The marine atmospheric boundary layer (MABL) is generally defined as a layer or cool, moist maritime air with the thickness of a few thousand feet immediately below a temperature inversion.
In the Mediterranean climate of the Los Angeles basin conditions favorable for the formation of a MABL inversion can occur during any month of the year, but are most common during the summer or "dry" season that runs from May through the middle of October. Natives of the area ("The Southland") use the terms "May Grey" and "June Gloom" to describe the frequent cool, damp, overcast days during this calendar period.
According to Filonczuk, et al (1995) there is a 15-percent probability of fog on any given day in Los Angeles during the dry season, compared to a probability of around five percent on any given day during the rest of the calendar year.
Besides tarnishing southern California's "sunny" reputation, the presence of low stratus clouds and/or dense marine fog can cause costly slowdowns at Los Angeles International Airport, and other airports located on or near the coastal plain. For example, on November 27, 2007, U.S.A Today reported that heavy fog forced officials at LAX to close two runways to arriving planes for several hours.
For motorists, fog "rolling in" from the coast can creates zero visibility conditions during L.A.'s often problematic morning commute. Statistics from the California Department of Transportation (Caltrans) show that more fatalities result from fog-related traffic accidents than from any other type. It should be noted that this statistic refers to all fog types across the entirety of California and not just to those created by the marine layer.
The presence of a MABL inversion is also a key ingredient for the creation of stagnant air pollution (smog) in the Los Angeles basin. Smog, a noxious mixture of fog and carbon emissions cause respiratory and other health problems for Los Angeles residents and add millions or billions of dollars to health care costs.
Along the coast of California and Oregon, the MABL inversion is created when subsiding air is heated by compression associated with the North Pacific Subtropical High passes over the cold, equator-bound waters of the Pacific Current. As moist air rises from the ocean surface, it becomes trapped under dome of subsiding high pressure.
If the height of the marine layer is 4000m or less, the marine air is compressed into fog. If the height is greater than 4000m, a layer of stratus clouds often forms at the top of the layer. On average the layer reaches its peak for the day at around 7:00 a.m. (local time) near the coast and around noon over interior valleys. Because of solar heating, the layer often "retreats" toward the coast during the early afternoon and is at its thinnest point during the hours prior to sundown.
Although more sunlight is allowed in once this daily "retreat" begins, areas under the influence of the MABL still have a strong maritime character with lower average afternoon temperatures and higher dew points than those recorded concurrently at more elevated, continental locations only a few miles away.
With its sharp rise in elevation from the coast to the mountains and the contract between the cool, marine air mass along the coast and the dry, warm continental air mass over the mountains, the topography of the Los Angeles basin one of the factors that makes forecasting the marine layer more difficult for Los Angeles (and San Diego) than for locations further north along the coast.
The geography of the area also presents challenges since it does not conform to the normal west coast pattern and, as a result, fog and low clouds there are not strictly the result of the standard "textbook" process of advection.
For example, for much of the west coast including San Francisco Bay, advection fog is most often produced when the prevailing wind is from the north or northeast and when there is a strong upwelling of cold water from the California Current that runs parallel to the shore.
But the Los Angels basin is shielded from most northerly winds by both the San Gabriel Mountains and the Southern California Bight. The San Gabriel Mountains are an east-west oriented coastal chain in northern Los Angeles and San Bernardino Counties that separate the Los Angeles basin from the Mojave Desert. The Southern California Bight is a bend in the coastline that extends from Point Conception north of Santa Barbara to a few miles south of San Diego. In this area, the coast curves sharply to the east, creating number of natural harbors and putting it is sharp contract with the typical north-south orientation found along the west coast of North America.
The east-west orientation of the Bight causes the equator-bound California Current to branch to the east and eventually flow to the north along the coast as the South California Countercurrent. This often creates an "Eddie" in the waters off the coast of southern California which draws warmer waters up from the tropics. As a result, readings from buoys and from satellite derived measurements of sea surface temperatures (SST) routinely show that the waters are warmer within the Bight when compared with SST readings from the open Pacific west of the Channel Islands.
And, with the absence of strong northerly winds over the Los Angeles basin, most fog and status events occur when winds are from the south-southwest. Particularly impressive marine events often follow off-shore or "Santa Anna" events during which down slope winds from the Great Basin create periods of hot dry weather. When it drifts further east, the winds over Los Angeles (return flow) shift to the south-southwest, pushing the warm, maritime air onto the shore under a very warm, very dry inversion layer. Koracin et al (2005) noted that fog creation is often quite impressive when the temperature of the inversion is 10C greater than the temperature of the temperature of the marine layer.
Although dense fog events are most common after a period of unusually warm, dry weather, a "May Grey" or "June Gloom" can also be created by the approach of a low pressure system from the west or southwest.
When creating forecasts for morning fog in Los Angeles it is best to consult all the tools at the meteorologist's disposal. SkewT soundings for LAX, SAN and other coastal stations should be used to determine a number of key factors including wind direction and the presence of an inversion. (Remember, the rule of thumb is that fog it the most likely solution if the dew points are high enough and the inversion is at 4000m or lower.)
According to Lundquist et al, (1999), the 700mb chart is the most useful in determining whether a ridge or trough is present. Since subsidence is believed to be stronger under a ridge, this is when it is believed that fog is more likely to occur.
During the morning hours, visible satellite images can confirm the advance of fog and stratus into interior valleys, while during the afternoon; one can measure the marine layer's "retreat" using the same method.
Visible satellite images are also good for the formation of small cyclonic swirls of clouds above the warm water Eddies formed by the returning Pacific Counter Current. By plotting the wind direction from the Eddie to the shore in the late afternoon, one can get a good indication of where the densest fog may develop during the coming, overnight hours.
Of course, it is also important to consult all available model data and atmospheric charts when making the forecast, but since fog and stratus are so difficult to pin down it is also important to rely on expertise of the National Weather Service office for Los Angeles and Oxnard. Many of the tips in the above paragraphs were gathered from the research techniques described in their area forecast discussions. For a "tricky" forecast challenge like coastal fog, their expertise should not be overlooked.
Filocuzuk, M.K, D.R. Cayan and L.G. Riddle, 1995:
Variability of marine fog along the California coast
SIO, Reference No. 95-2
Goodman, J., 1977:
The microstructure of California coastal fog and stratus
JAM, Vol. 16, 1977
Koracin, D., Leipper, D.F, and Lewis, J.M., 2005:
Modeling sea fog on the U.S. California coast during a hot spell event
GEOGIZIKA, Vol. 22, 2005
Lundquist, J. and Bourcy, T.B., 1999
California and Oregon humidity and coastal fog
SIO, Reference No. 99-17
Lutgens, F.K. and Tarbuck, E.J., 2004
The Atmosphere, pp. 130-36
McKnight, T.L. and Hess, D. 2005
Physical Geography, pp. 220-223