Monthly Archives: August 2015

Dense Fog over Kansas

Screen Captures of NWS Pages at Dodge City (left) and Wichita (Right), 0700 CST 31 August 2015 (click to enlarge)

Dense Fog Advisories were issued by both Dodge City and Wichita WFOs on the morning of 31 August 2015. GOES-R IFR Probabilities in this case did a better job of outlining where IFR conditions were occurring, distinguishing those regions from regions of low stratus. Each of the paired images below shows Brightness Temperature Difference fields on top and IFR Probability fields on bottom, for four different times during the night (0530, 0715, 0915, 1115 UTC). IFR Probability fields show a signal that is confined mostly to regions where IFR conditions are developing (for the earlier times) or observed (at 0915 and 1115 UTC). Thus, IFR Probability in this case refines the brightness temperature difference signal, highlighting regions only where low clouds/fog are present (eastern Kansas), rather than regions where stratus clouds are present (western Kansas). The incorporation of near-surface saturation as predicted in the Rapid Refresh model is key to screening out regions of mid-level stratus when low stratus and fog are the more important field.

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 0530 UTC 31 August 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 0715 UTC 31 August 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 0915 UTC 31 August 2015 (Click to enlarge)

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 1115 UTC 31 August 2015 (Click to enlarge)

When the sun rises, solar radiation with a wavelength of 3.9 µm will alter the brightness temperature difference field. At night, a water-based cloud will not emit 3.9 µm radiation as a blackbody and be perceived as colder (compared to 10.7 µm). During the day, the relatively large amount of 3.9 µm radiation from the sun reflected off the cloud will make the cloud appear to be warmer (compared to 10.7 µm). Thus the signal in the brightness temperature difference field flips. In contrast, the IFR Probability field signal is not significantly perturbed by sunrise. The 1315 UTC image, below, is an example of this. The GOES-R IFR probability field also benefits from better cloud clearing during the day.

GOES-13 Brightness Temperature Difference (10.7µm – 3.9µm) (top) and GOES-R IFR Probabilities (bottom), 1315 UTC 31 August 2015 (Click to enlarge)

IFR Probability and fog over the Southeast United States

This blog post shows how IFR Probability fields can give an early alert to the development of IFR Conditions.

GOES-R IFR Probability fields, 0400 UTC 24 August 2015 (Click to enlarge)

GOES-R IFR Probability fields from 0400 UTC on 24 August 2015, above, show enhanced probabilities over interior southeast Georgia. Observations of surface visibilities and ceilings do not show widespread IFR conditions. But the presence of enhanced values in IFR Probability suggest careful attention should be paid to this region.

GOES-R IFR Probability fields, 0700 UTC 24 August 2015 (Click to enlarge)

Three hours later (0700 UTC), IFR Probabilities have increased somewhat, and some observations are closer to IFR conditions. This is consistent with the slow development of radiation fog overnight over the southeast part of the country.

GOES-R IFR Probability fields, 0915 UTC 24 August 2015 (Click to enlarge)

By 0915 UTC, above, IFR Conditions are observed in regions where IFR Probabilities continue to increase. The increase in IFR Probability values continues at 1100 UTC, below, with numerous regions with values exceeding 90%. IFR Conditions are widespread over southeast Georgia and parts of adjacent South Carolina.

GOES-R IFR Probability fields, 1100 UTC on 24 August 2015 (Click to enlarge)

IFR Probability fields are consistent through sunrise, with values exceeding 90% after the sun has risen, at 1300 UTC, above. The cloud-clearing that can be done with visible imagery at 1300 UTC means the edges of the fog field are more sharply defined at 1300 UTC than they were at 1100 UTC.

Dense Fog over central Nebraska

GOES-R IFR Probability (Upper Left), GOES-R Low IFR Probability (Lower Left), GOES-13 Brightness Temperature Difference Field (10.7 µm – 3.9 µm) (Upper Right), GOES-R Cloud Thickness (Lower Right), Times as Indicated (Click to enlarge)

GOES-R IFR Probability fields, above, suggest IFR conditions are developing over central Nebraska during the early morning of 18 August 2015. Several aspects of the field require comment. At the beginning of the animation, there are noticeable north-south oriented lines in the IFR and Low-IFR Probability fields. These are artifacts of determining the saturation in the lowest 1000 feet of the model as discussed here. As the model saturation deepened over the course of the night, those parallel lines disappeared.

The flat nature of the field announces that satellite data are not being used in the computation of IFR Probability because multiple cloud layers are present. Model fields lack the fine spatial resolution of GOES Satellite data. Where breaks in the clouds do occur, GOES-R IFR Probability fields increase in value (because cloud predictors can be used, and their use enhances the ability of the algorithm to predict whether fog/low clouds are present): Temper your interpretation of the magnitude of the IFR Probability with knowledge of presence of clouds.

Note that the Brightness Temperature Difference field at 0500 UTC has much larger values. Stray Light is beginning to impinge on satellite fields as the calendar gets closer to the Equinox. This issue will remain through mid-October. Multiple cloud decks over Nebraska on 18 August greatly hampered the ability of the brightness temperature difference field to identify regions of low clouds.

The GOES-R Cloud Thickness fields are displayed; these are derived from an empirical relationship between 3.9 µm emmissivity and cloud thickness derived from SODAR data off the west coast of the United States. When multiple cloud layers are present, this field is not computed (nor is it displayed around sunrise/sunset). Thus, you should not see Cloud Thickness fields in regions where GOES-R IFR Probability fields are very smooth (suggesting that only model data are being used). The regions where Cloud Thickness is displayed, above, correspond to regions in the IFR Probability field that are pixelated (that is, where satellite data are being used).

Fog formation is aided by increasingly long nights. Nights now are above 90 minutes longer in Hastings than they were at the Summer Solstice.

The text for the advisory is below:

URGENT - WEATHER MESSAGE
NATIONAL WEATHER SERVICE HASTINGS NE
608 AM CDT TUE AUG 18 2015

...DENSE FOG PERSISTING FOR A FEW HOURS THIS MORNING...

NEZ048-049-061>064-073>077-181400-
/O.NEW.KGID.FG.Y.0006.150818T1108Z-150818T1400Z/
MERRICK-POLK-BUFFALO-HALL-HAMILTON-YORK-PHELPS-KEARNEY-ADAMS-CLAY-
FILLMORE-
INCLUDING THE CITIES OF...CENTRAL CITY...STROMSBURG...OSCEOLA...
SHELBY...POLK...KEARNEY...GRAND ISLAND...AURORA...YORK...
HOLDREGE...MINDEN...HASTINGS...SUTTON...HARVARD...CLAY CENTER...
EDGAR...FAIRFIELD...GENEVA...EXETER...FAIRMONT
608 AM CDT TUE AUG 18 2015

...DENSE FOG ADVISORY IN EFFECT UNTIL 9 AM CDT THIS MORNING...

THE NATIONAL WEATHER SERVICE IN HASTINGS HAS ISSUED A DENSE FOG
ADVISORY...WHICH IS IN EFFECT UNTIL 9 AM CDT THIS MORNING.

* VISIBILITY...REDUCED TO 1/4 MILE OR LESS ACROSS MUCH OF THE
  ADVISORY AREA...INCLUDING THE INTERSTATE 80 AND HIGHWAY 6
  CORRIDORS. THE COMBINATION OF RAIN MOVING INTO PARTS OF THE
  AREA...ALONG WITH A COLD FRONT MOVING IN FROM THE WEST...SHOULD
  HELP TO GRADUALLY IMPROVE DENSE FOG ISSUES.

* IMPACTS...DENSE FOG WILL GREATLY AFFECT THE MORNING COMMUTE IN
  SOME AREAS...AND MOTORISTS ARE URGED TO DRIVE WITH CAUTION AND
  PREPARE FOR RAPID CHANGES IN VISIBILITY OVER SHORT DISTANCES.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A DENSE FOG ADVISORY MEANS VISIBILITIES WILL FREQUENTLY BE
REDUCED TO LESS THAN ONE QUARTER MILE. IF DRIVING...SLOW DOWN...
USE YOUR HEADLIGHTS...AND LEAVE PLENTY OF DISTANCE AHEAD OF YOU.

&&

$$

PFANNKUCH

Coastal California Fog

Toggle between Suomi NPP Day Night Band Visible (0.70 µm) Image and Brightness Temperature Difference (11.45 µm – 3.74 µm) , 1003 UTC 12 August 2015 (Click to enlarge)

Suomi NPP data from 1003 UTC on 12 August, above, shows evidence of a cloud bank hugging the northern California coast from Cape Mendocino to San Francisco Bay. It also penetrates inland to Santa Rosa in Sonoma County. (Note also how the fires burning in interior show up well in the Day Night Band — they are emitting visible light — and in the Brightness Temperature Difference band — because they are much warmer in the 3.74 µm image than in the 11.45 µm image).

Terra overflew the California coast at ~0600 UTC and Aqua overflew the coast at ~1000 UTC; MODIS-based IFR Probabilities could be constructed from these overpasses, and they are shown below. At 0609 UTC, High IFR Probabilities (>90%) are confined to coastal Sonoma County and along the coast from Humboldt county north. By 1023 UTC, high IFR Probabilities stretch along the entire coast from Cape Mendocino to the mouth of San Francisco Bay, with evidence of inland penetration along river valleys. (The Russian River, for example, and perhaps the Noyo River in Mendocino County)

Terra MODIS-based GOES-R IFR Probability fields, 0609 UTC, 12 August 2015 (Click to enlarge)

Aqua MODIS-based GOES-R IFR Probability fields, 1023 UTC, 12 August 2015 (Click to enlarge)

MODIS can give high-resolution imagery, but the infrequency of the scenes tempers its usefulness. In contrast, GOES-15 (as GOES-West) views the California coast every 15 minutes, and this excellent temporal resolution (that will improve in the GOES-R era) allows a better monitoring of the evolution of coastal fog. Hourly plots of GOES-R IFR Probability, below, computed from GOES-15 and Rapid Refresh Data show the slow increase in GOES-R IFR Probabilities along the coast as ceilings and visibilities drop.

GOES-R IFR Probability fields, 0400-1200 UTC 12 August 2015 (Click to enlarge)

In the animation above, note the general increase in GOES-R IFR probabilities at 0900 UTC relative to 0800 and 1000 UTC. We are close enough to the Solstice that Stray Light Issues are starting. The 0800, 0900 and 1000 UTC brightness temperature difference imagery, below, shows the large signal increase at 0900 UTC that can be attributed to stray light. GOES-R IFR Probabilities can tone down that increase somewhat — because the model data will now show low-level saturation in regions where stray light erroneously suggests low clouds/fog might exist. GOES-R IFR Probabilities also screen out the constant fog signal over the Central Valley of California (and over Nevada) that is driven not by the presence of low clouds but by soil emissivity differences.

GOESWBTD_12August2015_0800_0900_1000step

GOES-15 Brightness Temperature Difference (10.7 µm – 3.9 µm), 0800, 0900 and 1000 UTC 12 August 2015 (Click to enlarge)

======================== Added =========================GOES-14 in SRSO-R mode (see also this link) viewed the west coast starting at 1115 UTC today. The Brightness Temperature Difference field, below, (click here for mp4) shows the slow expansion/evaporation of the low stratus/fog. (GOES-R IFR Probabilities were not computed with the GOES-14 1-minute imagery). The rapid change in the field at sunrise occurs because solar radiation at 3.9 µm quickly changes the brightness temperature difference from negative to positive.

GOES14_BTD_12August2015_1115_1526UTCanim

GOES-14 Brightness Temperature Difference (10.7 µm – 3.9 µm), 1115-1526 UTC 12 August 2015 (Click to view animation)

Visible Imagery is below (Click here for mp4).

960x1280_AGOES14_B1_GOES14_W_animated_2015224_133000_182_2015224_170000_182_GRAY

GOES-14 Visible Imagery (1330-1700 UTC) (Click to animate)

Dense Fog over Kansas and Nebraska

GOESR_IFRPROB_05August2015_0400_1300anim

GOES-R IFR Probability fields computed from GOES-13 and Rapid Refresh data, hourly from 0400 through 1315 UTC, 5 August 2015 (Click to enlarge)

Dense fog developed over the High Plains of Kansas and Nebraska overnight on 4-5 August 2015, and Advisories were issued by the North Platte, NE, Goodland KS and Dodge City KS WFOs. The animation above, of GOES-R IFR Probability fields computed from GOES-13 data, shows a slow expansion in the area of highest probabilities. The IFR Probability field has less spatial variability over eastern Kansas and eastern Nebraska where strong convection prevented the satellite from detecting low clouds; there, only model data predictors (from model fields that vary smoothly) could be used in the computation of the GOES-R IFR Probability fields and the IFR Probability field therefore has a smoother look.

GOES-13 Brightness Temperature Difference Fields (10.7 µm – 3.9 µm) at 0700, 0915 and 1100 UTC on 5 August 2015 (click to enlarge)

The GOES-13 Brightness Temperature Difference field (10.7 µm – 3.9 µm), above, at 0700, 0915 and 1100 UTC, similarly shows an expansion in the detection of water-based clouds over the High Plains. However, the field overpredicts the region of IFR conditions. The toggle between 1100 UTC IFR Probability and Brightness Temperature Difference, below, highlights how the IFR Probability can screen out regions (Southwestern Kansas, eastern Colorado) where low clouds are present, but IFR Conditions may not be. The toggle also shows how GOES-R IFR Probability can give information in regions where the Brightness Temperature Difference field has a signal for high clouds only (that is, under the convection in eastern Kansas)

Toggle between GOES-R IFR Probability and GOES-13 Brightness Temperature Difference (10.7 µm – 3.9 µm), 1100 UTC, 5 August 2015 (Click to enlarge)

Suomi NPP overflew Kansas around 0800 UTC on 5 August, and the Day Night Band imagery, below, showed both the strong convection (complete with streaks associated with lightning) and the developing low clouds. Brightness Temperature Difference (11.45 µm – 3.74 µm) fields from Suomi NPP (Link) confirm the presence of water-based clouds (yellow and orange in the enhancement used). The strong convection over eastern Kansas has multiple overshooting tops still at 0854 UTC (Link).

Suomi NPP Day Night Visible (0.70 µm) band, 0854 UTC 5 August 2015 (Click to enlarge)

Dense Fog in South Carolina and Georgia

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)

GOES-R Fog Product Examples

Fog detection fusing GOES, Terra/Aqua or Suomi/NPP Satellite data with Model output