Punta Eugenia, The Boundary Between Waters, And The "Twomey Effect"

Returning to Los Angeles on the last day of the Princess Sapphire's cruise to the Mexican Riviera ports of Puerto Vallarta, Mazatlan, and Cabo San Lucas - specifically Friday, December 3, 2010, at about 10:30 a.m. ship time (MST; 9:30 a.m. PST) - I was lazing around my cabin in the ship's interior when I turned the cabin's TV to the channel where they featured ship-specific information, like where the ship happened to be located at that instant. We were located not far west of Punta Eugenia, the big cape that extends off the west coast of Baja California. That realization made a big bell go off in my head!

Back in graduate school in the early 1980's, like many others in the Department of Atmospheric Sciences at the University of Arizona in Tucson, I used to gather in the hallway where they posted the weather maps and satellite photos transmitted by facsimile machine, and ponder the day's coming weather.

One of the other people pondering the satellite photos was one of my professors, aerosol scientist Sean Twomey. Dr. Twomey was serious, and rather laconic for an Irishman, except on those occasions when a subject excited him (like when he and lightning expert E.P. Krider gathered at an off-campus watering hole to calculate the rate at which bubbles rose to the surface of beer). Like the very best of the post-WWII generation of scientists, he kept a highly-schematic vision of the world foremost in his mind, and tried to keep calculations just as simple as possible (the back of the envelope calculation was everyone's Holy Grail).

I helped Dr. Twomey grade freshman-level tests one semester, and he never provided me with an answer key, reasoning that the answers should be self-evident to a graduate student. I found that the answers weren't THAT self-evident a number of times, and I slowly realized that I knew a lot less than I thought. I liked the way he gave advice: as simply as possible. "Have you looked in the Big Black Book?" he'd say when I asked a microphysics-related question (meaning reference book Pruppacher and Klett). "Have you looked in the Big Red Book?" (Abramowitz and Stegun, for math questions), or: "Have you looked in the Big Brown Book?" (Gradshteyn and Rhyzik, for yet harder math questions).

In the 1970's, Sean Twomey had studied satellite photos, and like others, noted the presence of ship tracks in the vast banks of stratus clouds that often formed off the California coast. Smoke from ship plumes contributed many more cloud condensation nuclei to the air than would typically be found in a place so remote from the land, creating clouds with smaller droplets that were more effective at scattering sunlight back to space. As a result, when observed from space, the clouds were whiter where ship plumes were located, and thus could easily be spotted from satellites. Based in part on these observations, Sean Twomey formulated what he called his "indirect aerosol effect" theory, which is now often called the "Twomey Effect". At this link, friend Peter Pilewskie describes the Twomey Effect in more detail, as part of Sean Twomey's nomination for the 2004 Haagen-Smit Award:

This relationship between increased cloud albedo and pollution, today widely known as the Twomey effect or the first aerosol indirect effect, was given little attention when it was first proposed in 1974. However, in the 1980's the climate community recognized that ship tracks, long identified but now more frequently observed in satellite imagery, were visible manifestations of the consequence of injecting additional particles into a cloud layer. Twomey elaborated on his theory in a 1984 paper in Tellus. In a 1991 paper in Atmospheric Environment, Professor Twomey introduced the term cloud susceptibility, a quantification of the sensitivity of enhancing cloud reflectance for a given droplet number concentration and cloud reflectivity. For example, if a cloud was produced in an environment of high concentration of condensation nuclei, introducing additional particles would do little to make the cloud even brighter. Conversely, a cloud forming in a pristine environment with sparse available nuclei would be more prone to albedo change when more nuclei were added. This is why ship tracks are readily observed in some environments, such as the California Pacific coast region, but less frequently in others, for example, west of the Namibian coast in southern Africa. And this is also why quantifying the magnitude of this effect globally is so challenging: it is difficult to identify a background state above which the influence of pollution can be measured.

Looking at the satellite pictures, there always seemed to be something a little different about Punta Eugenia. Oftentimes there was a boundary of a fog bank or stratus cloud there. Sometimes, even if no clouds were present, there seemed to be something subtly different about the way the water looked south of Punta Eugenia, compared to the water north of Punta Eugenia. There was some kind of transition zone there, where the cold currents from the north broke free from the coast, and allowed warmer water to infiltrate northwards along Baja California Sur's coast. Alternatively, perhaps there was cold water upwelling on the north side of Punta Eugenia, but not the south side. Certainly Punta Eugenia always seemed to be a killing zone for tropical storms. San Diego, Tijuana, and Ensenada almost never see tropical storms, but Cabo San Lucas sees them quite often. Waters abruptly turn cold in-between! Where? At Punta Eugenia!

Whatever the reason, sitting in my cabin, I realized we were about to pass through some kind of transition zone within a few minutes. Whether that transition was subtle or abrupt would depend on what kind of day it was. Perhaps if there was fog or stratus clouds, we might generate a bright ship track ourselves, and I could watch that process in situ. And, if not, I still might see subtle changes in the water. I hustled upstairs with my camera to find out what kind of day it was.

Meanwhile, the day had started warm and sunny, and optimistic sunbathers had already started finding places on the upper deck of the ship, particularly around the lavish "Neptune's Pool". My cabin mate, John, was doing his morning constitutional, and walking laps around the track that circled the forward part of the deck. Scattered patches of kelp off the ship, and the presence of birds and occasional dolphins, suggested we were passing over vast beds - vast forests - of kelp. A good place for living things!

The Baja California coast can be seen in the distance to the east; specifically Sierra Vizcaino.

There was a fog bank looming ahead. Off starboard, to the east, a mountain could be seen above the fog. This mountain was either part of the Sierra Vizcaino, or Isla Cedros.

Looking forward off the starboard side, and away from the sun, a diffraction effect called a glory can be seen. This kind of glory is sometimes called a "Specter of the Brocken", or a "Brocken Bow" (ref. Robert Greenler's "Rainbows, Halos, and Glories"). On deck level, we are located just below fog top, with most of the fog ranging below us. Here the fog is quite thin, but the Brocken Bow is still quite evident.

Brocken Bow.

More Brocken Bow as we plunge into the fog.

Brocken Bow, as seen from the bow of the ship.

Looking aft from near the bow, a shroud can be seen over the ship's superstructure where the stacks are located. A nearly fog-free strip is evident immediately besides the ship's track. Presumably the ship generates something like a gravity wave as it passes through the fog, with adiabatic cooling enhancing cloud formation as air is pushed across the top of the ship (quite apart from any aerosol effect), with countervailing adiabatic warming immediately beside the ship.

A closer look.

Meanwhile, the sunbathers had become very distressed. Not only had the day turned abruptly cold (and this mattered, because they were nearly naked), but the ship began using its god-awful loud foghorn, which deafened everyone gathered near Neptune's Pool. They were not happy.

Sure enough, that fog shroud extends forwards of the ship's superstructure.

Making my way aft.

Now the ship is passing into a nearly fog-free patch of air (looking aft).

Looking aft, the smoke plume can be seen, plus that portion of the fog directly affected by the plume. The plume extends forward from the fog bank, and is presumably a bit more persistent than the rest of the fog (maybe because of a higher liquid water content?). The nearly fog-free strips on either side of the plume are evident too.

To me, these pictures suggest the formation of ship tracks may be more complicated than first contemplated, and depend a lot where the plume is injected into the cloud. Here the plume is injected at the top, or just above the top, of the cloud, and cloud base is at the surface of the water. Maybe it's a simpler process when cloud base is above plume level.

Left: Detail of map in the Punta Eugenia area.

What do satellite pictures look like on this date? Here is the GOES West picture for 11 a.m. ship time....

Taking a closer look at Punta Eugenia, a fog bank is present offshore west of Isla Cedros (likely where we were located). No ship track is evident (the fog is a bit too small and patchy today), but a nearly fog-free zone hugging the coast south of Punta Eugenia is evident (probably where warmer water infiltrates northwards along Baja California Sur's coast).

On this morning of scientific serendipity, despite the cold and the foghorn, I was ecstatic. The geophysical transition I was looking for had been much more abrupt than even I had hoped for!

Looking aft at a patch of kelp (presumably the uppermost tippy-top of a kelp stalk attached to the ocean bottom far below) I saw a dolphin leaping in a perfect circle around the kelp (presumably a nervous tuna was hiding in that kelp). That ecstatic leaping dolphin perfectly-caught my mood!