Monthly Archives: July 2012

Isolated Fog/Low Stratus in Texas

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GOES-R IFR Probabilities computed using MODIS data (upper left), MODIS Brightness Temperature Difference (the ‘traditional’ Fog Product) (upper right), Lowest 1km Relative Humidity from the NAM (lower left), MODIS IR Window Channel (lower right)

The MODIS-based GOES-R IFR probability image, above, showed a region of high probabilities of IFR over Bandera and Real Counties in Texas west of San Antonio.  This signal is driven by the brightness temperature difference (shown, upper right) and the relative humidity in the Rapid Refresh.  The NAM relative humidity is shown in the lower left image, and the signal in the GOES-R IFR field suggests the RAP relative humidity is similar.  Note how the brightness temperature difference signal farther west in the image does not lead to a signal in the IFR Probabilities;  model relative humidities there are lower.

Does the high probability of IFR signal verify?  In other words, when you see an isolated signal like this, how much credence can you give it?  Hondo, TX (HDO), just southeast of the higher IFR probabilities, does not show IFR conditions.  How do things evolve with time?  GOES-based imagery, below, show the expansion of the IFR probabilities from 0800 UTC, the approximate time of the MODIS pass, above, to 1030 UTC.  The expansion is typical of what would occur with radiational fog formation overnight.  By 1100 UTC, ceilings at Hondo (HDO) and Rocksprings (ECU) are near IFR conditions.  It appears that the IFR probability signal is correctly diagnosing the slow development of a  fog/stratus deck.

GOES-R IFR Probabilities (Upper left), Ceilings and Visibility plotted over GOES-R Cloud Thickness (upper right), GOES-East enhanced Window Channel brightness temperature (bottom left), GOES-East Brightness Temperature Difference (bottom right) from 0800 UTC

GOES-R IFR Probabilities (Upper left), Ceilings and Visibility plotted over GOES-R Cloud Thickness (upper right), GOES-East enhanced Window Channel brightness temperature (bottom left), GOES-East Brightness Temperature Difference (bottom right) from 1030-1100 UTC

There was a fortuitous pass of the Suomi/NPP satellite over this region as the fog/low stratus developed.  Does that satellite give any more information about the presence of fog?  The loop below toggles between the GOES-R IFR probability from GOES-East data and the Day/Night Band from VIIRS.  Because the Moon is nearly new, very little moonlight is illuminating the cloud field so it is difficult to determine if fog is actually present at 0832 UTC over the region.  The parts of the counties over which the fog is developing are sparsely populated, so there are no city lights from which to glean information.

Signals of IFR Day and Night from GOES-R IFR Probability

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Brightness Temperature Difference (11 micrometers – 3.9 micrometers) at 0745 UTC on 16 July

GOES-R IFR probability product at 0745 UTC on 16 July 2012

The two images above show the ‘traditional’ GOES fog product — the brightness temperature difference between 11 and 3.9 micrometers — at 0745 UTC on 16 July 2012 (top) and the GOES-R IFR probabilities at the same time.  In addition, ceilings and visibilities at stations in North Dakota are plotted.  Both products accurately capture the fog/low stratus along the North Dakota/Canada border.  The IFR probabilities do a better job at suggesting the development of IFR at Rugby, ND (with 2-1/2 mile visibility).  The IFR probabilities are also correct over southwestern North Dakota: IFR conditions do not exist there despite the brightness temperature difference.

Brightness Temperature Difference (11 micrometers – 3.9 micrometers) at 1101 UTC on 16 July

GOES-R IFR probability product at 1102 UTC on 16 July 2012

At 1100 UTC (above), both products accurately portray the existence of fog/low stratus over north central North Dakota, but the traditional brightness temperature difference product continues to suggest fog/low stratus over southwestern North Dakota, where IFR conditions do not exist, and where the IFR probability has little signal.

Brightness Temperature Difference (11 micrometers – 3.9 micrometers) at 1215 UTC on 16 July

GOES-R IFR probability product at 1215 UTC on 16 July 2012

When the sun rises, reflected solar 3.9-micrometer radiation causes the brightness temperature difference between 11 and 3.9 micrometers to flip sign, and the color enhancement used at night loses value in day.  In contrast, the GOES-R IFR probability maintains a robust signal from nighttime through twilight to daytime (the terminator is visible in the IFR probability image at 1215 UTC 16 July, above, running north-northwest from south-central North Dakota to western north-central North Dakota).  Higher IFR probabilities continue to overlap the region where IFR conditions are reported.  By 1445 UTC (below), the region of IFR conditions is breaking apart;  stations that persist in reporting IFR conditions (or near IFR) are within the highest IFR probability in the field — Minot and Harvey in ND, Estevan in Saskatchewan and Portage in Manitoba.

GOES-R IFR probability product at 1445 UTC on 16 July 2012

GOES-R Cloud Depth

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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

Fog and Low Stratus where it rains

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Fog and low stratus is not rare in regions of precipitation, but the brightness temperature difference algorithm used historically to infer fog will not highlight such areas as those where IFR conditions are likely, usually because the emissivity properties of the precipitating clouds differ from those of fog/stratus decks.  The GOES-R Fog/Low Stratus product nevertheless will produce a signal in these regions because it uses input from numerical models in regions where a satellite signal cannot provide information.
GOES-East IFR Probabilities from the GOES-R Fog/Low Stratus algorithm (upper left), GOES-East cloud phase (upper right), GOES-East brightness temperature difference (11 microns – 3.9 microns) (lower left), GOES-East visible imagery (enhanced for low light conditions) (lower right), all for 1100 UTC on 12 July 2012.

Several things require explanation in these images.  In the IFR probability mapping, the SSE to NNW boundary extending from near Jacksonville to Chattanooga is the terminator, the boundary between using nigthtime and daytime values in the look-up tables that are used to relate model and satellite fields to IFR probabilities.  Note that the daytime values generally yield higher probabilities than the nighttime values, especially for regions where IFR probabilities are determined mostly from model output.  Regions where that occurs — where model output drives the IFR probability output — are typically underneath widespread ice clouds, and the probability fields have a more uniform look to them.  In the image above, more satellite data are being used over South Carolina, and the resultant IFR probability field has a more pixelated character.  Note, however, how little information about fog and low stratus is present in the traditional brightness temperature difference field in the lower left.  IFR flight rules are common from south central Georgia northeastward into South Carolina and over northern Mississippi.

Cirrus Effects in GOES-R Fog/Low Stratus Prediction

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MODIS-based IFR Probabilities (Upper Left), Cloud Thickness of the Highest Liquid Layer (Upper Right), 3.7 micron brightness temperatures (Lower Left) and MODIS Cloud Phase (Lower Right) from 1100 UTC on 12 July 2012.

A benefit of the GOES-R Fog/Low Stratus product is that it provides a signal even in the presence of higher clouds that make fog detection via brightness temperature difference methods impossible.  In this case from 1100 UTC on 12 July, a thin cirrus shield off the coast of Oregon (the bright white wisp in the 3.7 micron imagery in the bottom left) shows up (correctly) as an ice cloud in the cloud phase field (bottom right).  Note that low cloud thickness is not computed when higher clouds overlay the low cloud field — so data are available underneath the cirrus deck, although you might infer the thickness from the surrounding fields.  The GOES-R IFR probabilities show values exceeding 50% underneath the cirrus deck, in a region where the brightness temperature difference gives no information.

This effect is also seen in GOES-based imagery, below, from 1200 UTC.  The Cirrus cloud has moved onshore just south of KONP (Newport, OR) where IFR conditions exist.  The IFR probabilities suggest fog/low clouds are likely present in a region where the brightness temperature difference field gives no information because of cirrus clouds.  Although this cirrus shield is small, it should be easy to envision a larger cirrus shield and what that impact might be.

Note also the number of false positives in the brightness temperature difference field that do not show up in the IFR Probabilities.

GOES-West-based Cloud Thickness of the Highest Liquid Layer (Upper Left), IFR Probabilities (Upper Right), 11 micrometer – 3.9 micrometer brightness temperature difference (Bottom Left) and Cloud Phase (Bottom right)

Benefit of Fused product

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The brightness temperature difference product that can be used to infer the presence of fog/low clouds exploits emissivity differences in water clouds at 3.9 micrometers vs. 11 micrometers.  When the two bands have a co-registration error, as documented here, however, a false signal can arise.  A benefit of using a fused product is that the false signal is checked against a cloud mask and model data so that false positives can be identified and ignored.

Pacific NW Fog Event

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The image above shows IFR probabilities (upper left), Cloud depth (upper right), Cloud type (lower left) and enhanced brightness temperature differences (11 microns – 3.7 microns) for a Puget Sound Fog Event on 11 July 2012.  Heightened IFR probabilities occur where surface observations show IFR conditions are present.

Cloud depths at KUIL, on the coast of Washington are around 800 feet.   The sounding from 72797, below, shows a saturated region between 962 mb and 949 mb.  This is equivalent to a depth (using the hypsometric equation and a mean Temperature in the layer of 13.4 C) of 114 m.  If the cloud bases extend down to 980 mb — a possibility — then the layer depth would be about 270 m.  Note that the MODIS imagery is not at 1200 UTC, the time of the sounding.

GOES data can also be used to produce Fog/Low stratus products.  The GOES Image from the same time, below, shows similar patterns as the MODIS imagery, with the expected differences due to GOES’s inferior resolution.  In particular, GOES has difficulties detecting small spatial variability in fog that arises due to river valleys.  Note also that the cloud depth at 1015 UTC from GOES shows good agreement with the MODIS product:  values are around 900-1000 feet.  These values persist through 1200 UTC (the nominal time of the sounding) and beyond.

Note in the Cloud Phase product at night the difficulties inherent in detecting low water clouds over the ocean.  Those regions off the coast of Washington are not in fact clear, and both the brightness temperature difference and GOES-R Fog products correctly show fog or low stratus in those areas.

After sunrise, above (the images at 1430/1500 and 1630/1700 UTC), a couple of things change and are noteworthy.  First, the Cloud Top Phase product becomes more complete over the ocean as visible data is incorporated into the algorithm.  Incorporation of visible data reduces Fog/Low Stratus probabilities over the Puget Sound however, even as visible imagery shows persistent clouds there, and adjacent observations (KCLM/Pt. Angeles, for example) show continued IFR conditions.  This may be related to non-uniformity in the horizontal in the visible imagery.