Fog developed over western Kansas during the early morning of 14 August 2020 (helped along by above-normal precipitation in the past 30 days — shown here in an image created at this website). Low IFR Probability fields, above, show greatest probabilities of low ceilings and visibility restrictions in regions where they were observed: over western Kansas, with a sharp cut-off at the Colorado/Kansas border, and over western Oklahoma and the north Texas panhandle.
Compare the evolution of the Low IFR Probability field, above, to the evolution of the Night Fog brightness temperature difference (10.3 µm – 3.9 µm) field below.
The Night Fog field, below, has a region of strong return over western Kansas, but also two regions of weaker signals over central Kansas (where low clouds/fog are observed; note the regions in the brightness temperature difference field where the signal is very small, small grey pockets within the cyan, corresponding to the locations of towns in central Kansas) and over eastern Colorado (where low clouds/fog are not observed). Low IFR Probability fields are able to distinguish between the central Kansas and eastern Colorado because model predictions of low-level saturation are used to modulate the satellite-based signal: Low IFR Probability values are very small over Colorado (where the Rapid Refresh model is not predicting low-level saturation); values are larger over Kansas where ceiling and visibility reductions are occurring and where the Rapid Refresh model is suggesting low-level saturation is present). The brightness temperature difference field in Colorado might be driven by dry soils rather than low clouds. A brightness temperature difference signal can emerge at night because of soil emissivity differences (as noted earlier in this blog here).
Low IFR Probability fields are augmented underneath the convection that is apparent in the brightness temperature difference field over northwestern Arkansas. Satellite detection through the deep convection of low stratus in this region is impossible; the signal is driven by low-level saturation predicted by the Rapid Refresh model output.
As the sun rises, the brightness temperature difference field loses obvious cloud-detection signal because increasing amounts of reflected solar radiation (at 3.9 µm) overwhelm the emissivity-driven difference over low clouds at 3.9 µm and 10.3 µm. At some point after sunrise, the brightness temperature difference flips sign (and appears dark in the enhancement) because there is far more reflected solar radiation at 3.9 µm than at 10.3 µm.
The Low IFR Probability field by design includes temporal continuity around sunrise and sunset. This is most noticeable over central Kansas. The terminator sweep is noticeable in the field, but the values change only slowly in the hour surrounding the terminator. This temporal continuity is necessary because of the quick changes in detected 3.9 µm radiation that are occurring as solar reflectance changes occur.
Most of the posts on this blog discuss IFR Probability: The probability that IFR conditions are occurring. IFR, or Instrument Flight Rules conditions are defined as ceilings between 1000 and 3000 feet and/or visibilities between 1 and 3 miles. Two other Probability fields are created: MVFR Probabilities (MVFR, or Marginal Visual Flight Rules, are defined as ceilings between 3000 and 5000 feet and/or visibilities between 3 and 5 statute miles) and Low IFR Probabilities (LIFR, ceilings below 1000 feet and/or visibilities less than 1 mile). The animation above steps through the three fields from one time: MVFR Probability, IFR Probability and Low IFR Probability. As might be expected, MVFR Probability > IFR Probability > LIFR Probability.
Cursor readouts in AWIPS imagery are shown below; LIFR Probability fields are shown with the other two Probability fields are loaded underneath. The cursor readout (for the point just north and west of the upper left corner of the readout values) shows the relationship between the three fields. Low IFR Probability is shown in coral, IFR Probability in green, MVFR Probability in white. MVFR Probability values > IFR Probabilty values > LIFR Probability values.
GOES-16 Brightness Temperature Difference field (10.3 µm – 3.9 µm) from 0417 to 1357 UTC on 28 August 2017 (Click to animate)
GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing.
GOES-16 Brightness Temperature Difference fields (10.3 µm – 3.9 µm), above, show the development of stratus clouds (made up of water droplets) over the Plains during the morning of 28 August 2017. The Brightness Temperature for 10.3 µm is warmer than that for 3.9 µm during the night because cloud water droplets do not emit 3.9 µm radiation as a blackbody but those same cloud water droplets do emit 10.3 µm radiation more nearly as a blackbody would. The conversion from sensed radiation to brightness temperature does assume blackbody emissions; thus, the 3.9 µm brightness temperature is cooler where clouds made up of small water droplets exist. The animation above shows stratus clouds developing over Missouri and adjacent states. Dense Fog Advisories were issued near sunrise for much of the region (see image at bottom of this blog post) and IFR Conditions were widespread.
The animation above shows a positive signal over the western High Plains from Kansas northward to North Dakota. (Click here for the view at 1132 UTC on 28 August).
GOES-R IFR Probabilities are computed using Legacy GOES (GOES-13 and GOES-15) and Rapid Refresh model information; GOES-16 data will be incorporated into the IFR Probability algorithm in late 2017
How did GOES-R IFR Probability fields capture this event? The animation showing the fields every 30 minutes from 0215 through 1345 UTC on 28 August 2017, below, shows the development of High Probabilities in the region where Dense Fog was observed. There is a signal along the western High Plains, but it has low Probability; a conclusion might be that thin stratus has developed but that the Rapid Refresh model does not suggest that widespread low-level saturation is occurring. As the sun rises, the signal over the western High Plains disappears. Click here for a toggle between the GOES-R IFR Probability and the GOES-16 Brightness Temperature Difference field at 1115 UTC.
GOES-R IFR Probability fields, 0215-1345 UTC, 28 August 2017 (Click to enlarge)
Screen Capture of http://www.weather.gov at 1300 UTC on 28 August 2017 (Click to enlarge)
GOES-R IFR Probability that uses present GOES (GOES-13 and GOES-15) data in the computation of GOES-R IFR Probability fields was designed in anticipation of GOES-16 data that are now flowing to National Weather Service Forecast offices. Click here for a description of the Brightness Temperature Difference field values that are available now in AWIPS from GOES-16.
GOES-R FLS products are currently derived from GOES-13 and GOES-15 data. A GOES-16 version of the GOES-R FLS products will not be available until later in 2017.
GOES-13 Brightness Temperature Difference (3.9 µm – 10.7 µm), GOES-R IFR Probability and GOES-R Cloud Thickness at 1115 UTC on 17 January 2017 (Click the enlarge)
Dense Fog Advisories (National Weather Service Website) and IFR SIGMETs (Aviation Weather website) were issued early in the morning for dense fog over the southeastern United States. The toggle above from 1115 UTC on 17 January shows the Brightness Temperature Difference field (3.9 µm – 10.7 µm), the GOES-R IFR Probability Field, and the GOES-R Cloud Thickness fields associated with this dense fog event. Note the presence of high clouds over northern South Carolina and western North Carolina — the dark region in the Brightness Temperature Difference enhancement — prevents the brightness temperature difference field from highlighting that region of reduced ceilings/visibilities. The GOES-R Cloud Thickness field is not computed under cirrus either, as it relates 3.9 µm emissivity of water-based clouds to cloud thickness (based on a look-up table generated using data from a SODAR off the West Coast of the United States). If cirrus blocks the view, then, neither the Brightness Temperature Difference field nor the GOES-R Cloud Thickness field can give useful information about low clouds.
In contrast, the GOES-R IFR Probability field does give useful information in regions where cirrus clouds (and low clouds/fog) are present — because Rapid Refresh information about the lower troposphere can be used. IFR Probability values will be smaller in those regions because satellite predictors are unavailable, and the Probability incorporates both predictors from satellites and from Rapid Refresh model output — if the satellite predictors are missing because of cirrus, the IFR Probability values will be affected. Despite the smaller values, however, the IFR Probability fields in regions of cirrus are giving useful information for this event.
GOES-R Cloud thickness fields can be used to estimate Fog dissipation using the last GOES-R Cloud Thickness field produced before twilight conditions at sunrise (shown below for this case). (GOES-R Cloud Thickness is not computed during twilight conditions because of rapidly changing 3.9 µm emissivity related to the reflected solar radiation as the sun rises, or as it sets). This scatterplot gives the relationship between thickness and dissipation time after the Cloud Thickness time stamp (1215 UTC in this case). In this case, the thickest fog is near Athens GA; the algorithm predicts that clearing should happen there last, at about 1515 UTC.
GOES-R Cloud Thickness, 1215 UTC on 17 January 2017 (Click to enlarge)
Screenshot from Charleston WFO, 1230 UTC 4 August 2015
GOES-R IFR Probability fields computed from GOES-13 and Rapid Refresh Data, hourly from 0400 through 1215 UTC 4 August 2015 (Click to enlarge)
Dense fog developed over the piedmont of South Carolina/Georgia on 4 August 2015 in the wake of departing convection. The GOES-R IFR Probability fields, shown above hourly from 0400 to 1215, do parallel the development of the reduced ceilings and visibilities. Brightness Temperature Difference fields, below, from 0615 to 1215 UTC, do not show a strong fog signal until after 0800 UTC, yet IFR conditions at that time stretch from Walterboro SC (KRBW) southeastward to Eastman GA (KEZM) and Baxley GA (KBHC). GOES-R IFR Probabilities therefore give a better head’s up to a forecaster tasked with monitoring ceilings and visibilities.
Suomi NPP overflew the Southeast United States at ~0730 UTC on 4 August. Ample illumination from the waning three-quarter moon showed cloudiness over southeastern coastal South Carolina and adjacent parts of Georgia but the brightness temperature difference field does not suggest that these are all water-based clouds (such clouds generally fall in the yellow or orange part of the enhancement).
Suomi NPP Day Night Band visible (0.70 µm) image, 0732 UTC 04 August 2015 (click to enlarge)
Suomi NPP Brightness temperature Difference field (11.45 µm – 3.74 µm), 0732 UTC on 4 August 2015 (click to enlarge)
MODIS data from Terra and Aqua satellites can also be used to compute GOES-R IFR Probability fields, and two MODIS swaths were produced over South Carolina/Georgia early on August 4. Toggles between the 0337 Terra-based GOES-R IFR Probability Field and the 0755 UTC Aqua-based GOES-R IFR Probability fields are below. The larger values from MODIS — especially at 0755 UTC — suggest the fog was initially at small-scale horizontally. The 1-km resolution pixels from MODIS better capture any small-scale features.
MODIS-based (Terra) and GOES-based (GOES-13) GOES-R IFR Probability fields at ~0340 UTC on 04 August 2015 (click to enlarge)
MODIS-based (Aqua) and GOES-based (GOES-13) GOES-R IFR Probability fields at ~0800 UTC on 04 August 2015 (click to enlarge)
Multiple cloud decks — shown in the toggle, below, of Suomi NPP Day Night Band and Brightness Temperature Difference (11.45 µm – 3.74 µm) — prevented the traditional brightness temperature difference product from providing useful information. GOES-R IFR Probabilities, shown ~hourly in an animation above do highlight the region of developing IFR conditions. Low ceilings and reduced visibilities are commonplace in regions where IFR Probabilities are increasing over night. The predictors that are included to compute the IFR Probabilities are mostly model-based because of the multiple cloud layers that are present, and the IFR Probability field is somewhat flat as a result. Note that GOES-R IFR probabilities increase at the very end of the animation; when daytime predictors are used, probabilities are a bit higher than when nighttime predictors are used.
Suomi NPP Day Night Band visible imagery and Brightness Temperature Difference (11.45 µm – 3.74 µm) at 0818 UTC, 19 May 2015 (Click to enlarge)
GOES-R IFR Probabilities, hourly from 0200 through 1300 UTC on 7 April 2015 (Click to enlarge)
Denver International Airport had a period of restricted visibility during the morning of 7 April, starting around 0830 UTC, when northeast winds ushered in low ceilings and reduced visibilities. High Probabilities in the IFR Probability fields shift west and south with time, demonstrating how the fields can be used to anticipate the development of IFR conditions.
Webcam image of South Padre Island, TX, 11 AM 1 April 2015 (click to enlarge)
Webcam imagery from South Padre Island between 10 and 11 AM on 1 April (above, from this site) showed dense fog that had rolled in from the sea. This is likely an advection fog formed as humid air over the Gulf of Mexico moved over relatively cooler shelf water. SSTs in the region were in the upper 60s (Fahrenheit) as depicted by the image below.
Sea-surface Temperatures, early morning 1 April 2015 (Click to enlarge)
GOES-R IFR Probabilities, below, suggested the presence of the IFR conditions that existed at the coast. Very high probabilities are concentrated near South Padre Island, and spread north and northeastward, with highest values hugging the shoreline.
GOES-R IFR Probabilities computed from GOES-East and Rapid Refresh Data, 1545 UTC on 1 April 2015 (Click to enlarge)
Brightness Temperature Difference fields at the same time gave little surface information because of the presence of high clouds.
GOES-East Brightness Temperature Difference Field, 1545 UTC on 1 April 2015 (Click to enlarge)
A similar event occurred in March 2015 along the Florida Atlantic Coast. (Link).