Field measurements of urban roughness-sublayer turbulence Andreas - PowerPoint PPT Presentation

Field measurements of urban roughness-sublayer turbulence Andreas Christen Department of Geography, Atmospheric Science Program NCAS Urban Meteorology Workshop University of British Columbia NCAS Urban Meteorology Workshop 30-31 March 2009, University of Reading, UK A. Christen / 30-31 March 2009

Outline • Turbulence measurements in the urban roughness sublayer (URSL) - historical developments, approaches, technology. • Vertical profiles of mean flow and integral turbulence statistics - towards a conceptual division of the URSL. • The nature of exchange processes in the URSL - scales, coherent structures, and the dissimilarity of exchange. NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Turbulence measurements in the URSL Historical developments, approaches, technology NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

The challenge of measuring turbulence in cities • No control on flow and geometry. • The large variety of three-dimensional realizations of the surface-atmosphere interface (morphometry, source distribution, human activity cycles, irradiance and shadowing cycles). • The resulting micro-scale inhomogeneity of the flow field around groups of obstacles including flow caused by moving obstacles. Can we sample the flow representatively? • Technical restrictions and safety concerns of operating towers and instrumentation in densely populated areas. NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Experimental approaches in URSL field studies Single point or Line or volume Lagrangian Dispersion array measurements averages sensors experiments e.g. Ultrasonic e.g. Scintillometry, e.g. micro- e.g. tracer gas anemometers mini-LIDAR tetroons releases NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Vancouver-Sunset, 1978 Photo: D. Steyn NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Vancouver-Sunset, 1989 NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009 Photo: M. Roth

Zürich, 1986 NCAS Urban Meteorology Workshop Photo: M. W. Rotach A. Christen / 30-31 March 2009

Basel, 1993-2002 NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009 Basta / BUBBLE - Photo: A. Christen (2001)

Goteborg, Sweden, 2003 NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009 Photo from: Eliasson et al. 2006

Oklahoma City, USA, 2003 NCAS Urban Meteorology Workshop Photo from: Klein and Clark, 2007 A. Christen / 30-31 March 2009

The horizontal averaging approach Spatial averaging a ( x , t ) = � a � + a ′′ ( x ) + a ′ ( x , t ) horizontal layers in URSL Raupach and Shaw (1982) From an experimental view, the horizontal averaging concept forms a significant challenge, and information z = 4 z = 3 on possibly resulting z = 2 dispersive terms is even z = 1 more difficult to obtain in the field. NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Restrictions of field studies Most studies use ultrasonic anemometers either at a single location or arrayed vertically (barely horizontally) - in street canyons and above roof level - mounted on towers, masts or tripods. Problem: Single point measurements do not provide horizontally averaged statistics, nor can they be used directly to infer the 3D flow field or quantify dispersive fluxes. Even for a single point, it requires long measurement periods to retrieve statistically representative measurements. NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Tower Rooftop n o y n a C NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Scintillometry and remote sensing? Three experiments used scintillometers in Scintillometer the urban surface layer (one experiment within the URSL) and tried to overcome the problem of spatial averaging. Problem: the underlying theory (MOST) is not fulfilled in the URSL (see discussions in Roth et al., 2006), dispersive fluxes not measured. Remote sensing (mini-LIDAR) might be promising, but no technology / studies yet at the scale needed. Tokyo 2000 (Photo: M. Roth) NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Lagrangian approaches Several typical trajectories of small balloons released in a Chicago core canyon. So far, only applied to the study of mean flow. NCAS Urban Meteorology Workshop De Paul, F.T., Shieh C.M., 1986, Atmos. Environ. , 20, 455-459. A. Christen / 30-31 March 2009

URSL turbulence experiments 1985 - 2000 Code City (Project) UCZ 1 Comments Selected References Zu86 Zürich, Switzerland 2 Tower with 1D / 3D sonic at different Rotach (1991a,b, 1995) locations above and upper canyon. Vancouver, Canada 5 Tower with 1D and 2D sonic above suburban Roth (1991, 1993), Va89 surface. Roth and Oke (1993) Sa92 Sapporo, Japan 3 Three 3D Sonics on masts and 45m crane Oikawa and Meng (1995) with residential surface upwind. Hannover, Germany Single 3D Sonic and air pollutant Schatzmann et al. (1999) Hv94 concentrations in 4-lane street canyon Kastner-Klein et al. (2003) Ba95 Basel, Switzerland 2 Tower with 3D sonics above roof level (1.5 < Feigenwinter et al. (1999, (Basta) z / z h < 3.2) 2005) A farm, UK - Urban-like canyon between farm buildings, Louka et al. (2000) Rf97 3D sonic at different heights Na99 Nantes, France 2 Instrumented street canyon with 4 3D sonics, Vachon et al. (1999), (URBCAP) focus: air pollution and traffic TKE. Louka et al. (2002) 1 Urban tracer experiment, with near-field Allwine et al. (2002) Sl00 Salt Lake City, USA releases and 9 2D and 2 3D sonics. (URBAN 2000) NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009 1 Urban Climate Zones (Oke, 2004, WMO)

URSL turbulence experiments 2001 - 2009 UCZ 1 Comments Code City (Project) Selected References Marseille, France 2 Tower with two 3D sonics at different positions. Grimmond et al (2004), Ms01 (ESCOMPTE) CO 2 turbulence, scintillometry. Salmond et al. (2006) Tk01 Tokyo, Japan 3 Long-term tower with one to four 3D sonic Kanda et al. (2006), Moriwaki above roof level (2 < z/z h < 4) and Kanda (2005, 2006) Manchester, UK 2 3D sonic on telescopic mast with varying Longley et al. (2004) Mc01 (SCAR) heights in asymmetric canyon. Ba02 Basel, Switzerland 2 Three towers with 3-6 3D sonics each (1.5 < z / z h Christen et al. (2007, 2009), (BUBBLE) < 3.2). Tracer release above roof. Scintillometry. Roth et al. (2006) Goteborg, Sweden 2 Horizontal and vertical array of 14 3D sonics in a Eliasson et al. (2006) Go03 street canyon and above roof. Oklahoma City, USA 1 Instrumented canyon with >40 sonics in total, at Nelson et al. (2007a,b), Oc03 (JU2003) canyon floor or roof level. Tracer releases. Hanna et al. (2007) Arnold et al. (2004), Lo04 London, UK 2 Instrumented intersection with 7 3D sonics, at Dobre et al. (2005) (DAPPLE 2004) street or roof level. Tracer release experiments. New York, USA 1 Mainly tracer experiments. Instrumented area Hanna et al. (2007), Allwine Ny05 (MSG05 / MID 05) with 16 3D sonic sites, most at 3m or roof level. and Flaherty (2007) Lo07 London, UK 2 Short range tracer releases (<500 m), 8 sonics in Wood et al. (2009, accept.) (DAPPLE 2007) UCL, 1 reference sonic at rooftop. NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009 1 Urban Climate Zones (Oke, 2004, WMO)

URSL turbulence studies Morphometry of the urban surface 0.8 high density UCZ 1 (’highrise core’) 0.7 Plan area ratio of buildings λ P UCZ 2 (’compact core’) UCZ 3 (’dense residential’) Na99 0.6 Ms01 Ba95 UCZ 5 (’suburban’) Ba02u1 0.5 Go03 Lo07 Ny05 Oc03 Z86 Ba02u2 0.4 Tk01 0.3 Ba02s Sa92 low density 0.2 Va89 0.1 0 1 10 100 2 3 5 20 30 50 Average building height (m) NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Turbulence studies in street canyons Canyon aspect ratio z h /W shallow canyon deep canyon 3 Oc03 narrow canyon Trees in canyon Street canyon aspect ratio z h /W Oc03 No trees in canyon 2.5 Skimming flow Go03 2 1.5 Na99 Ba02u2 Z86 Mc01 1 wide canyon Ba02u1 Hv94 Rf97 0.5 Ba02u2 WIF 0 1 10 100 2 3 4 20 30 40 NCAS Urban Meteorology Workshop Average building height (m) A. Christen / 30-31 March 2009

Measured vertical profiles of mean flow and integral turbulence statistics Towards a conceptual division of the URSL NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Ba02u1 NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009 Basel-Sperrstrasse, 2002

Variability of wind profiles with direction Christen et al., 2009 NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

‘Directionally’ averaged wind profiles 4 Zu86 (Rotach 1991) Ba02u1 (Christen 2005) 3.5 Ba02u2 (Christen 2005) Ba02s (Christen 2005) 3 2.5 z/z h 2 1.5 1 0.5 neutral conditions only 0 0 0.5 1 1.5 u ( z ) /u (2 z h ) NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009

Vertical profiles of Reynolds stress SL Zu86 (Rotach 1991) 4 Sa92 (Oikawa and Meng, 1995) Ba95 (Feigenwinter et al. 1999) 3.5 Ba02u1 (Christen et al. 2009) Ba02u2 (Christen et al. 2009) 3 Ba02s (Christen 2005) 2.5 z/z h 2 1.5 z h 1 Buildings Building volume Air 0.5 fraction (m 3 m -3 ) at Ba02U1 0 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 u ′ w ′ /u ∗ NCAS Urban Meteorology Workshop A. Christen / 30-31 March 2009