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 Wood 1 , Christopher Bretherton 1 , Peter Caldwell 1 , Martin Köhler 2 , Rene Garreaud 3 , and Ricardo Muñoz 3 University of Washington, Seattle, USA ECMWF, Reading, UK - PowerPoint PPT PresentationTRANSCRIPT
A continental gravity wave influence on remote marine SE
Pacific cloud
Robert WoodRobert Wood11, Christopher , Christopher BrethertonBretherton11, ,
Peter CaldwellPeter Caldwell11, Martin Köhler, Martin Köhler22, , Rene GarreaudRene Garreaud33, and Ricardo , and Ricardo
MuñozMuñoz33
1.1. University of Washington, Seattle, USAUniversity of Washington, Seattle, USA2.2. ECMWF, Reading, UKECMWF, Reading, UK
3.3. Department of Geophysics, Universidad de Department of Geophysics, Universidad de Chile, ChileChile, Chile
EPIC Stratocumulus
2001• East Pacific Investigation of Climate East Pacific Investigation of Climate
(Bretherton et al. 2003) (Bretherton et al. 2003)
• Shipborne observations with NOAA Ronald Shipborne observations with NOAA Ronald H BrownH Brown
• Instruments include….MMCR, C-band Instruments include….MMCR, C-band radar, microwave radiometer, ceilometer, radar, microwave radiometer, ceilometer,
radiometers, met towerradiometers, met tower• Special MM5 runs performed by Rene Special MM5 runs performed by Rene
Garreaud and Ricardo Muñoz (Universidad Garreaud and Ricardo Muñoz (Universidad de Chile, Chile)de Chile, Chile)
• Special ECMWF run performed using new Special ECMWF run performed using new vertical wind diagnostic by Martin Köhler vertical wind diagnostic by Martin Köhler
(ECMWF, UK)(ECMWF, UK)
Low Low cloud cloud
ubiquitouubiquitous over s over the SE the SE PacificPacific
Important climatological effects…strong SW
cloud forcing but weak LW forcing….
…net cooling effect
Diurnal cycle –The view from spaceSE Pacific has similar mean LWP, but much stronger diurnal
cycle, than NE
Pacific….…Why?
A=LWP amplitude
/LWP mean
From Wood et al. (2002)
[cm s-1] ECMWF VERTICAL VELOCITY
[dBZ] 4
0
2
PRECIPITATION RATECloud-base
Surface
[mm day-1]
LOCAL HR 18 0 6 12 18
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
EPIC 2001 [85W, 20S]Diurnal cycle of subsidence ws, entrainment we, and zi/t
NIGHT DAY NIGHT DAY
ws
we
dzi/dt
we=0.24 cm s-1
ws=0.26 cm s-1
zi/t=0.44 cm s-1
zi/t + u•zi = we - ws
0.05 cm s-1
Conclusion: Subsidence and entrainment contribute equally to
diurnal cycle of MBL depth
Quikscat mean and diurnal divergence
Mean divergence Diurnal difference (6L-18L)
• 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
Cross section
through SE Pacific
stratocumulus sheet
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
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.
Vertical-local time contours (MM5)
• 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 < 5-6 km
Ascent peaks later further out into the SE Pacific
Hei
ght [
m]
17S-73W 22S-71W 21S-76W
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
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
22-18S, 78-74W
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
MLM results• Entrainment closure from Nicholls and Turton – results
agree favourably with observationally-estimated
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
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).
AcknowledgementsWe 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.
ReferencesBretherton, 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