Category Archives: Mid-Atlantic

Interpreting the GOES-R IFR Product in regions where Model and Satellite Predictors are used

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GOES-R IFR Probabilities computed from GOES-East and from the Rapid Refresh, hourly from 2202 UTC on 19 August through 1402 UTC on 20 August

The animation above demonstrates the different ways in which GOES-R probabilities can be detected, and the values that can occur.  Much of the East Coast was under a multi-layer cloud deck (as shown below in the color-enhanced 0632 GOES-East 10.7 µm image).  In those places, IFR probabilities will be computed using Rapid Refresh model output, and the character of the IFR probability field will differ from how the field looks when satellite data are used as IFR predictors.

For example, the smooth IFR probability field over North Carolina in this loop (especially from 0300 UTC to 0700 UTC) strongly indicates model-only predictors.  Before and after that time there are small regions that are more pixelated, suggesting satellite input.  At 0600 UTC, GOES-R IFR probabilities start to increase over West Virginia as overnight cooling allows temperatures to approach the dewpoint, and fog starts to develop.  Probabilities become quite high by sunrise over and near West Virginia, and IFR conditions are evident.  IFR probabilities where satellite and model data are used as predictors — as over West Virginia — are generally higher than regions where model data only are used (over North Carolina).  Model data will always be included in the computation of IFR probabilities.  In regions of multiple cloud layers, such as over North Carolina, it is used alone and IFR probabilities can be high.  In regions of low clouds, such as over West Virginia, the model underscores what the satellite data are telling and IFR probabilities will be even higher.  There can also be regions where the Satellite and Model predictors give conflicting information on the presence of fog/low stratus.  In these regions, IFR probabilities will be somewhat lower.

Note that the terminator is present in this loop, both at sunset (2345 UTC) and at sunrise (the 1100 UTC imagery).  The change in IFR probabilities that occur is evident as predictors that are used during the day change to or from those used at night.  At 1100 UTC, probabilities are increasing by about 18% over North Carolina.

GOES-13 10.7 µm imagery at 0632 UTC on 20 August 2012.

The Challenge of Identifying Fog in River Valleys

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Animation of GOES-R IFR probabilities computed from GOES-East (upper left), Brightness Temperature difference (11 – 3.9) from GOES-East (upper right), Surface visibilities and ceilings and GOES-East Visible imagery (lower left), GOES-R IFR probabilities computed from MODIS (lower right)

Radiation fog that forms first — or only — in river valleys is a challenge to detect.  In the example above from Pennsylvania for the morning of August 8 2012, the satellite signal starts to appear over the West Branch of the Susquehanna around 0500 UTC.  It is very unlikely that a numerical model with a horizontal resolution of (only) 13 km — such as the Rapid Refresh — will be able to forecast the development of such a small-scale feature, so the satellite observations are key.  Nominal 4-km resolution of the infrared channels is the principle limiting factor in detection.

The presence of high clouds over southeast Pennsylvania precludes the detection by satellite of low clouds/fog, so the GOES-R IFR product there is driven primarily by model output, and the scale of the river fogs are simply too small to be simulated in the model.  Fog probabilities do increase near Selinsgrove (KSEG) around daybreak because the model relative humidity does reach high enough values.  This also happens over southeast Pennsylvania.  Typically, model relative humidities in the lowest kilometer of the model must be greater than 80-85% for a strong GOES-R IFR signal to be present.

GOES-R IFR probabilities computed from GOES-East (upper left), Brightness Temperature difference (11 – 3.9) from GOES-East (upper right), Surface visibilities and ceilings and GOES-East Visible imagery (lower left), GOES-R IFR probabilities computed from MODIS (lower right), all valid at 0700-0715 UTC 8 August 2012

Note that at 0715 UTC there is a comparison between the better resolution of the MODIS imagery.  When GOES-R is operational, resolution will be in between that of current GOES and MODIS.  The MODIS GOES-R IFR probabilities are much higher, and show the different river valleys far more cleanly than present GOES.

How to validate GOES-R IFR in regions with no surface observations

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Animation of GOES-W IFR Probabilities over Arizona, half-hourly from 0430 UTC to 0800 UTC 31 July 2012

The animation of GOES-R IFR Probabilities over north-central Arizona shows the development of relatively high probabilities as the night progresses, an evolution that is consistent with the formation of radiation fog.  However, there are no surface observations routinely taken in the region to verify the presence of IFR conditions, or even clouds.  How much credence should be given to such a field?

GOES-R IFR Probabilities from GOES-West (upper left), Suomi-NPP Day/Night band imagery (upper right), Surface observations (lower left), GOES-West color-enhanced window channel (lower right), times as indicated.

A serendipitous Suomi-NPP overpass shows a region of clouds neatly outlined by the GOES-W IFR probabilities. Although the Day/Night band cannot give ceiling heights or visibilities (that is, it cannot alone determine if IFR conditions are occurring), it does show the presence of low clouds.

An earlier Suomi/NPP overpass over the eastern United States overflew a second developing region of enhanced IFR probabilities, over the piedmont from North Carolina southwestward into South Carolina (below).  The city lights of the Carolinas make it more difficult to detect cloud edges, although evidence of one does exist between Camden SC (KCDN) and Fairfield CO Airport (KFDW).  This is a also a region where IFR probabilities quickly drop from high values (near KCDN, where there is fog with 100-foot ceilings) to low (near KFDW, which reports clear skies).

GOES-R IFR Probabilities from GOES-East (upper left), Suomi-NPP Day/Night band imagery (upper right), Surface observations (lower left), GOES-West color-enhanced window channel (lower right), times as indicated.