Departamento de Geofísica Universidad de Chile Field campaign vocals chilean Upwelling Experiment (CUpEx)


НазваDepartamento de Geofísica Universidad de Chile Field campaign vocals chilean Upwelling Experiment (CUpEx)
Дата конвертації05.05.2013
Розмір445 b.
ТипПрезентации


Daytime coastal jet maximum in central Chile (30°S) during VOCALS-CUpEx

Dave Rahn, René Garreaud,

José Rutllant, and Ricardo Muñoz

Departamento de Geofísica

Universidad de Chile

Field campaign

  • VOCALS - Chilean Upwelling Experiment (CUpEx)

  • Data in a previously under sampled region.

    • Buoy, Ship, Aircraft, Land stations, Radiosonde, Remote Sensing
    • High resolution numerical simulations.
  • More details in Garreaud’s talk.

  • This work addresses the near shore coastal jet instead of the more-studied offshore features.



Coastal jet during CUpEx



Fluid System

  • Three main features:

    • Marine Atmospheric Boundary Layer (MBL)
    • Free troposphere above MBL
    • Coastal mountains
  • Result: Two layer fluid system with a lateral boundary.

  • Supports a wide range of features including:

    • Coastal Lows
    • Coastal Jet
    • Barrier Jet
    • Coastally trapped wind reversals
    • Trapped density current
    • Kelvin wave
    • Ageostrophic response to
    • topography


Numerical Simulation

  • Weather and Research Forecasting (WRF, v3.1.1) model.

    • Global Forecast System (GFS) analyses (1°x1°) for initialization/B.C.’s
    • 0000 UTC 20 October 2009 to 0000 UTC 6 December 2009
    • Mother Domain: 9 km
    • Inner Domain: 3 km
    • Vertical Levels: 56 sigma
    • ~60 m at 1 km
    • ~100 m at 2 km
    • Parameters
    • Thompson microphysics
    • Rapid radiative transfer model for longwave radiation
    • Dudhia for shortwave radiation
    • Monin-Obukhov (Janjic) surface scheme
    • Pleim land-surface model
    • Mellor-Yamada-Janjic boundary layer
    • Betts-Miller-Janjic cumulus scheme
    • Second order turbulence and mixing
    • Horizontal Smagorinsky first-order closure eddy coefficient.


Mean simulated 10-m wind

  • 1200 UTC

    • Minimum (~11 m s-1)
    • Acceleration (directional) into Tongoy Bay.
    • Large west-east gradient into bay.
    • Characteristics suggest an expansion fan.


Mean zonal cross section

  • Typical jet structure

    • Maximum at inversion base
    • MBL sloping down toward coast
  • Diurnal difference

    • Maximum daytime heating (~6 K) over the bay collocated with the valley to the south.
    • Maximum increase in jet (10 m s-1) concentrated north of point LdV.


Mean meridional cross section

  • General structure

    • Southerly wind under 1.5 km.
    • Westerly wind (onshore) in the bay.
    • Minimum in MBL depth in the southern part of the Bay.
  • Diurnal difference

    • Daytime heating extends far offshore and rides above the MBL in the bay.
    • Increase in the westward wind over the bay (~6 m s-1).


Pressure response

  • Over Tongoy Bay

    • Afternoon surface pressure drops rapidly in the bay under the warming temperatures.
  • Offshore

    • Small changes in temperature and surface pressure.
  • Result: a large localized gradient along coastal range axis.

  • 5 K warming in a 500 m column produces a surface pressure drop of ~1 hPa.



Diurnal pressure differences

  • Data from high wind period.

  • Mean removed from model and observations from both stations to remove offsets.

  • Range and diurnal cycle of observations and model agree well from 12 to 21 UTC.

    • Discrepancy at 0900 UTC.
  • From 12 to 21 UTC pressure difference decreases 1.2 hPa indicating pressure drops faster in TGY.

    • Remaining (~0.4 hPa) drop likely from regional diurnal tide that impacts both sites.


Ageostrophic Wind

  • Acceleration to the left of ageostrophic wind.

  • Advective component: Northwest ~20 m s-1

    • Large in the vicinity of large PGF gradients
  • Isallobaric component:

    • Large changes indicates change in pressure gradient force over time.
    • Morning: east, acceleration southward
    • Afternoon: west, acceleration northward


Topography Modification

  • Remove coastal range

    • Maximum change 8 m s-1
    • Greater northward extent of warming
  • Enhance coastal topography

    • Maximum change >10 m s-1
    • Smaller northward extent of warming


Summary

  • Local 10-m wind maximum (~16 m s-1) in the afternoon extending ~60 km north of LdV.

  • Induced by enhanced zonal gradient:

    • Small T/p changes offshore
    • Large T/p changes over the bay
    • Warm air (+4-6 K) advected above Tongoy Bay from the valley to the south.
    • Average pressure difference 1.6 hPa at TGY
      • ~1.2 hPa from DT
      • ~0.4 hPa from regional diurnal tides


Conclusions

  • Basic thermal differences dominate the forcing of the jet maximum in the afternoon.

    • Not dominated by expansion fan dynamics!
  • Topography sensitivity tests suggest wind maximum exists regardless if the terrain is present, but:

    • Coastal range enhances the local temperature (pressure) gradient that drives the daytime maximum north of LdV.
    • Topography inversely proportional to northward extent of high winds.
    • High topography blocks southerly wind.


Pressure contribution calculated from DT

  • Pressure profile calculated using the hypsometric equation and iterating down.

    • Same starting point, but using the different temperature profiles.
    • Note: Maximum change at coast! Farther north into the bay it decreases (especially were the MBL). Farther into the bay/offshore, more like 1 hPa.
  • While the path is not equal, model and observations reach the same surface pressure change.

  • Resulting pressure difference from morning to night: ~1 hPa.



Froude Number

    • Supercritical (>1) can support hydraulic features.
  • Both model and observations indicate supercritical numbers during the high-wind period.



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