Ideal conditions for the development of heavy freezing spray developed across the coastal waters of the northern Gulf of Alaska, including Cook inlet, on the back edge of a low pressure system during the afternoon/evening of 10 Feb 2020. Analysis of GOES-West water vapor imagery reveals the associated upper level trough exiting to the east as the next system approaches from the west (Fig 1).
Given the expected surface conditions, NWS Anchorage issued a Heavy Freezing Spray Warning for the associated offshore waters zone (Fig 2).
During the late afternoon and early evening of the 10th, surface observations indicated the development of gusty winds, temperatures well below freezing, and rough seas; all conditions necessary for the development of freezing (salt water) spray. Gusty winds and temperatures well below freezing were reported at Stations AUGA2 and AMAA2 (Fig 3), with wave heights of 10-15 ft reported at 46080. This is the region within which heavy freezing spray was expected and likely occurred based on analysis of VIIRS and ABI satellite imagery.
Given the relatively high latitude of the region, three VIIRS passes (one from NOAA-20, two from SNPP) were available during the day within 2.5 hours of each other. The five 375 m I bands from SNPP VIIRS for 2123 UTC are shown in Fig 4, centered over the region of freezing spray. A modified gray scale color table was created to focus on the reflectance values of the spray. The higher spatial resolution of the VIIRS imagery (vs GOES) captured the phenomenon in enhanced detail, allowing for easier diagnosis as to where freezing spray was occurring at that moment. The spray is observed as a region of relatively high reflectance (lighter gray) vs lower reflectance open sea extending from station AUGA2 through Cook Inlet and station AMAA2 into western portions of the broader Gulf of Alaska. Viewing VIIRS 375 m channels I1 – I3, it is obvious that the spray is most apparent in channel I2 given the relatively high reflectance of the lofted sea particles over the very low reflectance ocean surface.
Now viewing I4 and I5, the spray has a higher brightness temperature (darker gray) in the I4 (3.9 um) channel vs I5 (11.45 um) channel as a result of added reflectance component during the daytime due to scattering of the airborne particles. The brightness temperatures are similar in areas of clear sky with no spray over the ocean. Taking the difference between the two channels provides a clear view of where the spray is occurring. Sea current patterns are also apparent in bands I4 and I5, and are differentiated from the sea spray, particularly in band I4 where the spray has a higher brightness temperature, and currents have a lower brightness temperature.
Figure 4: 2123 UTC 10 Feb 2020 SNPP VIIRS I bands 1-5 and band 4 minus 5 difference (left to right, top to bottom). Higher res: I1 (0.64 um), I2 (0.865 um), I3 (1.61 um), I4(3.74 um), I5 (11.45 um), I4 – I5. Color table: low reflectance and warmer brightness temperatures is dark gray, high reflectance and cooler brightness temperatures is light gray. For the I4-I5 difference, dark gray represents a greater positive difference, light gray is near 0 difference.
Imagery from the three daytime VIIRS passes (2038 UTC from NOAA-20, 2123 UTC and 2306 UTC from SNPP) provides a sense of evolution of the spray during the day at high spatial resolution (Fig 5). Clouds appear to have developed within the region of spray by 2306 UTC.
GOES-West Full Disk sector provided high temporal resolution imagery over the region of lofted sea particles. At such a high latitude, the imagery spatial resolution is degraded (pixel area increases by roughly 4x), but still useful for detecting the spray in a broad sense. Given our analysis of the VIIRS imagery, we take a look at the 1 km (at nadir) 0.86 um band from GOES-West, and are able to track the evolution of the sea spray through the afternoon (Fig 6). A similar color table is used to that presented with the VIIRS imagery.
The CIRA Snow/Cloud product was very effective at capturing the spatial extent of the sea spray without needing to make any modifications (Fig 7). The integrity of the RGB for tracking other features is, therefore, maintained.
The ability to detect and track sea spray could be useful to NWS forecasters in verifying forecast products such as a Heavy Freezing Spray Warning, and for issuing new forecast products.
Bill Line (NESDIS and CIRA), Louie Grasso (CIRA)
