Increasing the Utilization of Weather Data for Safety Applications and Traveler Information

Fog Depth Algorithm

The following examples show that it is possible to identify ground fog based on satellite imagery. An experimental algorithm developed by the NOAA/NESDIS Forecast Products Development Team produces a fog depth color enhancement based on the temperature difference between GOES IR Band 2 (3.9 μm) and Band 4 (10.7 μm). This algorithm identifies the approximate depth of low level clouds and fog. More information is available here. Weinrab and Han (2011) provide the equations for the conversion of GOES Variable Format (GVAR) infrared data to both scene radiance and brightness temperature. The UNC Charlotte–UNC Asheville team has adopted this algorithm to produce real-time fog depth imagery and associated data files.

Examples

Two fog events below illustrate the fog depth product. The color steps show increasing cloud depth, from green (0 - 200 m) to black (> 500 m). Light blue areas are cirrus clouds and gray areas are cloud-free. The cloud depths obtained from this product are valid only for single cloud layers. After 1500 UTC, (i.e., during daylight hours), the fog detection algorithm can no longer identify fog due to the strong influence of reflected solar radiation on the radiance measured in GOES IR Band 2 (shortwave infrared).

23 February 2017 Fog Product

On the morning of 23 February 2017, the UNC Asheville investigator noted dense fog in the French Broad River valley near Arden, NC in Buncombe County. The plot above (click it for a larger, more readable image) indicates that fog with a depth of less than 200 m is present in the valley, with deeper pockets of fog from I-40 in Buncombe County northward through Madison County. The 1144 UTC observation at the nearby Asheville Regional Airport (KAVL) corroborates this satellite-derived estimate. The ASOS station reports calm winds, fog, visibility of 1/4 statute miles, and an indefinite ceiling through the fog with a vertical visibility of 200 feet.

20 April 2017 Fog Product

On the morning of 20 April 2017, the National Weather Service issued a dense fog advisory for much of the Piedmont region of North Carolina and portions of the western mountains. The fog product above indicates widespread fog throughout the region. Again, click here for a larger image. The Charlotte/Douglas International Airport (KCLT) is within the blue region in the plot. The 1014 UTC ASOS observations here indicate fog, visibility of 1/4 statute miles, calm winds, and an indefinite ceiling through the fog with a vertical visibility of 100 feet. The Piedmont Triad International Airport (GSO) near Greensboro is in the green region of the plot. The 1027 UTC observation here indicates a 5-kt wind from 250°, visibility of 1.5 statute miles, mist (i.e., light fog), and overcast skies at 200 feet.

8 February 2017 Fog Product

Lastly, the UNC Asheville investigator noted dense fog in the French Broad River valley near Arden, NC on the morning of 8 February 2017. The 1246 UTC observations at the Asheville Regional Airport (KAVL) indicate calm winds, visibility of 1/4 statute miles, and fog with vertical visibility of 100 feet. The plot above shows the fog product for this event and indicates that cirrus clouds over Buncombe County are obscuring the low-level fog. This example shows that the fog product is only valid for single cloud layers. The 1235 UTC observations at the Smith-Reynolds Airport (KINT) near Winston-Salem, however, also indicate a reduced visibility of 1.75 statute miles in mist (i.e., light fog) and a ceiling of 200 feet. The fog depth product shown above indicates a low-level cloud depth of 300-400 meters at this location.


We would like to thank Ken Pryor with the NOAA Center for Satellite Applications and Research (STAR) for assistance with the fog detection algorithm and Ken Knapp with the NOAA National Centers for Environmental Information (NCEI) for generously providing IDL routines to read McIDAS AREA files.


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