/NPP has a signal in that region, and the MODIS-based IFR Probability (lower right) field is also consistent with a developing fog/low stratus field.  The low IFR Probabilities in the GOES-based signal should alert to the possibility of developing fog.  Note that both the GOES-Based and MODIS-based IFR probabilities show a narrower strip of fog/low stratus south of Jacksonville than is represented in the GOES and Suomi/NPP Brightness Temperature Difference fields.  In this region, the Rapid Refresh Model data are refining the satellite predictors to represent more accurately the distribution of fog/low stratus in the Rapid Refresh model output.   The IFR Probabilities at 1000 UTC (below) shows that IFR conditions did indeed develop in that region of interior Florida to the east of Tampa.

GOES-R IFR Probabilities at 1002 UTC, computed from GOES-East and the Rapid Refresh Model

The Traditional Brightness Temperature Difference field shows a signal that parallels the west-facing Gulf coasts of Florida.  This signal arises from a co-registration error between the shortwave IR and longwave IR sensors on GOES (See this post for more examples).  This co-registration error is diurnally varying and typically peaks between midnight and sunrise.

Fog over the lower Mississippi River Valley

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GOES-East IFR Probabilities (upper left), GOES-East Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) (upper left), VIIRS Day/Night Band from Suomi/NPP (lower left), VIIRS Brightness Temperature Difference (10.80 µm – 3.74 µm) from Suomi/NPP (lower right) at 0315 UTC on 6 November 2012

IFR Conditions developed over the deep south overnight on the 5th/6th of November.  How did the GOES-R IFR Probabilities capture this event, and how do the fields compare to the traditional brightness temperature difference fields?  At 0315 UTC, near-IFR conditions have developed over central Mississippi and over southwest Missouri, the two regions where IFR probabilities are diagnosed to be highest.  Rapid Refresh Model data are appropriately de-emphasizing the satellite signal in regions where IFR conditions are not reported (northeast Arkansas, for example).

Four hours later (below), at 0715 UTC, the area of IFR conditions over Mississippi has expanded somewhat, and the IFR Probability field continues to suggest — strongly — that IFR conditions are present.  Both the traditional brightness temperature difference field and the IFR probability field suggest a sharp western edge to the fog/low stratus over north-central Louisiana, and that sharp edge is confirmed in the Day/Night band from VIIRS on Suomi/NPP.  The Brightness Temperature Difference fields from both GOES and from VIIRS suggest one large field of fog, but the IFR probability field has two separate fields:  one over Mississippi/extreme southern Arkansas and Louisiana, and one over southern Missouri.

GOES-East IFR Probabilities (upper left), GOES-East Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) (upper left), VIIRS Day/Night Band from Suomi/NPP (lower left), VIIRS Brightness Temperature Difference (10.80 µm – 3.74 µm) from Suomi/NPP (lower right) at approximately 0715 UTC on 6 November 2012

IFR Conditions under Cirrus in Florida

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GOES-R IFR Probabilities from GOES-East (Upper Right), GOES-East traditional ‘fog product’ (Brightness Temperature Difference 10.7 µm – 3.9 µm), GOES-R Cloud Thickness, GOES-East Water Vapor (6.5 µm) imagery

Fog developed over central Florida overnight underneath a thin cirrus (as indicated by both the water vapor and brightness temperature difference imagery).  Cirrus clouds prevent the traditional brightness temperature difference field from identifying low fog/stratus because the high ice clouds are detected rather than the developing low-level water clouds.  This is a case, then, when a fused product gives needed surface information to help diagnose the development of fog and low stratus.  The brightness temperature difference product, the traditional method to detect fog and low stratus, is giving no information where dense fog is forming.

On this date, the development and expansion of the higher IFR probabilities over central Florida neatly matches the development of IFR conditions at the observing stations.  Probabilities are not high because the satellite predictors are not contributing to the algorithm.  It is important when interpreting the IFR probabilities to be aware of the presence of high clouds that will influence IFR probability values.  Where the cirrus clouds are not present, notably over northeast Florida, IFR probabilities are much higher because satellite predictors there are contributing to the final probability.

This case also shows that the Cloud Thickness is only computed where the highest clouds detected is a water-based cloud.  Underneath the cirrus shield, except for a few regions where there are apparently holes, cloud thickness is not computed.

GOES-R IFR Probabilities and Day/Night Band from VIIRS on Suomi/NPP, 0715 UTC 5 Nov

The toggle above flips between the Day-Night band from VIIRS on Suomi/NPP and the GOES-R IFR probability at the same time.  The thin cirrus shield is readily apparent, and the regions of fog are also visible in the Day/Night band over north-central Florida and over coastal South Carolina.

A MODIS-based IFR Probability (shown below) was also created at 0715 UTC, and it shows a pattern similar to that above.  The pixelated part of the image corresponds to where satellite data are being used.  The region with lower values, and a flatter field, was created using only model predictors and, as noted above, is characterized by lower probabilities.  The highest probabilities are in regions where both satellite and model predictors are very confident that IFR conditions are present.

MODIS-based GOES-R IFR Probabilities, Monday 5 Nov 2012, 0714 UTC