AERMET - AERMET Meteorological Preprocessor for AERMOD...

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9/13/11 Aerosol & Particulate Research Lab AERMET Meteorological Preprocessor for AERMOD Purpose - Use meteorological measurements to computer terrain boundary layer parameters to estimate profiles of wind, turbulence and temperature: Monin-Obukhov length ( L ), surface friction velocity ( u* ), surface roughness length ( zo ), surface heat flux ( H ), convective velocity scale ( w* ), convective and mechanical mixing layer heights ( zic and zim ) Defines stability of the PBL by the sign of H (convective for H > 0 and stable for H < 0) Passes all measurements of wind, turbulence and temperature in a form AERMOD needs. 11
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9/13/11 Aerosol & Particulate Research Lab Energy Balance in PBL Surface sensible heat flux, H 22 ( 29 0 0 / 1 1 9 . 0 / 1 . 0 B R H E H B R G R G E H n n n + = = = = + + λ λ B0 : Bowen ratio – for available surface moisture latent heat flux Soil heat flux net radiation
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9/13/11 Aerosol & Particulate Research Lab Net Radiation If measured values for Rn not available, estimate from insolation and thermal radiation balance is used Solar Radiation Corrected for Cloud Cover 33 { } ( 29 3 2 4 6 1 1 1 c n c T T c R r R ref SB ref n + + - + - = σ ϕ C1 =5.31×10-13 Wm- 2K-6 C2 =60 Wm-2 C3 =0.12 9 SB : Stefin Boltzman constant (5.67×10-8 Wm- 2K-4) Tref : ambient air temperature at the reference height { } ( 29 { } o 2 90 ' ' 1 5 . 0 1 . 0 ) exp( ) ' 1 ( ' = = - - = - = + - + = ϕ ϕ ϕ r r r b a b a r r r albedo ( 29 { } { } 2 30 sin 990 75 . 0 1 0 4 . 3 0 t t R n R R p ϕ ϕ ϕ ϕ + = - = - =
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9/13/11 Aerosol & Particulate Research Lab Transition between the CBL and SBL When the PBL transitions from convective to stable, the heat flux changes sign from positive to negative Critical solar elevation angle For clear and partly cloudy conditions, φcrit ~ 13o; for overcast conditions, φcrit ~ 23o. If solar radiation measurements are available, determine φcrit from cloud cover. 44 Q: Does the transition occurs at sunset? { } ( 29 ( 29 + - - - + - = 30 75 . 0 1 1 990 1 sin 4 . 3 2 4 6 1 n r n c T T c SB crit ϕ σ ϕ 4 . 3 / 1 0 75 . 0 / 1 - = R R n eq
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9/13/11 Aerosol & Particulate Research Lab Surface Friction Velocity 55 Stress (τ) is the vertical transfer of horizontal momentum, per unit area, though the air-land/water interface. It is not a function of height, and does not require adjustment dependent on atmospheric stability class. Kinematic stress (τ/ρ) is stress per unit density (of air). Surface friction velocity: ρ τ / * = u http://www.knowledgerush .com/kr/encyclopedia/Visc osity/ Monin-Obukhov Length A parameter in the Monin–Obukhov similarity theory that relates surface friction velocity to potential temperature
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9/13/11 Aerosol & Particulate Research Lab Surface Friction Velocity ( u* ) and  Monin-Obukhov Length ( L ) in CBL 66 Initial guess for iteration: ψm = 0 (neutral) and u* = uref . k : von Karmon constant (=0.4) uref : wind speed at reference height zref: reference measurement height z0 : roughness length
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9/13/11 Aerosol & Particulate Research Lab uref : typically 10 m can be chosen. However, for rough surface (e.g. urban areas), AERMOD accepts wind speed between 7 z0 and 100 m. For temperature, the lower limit is 1 z0 .
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