1. Stratus Background
Stratus type clouds including stratocumulus, stratus, and fog persist in various
regions across the earth. Their impact on regional climatology is great due to their
complete coverage of a given level of the sky and the effects on the radiational balance of
the earth. Results from studies of the effect of stratus clouds on the total radiative flux
show that during the day, the high albedo values of the cloud tops along with radiational
cooling decreasing the radiation, and at nights the high water content acts to absorb and
radiate longwave radiation back to the surface (Hanson and Gruber 1981; Gruber et. al.
1994).
Based on the very numerous studies on stratus formation, it appears that the
existence of an inversion capping the boundary layer must be present. The inversion
layer forms due to subsidence associated with a high pressure area. Air being forced
down undergoes adiabatic heating until its buoyancy equalizes the downward forcing. If
the temperature below the now steady air is cooler, a sharp temperature inversion and a
vertical region of stable air develops in the lower troposphere. The subsidence inversion
acts as both a cap for vertical motions and an indicator of cloud top for most stratus
forming clouds (Klein and Hartmann 1993).
Subsidence inversions act as a boundary between lower moist atmosphere,called
the marine layer, and drier atmosphere above. Over oceans, where water is easily
evaporated into the air, the inversion acts as a cap for upward mixing water vapor. The
atmosphere in the lower troposphere over oceans has constant upward flux of water
vapor, continually increasing the vapor pressure and helping to mix the marine layer.
Stratus clouds are persistant throughout the globe and found over both ocean and
land. Seasonal maxima and minima of stratus cover have been observed (incomplete)
2. California Stratus
The US west coast experiences a summer season of deep coastal stratus, as during
summer (May to October), a broad area of high pressure develops in the North Pacific
Ocean. Hilliker and Fritsch (1999) explain that the clockwise rotation associated with
this semi permanent global circulation feature brings a northwesterly flow to the US West
coast. Colder ocean waters flow along the coast and relatively cooler air masses flow
onshore during this time due to the cyclonic rotation around the high and the
corresponding northerly winds. As the high pressure extends inland, advection of
warmer air from the California landmass moves westward and offshore over the cooler
ocean air mass and the coastal region is primed for synoptic scale subsidence. A moist
inversion layer resulting from the synoptic scale subsidence can be observed within the
lowest 500 meters of the boundary layer, and is referred to as the California Marine Layer
inversion (Koracin 2000).
Stratus cloud tops are commonly seen by satellite to extend eastward from the low
level boundary of the two colliding air masses. A diurnal variation of the east to west
propagation of the clouds edge the strengthening of onshore winds in afternoon, and
offshore winds during the night time. During the onshore period, the moist marine layer
moves inland allowing strong radiational cooling during the night. Clouds form as the
cooling atmosphere drops the level of the LCL, deepening the inversion layer. This
diurnal wind variation ultimately leads to a general maximum of observed inland stratus
along the California coast between 1400-1600 UTC, just before daytime heating
commences.
Stratus depletion varies by location and day due to the many small factors that are
involved with both the inland extension of the clouds during the evening, and the daytime
atmospheric conditions leading to the recession of the marine layer. The morning brings
solar heating and an offshore flow, which helps to mix the air below the inversion layer,
causing an increase in the height of the LCL(Dorman et. al. 2000; Farley,). The Koracin
(2000) study of divergence fields involving near-shore clouds concluded that marine
atmospheric boundary layer (MABL) divergence field analysis would help determine the
depth of the marine layer and subsequent cloudiness near the coast. Fig. 3 shows the
correlation of weak convergence along with strong albedo during mornings, and strong
convergence along with weaker albedos during the afternoon. The study only applied to
the near coastal clouds. Clouds that were inland or further offshore were more impacted
by radiational cooling or topography.