Impact of the Large-Scale Environment on Stratocumulus Clouds

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Update on stratocumulus simulations by the UCLA
AGCM

• C. R. Mechoso, I. Richter, G. Cazes, and R. Terra
• University of California, Los Angeles
• OUTLINE
• Sensitivity of Sc incidence to African orography
• A comparison with similar results for South
  American orography
• Aspects of PBL parameterization in AGCMs
• Work in progress
• Mechoso, C. R., J. -Y. Yu and A. Arakawa, 2000
  A coupled GCM pilgrimage From climate
  catastrophe to ENSO simulations. General
  Circulation Model Development Past, Present and
  Future. D. A. Randall Ed., Academic Press, 539-575

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Why Stratocumulus Matter

• Stratocumulus cover a large portion of the
  worlds oceans
• Impact on global radiation budget is significant
  (e.g. Slingo 1990, Hartmann et al. 1992)
• Climate of tropical regions strongly depends on
  subtropical marine stratocumulus position of the
  ITCZ, SST gradients (e.g. Philander et al. 1996,
  Ma et al. 1996)
• AGCMs difficulties with stratocumulus lead to
• ? large uncertainties in global warming
  estimates
• ? severe problems in coupled GCMs (double ITCZ,
  warm SST bias, weakened trade winds etc.)

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Overall Goal of this Study

• Increase understanding of the interplay between
  the large-scale environment and subtropical
  marine boundary layer clouds concerning their
  seasonal cycle in different regions of the world
  oceans.
• A first stage of the study focuses on the role
  that orography plays on the flow over the eastern
  tropical oceans.

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Seasonal Cycle of Stratocumulus

• Surface observations of the five major marine
  stratocumulus regions (from Klein and Hartmann,
  1993)

Peruvian and Namibian stratus peak in October
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Model Description

• UCLA AGCM, version 7.1
• Resolution 2.5ºlon x 2ºlat x 29 ? levels
• Harshvardhan (1987) radiation scheme
• Prognostic version (Pan and Randall 1998) of the
  Arakawa-Schubert (1974) cumulus parameterization
• Mixed-layer PBL parameterization based on
  Deardorff (1972), as designed by (Suarez et al.
  1983) and revised by Li et al. (1999, 2002). The
  PBL top is a coordinate surface a cloudy
  sublayer develops is this top is above
  condensation level.
• Climatological monthly-mean SSTs prescribed

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Experiment Design

• Test the impact of orography on stratocumulus by
  using the UCLA AGCM
• Contrast pairs of simulations
• Control realistic orography everywhere
• No-Orography orographic surface heights set to
  sea-level over the African (South American)
  continent
• Control is 20-year long. No-Orography runs are
  3-year long.

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African Orography
Contour Interval 500m
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Stratocumulus Incidence in Control
AGCM v7.1 2.5x2x29L
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Verification using NCEP Reanalysis
Control
NCEP
Contour Int. 2 K
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Impact on TOA Radiative Budget August
CI 20 W/m2
SW LW ? positive
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Annual Cycle in the Namibian Stratus Region
Stratocumulus Incidence
Lower Tropospheric Stability K
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Longitude-Height Section of TemperatureDifference
Control - No-OrographyAverage 20S-10S
Pressure mb
Longitude
Contour Int. 1K
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Thermodynamic Energy Equation
1
2
3
4

• 1 Temperature Tendency
• 2 Diabatic Effects
• 3 Vertical Advection
• 4 Horizontal Advection

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Calculation of terms in the thermodynamic equation

• Monthly accumulated value of diabatic effects is
  provided by the model.
• Monthly temperature tendency is provided by the
  instantaneous model output.
• Horizontal advection is computed off-line from
  monthly-mean model output.
• Vertical advection is obtained as a residual.

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Horizontal Temperature Advection at 700 mbAugust
Control
Difference
NAfO
Contour Interval 0.5 K/day
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Annual Cycle of Thermodynamic Balance Terms700
mb Level
NAfO
Diabatic Heating
Control
Vertical Advection
Horizontal Advection
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Anti-Cyclonic CirculationWind and Temperature at
700 mb, August
Control
Difference
NAfO
Contour Interval 0.5 K
Contour Interval 2 K
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Difference Control minus NAfO900 mb
Contour Interval 1 K
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Thermodynamic Balance TermsPeruvian Stratus
Region
Control
Diabatic Heating
NSAO
Vertical Advection
Horizontal Advection
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Linear vs. Non-Linear Mountain Effect(after
Rodwell and Hoskins 2001)

• Linear Response
• Anti-cyclone over the
• mountain

Non-Linear Response Anti-cyclones to the west
and east of the mountain
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Orographic Effects on Marine Stratocumulus

• Peruvian case (nonlinear)
• West of the Andes, conservation of potential
  vorticity for parcels descending equatorwards
  along the isentropes results in increased static
  stability at lower levels.
• Namibian case (linear)
• West of the African mountains, warm air advected
  polewards results in increased static stability
  at lower levels. The warm advection is a
  component of the anti-cyclonic circulation
  centered above the mountains.
• In both cases, mountains contribute to cold
  advection near the surface of the ocean.

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Seasonal Cycle of Stratus

• California stratocumulus peak in the northern
  summer, under the subsidence associated with the
  North American monsoon
• Peruvian and Namibian stratocumulus have broad
  peaks in the austral spring.
• Continental orography seems to contribute to the
  early start by increasing the temperature in the
  lower troposphere.
• Continental orography also seems to contribute to
  the late end by advection of cold air near the
  surface.
• Convection over the adjacent continents appears
  to play a minor role

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Annual Cycle of Simulated Stratocumulus (after
AGCM revisions)
3/19/04
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Work in Progress

• Explore role of convection over continents on
  marine stratocumulus i.e., by modifying
  continental convection through surface boundary
  conditions on land surfaces.
• Assess the sensitivity of AGCM simulations to
  different, yet realistic, orographic
  distributions.
• Explore these sensitivities in the context of the
  coupled atmosphere-ocean system.
• Explore these sensitivities in the context of the
  PBL parameterization of PBL clouds.