Category Archives: Dissipation Time

Radiation Fog over Wisconsin

GOES-East visible imagery over Wisconsin, 1232 UTC 1 August 2012

Fog developed over southern Wisconsin overnight on 1 August under clear skies and light winds.  The Wisconsin River, the Mississippi River and the Kickapoo River are starkly outlined by the fog that formed.  How well did the GOES-R IFR products do in diagnosing this event?

GOES-R IFR Probabilities (Upper Left), Traditional Brightness Temperature Difference Fog Product (upper right), Visible Imagery (lower left), GOES-R Cloud Thickness (lower right), all from 1045 UTC on 1 August 2012

Note that the IFR Probabilities (above, upper left) are highest over south-central Wisconsin.  In addition, a ribbon of higher values snakes down the Wisconsin River, and down the Mississippi River, in accordance with observations at 1232 UTC.  In contrast, the brightness temperature difference field shows returns suggestive of fog over most of Illinois and eastern Iowa, where fog was not observed just after sunrise.  The curious lack of fog signal over the Mississippi and Illinois Rivers likely arises from the co-registration error (discussed here) that also causes the spike in brightness temperature difference signal along the southeastern shore of Lake Michigan.

The thickest clouds are diagnosed at 1045 UTC (the last such image made before twilight conditions make the product unreliable) show the thickest clouds over central Wisconsin.  The 1415 UTC visible image, below, shows the region where fog/low clouds have lingered longest:  over central Wisconsin.

Visible Imagery from GOES-13, 1415 UTC on 1 August 2012

Again, GOES-R IFR Probabilities accurately outlined the region where fog was present (and equally importantly, where it was not).  The thickest clouds were the last to erode.  The relationship between fog thickness and dissipation time is given here.

Data from the GEOCAT browser at CIMSS shows how the GOES-R IFR Probabilities field evolved with time in the early morning hours of the 1st (below)

Fog Dissipation

GOES-R IFR Probability (upper left), GOES-R Cloud Thickness of highest Liquid Cloud Layer (upper right), Traditional brightness temperature difference (lower left) and Visible Imagery (lower right) over southern Georgia and northern Florida near 1100 UTC on 24 July 2012

GOES-R Cloud thickness at the last valid time before sunrise (recall that Cloud Thickness is not available during twilight times) can be used as a predictor for where low clouds/fog (that formed via radiative processes) will linger longest after sunrise.  In the image above, the thickest cloud — about 1000 feet thick — is diagnosed to be near KMGR (Moultrie, GA).  It is in this area that the fog should be last to dissipate.  The relationship between cloud thickness and time to dissipate was derived from Spring-time observations and is shown in the plot below.  (You can download the figure here as a png or here as a pdf).

This figure, from the GOES-R Fog/Low Stratus Training developed for the National Weather Service, shows dissipation time after sunrise as a function of Cloud Thickness
GOES-R IFR Probability (upper left), GOES-R Cloud Thickness of highest Liquid Cloud Layer (upper right), Traditional brightness temperature difference (lower left) and Visible Imagery (lower right) over southern Georgia and northern Florida near 1330 UTC on 24 July 2012

The visible imagery shows that fog is lingering as expected in the region between Moultrie and KVLD (Valdosta).  By 1401 UTC, below, the fog has largely dissipated as cumulus convection starts to develop surrounding the region where fog had existed.

GOES-R IFR Probability (upper left), GOES-R Cloud Thickness of highest Liquid Cloud Layer (upper right), Traditional brightness temperature difference (lower left) and Visible Imagery (lower right) over southern Georgia and northern Florida near 1415 UTC on 24 July 2012

GOES-R Cloud Depth

GOES-East Brightness Temperature Differences (upper left), GOES-R Fog/Low Stratus IFR Probabilities (upper right), GOES-R Cloud Thickness (lower left) and Cloud-top Phase (lower right) on Friday 13 July 2012 at 10:15 UTC

 The figure above shows cloud thicknesses around 1000 feet near Omaha, Nebraska, in a region where cloud phase products suggest water clouds and supercooled clouds with small patches of cirrus, suggestive of clouds that might not be stratiform.  The cloud thickness algorithm (the algorithm predicts the depth of the highest liquid layer) works night and day, although not in times of twilight.

Note also in the image the many false positives in the traditional brightness temperature difference product over South Dakota.  This region shows very low IFR probabilities, in contrast to the region over North Dakota where IFR probabilities are higher and where fog/low stratus is more likely, given the satellite image from 1125 UTC below.  None of the widely-spaced stations in the Dakotas reported IFR conditions;  there were some reports in northwestern Minnesota, however.

Visible GOES-East image, 1125 UTC on 13 July, with a low-light enhancement applied.

 At 1315 UTC, during daytime, cloud thickness products show somewhat thinner low clouds in a region of very low IFR probability.  The sounding from Omaha, bottom, suggests convective, not stratiform, clouds are present.  The GOES-R Cloud Thickness product is produced assuming a stratiform cloud, and results are more likely to be erroneous for situations with clouds that are more cumuliform.  Cloud thickness is derived in part by dividing the liquid water path by liquid water content:  for fog and low stratus, the liquid waer content value is 0.06″ per the meteorological literature.  If the clouds are actually non-stratiform, the assumed liquid water content may not be accurate.  Remember that the cloud depth product was designed to augment the fog/low stratus probability to assist in determining how thick the fog is, and therefore how long it will take to dissipate during the day.  Use the cloud depth with caution if the clouds in question are not stratiform.

GOES-East Brightness Temperature Differences (upper left), GOES-R Fog/Low Stratus IFR Probabilities (upper right), GOES-R Cloud Thickness (lower left) and Cloud-top Phase (lower right) on Friday 13 July 2012 at 13:15 UTC
Skew-T/Log-P thermodynamic diagram from Omaha, Nebraska (KOAX) at 1200 UTC on 13 July 2012