Update on stratocumulus simulations
by the UCLA AGCM
C. R. Mechoso, I. Richter, G. Cazes, and R. Terra
University of California, Los Angeles
OUTLINE
1.
Sensitivity of Sc incidence to African orography
2.
A comparison with similar results for South American orography
3.
Aspects of PBL parameterization in AGCMs
4.
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, 539575
Why Stratocumulus Matter
• Stratocumulus cover a large portion of the world’s
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.)
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.
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
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
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.
African Orography
Contour Interval = 500m
Stratocumulus Incidence in Control
AGCM v7.1 2.5x2x29L
Verification using NCEP Reanalysis
Control
NCEP
Contour Int. = 2 K
Impact on TOA Radiative Budget
August
SW + LW
 positive
CI = 20 W/m2
Annual Cycle in the Namibian Stratus
Region
Stratocumulus
Incidence [%]
Lower Tropospheric
Stability [K]
Longitude-Height Section of Temperature
Pressure [mb]
Difference Control - No-Orography
Average 20S-10S
Longitude
Contour Int. = 1K
Thermodynamic Energy Equation
1
2
3
4
1: Temperature Tendency
2: Diabatic Effects
3: Vertical Advection
4: Horizontal Advection
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 offline from monthly-mean model output.
• Vertical advection is obtained as a
residual.
Horizontal Temperature Advection at 700 mb
August
Control
NAfO
Contour Interval = 0.5 K/day
Difference
Annual Cycle of Thermodynamic Balance Terms
700 mb Level
NAfO
Diabatic Heating
Control
Vertical Advection
Horizontal Advection
Anti-Cyclonic Circulation
Wind and Temperature at 700 mb, August
Control
Difference
NAfO
Contour Interval = 2 K
Contour Interval = 0.5 K
Difference Control minus NAfO
900 mb
Contour Interval = 1 K
Thermodynamic Balance Terms
Peruvian Stratus Region
Control
Diabatic Heating
NSAO
Vertical Advection
Horizontal Advection
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
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 anticyclonic circulation centered above the mountains.
•
In both cases, mountains contribute to cold advection
near the surface of the ocean.
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
Determination of Surface Fluxes
Konor and Arakawa (2001)
Randall and Moeng et al. (1998, unpublished)
Surface flux of a quantity 
(FY )S = r PBL CY (ePBL)1/2 (Y S - Y PBL)
r PBL
CY
ePBL
YS
Y PBL
: Mean density of air within PBL
: Surface transfer coefficient of Y
: Mean TKE within PBL
: Value of Y at the surface (ground)
: Mean value of Y within PBL's subcloud layer
Annual Cycle of Simulated Stratocumulus
(after AGCM revisions)
3/19/04
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.