Ship Tracks Around Bermuda

2018-02-23 060220 UTC
GOES-East Geocolor imagery centred on Bermuda at 06:02 UTC, 23 February 2018. This multi-spectral product highlights low clouds (e.g. stratocumulus) as light blue shades. The brighter light blues indicate thicker low cloud.

[Watch animation]

High pressure over the western Atlantic over the last few days has been associated with persistent stratocumulus cloud cover. This typically thin layer of low-altitude cloud is a fairly common feature over the eastern half of ocean basins, and its occurrence around Bermuda is not uncommon – particularly in the cooler months.

Strong high pressure comes with subsidence. Air is compressed and warms as it descends. The descending air, with origins in the mid/upper-atmosphere is also quite dry.  This warm, dry air cannot make it all the way to the surface because it doesn’t have enough momentum to push through the cooler layer near the surface (the boundary layer). This leads to a stable layering of air that manifests as warmer drier air over cooler more humid air.

The boundary layer is often well mixed with an even distribution of heat and moisture (and can become increasingly humid over the ocean). Meanwhile, the warm air above acts like a lid over the cooler boundary layer preventing mixing between the two layers. Moisture trapped in the well mixed layer can form a sheet of stratocumulus cloud given the right conditions.

Potential temperature (the temperature that dry air would theoretically have if it was brought to the surface without external heating), Specific humidity (the mass of water vapour for every kilogram of air), and Relative Humidity (the ratio between the actual water vapour content and the water vapour content needed for saturation). Profiles measured by weather balloon at 00:00 UTC 23 February 2018 at Bermuda.

Exhaust from ships contains aerosols that act as cloud condensation nuclei. These are airborne particles that water can condense onto to form cloud droplets. The aerosols from the ships tend to form more, and smaller, cloud droplets making the clouds contaminated with ship exhaust appear brighter and thicker to satellite instruments.

The ship tracks in the animation above appear to advance from east to west or west to east, following the path of the ships. Meanwhile, another cloud enhancement appears to advance southeastward, starting from Bermuda. This track follows the wind direction in the boundary layer (unlike the ship tracks). It could be the result of aerosols from BELCO or the Incinerator acting to enhance the cloud brightness through the same mechanism as ship tracks.

High pressure and the settled weather it brings has been a theme for much of February 2018. Few mid-latitude cyclones have impacted the island so far this month. As a result, the cold air has stayed away and temperatures have remained far above normal. The lack of cyclones has also meant that precipitation totals are below normal and no days this month have experienced gale force winds. Sea surface temperatures are also far above normal as a result.

The month is expected to end with changeable weather as high pressure gives way. A cold front then takes the opportunity to push southeastward across Bermuda with some rain and showers followed by cooler air.

Further Reading:
Coakley, J. A., R. L. Bernstein, and P. A. Durkee, 1987: Effect of Ship-stack Effluents on Cloud Reflectivity. Science, 237, 1020-1022.
Hudson, J. G., 2000: Cloud Condensation Nuclei and Ship Tracks. J. Atmos. Sci., 57, 2696-2706.
Wood, R., 2012: Stratocumulus Clouds. Mon. Wea. Rev., 140, 2373-2423.


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