Category Archives: Multiple Cloud Layers

Model vs. Satellite Predictors in the GOES-R IFR Probability algorithm

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GOES-East Enhanced 10.7-micrometer imagery (Upper left), GOES-R IFR Probability (upper right), GOES-R Cloud Phase (lower left), GOES-East Visible imagery (lower right)

This blown-up version of satellite-based (and fused) products over central Wisconsin on the morning of 18 July 2012 shows how the use of different predictors in the GOES-R IFR probability field can be discerned from the character of the field produced.  This was a morning with MVFR/IFR conditions over central Wisconisin (600-foot ceilings at Marshfield (KMFI) and 700-foot ceilings at Wisconsin Dells (KDLL), for example).   IFR probabilities were high in the regions where IFR conditions were observed, but note how smooth the field is in the northwest and southeast part of the GOES-E IFR probability image.  In the regions under the anvil cirrus (cold cloud-tops as depicted in the 10.7 micrometer image, upper left), the GOES-R IFR Probability algorithm will rely on model data.  In this case, the relative humidity in the RAP forecast has more influence because the high clouds mean the satellite signal is not from a fog/low stratus layer and therefore does not influence the IFR probabiltiies.  The layer relative humidity in the model is likely higher in the northwestern part of the image (where GOES-R IFR probabilities are in the 70% range) than in the southeastern part of the image (where GOES-R IFR probabilities are in the 50% range).  Over the central part of the GOES-R IFR Probability image, the absence of high clouds allows satellite information to be used, and a more variable field results that has a mirror in the variability of the satellite observations in that area.  This region is also where Cloud Thickness diagnoses can be made.

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)