1 Tim Oke and the extension-upwards of UHI-observations into the PBL Prof. Robert Bornstein Dept. of Meteorology, San Jose State University San Jose, CA, USA, Prof. Julian Hunt University College, London, UK Presented at the T. R. Oke Symposium AMS Annual Meeting, Phoenix AZ Jan 2009 Funding source: NSF

2 OVERVIEW URBAN CLIMATE ELEMENTS: a review URBAN CLIMATE ELEMENTS: a review TIM OKE’S (and his group) PBL-UHI work TIM OKE’S (and his group) PBL-UHI work –Observations –Explanations –Impacts on other urban-PBL climate elements Synthesis of ideas from Synthesis of ideas from –PBL observations (Oke and his group) –Analytical fluid dynamics (work of Prof. Hunt) –Urbanized meso-met models (Bornstein & many other groups)

3 uPBL CLIMATE ELEMENTS: battles between conflicting effects Monatomic effects VISIBILTITY: decreased VISIBILTITY: decreased TURBULENCE: increased (mechanical & thermal) TURBULENCE: increased (mechanical & thermal) PBL NIGHT STABILITY: neutral PBL NIGHT STABILITY: neutral FRONTS (synoptic & sea breeze): slowed FRONTS (synoptic & sea breeze): slowed More complex effects TEMP: increased (UHI) or decreased TEMP: increased (UHI) or decreased MOISTURE: increased or decreased MOISTURE: increased or decreased WIND SPEED (V): increased or decreased WIND SPEED (V): increased or decreased WIND DIRECTION: convergence or divergence WIND DIRECTION: convergence or divergence PRECIP: increased or decreased PRECIP: increased or decreased THUNDERSTORMS: triggered or split THUNDERSTORMS: triggered or split

4 PBL scales & layers (modified after Oke, 1997) UBL UCL RSL

5 uPBL sub-layers Urban mixing layer Urban mixing layer –Non-homogeneous –Non-stationary Urban SfcBL: has several sub-layers (next slide) Urban SfcBL: has several sub-layers (next slide) Urban surface: where is it in urban meso-met models: Urban surface: where is it in urban meso-met models: –ground –roughness length, z o –rooftop –displacement height, d –top of roughness sub-layer (RSL) Urban sub-surface: consists of Urban sub-surface: consists of –ground –walls –roofs

6 Integration of all surface temperatures gives ‘the’ SUHI “Surface” temperatures and ‘the’ Sfc UHI (SUHI): from Oke 2008

7 uSfcBL sub-layers Urban Canopy Layer (UCL) Urban Canopy Layer (UCL) –Between buildings (extends from 0-h) –Flow pattern is f (W/h ratio)  skimming, vortex, or isolated-obstacle flow Roughness sub-layer, RSL Roughness sub-layer, RSL –Flux-blending layer (extends from h to 3h) –M-O theory not valid, as u * (z) Inertial sub-layer (Oke’s tower obs) Inertial sub-layer (Oke’s tower obs) –Fluxes have blended (>3h) –M-O theory is valid, as u * not f(z)

8 St. Louis nocturnal windy PBL: warm near-neutral, polluted urban-plume vs. rural stable surface-inversion urban-plume Clark & McElroy (1970): rural inversion F0FF0F T T min T T max

9 or NYC nocturnal UHI-dome (Bornstein, 1968):  in calm conditions or  along cross-wind direction during windy conditions _____ uPBL N S

10 NYC AVERAGE NOCTURNAL UHI (z): note cross-over layer (UHI < 0) aloft due to RFD, mixing, and/or sinking rural air, ?? Bars = ± σ

11 AVERAGE MONTREAL NOCTURNAL UHI (z) AS A FUNCTION OF WIND SPEED: HIS LOOKING AT PROCESSES (OKE AND EAST, 1971)  CROSS OVER (W/ SLOW SPEEDS) NO CROSS-OVER  (W/ FAST SPEEDS)

12 HIS CLEAR LINKAGE OF SFC (left) & PBL (right) UHIs

13 HIS CLEAR SYNTHESES WITH THE WORK OF OTHERS

14 HIS CLEAR SYNTHESES OF UHI-EFFECTS ON OTHER URBAN PARAMETERS (more follows) T SO 2 θ

15  Urban-induced nocturnal elevated inversion-I traps home-heating emissions  Power plant plume is trapped b/t urban-induced inversions I & II  Inversion III is regional inversion  over-estimate of mixing depth Home-heating Sources Plume

16 NOCTURNAL UHI-INDUCED SFC-CONFLUENCE: otherwise-calm synoptic flow  confluence-center over urban center of Frankfurt, Germany

17 NYC TETROON-DERIVED w-VELOCITIES: Note (a) larger during unstable daytime-hours (b) Smaller during more stable nighttime-hours (c) thin, weak nocturnal urban elev-inversion layer-base stops w γ γ (trapping)

18 NYC inversion pattern several hrs after previous x-section: Note: flow hit urban upwind edge  roughness-deceleration  up-motion  inversion raised up-motion  inversion raised V 

19 NYC URBAN EFFECTS ON ρ (g/cm): NYC URBAN EFFECTS ON ρ v (g/cm 3 ): Large night UHI, ρ v -island, & RH-deficit Day Nite

20 URBAN IMPACTS ON PRECIP INITATION BY THERMODYNAMICS (at SJSU) INITATION BY THERMODYNAMICS (at SJSU) –LIFTING FROM UHI CONVERGENCE UHI CONVERGENCE THERMAL & MECHANICAL CONVECTION vs. THERMAL & MECHANICAL CONVECTION vs. –DIVERGENCE FROM BUILDING BARRIER EFFECT AEROSOL MICROPHYSICS AEROSOL MICROPHYSICS –SLOWER SECONDARY DOWNWIND ROLE –METROMEX & PROF. D. ROSENFELD (HUJI)

21 NYC splitting thunderstorm (via precip radar-echoes) Should be due to dynamics & aerosols?

22 ATLAN UHI-INITIATED STORM: OBS GOES-SATELLITE & PRECIP (UPPER) & MM5 w’s & precip (LOWER)

23 Fluid-dynamics meso-scale PBL-UHI work of J. Hunt (part 1) Uses Uses –Linearized Navier-Stokes equations –Scale arguments –Froude No. scaling –Analytical solutions Compares effects of Compares effects of –Roughness-change –Coriolis turning –UHI magnitude –Building heights –City size –Time of day

24 Fluid-dynamics meso-scale PBL UHI-work of J. Hunt (part 2) Urban-climate parameters studied Urban-climate parameters studied –Mixing-depth variation –Mean velocity fields –Thermal & mechanical turbulence –Precipitation distribution Goals: Goals: –Synthesis of three sources of information (PBL obs, fluid dynamic scale arguments, & meso-met models) –Two papers: Weather (qualitative) & QJRMS (quantitative)

25 Some important outstanding science questions What is the cause of UHI cross-over effect What is the cause of UHI cross-over effect What are the effects of radiative flux divergence from black carbon aerosols on the thermal structure of the uPBL What are the effects of radiative flux divergence from black carbon aerosols on the thermal structure of the uPBL What are the relative roles of UHI and barrier dynamics versus urban aerosols on urban precip patterns What are the relative roles of UHI and barrier dynamics versus urban aerosols on urban precip patterns What fraction of PBL flow goes around, over, and through the city as a function of stability What fraction of PBL flow goes around, over, and through the city as a function of stability How can PBL obs be made in urban areas How can PBL obs be made in urban areas How can research meso-met models be better urbanized How can research meso-met models be better urbanized How can the urban morphological data needed as input to urbanized meso-met models be obtained How can the urban morphological data needed as input to urbanized meso-met models be obtained How will urban climate effects in various climatic regions be impacted by a changing global climate How will urban climate effects in various climatic regions be impacted by a changing global climate

26 Questions?