1. Aircraft cross-section flights Advanced Regional Prediction System (ARPS) –Large-eddy simulation mesoscale atmospheric model –Developed at the Center for Analysis and Prediction of Storms, University of Oklahoma Improved turbulence models –Reconstruction modeling for LES using explicit filtering (see Gullbrand and Chow 2003, Chow and Street 2002) High grid resolution –Down to 150 m horizontal spacing High-resolution surface data –100 m resolution land use data –Hydrologic model (WaSiM-ETH) for soil moisture –Topographic shading for radiation budget (Colette, Chow and Street 2003) Large-eddy simulations of the atmospheric boundary layer in a steep Alpine valley Fotini Katopodes Chow 1, Andreas P. Weigel 2, Robert L. Street 1, Mathias W. Rotach 2, Ming Xue 3 1 Environmental Fluid Mechanics Laboratory, Stanford University 2 Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich 3 Center for Analysis and Prediction of Storms, University of Oklahoma Riviera Valley To understand evolution of atmospheric boundary layer over steep terrain –e.g. thermal inversion layers trap air pollution in valleys Most surface parameterizations and turbulence models based on theory/observations over flat terrain Extensive field data available for comparison –Mesoscale Alpine Programme (MAP), MAP-Riviera Project, 1999 (Rotach et al. 2003) Motivation High resolution necessary –Small valley (~2 km wide at valley floor, ~15 km long) –Steep slopes (~35°) –Highest surrounding peaks ~2500m above valley floor Major North-South traffic route (connects to Gotthard tunnel) –4000 trucks/day, 14000 cars/day, 150 trains/day Local Time = UTC + 2 hours Riviera valley specifics Dynamic reconstruction + Near-wall model Smagorinsky Log law What grid resolution and initial conditions are necessary to match observations and model results? How does the valley wind transition happen? Why does the stable layer persist throughout the day? What is the effect of turbulence modeling? Surrounding mountain heights Stable layer persists throughout the day, despite strong convective conditions. 0630 UTC0830 UTC0730 UTC0930 UTC Shaded surface shows incoming solar radiation – black (no sunlight), white (bright sun). m/s Clockwise circulation induced by pressure gradient setup by strong upvalley winds. Contours show along-valley winds, vectors show cross-valley winds. High-resolution simulations compared well to observations Captured transitions of valley winds Persistence of stability in valley atmosphere may be due to secondary circulation References Chow, F.K. and R.L. Street. 2002. Modeling unresolved motions in LES of field- scale flows. In 15th Symp. Boundary Layers and Turbulence, American Meteorological Society, pp. 432-325. Colette, A., Chow, F.K., and R.L. Street. 2003. A numerical study of inversion layer breakup in idealized valleys and the effects of topographic shading. J. Appl. Meteor. 42(9), 1255-1272. Gullbrand, J. and F.K. Chow. 2003. The effect of numerical errors and turbulence models in LES of channel flow, with and without explicit filtering. J. Fluid Mech., in press. Rotach, M.W., et al. 2003. The turbulence structure and exchange processes in an Alpine valley: The Riviera project. Submitted to Bull. Amer. Meteor. Soc. 0800 UTC1100 UTC1300 UTC1600 UTC2100 UTC K km ASL Onset of valley winds degrees Time [UTC] Down-valley Up-valley Field observations x 2 m agl x 13 m agl Tower on eastern slope Should observe upslope winds, but instead wind has shifted to upvalley, even towards downslope. Large-eddy simulations – comparisons with observations 9 km 150 m350 m1 km 3 km ECMWF and soundings Grid nesting - Initial conditions and lateral boundary conditions from ECMWF data, for August 25, 1999. Initial conditions also incorporate sounding observations. 50-60 vertical levels,  z min = 40-50 m Methods Surface wind direction 08/25/99 Questions to answer Topographic shading subroutine incorporates shade induced by neighboring mountains. Weak surface winds along valley floor, weak downslope winds on shaded slopes. Descriptions for onset of slope and valley winds also apply to this side valley. Upslope winds begin on east-facing slope.Winds on eastern slope shift to up-valley. Upvalley transition begins at foot of valley. Significant upslope winds on both slopes, with strong upvalley component as well. Local Time = UTC + 2 hours Future work Study effects of using improved turbulence models (Chow and Street, 2002). Current parameterizations are based on flat terrain; error in turbulence at surface affects entire boundary layer. Cross-valley secondary flow Surface radiation budget Downvalley Upslope Upvalley Downslope Reconstruction model captures log region in neutral boundary layer flow over flat terrain. Topographic shading allows good agreement with incoming radiation near sunrise and sunset. (lw = longwave, sw = shortwave) Upslope wind on sunlit east-facing slope only. Downvalley winds dominate core flow. Upslope winds on both slopes. Upvalley winds take over core flow. Eastern slope downslope winds due to clockwise secondary circulation. Upvalley jet shifted to the right (compares well to obs.). Upslope flows on sunlit west-facing slope. Upvalley winds and secondary circulation weaken. Both slopes see weak downslope winds. Valley flow shifts to downvalley. Conclusions Potential temperature soundings Up-valley wind jet shifted to the right. Strong vertical gradients in valley winds. katopod@stanford.edu http://www.stanford.edu/~katopod Location of cross-sections Location of tower Forecast started at 1800 UTC, 08/24/99 Weak upvalley wind in lower valley atmosphere, strong downvalley winds above Inversion persists throughout afternoon Wind direction Wind speed Potential temperature Cross-sections at same location as aircraft flights above. Acknowledgments National Defense Science and Engineering Graduate fellowship NSF, Physical Meteorology Program, ATM- 0073395 (R.R. Rogers, Program Director) NCAR Scientific Computing Division m/s Soundings at 1200 UTC km ASL degrees m/s Radiation flux [W/m 2 ] Time [UTC] degrees m ASL position perpendicular to valley axis [m] Wind direction, 08/25/99 Along-valley winds, 11.6 to 12.4 UTC km ASL K From valley floor, 08/25/99