Diurnal circulations in Southern California

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Diurnal circulations in Southern California

• Mimi Hughes and Alex Hall

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Outline

• Motivation
• Experiment design
• Description of climatological diurnal winds
• Explanation for and impact of diurnal winds
• Model Validation
• Conclusions and future work

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Motivation

• Why hindcasts of regional climate?
• Need to bring climate descriptions down to local
  scales for people to appreciate them.
• Better understanding of current climate can only
  lead to improved descriptions of how the climate
  could change in the future.
• Why diurnal mountain winds?
• A dominant feature of the climate in Southern
  California during the summer
• Persistent year round

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Experiment design

• MM5 forced by ETA Reanalysis run at high
  resolution (6km) from 1995-2004
• MRF Boundary layer, Simple ice microphysics, no
  convective scheme in innermost domain

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Diurnal winds
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Existing theory to explain diurnal mountain
circulations

• Two types of circulations plain/valley and slope
  winds
• Designed to explain circulations in river valleys
• Would work perfectly for very simple topography
  -- terrain in Southern California is too
  complicated for direct application

Taken from Whiteman (1990)
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Why the winds blow

• Slope winds as the air near the surface heats
  up, a pressure gradient force develops from the
  cool surrounding air toward the slope
• Valley winds there is less air contained within
  the valley than on the plain, and so the air
  within the valley heats up more quickly, causing
  flow from the plain to the valley

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Diurnal surface air temperature
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96
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96
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Diurnal composites of SAT

• EOF1 primarily determines the amplitude, with
  smaller amplitude at high elevations and near
  water
• EOF2 determines the shape and phase of the
  diurnal cycle, with high elevations and coastal
  areas tending to have reduced cooling at night
  and an earlier daytime peak

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Whats causing these two different types of
diurnal cycle?

• The air on the coastal side of the mountains
  remains stratified through the day, meaning at
  night the downslope flow brings warm air to these
  regions
• At high elevations, theres enhanced subsidence
  because of the downslope flows, causing warming
• The balance between these two advective warming
  mechanisms and radiative cooling determines the
  overall shape of the diurnal cycle of SAT

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Huh?
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An illustrative cartoon
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Model Validation
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EOFs of winds
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Diurnal surface air temperature
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Conclusions

• The diurnal winds are a major component of the
  summertime climate of Southern California -- they
  also persist year round
• There are two major modes associated with the
  diurnal winds
• The first mode of the winds is what wed expect
  given the dominant phase of the diurnal cycle of
  SAT, and impacts the amplitude of the diurnal
  cycle of SAT
• The second mode of the winds can be explained by
  the different shapes of the diurnal cycle,
  represented by the second EOF of diurnal
  temperature
• The alterations in the phase and shape of diurnal
  surface air temperature are caused by the balance
  between radiative cooling and advective warming

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Future/Related Work

• Have studied the impact resolution has on the
  diurnal winds by running 2km simulation for
  August 2002 and then turning off fine resolution
  domains -- modes are robust up to 18km
  resolution, but fine structures are removed
• Would like to see the impact diurnal winds have
  on ocean circulation by using just the diurnal
  component of the winds to force an ocean model
• Working on creating a longer time series to study
  different features of Southern Californias
  climate