Observations and Models of Boundary-Layer Processes Over Complex Terrain • What is the planetary boundary layer (PBL)? • What are the effects of irregular terrain on the basic PBL structure? • How do we observe the PBL over complex terrain? • What do models tell us? • What is our current understanding of the PBL and what are the outstanding problems to be addressed?
Overview of problems and possible future directions • Most theoretical, modeling and observational results are applicable to a horizontally homogeneous PBL and underlying surface. • Non-uniform surfaces predominate over land. • New tools are needed and are becoming available to address PBL structure over heterogeneous terrain.
Comparison of remote Doppler radial velocity statistics (30 m vertical resolution) from the Wyoming Cloud Radar with in situ measurements during DYCOMS-II obtained from 30 minute (180 km) circles. Variance Turbulence dissipation
daytime z = 40 m
Schematic of complex flows and effects on CO at forest-lake boundary with nocturnal stably-stratified flow
Even minimal terrain slope can affect e.g. surface wind direction in the stably-stratified PBL. Caughey et al., JAS, 1979 noted from the Minnesota experiment that surface wind rotation through the evening transition was affected by a terrain slope of 0.0014 (0.08°).
Vertical velocity at 8 levels on the 60 m CASES99 tower on the night of 18 Oct 1999. Strong turbulent mixing is associated with 3 different events: 1) a density current, 2) a solitary wave, and 3) an internal gravity wave. Vertical velocity is shifted by the amount given at the right of each time series. (From Sun, et al., BLM, 2003.)
Time-height cross- • sections of: • air temperature • specific humidity • wind speed • wind direction • vertical velocity • at 8 levels • relative variations • of CO at 5, 20, • and 40 m
FM-CW radar image between 1100 and 1200 UTC showing radar backscatter from refractivity fluctuations (moisture). (Univ. of Massachusetts S-band 11 cm FM-CW radar with 2.5 m resolution within a 3° beam.) From Sun et al., 50 m tower height time series of : (a) vertical velocity and air temperature (b) wind speed and direction
MAP-Riviera Project – summer-autumn 1999 – investigate boundary layer and hydrology, including turbulence exchange processes in one exemplary alpine valley (Weigel and Rotach, 2003).
Tower and Aircraft measurements Eco-Dimona HB 2335 (930 kg) Measurement of wind speed, temperature and moisture with temporal resolution of 10 Hz
How to determine zi ? • Temperature profiles often lack a clear inversion • Apply TKE-threshold criterium for boundary layer thickness: zi = where TKE < 0.4m2/s2
Main Reference Sources for these Lectures Belcher, S.E. and J.C.R. Hunt, 1998: Turbulent flow over hills and waves. Annu. Rev. Fluid Mech.. 30:507-538. Blumen, W., 1990: Atmospheric Processes Over Complex Terrain. American Meteorological Society, Boston, MA. Geiger, R., R.H. Aron and P. Todhunter, 1961: The Climate Near the Ground. Vieweg & Son, Braunschweig. Kaimal, J.C. and J.J. Finnigan, 1994: Atmospheric Boundary Layer Flows. Oxford Univ. Press, New York. Oke, T.R., 1987: Boundary Layer Climates. Routledge, New York. Venkatram, A. and J.C. Wyngaard, Eds.,1988: Lectures on Air Pollution Modeling. American Meteorological Society, Boston MA. Abstracts from the10th Conference on Mountain Meteorology, 17-21 June 2002, Park City, UT, American Meteorological Society, Boston. Suggestions for Further Reading