Category Archives: Suomi/NPP

Fog on Texas and Louisiana Gulf Coasts

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Fog was anticipated to develop over the coastal sections of Texas and Louisiana starting late on Oct 31 2012.  From the 0149 UTC 1 November Houston Forecast Discussion:    The 0921 UTC Forecast Discussion from Lake Charles (above) describes increasing fog possibilities — and the 0453 UTC AFD (below) mentions patchy fog.

For both WFOs, the GOES-R IFR Probability field shows a good picture of the evolving fog/low stratus as it develops.

000
FXUS64 KHGX 010149
AFDHGX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
849 PM CDT WED OCT 31 2012

.DISCUSSION...
CURRENT FCST IS ON TRACK. ONLY TWEAKS TO GRIDS WERE TO MOVE UP
TIMING OF FOG FORMATION. WOULDN`T DOUBT IF A DENSE FOG ADVSY
MIGHT BE REQUIRED FOR SOME LOCATIONS...ESP SW. WILL KEEP AN EYE ON
TRENDS. DIFFUSE WIND SHIFT AND SLIGHTLY LOWER DEWPOINTS WILL
PROBABLY MOVE INTO NE ZONES LATER TONIGHT THEN STALL/WASHOUT.
 

 
000
FXUS64 KLCH 010921
AFDLCH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAKE CHARLES LA
421 AM CDT THU NOV 1 2012

.DISCUSSION...TRAPPED LOW LEVEL MOISTURE AND CLEAR SKIES HAVE
ALLOWED AREAS OF FOG TO DEVELOP THIS MORNING ACROSS THE CWA. AT THIS
TIME PATCHY DENSE FOG HAS ALSO DEVELOPED IN CALCASIEU PARISH AND
JEFFERSON COUNTY. VISIBILITIES ARE SLOWLY DROPPING ELSEWHERE AND
IF CONDITIONS CONTINUE TO DETERIORATE A DENSE FOG ADV MAY BE NEEDED
THIS MORNING. 



The 0921 UTC Forecast Discussion from Lake Charles (above) describes increasing fog possibilities — and the 0453 UTC AFD (below) mentions patchy fog.

  

 
 
000
FXUS64 KLCH 010453
AFDLCH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LAKE CHARLES LA
1153 PM CDT WED OCT 31 2012

.DISCUSSION...
01/06Z TAF ISSUANCE.

&&

.AVIATION...
FEW CHANGES TO THE TAFS THIS EVENING WITH WINDS NEARLY CALM ACRS
THE AREA. T/TD SPREAD NARROWING AT BPT AND LCH AND COULD SEE SOME
PATCHY FOG DEVELOP AT THESE SITES WITHIN THE NEXT HOUR OR TWO.
CANNOT RULE OUT FOG AT OTHER TAF SITES...BUT EXPECT ONSET A LITTLE
LATER AS DEWPOINT DEPRESSIONS ARE SLIGHTLY LARGER. VFR CONDITIONS
EXPECTED TO PREVAIL WITH THE EXCEPTION OF PERIODIC MVFR OR BRIEF
IFR VISBYS BETWEEN NOW AND 14Z. LT WINDS WILL GRADUALLY BECOME
SWLY THURS AFTN. 24 


For both WFOs, the GOES-R IFR Probability field shows a good picture of the evolving fog/low stratus as it develops.  The every-hour loop below, starting at 0315 UTC, shows the steady increase in probabilities along the Louisiana and Texas Gulf Coasts.  Note the relatively low probabilities in and around Houston — an apparent break between IFR conditions to the north and east and those to the south.  The Houston airport observations did not fall to IFR criteria although those criteria were common to the north and south.  Also, the IFR probabilities downplay the brightness temperature difference signal over central Texas where IFR conditions do not occur.











GOES-R IFR Probabilities computed from GOES-East (upper left), Traditional Brightness temperature Difference product (10.7 µm – 3.9  µm) (upper right), GOES-R IFR Probabilities computed from MODIS (lower left), Suomi/NPP Day/Night Band (lower left)






The imagery below shows GOES-R and MODIS imagery at the same time (immediately below) and GOES-R and Suomi/NPP imagery at the same time (bottom).  Note that the fog that develops is not of sufficient thickness to block views of the city lights.











GOES-R IFR Probabilities computed from GOES-East (upper left),  Traditional Brightness temperature Difference product (10.7 µm – 3.9  µm)  (upper right), GOES-R IFR Probabilities computed from MODIS (lower  left), Suomi/NPP Day/Night Band (lower left)
















GOES-R IFR Probabilities computed from GOES-East (upper left),  Traditional Brightness temperature Difference product (10.7  µm- 3.9  µm)  (upper right), GOES-R IFR Probabilities computed from MODIS (lower  left), Suomi/NPP Day/Night Band (lower left)








 

					

		
		
	


	

				

			
						

				Stratus vs. Fog in Montana/North Dakota
			

										

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Suomi/NPP VIIRS Day/Night Band Visible imager (upper left), Traditional Brightness Temperature Difference (10.7 µm – 3.9 µm) image (upper right), GOES-R Cloud thickness algorithm (lower left), GOES-R IFR Probability (lower right)






Both the nighttime visible image from the Suomi/NPP VIIRS instrument (which uses moonlight as an illumination source) and the traditional brightness temperature difference product suggest the presence of water-based clouds over eastern Montana and northwest North Dakota on the morning of 31 October 2012.  However, there are no reports in Montana of IFR conditions.  GOES-R IFR Probabilities overlapping the cloud deck are very low.  It is likely that this cloud feature is elevated stratus, and that saturation is not occuring in the lowest levels of the model.  Fusing both model and satellite data yields a product that can be greater than the sum of the two.  Note, however, the IFR conditions that do exist in Bismarck, where IFR probabilities are very low and satellite information shows no fog signal.  This is likely very small scale river fog in the Missouri River Valley that is both too small to be resolved in the model or detected by the satellite.  In addition, very thin cirrus is interfering with the detection of low-level water clouds in much of central North Dakota.

					

		
		
	


	

				

			
						

				Marine Stratus in the Mid-Atlantic States
			

										

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GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-R Cloud Thickness in ft (upper right), Brightness Temperature Difference (10.8 µm – 3.74 µm) computed from Suomi/NPP VIIRS data, Brightness Temperature Difference (10.8 µm– 3.9 µm) computed from GOES East, all valid about 0700 UTC 25 October 2012







Marine Stratus has moved inland over Maryland and surrounding states overnight, and the GOES-R IFR probabilities capture the visibility restrictions that have accompanied it.  The imagery above includes the traditional brightness temperature difference fields computed from Suomi/NPP (horizontal resolution:  1 km at nadir) and from GOES (nominal horizontal resolution:  4 km at nadir).  The distinct leading edge of the marine layer is readily apparent in the Susquehanna River Valley.  Note, however, that the strong returns over the Hudson River Valley do not correlate well with reduced visibilities. The GOES-R IFR probabilities do a good job depicting the marine layer in the Susquehanna River Valley (shown by the relatively high probabilities), but return much lower probabilities over the Hudson River Valley where surface observations indicate hazardous low clouds are not present.  Again, this is a benefit of using a fused product.











GOES-R IFR Probabilities (Upper Left) and Cloud Thickness (Upper Right), Suomi/NPP Day/Night Band (Lower left) and MSAS-derived Dewpoint depression on top of Visible imagery.  Times as indicated






The temporal resolution of GOES-R IFR products allow for continuous monitoring of an evolving situation, making up for the relatively poor horizontal resolution (compared to polar orbiting platforms like Suomi/NPP or Terra/Aqua).  The IFR probability field neatly captured the relentless inland push of the marine stratus air, and as the probabilities increase at locations, IFR conditions become more likely.  The loop above includes a Day/Night band image that is reproduced below.  The day/night band, even in low lunar light cases, can distinguish between clear sky and clouds over ocean.  Over land, however, the interpretation is complicated by city lights shining through clouds.  Nevertheless, the cloudy region can be discerned over parts of eastern Pennsylvania and Maryland, where the light signal is more diffuse.





					

		
		
	


	

				

			
						

				Fog/Low stratus in California
			

										

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GOES-R IFR Probabilities (Upper left) and GOES-15 brightness temperature difference (10.7 µm – 3.9 µm) (upper right) from 0200 UTC






 GOES-R IFR Probabilities can be used to evaluate and monitor the evolution of marine stratus as it (nightly) moves inland from the Pacific Ocean to adjacent land over California, as well as the evolution of fog and low clouds/haze over the central Valley of California.  At 0200 UTC — near sunset — as shown above, IFR conditions are diagnosed from fused satellite and model data to be most likely offshore, despite a brightness temperature difference signal in California’s Central Valley, where airports are not reporting IFR conditions.  In other words, the GOES-R IFR algorithm is correctly suppressing the satellite signal inland.











GOES-R IFR Probabilities (Upper left) and GOES-15 brightness temperature difference (10.7 µm – 3.9 µm) (upper right) from 0700 UTC






 By 0700 UTC, ceilings and visibilities in the Salinas and Central Valleys have started to lower as IFR probabilities decrease.











GOES-R IFR Probabilities (Upper left) and GOES-15 brightness temperature  difference (10.7 µm – 3.9 µm) (upper right) from 0900 UTC, Suomi NPP Brightness Temperature Difference (10.8 µm – 3.74 µm) (lower left) 






 Ceilings and visibilities decrease further by 0900 UTC, as shown above.  Reasons for the difficulty in accurately diagnosing the probabilities over the Salinas Valley can be inferred from the figure above, which includes a (high-resolution) Suomi/NPP Brightness temperature difference plot that has a strong signal all along the Salinas Valley; such a strong signal is not so evident in the GOES brightness temperature difference field that uses data at coarser resolution.  It’s important that the satellite signal is strong in this fused product because the valley is not well-resolved in the Rapid Refresh Model.  When GOES-R is operational, infrared resolution will be 2 km — in between that of Suomi/NPP (1 km) and GOES (4 km).











GOES-R IFR Probabilities (Upper left) and GOES-15 brightness temperature  difference (10.7 µm – 3.9 µm) (upper right) from 1400 UTC, GOES-R Cloud Thickness (lower left) 






 The 1400 UTC image includes cloud thickness as diagnosed by the GOES-R algorithm.  Cloud thickness is well correlated with dissipation time for radiation fog — but not necessarily for advection fog.  Nevertheless, the regions diagnosed with thickest fog above continue to show fog in the visible imagery below, from 1600 UTC.











GOES-R IFR Probabilities (Upper left) and GOES-15 brightness temperature  difference (10.7 µm – 3.9 µm) (upper right) from 1400 UTC, GOES-15 Visible Imagery (lower left) 







					

		
		
	


	

				

			
						

				GOES-R Probabilities evolve as the night progresses
			

										

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GOES-R IFR Probabilities computed from GOES-14 (upper left), GOES-14 heritage brightness temperature difference product (upper right), VIIRS heritage brightness temperature difference product (lower left), MODIS heritage brightness temperature difference product (lower right), all from near 0700 UTC.






MODIS and VIIRS yield satellite information that can be used to detect fog/low stratus at high spatial resolution.  In the exa,ple above, the VIIRS brightness temperature difference product highlights many river valleys as possibly cloud-filled over eastern Kentucky, southern Ohio and southwestern West Virginia.  The coarser resolution of the GOES satellite pixel precludes such fine-scale detection.  Note, however, that both satellite platforms detect the presence of stratiform water clouds over north-central Ohio where surface observations show only mid-level cloudiness.  IFR probabilities are confined to the spine of the Appalachians from the Laurel Mountains near Johnstown (PA) southward towards southern West Virginia.  How do things evolve with time?











As above, but from near 0800 UTC

















As above, but from near 0915 UTC

















As above, but from near 1000 UTC

















As above, but from near 1100 UTC






The power of GOES imagery in this case is to show the evolution of the fog/low stratus field.  Even at this only-hourly timestep, the development of regions of IFR conditions is evident, and those developing conditions occur in tandem with increasing probabilities in the GOES-R IFR Probability field.  Throughout the night, the GOES brightness temperature difference field flags the unimportant (from an aviation standpoint) stratus deck over northcentral Ohio, and the IFR probability field, which field also uses Rapid Refresh Model data, discounts the satellite signal.  By 1100 UTC, the river valley signal has strengthened enough in the GOES imagery to appear, and a corresponding increase in IFR probability occurs.


IFR Probability can also be computed using MODIS data (below). The 0739 UTC MODIS data, shown at the top of this blog post, highlights — as does GOES — the stratus deck over northern Ohio.  The MODIS-based IFR probabilities, however, do not highlight that cloud-deck, by design.  Note also that the higher-resolution MODIS imagery, because it detects river valley fogs at 0739 UTC, also has a strong IFR probability signal there.  Pixel resolution on GOES-R will be intermediate between MODIS and present GOES.











As at top, for near 0800 UTC, but with MODIS-based IFR probabilities in the lower right





					

		
		
	


	

				

			
						

				Day/Night band and fog detection
			

										

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Day/Night band from VIIRS on Suomi/NPP from 0714 UTC on 4 October 2012






The day/night band imagery, above, shows fog/low clouds in the Hill Country west of San Antonio and Austin in south-central Texas.  Additionally, there are low clouds over the Gulf of Mexico, with cloud street structures that suggest a south-southeasterly wind bringing moisture inland from the Gulf.  The 0855 UTC day/night band image over the same domain, below, shows an expansion of the fog/low cloud signal.











Day/Night band from VIIRS on Suomi/NPP from 0855 UTC on 4 October 2012















Day/Night band from Suomi/NPP with observations overlain.






Observations suggest that the northern edge of the cloud streets over the extreme western Gulf of Mexico is the edge of a moisture gradient, and that that gradient extends inland to where the fog and low stratus are occurring.  How did the GOES-R IFR Probability field perform on this day?  The 0702 UTC (below) and 0915 UTC IFR probability fields show an increas in the areal extent of higher probabilities over the course of the night, consistent with the overnight cooling and the continued feed of moisture from the Gulf.  By 0900 UTC, IFR observations are common in and near the region where IFR probabilities are high.  This is a good example of how the Day/Night band and IFR Probabilities can be used in concert to understand the evolution of the fog/low stratus field over south Texas.









					

		
		
	


	

				

			
						

				FLS detection over the upper midwest
			

										

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0730 UTC GOES-R IFR Probabilities computed from GOES-14 (Upper left), Traditional Brightness Temperature difference (10.7 µm – 3.9 µm) from GOES-14 (Upper Right), Brightness Temperature Difference (11.35 µm – 3.74 µm) from VIIRS on Suomi/NPP (Lower left),  Thickness of highest Liquid Water Cloud Layer (Lower Right)






Low cloud formation over northern and northeastern Wisconsin early in the morning of October 3rd 2012 demonstrated some strengths of the fused GOES-R Fog/Low Stratus product.  Note that the 0732 UTC GOES-R IFR Probability has a pocket of higher probabilityes over north-central Wisconsin (near Vilas and Oneida Counties) in a region where the traditional brightness temperature difference has a weak signal, and in a region where surface observations indicate fog is present. This region is a good example of how the model RH data can amplify weak satellite signals where fog/low clouds are present but the satellite signal alone may not be strong enough to detect it.











0915 UTC GOES-R IFR Probabilities computed from GOES-14 (Upper left),  Traditional Brightness Temperature difference (10.7 µm – 3.9 µm) from GOES-14 (Upper  Right), Brightness Temperature Difference (11.35 µm – 3.74 µm) from VIIRS on Suomi/NPP (Lower  left),  Thickness Of highest Liquid Water Cloud Layer (Lower Right)






At 0915 UTC, the region of IFR conditions in north-central Wisconsin persists.  At this time, however, the satellite signal also increases showing a signature consistent with low clouds/fog, and therefore GOES-R IFR probabilities significantly increase.  Two things should be clear.  First, the GOES-R IFR probability predicted the presence of Fog/Low Stratus before satellite signal was strong enough to detect it alone (a benefit of using a fused data product) and could be used better to nowcast the evolving boundary layer.  Second,  GOES-R IFR Probabilities when the model signal (in this case, the Rapid Refresh) is strong and the satellite signal is weak are lower than when both model and satellite signals are strong.  This is always the case.











1145 UTC GOES-R IFR Probabilities computed from GOES-14 (Upper left),  Traditional Brightness Temperature difference (10.7 µm – 3.9 µm) from GOES-14 (Upper  Right), Visible Imagery (0.62 µm) from GOES-14 (Lower  left),  Thickness Of highest Liquid Water Cloud Layer (Lower Right)






The 1145 UTC shows the last pre-dawn estimate of Cloud Thickness over north-central Wisconsin (the terminator is apparent, running north-south through the Straits of Mackinac).  Estimated cloud thickesses over Vilas and Oneida counties are between 1000 and 1150 feet, vs. 1250 feet over Green Bay and more than 1300 feet over southern Upper Michigan.  Predictably, then, the fog/low stratus over north-central Wisconsin dissipates before the fog/low stratus over northeast Wisconsin and eastern Upper Michigan (see below).











1602 UTC GOES-R IFR Probabilities computed from GOES-14 (Upper left),  Traditional Brightness Temperature difference (10.7 µm – 3.9 µm) from GOES-14 (Upper  Right), Visible Imagery (0.62 µm) from GOES-14 (Lower  left),  Thickness Of highest Liquid Water Cloud Layer (Lower Right)







					

		
		
	


	

				

			
						

				GOES-R Fog and the Day/Night Band on VIIRS
			

										

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GOES-R IFR Probabilities (upper left), Suomi/NPP VIIRS Day/Night Band (upper right), Brightness temperature difference (10.7 micrometers – 3.9 micrometers) from GOES (lower left), Brightness temrperature difference (11.35 micrometers – 3.74 micrometers) from Suomi/NPP VIIRS (lower right), all around 0930 UTC on 31 August.






The presence of the Day/Night band on the VIIRS instrument on the Suomi/NPP satellite offers a unique method of validating the presence of fog or stratus at night.  During times near full moon (such as the Blue Moon on 31 August), the Day/Night band can detect low clouds using light reflected from the moon.  The GOES-R IFR probabilities show fog and low/stratus over southwestern Oregon;  a larger region of fog/low stratus stretched from just north of Crescent City, CA (where IFR conditions are reported) southward down the coast.  Note also a small patch over southwestern Washington and coastal northwest Washington (where IFR conditions are reported.  Cirrus clouds that prevent the detection of fog/low stratus from satellite are present stretching northeastward from the ocean off the central Oregon coast into central Washington.  There is a small signal in the GOES-R IFR Probability field underneath this upper cloud feature.













GOES-R IFR probabilties (Upper left), Suomi/NPP VIIRS day/night band (upper right), GOES-West Brightness Temperature Difference between 10.7 and 3.9 micrometer channels (Lower left), Observations (Lower right), all around 1200 UTC, 31 August






AT 1200 UTC, some benefits of the GOES-R IFR probability field are apparent.  The noisy signal over central and eastern Oregon is reduced, and a signal is present also underneath the thin cirrus streak that persists over extreme northwest Oregon.

					

		
		
	


	

				

			
						

				Excellent example of the importance of model data
			

										

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GOES-R IFR Probabilities computed using GOES-East data (Upper Left), GOES-R IFR Probabilities computed using MODIS data (Upper Right), Surface Observations and Cloud Ceilings Above Ground level (Lower Left), Suomi-NPP VIIRS Brightness Temperature Difference field, 10.8  µm – 3.74 µm (Lower Right).  Times as indicated.






Three different satellite sensors — the GOES Imager on GOES-East, MODIS on Aqua, and VIIRS on Suomi/NPP — viewed data from the occurrence of Valley Fog over the Appalachians (and surroundings) early in the morning of 21 August.  A shortcoming of the Brightness Temperature Difference field in the lower left is immediately apparent:  no fog/low stratus is indicated where high clouds exist, even though observations do show IFR conditions.  In contrast, the fused product does show heightened probabilities underneath that high cloud deck.  Probabilities are not as high as they are where both satellite and model predictors can be used to evaluate the presence of fog and low stratus, and the resolution of the field is different, obviously limited to the horizontal resolution of the Rapid Refresh, meaning that small river valleys, that are very obvious in the regions where satellite data are used (and even much more obvious when high-resolution MODIS or VIIRS data are used).  Note also how GOES-R IFR Probabilities de-emphasize the signal over western Ohio, where IFR conditions are not reported.  Brightness temperature difference fields from MODIS and from GOES both see a signal there, as also shown in the VIIRS field, but these stratus clouds are not obstructing visibility.


Bottom line:  MODIS data’s higher resolution observes the big differences between river valleys and adjacent cloud-free ridge tops.  GOES-East has difficulty in resolving those differences.  So MODIS IFR fields better highlight river fog.  Model data can help discern between fog on the ground, and stratus that is off the ground.

					

		
		
	


	

				

			
						

				How to validate GOES-R IFR in regions with no surface observations
			

										

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Animation of GOES-W IFR Probabilities over Arizona, half-hourly from 0430 UTC to 0800 UTC 31 July 2012






The animation of GOES-R IFR Probabilities over north-central Arizona shows the development of relatively high probabilities as the night progresses, an evolution that is consistent with the formation of radiation fog.  However, there are no surface observations routinely taken in the region to verify the presence of IFR conditions, or even clouds.  How much credence should be given to such a field?











GOES-R IFR Probabilities from GOES-West (upper left), Suomi-NPP Day/Night band imagery (upper right), Surface observations (lower left), GOES-West color-enhanced window channel (lower right), times as indicated.








A serendipitous Suomi-NPP overpass shows a region of clouds neatly outlined by the GOES-W IFR probabilities. Although the Day/Night band cannot give ceiling heights or visibilities (that is, it cannot alone determine if IFR conditions are occurring), it does show the presence of low clouds.


An earlier Suomi/NPP overpass over the eastern United States overflew a second developing region of enhanced IFR probabilities, over the piedmont from North Carolina southwestward into South Carolina (below).  The city lights of the Carolinas make it more difficult to detect cloud edges, although evidence of one does exist between Camden SC (KCDN) and Fairfield CO Airport (KFDW).  This is a also a region where IFR probabilities quickly drop from high values (near KCDN, where there is fog with 100-foot ceilings) to low (near KFDW, which reports clear skies).











GOES-R IFR Probabilities from GOES-East (upper left), Suomi-NPP  Day/Night band imagery (upper right), Surface observations (lower left),  GOES-West color-enhanced window channel (lower right), times as  indicated.