The Subtropical Sea Breeze

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The Subtropical Sea Breeze
John W. Nielsen-Gammon Texas AM University
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Outline

• Preconceptions
• Observations
• Rotunno (1983) Theory
• Niino (1987) Theory
• Reconciling with Observations
• Modeling Implications

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Preconceptions

• At what time of day (local standard time) does
  the sea breeze attain maximum strength?
• 00 LST (midnight) 12 LST (noon) Need more
• 03 LST 15 LST info
• 06 LST (sunrise) 18 LST (sunset) Dont know
• 09 LST 21 LST

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Land/Sea Breeze, Israel Coast
Peak sea breeze 5 PM
SEA
LAND
Source Newman, 1977, JAS
Peak land breeze 5 AM
Latitude 31.6 N
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Standard Conceptual Model
from Hsu 1970 MWR
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Observations

• Coordinate definitions
• u along-coast, land to left
• v toward land

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Surface Stations
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SRST2 C-MAN platform,Sabine, Texas

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SRST2 ubar,vbar (yellow, cyan)u, v (blue,
violet) August 2000
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42002, August 2000
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Sea Breeze
Land Breeze
Sunset
Sunset
Midnight
Sunrise
Midday
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Rotunno (1983)

• Linear theory
• Horizontal scale of sea breeze
• Dependence on f

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The Coriolis Force

• Caused by Earths rotation
• Accelerates air parcels to the right (in NH) of
  their current motion
• Force proportional to velocity
• If no other forces, parcel will trace a complete
  circle
• Force stronger ( circle faster) at high lats

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Equations of MotionLinear, hydrostatic,
boussinesq

• ut f v - px Fx
• vt f u Fy
• wt b -pz Fz
• bt N2 w Q
• ux wz 0

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Reduce to one equation for the two-dimensional
streamfunction

• N2 yxx yzztt f2 yzz - Qx
• Assume e-iwt time dependence
• N2 yxx (f2 w2) yzz - Qx

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N2 yxx (f2 w2) yzz - Qx

• Elliptic if (f2 w2) gt 0
• Latitudes greater than 30o
• Solution decays with distance from forcing
• Aspect ratio L NH/ (f2 w2)1/2

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Rotunno model imposed heating
-2
0
2
Sea
Land
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Streamfunction at middayhigh latitudes (f2
w2) gt 0
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N2 yxx (f2 w2) yzz - Qx

• Hyperbolic (wavelike solutions) if (f2 w2) lt
  0 within 30o of equator

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Streamfunction at dawn low latitudes (f2 w2)
lt 0
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Streamfunction at midday low latitudes (f2
w2) lt 0
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Streamfunction at sunset low latitudes (f2
w2) lt 0
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Why the difference?

• Role of f as damping at high latitudes
• Undamped oscillations at low latitudes

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Magic Latitudes

• (f2 w2)1/2 is normally of order 7x10-5
• For typical H and N, L 150 km
• At 30/- 1 degrees, (f2 w2)1/2 is of order
  2x10-5
• For typical H and N, L 500 km

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Phase relationships

• Inviscid case north of 30 In phase with heating
• Inviscid case south of 30 Out of phase with
  heating
• Add viscosity In quadrature with heating

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Phase relationships
midnight
Time of strongest sea breeze
Low latitudes
sunset
High latitudes
midday
Increasing friction ?
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Niino (1987)

• Heating produced by vertical diffusion
• Prandtl number unity
• All vertical diffusion terms remain at leading
  order
• Really ugly equation

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Niino (1987)

• (d/dt k d2/dz2) (d/dt n d2/dz2)2 f2
  d2b/dz2
• N2(d/dt n d2/dz2) d2b/dx2 0
• nmomentum diff., k heat diff.
• (reduces to Rotunno if k n 0)

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Niino (1987)

• Vertical scale (k/w)1/2
• Horizontal scale N/w (k/w)1/2 F(f)
• F(f) ranges from 0.7 (high latitudes) to 2.2 (low
  latitudes)

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Niino with frictionmaximum onshore wind
High latitude
30 North
Equator
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Niino without frictionmaximum onshore wind
0 North
29.7 North
50 North
15 North
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Compromise (2000) Theory

• Viscosity matters in neutral boundary layer
• Viscosity important over land, not water
• Internal inertia-gravity waves extending to sea
• Seaward scale much larger than landward scale

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Summary

• Observations show sea-breeze-like oscillations
  extending well into Gulf of Mexico
• Observations show wavelike wind oscillations
  above the boundary layer over land

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Summary (continued)

• Inviscid theory predicts large horizontal extent
  near 30o
• Viscous theory predicts limited (100 km)
  horizontal extent everywhere
• Compromise theory
• Viscous wavemaker over land
• Nearly inviscid waves over water

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Implications for Atmosphere

• Enhanced heat/moisture fluxes over water
• Diurnally-dependent transport over Gulf
• Layered diurnal dispersion of plumes over land
• Away from sea breeze front, simple oscillatory
  behavior

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Implications for Air Quality Modeling

• Model must resolve freely-propagating waves
  within and above the boundary layer
• Vertical grid spacing?
• Horizontal grid boundaries?
• Spinup time?

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Future Directions

• Horizontal structure with profiler data
• Time-dependent viscosity over land
• Full MM5 simulation of sea breeze
• PBL parameterization?
• Vertical resolution?
• Role of basin shape?

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Acknowledgments

• Supported by the state of Texas through the Texas
  Air Research Center (but what I said is not
  necessarily what they would say)
• Profiler data Dick McNider
• Buoy data National Oceanographic and Atmospheric
  Administration