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 cloud Low cloud ubiquitous ubiquitous over the over the

SE PacificSE Pacific

Important climatological

effects…strong SW

cloud forcing but weak LW

forcing….…net

cooling effect

Diurnal cycle –The view from space

SE 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 RATE

Cloud-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

Heig

ht

[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)

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).

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.

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