Daily Archives: October 25, 2012

Day/Night Transition artifacts

GOES-R IFR Probability, 1430 UTC on 25 October 2012

The predictors used by the GOES-R IFR Probability algorithms change as night turns to day and vice versa.  For example, the daytime predictors include the visible satellite imagery.  The change in predictors occurs when the solar zenith angle is less than 85 degrees (that is, when the sun is just rising). For solar angles between 85-90 degrees, temporal continuity has an increased importance and therefore the IFR probabilities look very similar to the previous temporal images.  When the solar zenith angle is between 80-85 degrees temporal metrics are used combined with current visible channel data (only if certain tests are passed that prove the data is of good quality). The cloud mask sometimes has difficulty detecting low clouds in this region so it still is not fully utilized until the solar zenith angle drops to below 80 degrees, where the cloud mask and the full array of daytime predictors are used. This leads to discernible changes in the probability fields, as shown above.  Two boundaries, extending southwest to northeast (roughly parallel to the terminator) are apparent, one from extreme northwest Utah through central Montana and into Canada (where the solar angle is around 85 degrees) and one from northwest Colorado through northwest South Dakota into Manitoba (where the solar angle increases above 80 degrees).   This transition zone is less noticeable in the Summer because of the Earth-Sun geometry.  It is very difficult to create the GOES-R Cloud Thickness product when the solar zenith angles are between 75-90 degrees so the product is currently not available in this region.  This is why the Cloud Thicknesses cut off over eastern Oregon and show back up over central Missouri.

GOES-R Cloud Thickness for 1430 UTC, 25 October 2012

Fog/Low Stratus along the Yukon River

GOES-R IFR Probabilities from GOES-West and surface observations (Upper Left), Brightness Temperature Difference (Upper Right), Topography (Lower Left), MSAS-derived dewpoint depression and surface observations (Lower Right)

Detection of fog and low stratus in Alaska present some unique challenges related both to the very low sun angle that is common there, and the large GOES-West pixel size.  Nevertheless, the fused product does give important information in a state where small planes are ubiquitous for transport between towns.  The loop above shows the development of IFR conditions within the Yukon River valley in east-central Alaska;  note the reduction in visibility that occurs at Fort Yukon (and only there) as the IFR probabilities increase.  IFR probabilities also increase in the Yukon Territory of Canada.  The lack of surface observations there highlights another challenge in Alaska:  Verification of predictions of IFR conditions.

Marine Stratus in the Mid-Atlantic States

GOES-R IFR Probabilities computed from GOES-East (upper left), GOES-R Cloud Thickness in ft (upper right), Brightness Temperature Difference (10.8 µm – 3.74 µm) computed from Suomi/NPP VIIRS data, Brightness Temperature Difference (10.8 µm– 3.9 µm) computed from GOES East, all valid about 0700 UTC 25 October 2012

Marine Stratus has moved inland over Maryland and surrounding states overnight, and the GOES-R IFR probabilities capture the visibility restrictions that have accompanied it.  The imagery above includes the traditional brightness temperature difference fields computed from Suomi/NPP (horizontal resolution:  1 km at nadir) and from GOES (nominal horizontal resolution:  4 km at nadir).  The distinct leading edge of the marine layer is readily apparent in the Susquehanna River Valley.  Note, however, that the strong returns over the Hudson River Valley do not correlate well with reduced visibilities. The GOES-R IFR probabilities do a good job depicting the marine layer in the Susquehanna River Valley (shown by the relatively high probabilities), but return much lower probabilities over the Hudson River Valley where surface observations indicate hazardous low clouds are not present.  Again, this is a benefit of using a fused product.

GOES-R IFR Probabilities (Upper Left) and Cloud Thickness (Upper Right), Suomi/NPP Day/Night Band (Lower left) and MSAS-derived Dewpoint depression on top of Visible imagery.  Times as indicated

The temporal resolution of GOES-R IFR products allow for continuous monitoring of an evolving situation, making up for the relatively poor horizontal resolution (compared to polar orbiting platforms like Suomi/NPP or Terra/Aqua).  The IFR probability field neatly captured the relentless inland push of the marine stratus air, and as the probabilities increase at locations, IFR conditions become more likely.  The loop above includes a Day/Night band image that is reproduced below.  The day/night band, even in low lunar light cases, can distinguish between clear sky and clouds over ocean.  Over land, however, the interpretation is complicated by city lights shining through clouds.  Nevertheless, the cloudy region can be discerned over parts of eastern Pennsylvania and Maryland, where the light signal is more diffuse.