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
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)
Low cloud ubiquitous over the SE Pacific Important climatological effects…strong SW cloud forcing but weak LW forcing…. …net cooling effect
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? A=LWP amplitude /LWP mean
Diurnal cycle – The view from EPIC 2001 (85 W, 20S) [cm s-1] ECMWF VERTICAL VELOCITY Surprising diurnal cycle in subsidence…. …results in strong diurnal cycle of cloud top height… …that enhances diurnal cycle of LWP 4 [mm day-1] [dBZ] PRECIPITATION RATE Cloud-base 2 Surface LOCAL HR 18 0 6 12 18
EPIC 2001 [85W, 20S] Diurnal cycle of subsidence ws, entrainment we, and zi/t zi/t + u•zi = we - ws 0.05 cm s-1 NIGHT DAY NIGHT DAY ws we dzi/dt swe=0.24 cm s-1 sws=0.26 cm s-1 szi/t=0.44 cm s-1 Conclusion: Subsidence and entrainment contribute equally to diurnal cycle of MBL depth
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)
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) 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 < 5-6 km Ascent peaks later further out into the SE Pacific
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 Diurnal vs. synoptic variability (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 References 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