Fog around Puget Sound

GOES_IFR_PROB_20131021loopGOES-15-based GOES-R IFR Probabilities (Upper Left), GOES-15 Brightness Temperature Difference Product (10.7 µm – 3.9 µm) (Lower Left), MODIS-based GOES-R IFR Probabilities (Upper Right), Suomi-NPP Day/Night Band (Lower Right), all times as indicated (click image to enlarge)

IFR Conditions developed around the Puget Sound during the night of 20 October. How did the GOES-R IFR Probabilities capture this event? The animation above includes imagery from 0500, 0900, 0945, 1115 and 1915 UTC. Higher-resolution polar orbiter data (from MODIS and Suomi/NPP) shows the value of higher-resolution in capturing fog that settles into valleys over southeast British Columbia and western Washington. GOES data are unable to resolve those features.

The Brightness Temperature difference fields have a strong signal over the Pacific Ocean and adjacent coastal areas (IFR Probabilities are high in those regions: both satellite data and Rapid Refresh data are consistent with a high likelihood of fog/low stratus). Over land, the signal is more noisy, perhaps because of differences in land emissivity. (That noise is not present when the sun is up — at that time the brightness temperature difference signal is determined by reflected solar radiation). Where the brightness temperature difference signal is smaller over land, the IFR Probability is also lower. That it is not even smaller suggests that model fields are at or near saturation over land. Note also a strength of the IFR Probability: A consistent signal both day and night. IFR Probabilities are high over Seattle where IFR conditions persist.

GOES_IFR_PROB_20131021DNBloopAs above, but for times with Day/Night band data at night only (click image to enlarge)

The Suomi-NPP Day/Night band can give a good indication of where clouds are present at night when, as occurred last night, the moon is near full. (The Day/Night band does not, however, by itself give any indication of surface visibility) In the example above, the clouds do not change much in the 90 minutes between overpasses. (The slight shift in the apparent location of snow-covered mountains is apparently due to parallax) GOES can just barely resolve the very thin fog features that are so evident in the Suomi/NPP data.