An intra-urban nocturnal cooling rate modelTwo-phase nocturnal cooling • Two-phase cooling (ex....
Transcript of An intra-urban nocturnal cooling rate modelTwo-phase nocturnal cooling • Two-phase cooling (ex....
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An intra-urban nocturnal cooling rate model
ICUC9
Toulouse, France
9th July 2015
Shiho Onomura
Björn Holmer
Fredrik Lindberg
Sofia Thorsson
Urban Climate Group, University of Gothenburg, Sweden
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modified from Oke (1986)
Phase 1
• site-dependent
• most intensive cooling
• driven by sensible heat and
longwave radiative
divergence (Holmer et al. 2007)
Phase 2
• site-independent
• gradually decreasing
• driven by raidative
divergence in a capping
inversion above the canonpy
layer (Holmer et al. 2007)
Two-phase nocturnal cooling
/dense
/open
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Objectives
To develop a nocturnal cooling rate model to simulate air temperature
investigating the temporal development of nocturnal cooling and the
relationship between cooling rates and meteorological variables and
building density.
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• Air temperature (2m) measured at an open site and four built-up
sites with little vegetation in Gothenburg during May-Sep in 1999.
• U (10m), Ta, clearness of the sky (K↓, RH), from a nearby reference
station (SMHI).
• Average sky view factor within a 25 m radius calculated using digital
surface model and ArcMAP 10.1.
Dataset for model development
SVF
① 0.98
② 0.39
③ 0.40
④ 0.53
⑤ 0.39
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A conceptual profile of nocturnal cooling rate
Phase 1 Phase 2
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A conceptual profile of nocturnal cooling rate
CR = α·cos (t) + β
CR = γ·cos (t) + δ
CR = ε·t + ζ
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A conceptual profile of nocturnal cooling rate
CR = α·cos (t) + β + ΔCR(w,t)
CR = γ·cos (t) + δ + ΔCR(w,t)
CR = ε·t + ζ + ΔCR(w,t)
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A conceptual profile of nocturnal cooling rate
ΔCR(w,t)
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A conceptual profile of nocturnal cooling rate
ΔCR(w,t)
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
𝑈𝑡 − 𝑈𝑡−1
𝑈𝑡 + 𝑈𝑡−1
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
Phase 1A starts when relative wind
speed change drops below -0.2.
-0.2
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
Phase 1A starts when relative wind
speed change drops below -0.2.
Phase 1B occurs when
the atmosphere turns from unstable
to stable.
tpeak = t1 + t2
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
Phase 1A starts when relative wind
speed change drops below -0.2.
Phase 1B occurs when
the atmosphere turns from unstable
to stable.
Phase 2 starts 1 hr after relative
wind speed change drops below -
0.5.
-0.5 tpeak = t1 + t2
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Determination of CRpeak
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Determination of CRpeak
• clearness of the sky (CI)
• wind speed (U)
• maximum daily air temperature (Tmax)
• sky view factor (SVF)
Cooling rate impact factor (CRIF)
𝐶𝑅𝐼𝐹 = 1 −𝑈
𝑈𝑐𝑟𝑖𝑡∙
𝐶𝐼 − 𝐶𝐼𝑚𝑖𝑛
𝐶𝐼𝑚𝑎𝑥 − 𝐶𝐼𝑚𝑖𝑛
Ucrit = 4 m/s
CImax = 1
CImin = 0.4
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cloudy
CRIF=0.20
semi-cloudy
CRIF=0.48
clear
CRIF= 0.72
The relationship between CRpeak and Tmax
Tmax (oC)
CR
peak (
K h
-1)
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The impact of SVF on cooling rate
clear
CRIF= 0.72
semi-cloudy
CRIF=0.48
cloudy
CRIF=0.20
𝐶𝑅𝑝𝑒𝑎𝑘,𝑏𝑢𝑖𝑙𝑡−𝑢𝑝𝐶𝑅𝑝𝑒𝑎𝑘,𝑜𝑝𝑒𝑛
=Sky View Impact Factor (SVIF)
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Determination of CRpeak
𝐶𝑅𝑝𝑒𝑎𝑘 = −0.2 − 0.083 ∙ 𝐶𝑅𝐼𝐹 ∙ 𝑇𝑚𝑎𝑥 + 13 ∙ 𝑆𝑉𝐼𝐹
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Determination of CR2
𝐶𝑅2 = 𝐶𝑅2_𝑚𝑖𝑛 + 𝐶𝑅2_𝑐𝑙𝑒𝑎𝑟 − 𝐶𝑅2_𝑚𝑖𝑛 ∙𝐶𝐼 − 0.57
1 − 0.57
𝐶𝑅2_𝑐𝑙𝑒𝑎𝑟 = −1.6 + 0.67 ∙ 𝑈2
𝐶𝑅2_𝑚𝑖𝑛 = −0.34 + 0.03 ∙ 𝑈2
Impact of wind speed change
∆𝐶𝑅𝑤 𝑡 = − α ∙ 𝑈 𝑡 − 𝑈𝑎𝑣𝑒,𝑝ℎ𝑎𝑠𝑒𝑥
1 3 , 𝑖𝑓 𝑥 = 1𝐴
+ α ∙ 𝑈 𝑡 − 𝑈𝑎𝑣𝑒,𝑝ℎ𝑎𝑠𝑒𝑥1 3
, 𝑖𝑓 𝑥 = 1𝐵 𝑜𝑟 2
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Fundamental equations
Phase 1A 𝐶𝑅 = 𝐶𝑅1 −𝐶𝑅𝑝𝑒𝑎𝑘 ∙ cos 𝑓 𝑡 − 𝑡1 − 1 2 + 𝐶𝑅1 + ∆𝐶𝑅𝑤 𝑡
Phase 1B 𝐶𝑅 = 𝐶𝑅2 −𝐶𝑅𝑝𝑒𝑎𝑘 ∙ cos 𝑓 𝑡 − 𝑡𝑝𝑒𝑎𝑘 + 1 2 + 𝐶𝑅𝑝𝑒𝑎𝑘 + ∆𝐶𝑅𝑤 𝑡
Phase 2 𝐶𝑅 =𝑡𝑒𝑛𝑑−𝑡
𝑡𝑒𝑛𝑑−𝑡2𝐶𝑅2 + ∆𝐶𝑅𝑤 𝑡
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Model evaluation
Observational data from
– A open (SVF=0.92) site and a built-up (SVF=0.40) site in Gothenburg,Sweden
– A complex built-up (SVF=0.46) site in London, UK
during May – Sep in 2012 and 2014. The sites have little vegetation.
SVF=0.4
SVF=0.92
SVF=0.46
Gothenburg London
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Model results
clo
udy
sem
i-clo
udy
cle
ar
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Summary
• A intra-urban nocturnal cooling model was developed based on the
concept of two-phase cooling.
• The model successfully estimates cooling rate under a wide range of
weather (CI and U) condition as well as at sites of different building
densities.
• To be tested for other cities, e.g. lower latitudes and/or with very different
site characteristics.
• To take into account the impact of anthropogenic heat and latent heat
fluxes.
• To be used in climate application, e.g. human thermal comfort estimation.
Future work
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Thank you for your attention.
CONTACT INFORMATION : [email protected]
Aknowledgement
• This project is financially supported by the Swedish Research Council Formas,
the Swedish Energy Agency, the Swedish Environmental Protection Agency, the
Swedish National Heritage Board and the Swedish Transport Administration.
• We appreciate Prof. Sue Grimmond, Dr. Simone Kotthaus, Mr. William Morrison
and Ms. Janina Konarska for providing observational data.
Relevant research paper
• Onomura S, Homer B, Lindberg F and Thorsson S (2015) Urban nocturnal
cooling rates: development and evaluation of the NOCRA Model. Manuscript.
• Konarska J, Homer B, Lindberg F and Thorsson S (2015) Influence of vegetation
and building geometry on the spatial variations of air temperature and cooling
rates in a high latitude city. In Review, Int J Climatol
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Two-phase nocturnal cooling
• Two-phase cooling (ex. Chow and Oke 2006)
Phase 1 : site-dependent
most intensive cooling
Phase 2 : site-independent
gradually decreasing
Radiative divergence
Sensible heat flux
Radiative divergence
in a capping inversion
above the canopy layer
(Holmer et al. 2007)
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Cooling rate (CR2) at the start of phase 2
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Impact of cloudiness on CRpeak
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Impact of U and Tmax on cooling rate
cloudy
semi-cloudy
clear
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
unstable
built-up open
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
unstable
Phase 1 starts when relative wind
speed change drops below -0.2.
built-up open
-0.2
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
Phase 1 starts when relative wind
speed change drops below -0.2.
Most intensive cooling occurs when
the atmosphere turns from unstable
to stable.
tpeak = (t1 + t2)/2
stable
built-up open
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Onset of phase 1A,1B and 2 (t1, tpeak, t2)
Phase 1 starts when relative wind
speed change drops below -0.2.
Most intensive cooling occurs when
the atmosphere turns from unstable
to stable.
Phase 2 starts about 1 hour after
relative wind speed change drops
below -0.5.
stable
built-up open
-0.5