observations and models of boundary-layer processes over complex terrain

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.

DYCOMS-II Platform and Payload

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

day

daytime z = 40 m

Sun, Desjardins, Mahrt, MacPherson, 1998, JGR

night

Sun, Desjardins, Mahrt, MacPherson, 1998, JGR

night

Schematic of complex flows and effects on CO at forest-lake boundary with nocturnal stably-stratified flow

dzcou L

z

sfc

]2[

L

dxcow z

L

]2[0

z

CO2 transport at night

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 levelson the 60 m CASES99 tower on the night of 18 Oct 1999. Strong turbulent mixing isassociated with 3 differentevents: 1) a density current, 2) a solitary wave, and 3) aninternal gravity wave. Vertical velocity is shifted by the amount given at the right of each time series. (FromSun, et al., BLM, 2003.)

Time-height cross-sections of:(a) air temperature

(b) specific humidity

(c) wind speed

(d) wind direction

(e) vertical velocity at 8 levels

(f) relative variations of CO at 5, 20, and 40 m

FM-CW radar image between 1100 and1200 UTC showing radar backscatter fromrefractivity 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 measurementsTower and Aircraft measurements

Eco-Dimona HB 2335 (930 kg)

Measurement of wind speed, temperature and moisture with temporal resolution of 10 Hz

Aircraft measurementsAircraft measurements

... strange... strange

How to determine zHow to determine zi i ??

• Temperature profiles often lack a clear inversion

• Apply TKE-threshold criterium for boundary layer thickness:

zi = where TKE < 0.4m2/s2

Suggestions for Further ReadingMain 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.

Do Mountains alter the atmosphere?