A continental gravity wave influence on remote marine SE Pacific cloud

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A continental gravity wave influence on remote
marine SE Pacific cloud

• Robert Wood1, Christopher Bretherton1,
• Peter Caldwell1, Martin Köhler2, Rene Garreaud3,
  and Ricardo Muñoz3
• University of Washington, Seattle, USA
• ECMWF, Reading, UK
• Department of Geophysics, Universidad de Chile,
  Chile

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EPIC Stratocumulus 2001

• East Pacific Investigation of Climate
  (Bretherton et al. 2003)
• Shipborne observations with NOAA Ronald H Brown
• Instruments include.MMCR, C-band radar,
  microwave radiometer, ceilometer, radiometers,
  met tower
• Special MM5 runs performed by Rene Garreaud and
  Ricardo Muñoz (Universidad de Chile, Chile)
• Special ECMWF run performed using new vertical
  wind diagnostic by Martin Köhler (ECMWF, UK)

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Low cloud ubiquitous over the SE Pacific
Important climatological effectsstrong SW cloud
forcing but weak LW forcing.net cooling effect
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Diurnal cycle The view from space
From Wood et al. (2002)
SE Pacific has similar mean LWP, but much
stronger diurnal cycle, than NE
Pacific.Why?ALWP amplitude/LWP mean
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Diurnal cycle The view from EPIC 2001(85 W,
20S)
Surprising diurnal cycle in subsidence.
results in strong diurnal cycle of cloud top
height that enhances diurnal cycle of LWP
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mm day-1
dBZ
PRECIPITATION RATE
Cloud-base
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Surface
0
LOCAL HR 18 0 6
12 18
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EPIC 2001 85W, 20SDiurnal cycle of subsidence
ws, entrainment we, and ?zi/?t
swe0.24 cm s-1 sws0.26 cm s-1 s?zi/?t0.44 cm
s-1
Conclusion Subsidence and entrainment contribute
equally to diurnal cycle of MBL depth
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Quikscat mean and diurnal divergence

• Mean divergence observed over most of SE Pacific
  Coastal SE Peru
• Diurnal difference (6L-18L) anomaly off
  Peruvian/Chilean coast (cf with other coasts)
• Anomaly consistent with reduced subsidence
  (upsidence) in coastal regions at 18L

Mean divergence Diurnal
difference (6L-18L)
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Cross section through SE Pacific stratocumulus
sheet
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Diurnal subsidence wave - ECMWF

• Daytime dry heating leads to ascent over S
  American continent Diurnal wave of
  large-scale ascent propagates westwards over the
  SE Pacific at 30-50 m s-1 Amplitude 0.3-0.5
  cm s-1 Reaches over 1000 km from the coast,
  reaching 90W around 15 hr after leaving coast

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Subsidence wave in MM5 runs (Garreaud Muñoz
2003, Universidad de Chile)

• Vertical large scale wind at 800 hPa (from
  15-day regional MM5 simulation, October 2001)
  Subsidence prevails over much of the SE Pacific
  during morning and afternoon (10-18 UTC) A
  narrow band of strong ascending motion originates
  along the continental coast after local noon (18
  UTC) and propagates oceanward over the following
  12 hours, reaching as far west as the IMET buoy
  (85W, 20S) by local midnight.

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Vertical-local time contours (MM5)
17S-73W 22S-71W
21S-76W
Height m

• Vertical wind as a function of height and local
  time of day contours every 0.5 cm/s, with
  negative values as dashed lines Vertical extent
  of propagating wave limited to lt 5-6 km Ascent
  peaks later further out into the SE Pacific

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Diurnal vs. synoptic variability (MM5)
Diurnal amplitude equal to or exceeds synoptic
variability (here demonstrated using 800 hPa
potential temperature variability) over much of
the SE Pacific, making the diurnal cycle of
subsidence a particularly important mode of
variability
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Seasonal cycle of subsidence wave (MM5)

• Wave amplitude greatest during austral summer
  when surface heating over S America is strongest.
  Effect present all year round, consistent with
  dry heating rather than having a deep convective
  origin MM5 simulations broadly consistent with
  ECMWF reanalysis data

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Effect of subsidence diurnal cycle upon cloud
properties and radiation

• Use mixed layer model (MLM) to attempt to
  simulate diurnal cycle during EPIC 2001 using
• (a) diurnally varying forcings including
  subsidence rate
• (b) diurnally varying forcings but constant
  (mean) subsidence
• Compare results to quantify effect of the
  subsidence wave upon clouds, MBL properties,
  and radiative budgets

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MLM results

• Entrainment closure from Nicholls and Turton
  results agree favourably with observationally-esti
  mated values Cloud thickness and LWP from both
  MLM runs higher than observed stronger diurnal
  cycle in varying subsidence run. Marked
  difference in MLM TOA shortwave flux during
  daytime (up to 10 W m-2, with mean difference of
  2.3 W m-2) Longwave fluxes only slightly
  different (due to slightly different cloud top
  temperature) Results probably underestimate
  climatological effect of diurnally-varying
  subsidence because MLM cannot simulate daytime
  decoupling

SW
LW
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Conclusions

• Reanalysis data and MM5 model runs show a
  diurnally-modulated 5-6 km deep gravity wave
  propagating over the SE Pacific Ocean at 30-50 m
  s-1. The wave is generated by dry heating over
  the Andean S America and is present year-round.
  Data are consistent with Quikscat anomaly.
• MM5 simulations show the wave to be characterized
  by a long, but narrow (few hundred kilometers
  wide) region of upward motion (upsidence)
  passing through a region largely dominated by
  subsidence.
• The wave causes remarkable diurnal modulation in
  the subsidence rate atop the MBL even at
  distances of over 1000 km from the coast.
• At 85W, 20S, the wave is almost in phase with the
  diurnal cycle of entrainment rate, leading to an
  accentuated diurnal cycle of MBL depth, which
  mixed layer model results show will lead to a
  stronger diurnal cycle of cloud thickness and
  LWP.
• The wave may be partly responsible for the
  enhanced diurnal cycle of cloud LWP in the SE
  Pacific (seen in satellite studies).

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Acknowledgements

• We thank Chris Fairall, Taneil Uttal, and other
  NOAA staff for the collection of the EPIC 2001
  observational data on the RV Ronald H Brown. The
  work was funded by NSF grant ATM-0082384 and NASA
  grant NAG5S-10624.

References
Bretherton, C. S., Uttal, T., Fairall, C. W.,
Yuter, S. E., Weller, R. A., Baumgardner, D.,
Comstock, K., Wood, R., 2003 The EPIC 2001
Stratocumulus Study, Bull. Am. Meteorol. Soc.,
submitted 1/03. Garreaud, R. D., and Muñoz, R.,
2003 The dirnal cycle in circulation and
cloudiness over the subtropical Southeast
Pacific, submitted to J. Clim., 7/03. Wood, R.,
Bretherton, C. S., and Hartmann, D. L., 2002
Diurnal cycle of liquid water path over the
subtropical and tropical oceans. Geophys. Res.
Lett. 10.1029/2002GL015371, 2002