15 th IUAPPA Clean Air Congress Vancouver, BC Optional Presentation Title Photo Slide Progress In Dispersion Model Evaluation And Prediction For Low Wind Speed Conditions (Control #19) Presenter’s Name September 16, 2010 September 16, 2010 p p , , Study Study conducted by: conducted by: Jeff Connors, Bob Paine Jeff Connors, Bob Paine and Carlos Szembek and Carlos Szembek Steve Hanna, Steve Hanna, subcontractor subcontractor (photo by James Shuepp, provided by Larry Mahrt) Study funded by API and UARG September, 2010
Outline of Presentation • Why are low wind speeds a concern? • Review of current AERMOD formulation • Evaluation study approach y pp • Meteorological evaluation results • Tracer study evaluation results T t d l ti lt 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 2
Concerns Noted by EPA and the Modeling Community • Brode USEPA 2007 Modeling Workshop from AERMOD Implementation Workgroup Highlights – “Mandatory Work: Light winds. Revise AERMOD’s “Mandatory Work: Light winds Revise AERMOD’s treatment of light winds to avoid unrealistically high concentrations” • Reported at USEPA’s 9 th Modeling Conference - Air & Waste Management Association Comments – Many investigators report that the worst-case AERMOD Many investigators report that the worst case AERMOD impacts occur for very low wind speeds at night, especially for low-level sources – AERMOD has limited evaluation for these conditions AERMOD has limited evaluation for these conditions – AERMOD needs supplemental evaluation to assess the accuracy of the model for these conditions y 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 3
Current AERMET/AERMOD Approach • AERMET computes the friction velocity (u * ), which is an important parameter for nocturnal hour estimates of an important parameter for nocturnal hour estimates of mixing height, sigma-z, and sigma-y • AERMOD approach in low winds is reasonably simple, AERMOD approach in low winds is reasonably simple, and involves a combined solution of a coherent plume (traditional Gaussian shape) and a random (pancake) plume plume • Weighting of the two solutions depends upon wind speed and turbulence provided to AERMOD speed and turbulence provided to AERMOD 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 4
Approach for this Study • We initiated a new evaluation study to understand AERMOD’s performance under low wind speeds • The evaluation study featured existing research-grade meteorological and low wind speed tracer databases • Current and alternate versions of AERMET/AERMOD were tested in this study • Collaboration with USEPA and AERMIC review was • Collaboration with USEPA and AERMIC review was important for this study • At the present time, AERMIC has provided limited p , p review and EPA attention has been diverted to the new NAAQS modeling issues 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 5
Meteorological Evaluation Study • Requested by EPA; evaluation focused upon u * • • Acceptable databases were selected for low wind Acceptable databases were selected for low wind speeds and sonic anemometer to get u * • • Evaluation focused upon nocturnal low wind Evaluation focused upon nocturnal, low wind conditions Cardington (flat, grassy site in the UK) was included Cardington (flat grassy site in the UK) was included • • in the evaluation • Other met databases (USA) were: Other met databases (USA) were: 1. Bull Run (mixed land use/terrain Tennessee site) 2. FLOSS II (Fluxes Over Snow Surfaces, Phase 2: flat open site in northern Colorado) in northern Colorado) 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 6
Initial Meteorological Evaluation Results • Single-level friction velocity predictions by AERMET were found to be too low for low wind nocturnal hours were found to be too low for low wind, nocturnal hours • An adjustment to the formulation was suggested by the data, and appeared to greatly improve the AERMET the data, and appeared to greatly improve the AERMET single-layer performance • This adjusted formulation was tested all three met This adjusted formulation was tested all three met databases 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 7
u* (y axis) vs. u for Bull Run Nocturnal Stable Hours 0.4 AERMET formulation is AERMET formulation is 0.35 0.35 underpredicting friction velocity underpredicting friction velocity 0.3 U * 0.25 Curves adapted from Curves adapted from Curves adapted from Curves adapted from u* (m s -1 ) AERMET formulation AERMET formulation 0.2 Observed values Observed values 0.15 <-- --Transition point where single layer Transition point where single layer p p g g y y i i in light blue in light blue li ht bl li ht bl quadratic equation has real solution quadratic equation has real solution 0.1 (transition point is arbitrary) (transition point is arbitrary) 1- -layer AERMET in beige layer AERMET in beige 0.05 z 0 = 0.51 0 0 0.5 1 1.5 2 2.5 3 3.5 4 U u (m s -1 ) L.I. Residential, Clear L.I. Residential, 50% Cloud Cover L.I. Residential; 100% Cloud Cover Single-Layer Model Observed 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 8
Recommended Improvement to Single-Layer Method Agreement with Agreement with New AERMET transition point New AERMET transition point obs is excellent obs is excellent for cloudy skies. This avoids for cloudy skies. This avoids the “dip” by connecting the th the “dip” by connecting the th “di ” b “di ” b ti ti th th origin to 1.25 times the origin to 1.25 times the U * transition wind speed � transition wind speed Avoid “dip” by connecting Avoid “dip” by connecting Avoid dip by connecting Avoid dip by connecting Agreement with observations Agreement with observations Agreement with observations Agreement with observations origin to 1.25 times origin to 1.25 times is much better! is much better! transition wind speed transition wind speed � current AERMET transition point for current AERMET transition point for p cloudy skies cloudy skies U 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 9
Comparisons for u* with Cardington data (low wind speed, stable hours) Modified AERMET Modified AERMET Current AERMET Current AERMET Single layer Si Single layer Si l l l l 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 10
Conclusions from Met Evaluation • Current AERMET formulation will likely underpredict u* in low wind speed, stable conditions • This would be expected to result in higher predicted concentrations (lower dilution speed and dispersion rate) t ) • This happens for both the single-layer and 2-layer (Bulk Ri) methods Ri) th d • Met model performance with the suggested improvements is better overall improvements is better overall • These changes were carried forward into the tracer evaluation phase of the study evaluation phase of the study 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 11
Tracer Database Evaluation Study focused on 3 databases: • 1. Bull Run, TN (tall stack, buoyant plume) 2. Idaho Falls, ID (low-level releases) 3. Oak Ridge, TN (low-level releases) 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 12
Tracer Model Evaluation Procedures • Model the plume trajectory toward the sampler with the max observation – avoid any misrepresentation of model performance do to error in wind directions • Compare arc-wise maximum predicted and observed concentrations • Model performance evaluated with a Quantile-Quantile plot – For each arc of samples/receptor the concentrations from all the modeled hour are sorted from largest to smallest independant of one another. i d d t f th • Good performing model +/- factor of 2 as compared to observed concentrations • Conservative results are better for regualtory model trying to protect public health and welfare. 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 13
Candidate Models • Candidate models based on changes to AERMET/AERMOD AERMET/AERMOD • Results presented for 3 cases: 1.Base AERMET 2.Modified u* formulation in AERMET 3.AERMET/AERMOD with minimum sigma-v = 0.4 m/s – Current minimum sigma-v = 0.2 m/s 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 14
Why Adjustments to Minimum Sigma-v? • After running AERMOD with current AERMET, we constructed Excel spreadsheet to replicate AERMOD predictions during stable hours (Oak Ridge and Idaho Falls) w/ model debug stable hours (Oak Ridge and Idaho Falls) w/ model debug output • Found sigma-v becomes very important under low-wind speed conditions when sigma-theta data is not available because it helps define: – lateral dispersion (sigma-y) – fraction of the random plume used to calculate total concentration • AERMOD is underestimating the lateral dispersion and fraction of the random plume f th d l • This was causing the model to overpredict significantly for VERY light winds (less than 0 5 m/s) VERY light winds (less than 0.5 m/s) 15 th IUAPPA World Clean Air Congress - Vancouver, BC September 12-16, 2010 Page 15
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