Georges Bank is a region relatively shallow waters oriented SSW to NNE in the western Atlantic Ocean just east of Cape Cod (Fig 1).

An upward flux of relatively cool subsurface water onto Georges Bank due to tidal mixing (high tide and low tide due to the moon) yields cool Sea Surface Temperatures (SST, relative to surrounding waters) during summer months. The process also results in a nutrient-rich aquatic environment, resulting in a rewarding fishing location for New England.

Given the relatively cool SSTs over Georges Bank during the summertime, northward advection of moist low-level air over the geographic feature results in the development of widespread low clouds and fog. This, of course, poses a hazard to marine interests, particularly fisherman. The NWS (BOX) is responsible for issuing forecasts to only 40 miles offshore (coastal waters forecast), but fog and low clouds do impact those waters too, so forecasters will mention fog conditions when expected/observed. They also receive and respond to inquiries from mariners regarding offshore conditions. Satellite imagery is an important observational tool in providing these forecast products and decision support services.

Low clouds and fog developed over Georges Bank during the day and evening of 28 July 2020. GOES-East SST derived product highlights the cool temperatures over Georges Bank, surrounded by warm waters to the north and south (Fig 2). SSTs sampled over the bank during the morning of the 28th were in the mid 60s F, with low-mid 70s to the north, and mid-upper 70s to the south.

Animating the hourly SST product with RAP SFC wind barbs and dew point temperatures overlaid, moist southwesterly flow was analyzed with upper 60s to low 70s dew point temperatures entrenched over the relatively cool, mid 60s, waters of Georges Bank (Fig 3).

The result of a warm, moist low-level airmass progressing over the cool waters of Georges Bank on 28 July was the development of low clouds and fog. GOES-East 10-min visible imagery reveals the development of clouds over Georges Bank, and advection to the northeast, during the daytime hours (Fig 4).

GOES-East Day Cloud Phase Distinction RGB (modified to enhance low cloud appearance: Green max to 40, blue max to 30) similarly shows the development and evolution, confirming low-liquid water clouds, or bright cyan colors, and differentiating the low clouds from other features (Fig 5).

Overnight, the low clouds continue to be detected using typical IR band combinations. The Night Fog Difference (10.3 – 3.9 um) highlights the low clouds as positive values (blue) due to a difference in emissivity from low clouds at the two wavelengths, contrasting well with the near-zero difference (light gray) from the background surface and upper (ice) clouds in the foreground (Fig 6).

Combining the Night Fog Difference with the IR-Window Channel (13) and Split Window Difference (12.3 – 10.3 um) results in the Nighttime Microphysics RGB, which provides more inclusive cloud detection at night (Fig 7). The evolution of the low (water) clouds and fog is captured in the RGB as aqua/light blue, along with other cloud layers such as mid-level (liquid) clouds (light green) and high thick (red) and high thin (black/dark blue) ice clouds.

There is a drawback to these nighttime IR detection methods in this region. Viewing the Fog Diff over a period surrounding sunset, one notices relatively static regions of constantly positive values (blue) prior to and following sunset in the cold water locations under clear skies (confirmed in visible imagery), especially along the Maine north coast and east around Nova Scotia in this example (Fig 8).

These static regions of consistently positive fog difference values during the day and night are not clouds, but are due to water vapor sensitivity differences between the 10.3 um and 3.9 um bands, abundance of water vapor in the low levels, and the presence of a low-level temperature inversion (which is especially strong over the cold water zones). Compared to at 3.9 um, moisture absorbs (and re-emits) energy better at 10.3 um. Therefore, the satellite is sensing a layer at a higher altitude at 10.3 um compared to at 3.9 um (especially in a very moist environment). Given the temperature inversion, the higher altitude environment sensed at 10.3 um is warmer than that closer to the surface at 3.9 um. As a result, taking the 10.3 um minus 3.9 um difference will yield a positive value, similar to that for liquid clouds, making it difficult to confidently assess the presence of low clouds and fog in these conditions.

A half-hourly animation of the Geocolor product from early on the 28th through the overnight hours shows the complete evolution of this low cloud event (Fig 9). Overnight, when the product utilizes the fog difference for highlighting low clouds (blue), the influence of the aforementioned false alarm on the Geocolor product are readily apparent. One must keep this phenomenon in mind when analyzing low clouds and fog at night in RGBs and other products utilizing the fog difference, particularly when a low-level inversion and moist layer is present.

Moving ahead to Aug 2, we again analyze a geocolor animation from overnight through the early daytime (Fig 10). The low cloud false alarm “blue” areas are present overnight, and are confirmed after sunrise when the cloudy areas are obvious vs the clear sky ocean.

A VIIRS Day Night Band Near Constant Contrast image during the overnight hours confirms the location of low clouds over Georges Bank and the eastern part of the scene vs false alarm around Nova Scotia and portions of Georges Bank (Fig 11).

Bill Line (NESDIS/STAR), Louie Grasso (CIRA), Eleanor Vallier-Talbot (retired NWS BOS)