Monthly Archives: December 2014

Sharp edge to Fog/Low Stratus over east Texas

US_11-3.9_Sat_20141229_2_14anim

Brightness Temperature Difference Fields (10.7µm – 3.9µm) and surface observations of ceilings and visibilities, Hourly from 0200 through ~1400 UTC [Click to enlarge].

Traditional method of fog/low stratus detection revealed a sharp edge to clouds over east Texas during the morning of 29 December 2014. The animation above reveals several difficulties inherent in using brightness temperature difference fields in diagnosing fog/low stratus. Where multiple cloud layers are present — such as along the coast at 0500-0600 UTC — the brightness temperature difference product cannot view the low clouds. At sunrise, increasing amounts of solar 3.9µm radiation causes the brightness temperature difference product to flip sign. The signal for low clouds is still there, however.

GOES_IFR_PROB_20141229_2_14anim

GOES-R IFR Probabilities and surface observations of ceilings and visibilities, Hourly from 0200 through ~1400 UTC [Click to enlarge]

The animation of GOES-R IFR Probabilities, above, created from GOES-13 data and Rapid Refresh Model data, shows high IFR Probabilities over east Texas where low ceilings and reduced visibilities prevailed, including metropolitan Houston. The algorithm suggests the likelihood of fog/low stratus underneath the cirrus debris that is over the coast around 0500-0600 UTC as well, because the Rapid Refresh model output in that region strongly suggests low-level saturation. In addition, the fields show only minor changes through sunrise (the effect of the terminator is present in the final image in the loop).

MODIS data from either Terra or Aqua can be used to produce IFR Probabilities. The data below is from 0442 UTC. Polar orbiter data is infrequent, however, so temporal monitoring of the fog/low clouds is more easily achieved using GOES data.  MODIS data, like the GOES data above, shows the effects of cirrus clouds on Brightness Temperature Difference fields and on IFR Probabilities.  Cloud predictors of low clouds/fog from satellite cannot be used in regions of cirrus, so IFR Probabilities are smaller in regions where multiple cloud layers exist, which regions are where only Rapid Refresh Data can be used as predictors.

MODIS_IFR_PROB_BTD_20141229_0442

0442 UTC MODIS-based brightness temperature difference and IFR Probability fields (Click to enlarge)

 

Brightness Temperature Difference fields can also be created from Suomi/NPP data and the orbital geometry on 29 December meant that eastern Texas was viewed on two sequential overpasses.  IFR Probabilities are not quite yet computed using Suomi NPP data, but the brightness temperature difference fields can be used to show where water-based clouds exist. They show a very sharp western edge to the clouds.

VIIRS_FOG_20141229_0723_0904

Brightness Temperature Difference Fields (11.35µm – 3.74µm) from Suomi NPP at 0723 and 0904 UTC on 29 December 2014 (Click to enlarge)

The Day Night Band on Suomi NPP produces visible imagery at night. When lunar illumination is strong, it can provide compelling imagery. On 29 December 2014, however, the moon set around 0600 UTC, so no lunar illumination was available, and fog/low clouds are very difficult to discern in the toggle below between the Day Night Band and the brightness temperature difference field at 0723 UTC.

VIIRS_FOG_DNB_20141229_0723

Day Night Band and Brightness Temperature Difference Fields (11.35µm – 3.74µm) from Suomi NPP at 0723 on 29 December 2014 (Click to enlarge)

 

Widespread fog and low clouds along the East Coast

US_Water_Vapor_20141223_2hrmovie

GOES-East Water Vapor imagery, every 2 hours, 0130 – 1330 UTC 23 December 2014 (Click to enlarge)

 A slow-moving storm system is producing widespread fog and low clouds on the east coast (and in the middle of the country as well). The water vapor animation above shows the cloud cover associated with the system. Water vapor imagery such as this suggests many different cloud layers, and in such cases the IFR Probability fields (below) rely on Rapid Refresh Data to provide information because Satellite signals of low clouds cannot occur in the presence of cirrus contamination. A simple Brightness Temperature Difference product would give little information about near-surface clouds over the Southeast.

GOES_IFR_PROB_20141223anim

GOES-East-based GOES-R IFR Probabilities and surface-based observations of ceilings and visibilities, hourly from 0145 through 1245 UTC 23 December 2013 (Click to enlarge)

 IFR Probability fields show a flat nature that occurs when satellite data cannot be used as a Predictor because of the presence of high clouds/multiple cloud layers. The Probability values are suppressed; interpretation of those values should be colored by the knowledge of the presence or absence of high clouds. In the example above, when high clouds briefly separate over central South Carolina around 0600 UTC, a region of higher IFR probability is shown. The algorithm is more confident that fog/low stratus exists because Satellite Predictors can also be used in that region. What changes is the ability of the GOES-R IFR Probability algorithm to assess the probability of IFR conditions because more predictors can be included; in the region where the high clouds part, satellite information about the low clouds can be included, and IFR Probabilities increase as a result.

IFR Probabilities in High Terrain

BlueCanyonIFR_18dec2014anim

GOES-15-based GOES-R IFR Probabilities, hourly from 0000 UTC through 1500 UTC on 18 December 2014 (Click to enlarge)

IFR Probabilities give information about IFR Conditions that occur when terrain ascends up into the clouds, as happens sometimes in the Sierras. A stratus deck might exist over the Central Valley of California, but as that stratus extends to the east, the terrain rises up into the cloud, and IFR conditions result. The animation above shows IFR Probabilities during the night of 17-18 December 2014. Stations at elevation are not common, but Blue Canyon (KBLU) in Placer County is at 1600 meters above sea level. IFR Probabilities in/around north central Placer County (where KBLU is sited) are highest with the visibility is most restricted. Note in the animations that Truckee, CA, to the east of Blue Canyon, does not experience IFR Conditions, perhaps because it is east of the crest of the Sierras.

MODIS data occasionally gives very high-resolution information. The data below from 2100 UTC on 17 December and shows the IFR Probability banked up against the Sierras.

MODIS_IFR_17December_2113

MODIS-based GOES-R IFR Probability, 2113 UTC on 17 December 2014, along with surface-based observations of ceilings and visibilities (Click to Enlarge)

Extended Period of Fog/Low Stratus over the Arizona

GOES13_AzFOG_6_13December2014visloop

GOES-13 Visible Imagery at 1600 and 2300 UTC from 6 December through 13 December 2014 (Click to enlarge)

Northern Arizona experienced a wet start to December (From 2-4 December, Flagstaff received 1.79″, Winslow received 0.80″ and Grand Canyon Airport received 0.41″). When High Pressure and an inversion then settled over the region (an animation of surface weather charts is here), the stage was set for a prolonged period of fog and low stratus, as noted here, for example. The visible imagery, above, testifies to the persistence of the low clouds and fog. It is apparent on 6 December during the day, and persists through the 12th. Visible imagery also shows the presence of high clouds; the presence of those high clouds makes ongoing detection of lower clouds difficult. In addition, both the visible imagery and brightness temperature difference product (10.7 µm – 3.9 µm on GOES-13, historically and still routinely used to detect water-based clouds) give information about the top of the cloud. There is great difficulty in using this information to infer a surface visibility or ceiling (that is, information about the bottom of the cloud).

Fused products have an advantage of incorporating surface-based data (assimilated into the model — the Rapid Refresh in this case) to provide information on whether saturation is occurring in the lowest kilometer of the atmosphere. If that is the case, IFR Probabilities will be larger. The animation below shows Brightness Temperature Difference products and IFR Probabilities at 3 different times (~0700, ~1700 and ~2300) during the days on 8-11 December. IFR Probability fields continually show a strong signal in the region of fog/low stratus over eastern Arizona; the brightness temperature difference field does not, as it is affected by cirrus clouds and by solar reflectivity during the day. Clear skies on 10-11 December at night did allow the brightness temperature difference product to highlight the low clouds over Arizona.

GOES13_BTD_IFR_8_11December

GOES-13 Brightness Temperature Difference (Left, 10.7 µm – 3.9 µm) and IFR Probability (Right) from 8 December through 11 December at ~0700, ~1700 and ~2300 UTC. The Brightness Temperature Difference is enhanced so that fog/low stratus are yellow/orange/red at night, black during the day. (Click to enlarge)

Fog over the Southern Plains

Fog developed over Texas, Oklahoma and Arkansas early in the morning of 9 December 2014. Multiple cloud layers made traditional satellite detection (that is, using brightness temperature difference field (10.7µm – 3.9µm)) problematic. How did the fused product, GOES-R IFR Probability perform? The animation below shows the hourly evolution of IFR Probability from 0215 UTC through 1415 UTC.

IFR_09Dec2014_02_14anim

GOES-R IFR Probabilities, hourly from 0215 through 1415 UTC on 9 December 2014, along with surface plots of ceilings and visibilities (Click to enlarge)

There are widespread reports of IFR conditions over southeast Oklahoma and northern Texas, as well as over Arkansas in the Arkansas River Valley. IFR Probability fields generally overlap the region of reduced ceilings and visibilities.

Note that the probabilities increased over west Texas between 1315 and 1415 UTC. The boundary between day and night predictors is also apparent at 1415 UTC as a SW to NE line over the Texas Panhandle. Probabilities change as night switches to day because different combinations of satellite predictors can be used. In particular, the use of visible imagery improves cloud clearing and therefore IFR Probabilities increase in regions where low clouds exist (because the possibility of clouds being present is more easily detected).

The toggles below show data from 0615, 1115 and 1415 UTC and demonstrate why a fused product can give better information than a satellite-only product. Intermittent high clouds over the southern Plains prevented GOES-13 from identifying regions of low clouds (Cirrus clouds in the enhancement below appear as dark regions). IFR Probabilities can give valid information in these regions because the Rapid Refresh Model gives information about the possibility of low-level saturation. There are large regions at 1415 UTC over west Texas that are covered by cirrus clouds; despite the inability of the satellite to detect low clouds, IFR Probability maintains a strong signal there where IFR Conditions are occurring. The 1415 Brightness Temperature Difference field, in contrast to the IFR Probability field, gives very little information because increasing amounts of solar radiation are changing the relationship between 10.7µm and 3.9µm radiation at 1415 UTC.

IFRPROB_BTD_0615_9Dec2014

GOES-R IFR Probabilities and GOES-13 Brightness Temperature Difference Fields (10.7µm – 3.9µm), 0615 UTC on 9 December 2014, along with surface observations of ceilings and visibilities (Click to enlarge)

IFRPROB_BTD_1115_9Dec2014

As above, but at 1115 UTC (Click to enlarge)

IFRPROB_BTD_1415_9Dec2014

As above, but at 1415 UTC (Click to enlarge)

A near-Full Moon on 9 December means that the Day Night Visible imagery from Suomi NPP produced great imagery of the clouds over the southern Plains. The toggle below shows the Day Night band, the brightness temperaure difference field (11.35µm – 3.74µm) and the topography. Very narrow fog banks are apparent over southeast Oklahoma and over Arkansas, nestled into narrow valleys. The Brightness Temperature Difference field distinguishes between water-based clouds (presumably low stratus or fog) in orange and ice clouds (cirrus) in black.

SNPP_BTD_DNB_0839UTC_09Dec2014

Suomi NPP Brightness Temperature Difference (11.35µm – 3.74µm) fields, Day Night band imagery and Color-shaded topography, 0839 UTC 9 December 2014 (Click to enlarge)

The fog event over Dallas was photographed from the air: Link.

More fog over South Carolina

Dense fog redeveloped over South Carolina overnight on 3-4 December 2014, and as noted in the Forecast Discussion below, its character was just a bit different than on the previous night.

000
FXUS62 KCHS 040239
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
939 PM EST WED DEC 3 2014

.SYNOPSIS…
UNDER A WEAKENING WEDGE OF HIGH PRESSURE…FOG WILL PERSIST TONIGHT.
ANOTHER AREA OF HIGH PRESSURE WILL BUILD FROM THE NORTH THURSDAY THROUGH
FRIDAY. A WARM FRONT WILL THEN LIFT ACROSS THE AREA ON SATURDAY…
BEFORE A COLD FRONT MOVES THROUGH SATURDAY NIGHT. AN INLAND WEDGE
OF HIGH PRESSURE WILL BECOME ESTABLISHED SUNDAY AND MONDAY…FOLLOWED
BY THE PASSAGE OF ANOTHER COLD FRONT MONDAY NIGHT. HIGH PRESSURE
WILL THEN PREVAIL INTO THE MIDDLE OF NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/…
WHILE THE SCENARIO IS QUITE DIFFERENT FOR FOG TONIGHT COMPARED TO
LAST NIGHT…DESPITE PLENTY OF CIRRIFORM CLOUDS /SOME OF WHICH ARE
OPAQUE/
…WE ARE STILL GETTING AREAS OF FOG TO FORM. SOME OF THE
FOG IS ALREADY DENSE…ESPECIALLY IN THE CHARLESTON QUAD COUNTY
AREA AND ALONG OUR COASTAL ZONES SOUTH INTO MCINTOSH. THIS IS A
MIX OF STRATUS BUILD-DOWN AND ADVECTIVE FOG FROM OFF THE ATLANTIC
.
SO WE LOOK FOR A FURTHER EXPANSION OF THE FOG INLAND TO THE WEST
OF I-95 THROUGH THE NIGHT. DENSE FOG ADVISORIES WILL THEREFORE
REMAIN IN EFFECT.

IFRProb_04Dec2014anim

GOES-based GOES-R IFR Probabilities, hourly from 2315 UTC 3 December through 1015 UTC on 4 December as well as observations of ceilings (AGL) and visibilities (Click to enlarge)

GOES-R IFR Probabilities, above, (click here for an animation with a faster dwell rate) once again capably outlined the region of IFR conditions over South Carolina. Probabilities are lower when Satellite data cannot be used as a predictor, as when cirrus clouds prevent the satellite from viewing water-based clouds closer to the surface. In such cases when only Rapid Refresh model data can be used as Fog Predictors, the fields take on a flatter, less pixelated character as above. There are a few regions where breaks in the cirrus cloud allow Satellite predictors to be incorporated in the IFR Probability fields, for example along the South Carolina coast at 0600 UTC. When high clouds are present, interpret the magnitude of the IFR Probability in a different way than when high clouds are absent. An IFR Probability of 55% in a region of cirrus clouds has a different meaning than an IFR Probability of 55% in a region of only low clouds.

Because of Cirrus clouds, the brightness temperature difference fields gave almost no information about the presence of low clouds. See the animation below (a loop with a faster dwell rate is here).

BTD_GOES13_4DEC2014anim

GOES-13 Brightness Temperature Difference Fields (10.7µm – 3.9µm), hourly from 2315 UTC 3 December through 1015 UTC on 4 December, as well as observations of ceilings (AGL) and visibilities (Click to enlarge)

Dense Fog over South Carolina

Dense fog devloped over South Carolina overnight, in an event that was well-anticipated: From the Charleston, SC, National Weather Service Office AFD (emphasis added):

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
648 PM EST TUE DEC 2 2014

.SYNOPSIS…
WEAK HIGH PRESSURE WILL PERSIST THROUGH WEDNESDAY. A COLD FRONT
WILL ADVANCE SOUTH THROUGH THE AREA THURSDAY…THEN HIGH PRESSURE
WILL BUILD FROM THE NORTH AND WILL PREVAIL THROUGH FRIDAY. LOW
PRESSURE WILL TRACK THROUGH OR JUST NORTH OF THE REGION SATURDAY…
FOLLOWED BY COLD FRONTS SATURDAY NIGHT AND AGAIN LATE MONDAY.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/…
A FEW LINGERING STRAY SHOWERS WEST OF I-95 WILL DISSIPATE THIS
EVENING WITH THE ONSET OF NOCTURNAL COOLING AND STABILIZATION.

OUR MAIN FOCUS FOR TONIGHT IS IN REGARDS TO FOG AND THE BUILD DOWN
OF STRATUS
…SOME OF WHICH COULD BE DENSE. THE HIGH PRESSURE WEDGE
WILL REMAIN ANCHORED OVER THE INLAND PARTS OF THE SE AS IT
PROGRESSES A LITTLE SOUTHWARD THROUGH THE NIGHT…WITH A WEAK
COASTAL TROUGH OFFSHORE. A STRENGTHENING NOCTURNAL INVERSION WILL
DEVELOP AND TRAP ABUNDANT MOISTURE WITHIN THE PLANETARY BOUNDARY
LAYER UNDERNEATH. THIS WILL LEAD TO THE DEVELOPMENT AND SPREADING
OUT OF FOG AND STRATUS ENCOMPASSING MOST IF NOT ALL OF THE CWFA
DURING THE LATE NIGHT HOURS. BUT ALREADY WE/RE SEEING A FOG BANK
WITH IT/S ORIGINS NORTH OF THE SANTEE RIVER THIS AFTERNOON IS
PROGRESSING SOUTH/SW AND STARTING TO SPREAD A LITTLE INLAND. THIS
IS CLEARLY DEPICTED BY THE 11-3.9 MICRON SATELLITE IMAGERY AS WELL
AS COASTAL METAR SITES. THE COMBINATION OF THE MARINE INDUCED FOG
AND STRATUS WILL COMBINE WITH RADIATION FOG THAT FORMS OVER INLAND
SECTIONS TO RESULT IN AREAS OF FOG ACROSS THE FORECAST ZONES.
THE
WORST CONDITIONS WILL BE OVER CHARLESTON COUNTY THIS
EVENING…SPREADING INTO OTHER COASTAL SC ZONES LATER THIS EVENING
AND INTO THE GA COASTAL ZONES BY MIDNIGHT OR SO. INLAND THE BULK
OF THE GREATER FOG/STRATUS COVERAGE WILL WAIT UNTIL WE REACH OUR
CROSS-OVER TEMPS BY 2-3 AM. DENSE FOG ADVISORIES ARE CERTAINLY
POSSIBLE. ANY NEGATING FACTORS AGAINST THE WIDESPREAD DENSE FOG
WOULD BE THE RELATIVELY WARM GROUNDS. BUT GIVEN FOG STABILITY
INDICES IN THE TEENS AND LOWER 20S…MOST PLACES REACHING THEIR
CROSS-OVER TEMPS OF 55-60 AND SOME MARINE FOG WE/D LEAN MORE
TOWARD DENSE FOG BECOMING A CONCERN…THAN NOT.

GOESR_IFRProb_03Dec2014loop

GOES-based GOES-R IFR Probabilities, hourly from 2215 UTC 2 December through 1315 UTC on 3 December as well as observations of ceilings (AGL) and visibilities (Click to enlarge)

The GOES-R IFR Probability fields, shown at hourly intervals above, nicely capture the spread of the extensive fog as the expanding marine fog bank joins up with the developing radiation fog over northwestern South Carolina. (A loop with a shorter dwell rate is here). GOES-13 Brightness Temperature Difference Fields are shown below. (A loop with a shorter dwell rate is here). Brightness Temperature Difference is the traditional method of detecting fog and low stratus, and it is referenced in the AFD above. However, the method cannot operate in regions with high cirrus (such as over Tennessee and northwest South Carolina after 0900 UTC); the brightness temperature difference signal flips sign as the sun rises (as at 1315 UTC in the animation below); the method cannot distinguish between elevated stratus and visibility-restricting fog.

GOES13BTD_03Dec2014loop

GOES-13 Brightness Temperature Difference Fields (10.7µm – 3.9µm), hourly from 2215 UTC 2 December through 1315 UTC on 3 December, as well as observations of ceilings (AGL) and visibilities (Click to enlarge)

In comparing the two animations, note how the Brightness Temperature Difference field, for example, has strong returns around 0215-0315 UTC over central South Carolina in regions where widespread IFR conditions do not yet exist. In these regions, the Rapid Refresh model data used as a predictor in the GOES-R IFR Probability is not yet showing saturation, so IFR Probabilities aren’t quite so high as they are along the coast. (Click here for a toggle between the GOES-R IFR Probability and the Brightness Temperature Difference fields at 0215 UTC; a toggle at 0315 UTC is here). Note also in the toggles how the IFR Probability fields have little signal over Georgia despite the Brightness Temperature Difference field signal. This demonstrates the power of using fused data products: both the satellite and the model signal must be in accord for very high probabilities to occur.