NUMERICAL SIMULATIONS OF THE DYNAMICS OF MONTEREY BAY AND THE ERITREAN SOUTHERN COAST

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NUMERICAL SIMULATIONS OF THE DYNAMICS OF MONTEREY
BAY AND THE ERITREAN SOUTHERN COAST
A Paper Presented to The Department of
Meteorology San Jose State University By Bereket
Lebassi May 13, 2005
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• Outline
• Introduction
• RAMS
• General assumptions
• Untransformed Equations
• Transformed Equations
• Diffusion
• Monterey Simulation
• Background
• Results and Comparisons
• Eritrea Simulation
• Background
• Previous Work

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Introduction

• RAMS model will be used to solve two research
  problems
• For the Monterey case, previous observational
  research has not been able to detect a sea breeze
  return flow aloft in the case of a prevailing
  synoptic offshore flow
• For the Eritrean case, a pressing need for the
  country is the development of new energy supplies

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Case comparisons
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RAMS Model General Assumptions

• Reynolds decomposed
• Quasi-Boussinesq
• Molecular effects ignored
• Modified Kuo cumulus parameterization
• Explicit microphysics
• Parameterized radiation, soil, and vegetation
• Curvature of earth ignored
• Two terms dropped from momentum equations
• Exener function used to scale pressure.

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Specific Assumptions

• Motion
• Pressure gradient term cast in terms of ? and ?
• Shallow Boussinesq
• u dropped from the momentum eqs.
• -w dropped from the horizontal eqs.
• Constant Coriolis
• Thermodynamic
• Diabatic heating through radiative forcing,
  latent heating, and microphysics
• Aerosol radiative effects ignored

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• Water species
• Gaseous, liquid, solid water, and chemically
  inert tracers substance
• Phase change allowed
• Sources and sinks water substances parameterized
• Mass continuity
• Quasi-Compressible
• Deep Boussinesq anelastic
• Ideal gas Poisson eqs.
• Virtual temperature assumed

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Cartesian Equations
Motion
(1) (2) (3)
Thermodynamic
Water Species
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(4) (5) (6)
Continuity
Ideal Gas
Poisson
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Transformed Equations

• Notes
• Motion
• No change on horizontal Advection, Diffusion,
• pressure gradient, and Corioles
• Terms with w have new transformation multiplier
• Vertical eq. has become more complicated with new
  terms on pressure, w and g.
• Thermodynamic water species
• Only w in advection changes
• Mass Continuity
• two new multipliers from the transformation
  coordinate vertical derivative
• Ideal Gas Poisson
• No change

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Transformed Equations
Motion
(7) (8)
Thermodynamic
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Water Species
(9) (10) (11) (12)
Continuity
Ideal Gas
Poisson
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General Assumptions
Diffusion

• Horizontal grid spacing is large compared to the
  vertical spacing
• Vertical diffusion modified 2.5 Mellor Yamada
• Horizontal diffusioncomputed as the product of
  the horizontal deformation rate, based on the
  original Smagorinsky formation
• Horizontal diffusion in such cases is normally
  required for numerical damping
• Reynolds-averaged flow cannot resolve
  convection, parameterized convection performs
  all vertical transport
• The fields of wind potential temperature , and
  TKE from prognostic fields

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• Specific Assumptions

• The first term in the prognostic TKE equation is
  the
• local derivative
• First three terms on the right hand side of the
  TKE equation are advection terms
• The next three terms are the diffusion terms and
  Ke is parameterized
• Ps is the shear production term
• Pb is the buoyancy production term
• e is the dissipation term
• Wind and temperature enter these calculations in
  the form of no dimensional vertical gradients
• Turbulent length scale, l, assumed after Mellor
  and Yamada (1982)
• functions Sm and Sh depend on nondimentional
  gradients of wind and potential temperature
• Empirical constants are assigned values following
  Mellor and Yamada (1982)

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• Diffusion Equations

(13) (14) (15) (16)
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Boundary conditions

• Vertical Boundary
• Z H (model top 26 km)
• Ridged Lid with Rayleigh friction layer up to 4
  km
• W 0
• Z h (SBL top)
• Continuity of fluxes, gradients and profiles
• Z0
• Soil model (LEAF2)
• No slip boundary condition for V
• ZHs (bottom soil layer 1 m)
• Constant temperature from large scale model

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• Lateral Boundary

(9)

• For normal velocity component
• Klamp-Wilhelmson lateral boundary condition
• c 20 m/s in (Eq. 9)
• For variables other than normal velocity
  component
• Constant Inflow out flow boundary condition

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Numerics

• Hybrid scheme
• Forward time differencing for thermodynamic
• Leapfrog for velocity components
• Time-split
• Time step 10s, 5s, and 5s for each grid

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(17) (18) (19) (20) (21)
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RAMS Simulation of Monterey Bay sea breeze
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Background
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Contribution to basic thermal forcing of sea
breeze

• Time of the year and latitude
• Factors that alter the land surface energy
  balance Ocean
• temperature just offshore
• Depth and stability of planetary
• boundary layer (PBL)

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Modifying Effects

• Coastline Shape
• determines
• direction of loc-
• al acceleration
• sets up regions
• of enhanced
• convergence/
• divergence

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continued

• Stratification and PBL structure
• Topography
• Clouds

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Return flow

• Examination of the LASBEX dataset, which was
  gathered during offshore synoptic conditions,
  showed that there was no compensatory return flow
  above the local sea breeze (Banta 1990).
• 1) Weak return flow (too weak to be detected)
• 2) Return flow distributed in the vertical in an
  undetectable way
• 3) Return flow superimposed on strong large-scale
  flow
• 4) Return flow does not exist.

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Case Selection

• we selected two simulation periods based on the
  following
• Existence of clearly identifiable sea breeze
  flows.
• A strong, stable stratification of the PBL
• Clear sky conditions around the Monterey Bay area
• A clear sea breeze surface wind at Monterey (i.e.
  onshore during the day, and offshore at night).
• Synoptic conditions for our simulation periods
• strong offshore case
• weak offshore case
• The selections were made based on the Fort Ord
  profiler data

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Grid Configuration
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Grid Configuration

• Horizontal Grid
• Arakawa type C staggered grid
• Three nested grids

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• Vertical Grid
• Model top at 26 km
• Lowest grid w at 50 m and for v, u, and T at 25
  m
• Grid stretch ratio of 1.12
• Soil Levels 0.0, 0.1, 0.3, 0.6, and 1.0 m.

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Initial Condition

• Model initialized
• 0000 UTC 24 October 2003 (Case 1A)
• 0000 UTC 15 March 2004 (Case 1B)
• 12 h allowed to spin up
• Total of 4 days simulation
• Analysis fields produced by RAMS isentropic
  analysis package for every 12 h based on girded
  dataset from US NCEP global model.
• Four dimensional data assimilation (4DDA) by
  Newtonian relaxation (nudging).
• Grid 1 and Grid 2 are nudged towards their
  respective nudging fields , with a nudging time
  scale of 6h.

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Site Map
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Synoptic Condition

• Case 1A
• Shows a broad Eastern Pacific high pressure ridge
  with a height of 3200m centered at (39N, 129W)

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24 Oct 00Z 25 Oct 00z
700 mb
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24 Oct 00Z 25 Oct 00z
1000 mb
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Synoptic Condition

• Case 1B
• The synoptic pattern changed very little over the
  Monterey Bay region during the weak offshore case
• Generally the 700 mb fields were stable and
  consistent during the weak offshore period of
  simulation.

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15 March 00Z 16 March 00z
700 mb
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15 March 00Z 16 March 00z
1000 mb
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Analysis

• background flow the relatively calm wind field
  that exists early in the morning before solar
  heating effects have had time to induce mesoscale
  winds.
• weighed average of 0700 LST wind fields
  immediately before and after that particular
  time.
• For any time t At/24 B(24-t)/24

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Case 1A (strong offshore)
1000 mb
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1000 mb
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1000 mb
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925 mb
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925 mb
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925 mb
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700 mb
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700 mb
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700 mb
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1000 mb
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Case 1B (Weak offshore)

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Summary

• The Monterey Bay sea breeze has return flows that
  are diverted to two major areas.
• return flows come from the Sierra Nevada
  Mountains.
• As the air ascends off the western slopes of the
  Sierra Nevadas, it produces a deep, weak sea
  breeze of 1-3 m s-1 from 1 to 4 km in elevation.
• The second are return flows that come from the
  complex mountain/valley circulations in the
  coastal range.

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Summary

• Banta did not see the Monterey Bay sea breeze
  circulation because it consisted of a weak thick
  return flow of 1-3 m s-1 in a layer from 1 to 4
  km in elevation, that would have been extremely
  difficult to detect when there is a 5-10 m s-1
  synoptic background flow at that level.
• In addition, the Banta study was able to take
  measurements only over a limited area near
  Monterey, when the return flow structure is
  dispersed over a wide geographic area extending
  to the Sierra foothills.

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Model Validation

• To have confidence in the simulations, validation
  of the model results against available
  observations was carried out.
• The synoptic scale forcing
• Temperature
• Wind

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Monterey (Case 1A)
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Sacramento (Case 1B)
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Part two Eritrea Simulation
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Background

• In providing for the rapidly expanding
  electricity needs of its population, Eritrea
  faces two major constraints environmental
  impacts, and cost.
• Wind energy holds the potentially of providing a
  significant contribution to Eritrea's electricity
  needs that is both environmentally friendly and
  low cost

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Previous Study

• collecting and analyzing ground station data on
  wind speeds with some preliminary efforts at
  performing mesoscale simulations for the
  Southeastern coast
• Van Buskirk et.al.
• Lehremeyer mesoscale model KLIMM
• SWECO WaSP
• In this study, we apply Regional Atmospheric
  Modeling System (RAMS).

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Simulation Site For The Eritrean Study
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Grid Configuration

• Horizontal Grid
• Arakawa type C staggered grid
• Three nested grids

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• Vertical Grid
• Model top at 26 km
• Lowest grid w at 50 m and for v, u, and T at 25
  m
• Grid stretch ratio of 1.12
• Soil Levels 0.0, 0.1, 0.3, 0.6, and 1.0 m.

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Boundary conditions

• Vertical Boundary
• Z H (model top 26 km)
• Ridged Lid with Rayleigh friction layer up to 4
  km
• W 0
• Z h (SBL top)
• Continuity of fluxes, gradients and profiles
• Z0
• Soil model (LEAF2)
• No slip boundary condition for V
• ZHs (bottom soil layer 1 m)
• Constant temperature from large scale model

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• Lateral Boundary

(9)

• For normal velocity component
• Klamp-Wilhelmson lateral boundary condition
• c 20 m/s in (Eq. 9)
• For variables other than normal velocity
  component
• Constant Inflow out flow boundary condition

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Initial Condition

• Model initialized 300 UTC 10 February 2002
• 12 h allowed to spin up
• Total of 4 days simulation
• Analysis fields produced by RAMS isentropic
  analysis package for every 12 h based on girded
  dataset from US NCEP global model.
• Four dimensional data assimilation (4DDA) by
  Newtonian relaxation (nudging).
• Grid 1 and Grid 2 are nudged towards their
  respective nudging fields , with a nudging time
  scale of 6h.

69
Numerics

• Hybrid scheme
• Forward time differencing for thermodynamic
• Leapfrog for velocity components
• Time-split
• Time step 10s, 5s, and 5s for each grid

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Site Map
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Synoptic Condition

• The synoptic patterns change very little and are
  characterized by a south west monsoon.
• There is a steady south west LLJ coming from the
  strait of Ba El Mendeb. As a result we have a
  mass of air channeled over the Red sea and
  constrained by the topographies on each side of
  the Red sea. This makes the period easier to
  simulate.

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Continued
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Results -- Surface Winds
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High Resolution Map
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Average wind speeds at 60m
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Wind Speeds at 60m
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Higher resolution at 60m
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Vertical XZ Crossection
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Vertical XZ Crossection
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Model Validation Surface Winds
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Roughness Adjusted Surface wind at Aseb
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Surface Temperature
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Surface Temperature
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Conclusion

• Summary and Conclusion
• For roughness adjusted surface winds and
  temperature model values agree with observations
• In this research paper, it was shown that the
  regional mesoscale model RAMS had proven to model
  the important dynamics of the complex coastal
  environment of the Southern coast of Eritrea.
• This research has helped in proposing the recent
  wind energy project between the Eritrean
  Government and Dr. Robert Vanbuskirk team.
• A future research work could also be modeling of
  the wind resources in the highlands of Eritrea.

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Recommendation

• In areas of complex costal environment
  environment model study not only depends on local
  surface fluxes but also on elevated layers
  transported from remote areas
• Soil temperature and moisture values are very
  important and should be initialized as correct as
  possible
• Care Must be taken in areas were fog appeared
  frequently because RAMS has no fog model except
  the Microphysics schemes

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References

• Arritt, R. W., 1993 Effects of the large-scale
  flow on characteristics features of the sea
  breeze. J. Appl. Meteor., 32, 116-125.
• Atkinson, B. W., 1981 Mesoscale
  Atmospheric Circulations. Academic
• Press, 495 pp.
• Banta, R. M., 1986 Daytime Boundary Layer
  Evolution over Mountainous Terrain. Part II
  Numerical Studies of Upslope Flow Duration. Mon.
  Wea. Rev., 114, 11121130.
• Banta, R. M., 1995 Sea breezes shallow and deep
  on the California coast. Mon. Wea. Rev., 3,
  3614-3622.
• Banta, R. M., L. D. Olivier, and D. H. Levinson,
  1993 Evolution of the Monterey Bay sea-breeze
  layer as observed by pulsed Doppler lidar. J.
  Atmos. Sci., 50, 39593982.
• Bridger, A. F. C., W. C. Brick, and P. F. Lester,
  1993 The structure of the marine inversion layer
  of the central California coast Mesoscale
  conditions. Mon. Wea. Rev., 121, 335-351.
• Burk, S. and W. Thompson, 1996 The summertime
  low-level jet and marine boundary layer structure
  along the California coast. J. Atmos. Sci., 50,
  3959-3982.