Ship Tracks on Visible Satellite Imagery
The unusual features in the lower left corner of the visible satellite below are referred to as ship tracks or ship plumes. Considering the number of large ships departing from the ports of San Diego, Los Angeles and San Francisco, it’s not surprising that such features, which can reach 400km in length, appear on visible satellite images of the eastern Pacific. Although similar to aircraft contrails, the appearance of ship tracks is far rarer, occurring on average only a couple of times a week off the California coast.
Scientists have long known that an atmospheric profile comprised of a shallow marine boundary layer and stratiform clouds is very supportive of the formation of ship tracks. A study conducted in the mid 1990s by the Los Alamos National Laboratory attributed this relationship to the lack of both condensation nuclei and convective turbulence in areas supporting marine stratiform clouds. This shortage of condensation nuclei is principally due to the relatively unpolluted nature of the air over the eastern Pacific Ocean. Cloud drops that form in this nuclei scarce environment tend to be larger than those that form over urban areas. The same study also noted that marine stratiform clouds are an important mechanism in cooling the earth’s climate, and a small increase (4%) in their world-wide coverage could offset a doubling of carbon dioxide in the atmosphere.
Ship tracks are more likely to form when the combustion of bunker fuel (a container ship may consume as much as forty tons of this heavy fuel each day) introduces relatively warm and moist air along with condensation nuclei (sulfate particles) into a stable and relatively calm shallow marine boundary layer. In addition, the turbulence (ship air wakes) created by the ship passing through this stable environment (most container and other large ships travel faster than twenty knots) may also promote the development of the tracks. The Los Alamos study hypothesized that these factors modify the existing stratus clouds near the top of the marine boundary layer.
The infusion of additional condensation nuclei (from combustion) provide an opportunity for the formation of more, and relatively smaller cloud drops. A cloud's ability to backscatter sunlight increases as the number of cloud drops increase, even if those drops are somewhat smaller, as demonstrated in the animations below.
The image on the left shows a boundary layer comprised of fewer and relatively larger cloud drops when compared to the image on the right. A cloud containing a greater number of smaller drops has an increased capacity for backscattering sunlight as demonstrated in the image on the right.
The buoyancy of the air parcels in the ship's wake may also be enhanced by the warmth associated with the ship's exhaust. The bottom line? The clouds above the ship's path are brighter, and therefore more noticeable, than the background stratiform cloud deck on visible (and some types of infrared) satellite imagery.
The marine boundary layer, more commonly known as the marine layer, may form over any large body of water that is significantly colder than the air above it. Instead of an atmospheric temperature profile that progressively cools with increasing height, a marine layer exists within an environment where a layer of the atmosphere is warmer than the layer below it. Meteorologists refer to this situation as a temperature inversion. The height above the surface where this inversion exists defines the upper limit of the marine layer. Typically, a marine layer is quite shallow, extending from several hundred to a few thousand feet above the surface.
Although more dominant during the summer months, the marine layer is a regular facet of California’s weather. The prevailing winds and currents along the coast contribute to the development of the marine layer by promoting upwelling, a process where colder water from below is brought to the surface. This steady flow of cold water cools the layer of air above the ocean surface via conduction. In combination with a persistent area of high pressure in the eastern Pacific, the stage is set for the development of a temperature inversion and a layer of stratiform clouds just below it.
The visible satellite image (above) in which the ship tracks appear is from 2230Z on December 5, 2007. The combined surface analysis and infrared image (below left) from 0Z on December 6, 2007, was valid 1 1/2 hours later. The analysis shows a broad area of surface high barometric pressure (1022mb) in the eastern Pacific just off the coast of California. Note that this region was free of high-level clouds, in stark contrast to the area associated with the frontal boundary to the northwest.
The trace of wind speed, gust speed and surface barometric pressure (above right) at NDBC buoy 46059, located 357 nautical miles west of San Francisco, shows the falling pressure associated with an approaching low pressure system. The surface winds at the time of the visible satellite image were less than 10 knots.
The upper-air observation network of the United States is very sparse and, although a few hundred miles away, Oakland California is the closest station to our area of interest. The Oakland soundings at 12Z on December 5, 2007 (above left) and at 0Z on December 6, 2007 (above right) clearly indicate the existence of a near-surface temperature inversion. Although the surface inversion had eroded by 0Z on December 6th, both soundings support the suggestion that a shallow marine layer was resident over the eastern Pacific.
The presence of a marine layer off the California coast is also supported by the meteogram for Oakland on December 5, 2007. By 13Z, all the hallmarks of a marine layer are represented -- cooler temperatures, fog and the resulting reduction in visibility. The generally westerly winds beginning around 15Z also confirm the arrival of a cool and stable air mass from the Pacific. Based upon the details of the METARS, visibility at Oakland was a mere 1/8 mile from 20Z to 22Z.
The surface analysis with visible satellite overlay at 0Z on December 6, 2007 shows the clear correlation of the vast area of surface high pressure and the ship tracks. It is interesting to follow the tracks of ships as they cross from the tranquil area of the marine layer (stratus clouds) to the frontal boundary (cumuliform clouds) associated with the approaching approaching low pressure system to the northwest, and vice versa.
The surface analysis with visible satellite overlay (above) at 18Z on December 6, 2007 shows an absence of ship tracks off the California coast. The area of high pressure and atmospheric stability which was supportive of their development had given way to the instability (as suggested by the surface trough and cumuliform clouds) associated with a low pressure system. The observation trace for NDBC buoy 46059 shows the surface wind jumped from 5 knots to over 40 knots in just a couple of hours as this system arrived mid-day on December 6. It was quite a dramatic transition for westbound vessels during these two days.