Post-Processing of Numerical Ozone Model Forecasts: The Land-Sea Problem Bill Ryan Department of Meteorology The Pennsylvania State University 2010 International Workshop on Air Quality Forecasting Research Quebec City, Quebec

The Problem: False Alarms of High O 3 Operational air quality forecasters along the I-95 Corridor in the northeastern US are increasingly reliant upon photo- chemical models for O 3 forecast guidance. – Significant recent changes in regional emissions have reduced skill of guidance methods that require training period. Overall model performance in this region is good but suffers from frequent “false alarms” of unhealthy O 3 levels. The false alarm rate is primarily a function of strong forecast O 3 gradients along land-sea boundaries. Numerical guidance output (1200 UTC initialization) arrives close (within minutes in some cases) to forecast deadlines. Forecasters require a simple, quick method to deal with this issue.

Example of the Land-Sea O 3 Gradient NOAA-EPA Numerical Ozone Forecast Model (

Pattern Recurs in Certain Locations

Not an Issue with PM 2.5 Forecasts Daily maximum 8-Hour Average Ozone (ppbv), NOAA-EPA Model 24-hour (midnight-midnight) average PM 2.5 (µg/m 3 )

Complicating the Issue: Sea Breezes Can Drive Steep Coastal O 3 Gradients In this case (August 11, 2010) sea breeze circulations developed north of a frontal boundary and re-circulated the previous day’s polluted air mass. MODIS/Aqua ~ 1730 UTC

Simplest Application: Philadelphia Forecast Area The Philadelphia Metro Forecast Area, roughly enclosed in rectangle, is peripherally affected by modeled sea breeze-O 3 effects, primarily in southern NJ

How Do US Operational Forecasters Receive Local Scale Model Forecast Information? Model forecast output is automatically generated by NOAA-EPA by extracting peak O 3 concentrations over land areas within designated warning areas (using zipcodes) that are used for /Web notification. This is posted in timely fashion at AirNow Tech ( Millville, NJ monitor location shown at left. Interstates and zip code boundaries shown. Millville

Forecast Peak O 3 Frequently Skirts Coastal Areas Simplest post-processing method to remove land-sea effects is to extract O 3 forecast at locations near monitors and/or away from the boundary. This can be quickly done using point data extraction from the NOAA forecast web site.

Results of Simple Method (2010) Removing coastal zip codes from the Millville forecast reduces bias at that location from 17 ppbv to 9 ppbv. Because this location is often the location of peak modeled O 3 in the Philadelphia metro area, Code Orange model forecast frequency in PHL is reduced from 44 to 20 days (Threshold for Code Orange - Air Quality Alert is 76 ppbv 8-hour average). Of the 20 forecast Code Orange cases, 16 observed Code Orange in PHL in 2010.

Effect on Metropolitan Scale Forecasts (2010) When this simple post-processing is applied to the PHL metro domain, numerical model skill equals, or exceeds, expert forecast skill and is a large improvement on earlier forecast methods (not shown).

Simple Approach Less Useful When Monitors are Located Immediately Along Sea-Land Boundary Southern Delaware forecast area includes monitor along coast at Lewes.

Bias Correction Post-Processing Use of Sonoma Tech Air Quality MOS (AQMOS) – Not a true MOS, method is long term bias removal with additional correction in higher O 3 cases (hierarchal bias correction method). Applied to southern Delaware in 2010, reduces overall forecast bias by 4.6 ppbv. Also, reduces number of false alarms of Code Orange O 3 (76 ppbv) from 19 to 5. At cost of 4 additional missed Code Orange cases although “close” forecasts (71,74,75 ppbv).

More Difficult Application: Baltimore Metropolitan Area In 2010, the monitor at Edgewood, located northeast of Baltimore, reached the Code Orange range on 75% of all Code Orange days in the Baltimore region. On half of the Code Orange days, the three bayside monitors were the only locations reaching the alert threshold.

An Example of the Bay Breeze Effect on O 3 in the Baltimore Metro Area 2200 UTC hourly O 3 observations and wind barbs, June 25, 2009.

Note on Determining Air Quality Forecast Skill in the mid-Atlantic False alarms can be due to factors other than water-land effects. A frequent problem is forecasting the timing and extent of convection. At right, a late day thunderstorm on June 3 limited O 3 to 74 ppbv. A set of 7 convection cases excluded in further analysis of 2010 results.

Model Forecast O 3 in Baltimore The problem is solving the “false alarm” issue in non-convective cases. Using raw model output, 26 false alarm cases in Using the simple method of excluding near-water zipcode locations reduces false alarms to 19.

Further Post-Processing Approaches Due to the close proximity of emissions sources to the sea-land boundary, improvement by removing bay side grid points (simple method) is useful but limited. Skill decreases quickly if further exclude near-bayside locations. Analysis of 2010 Cases (a very active O 3 season) gives some hints: – On average, observed Code Orange cases feature a stronger modeled gradient from bay side to inland, but too much variability to use operationally (CART method). – Most useful method combines the magnitude of modeled bayside O 3 and the mean domain modeled O 3.

Forecast Results for Code Orange Threshold (Air Quality Alert, 8-hour O 3 ≥ 76 ppbv, 2010) Blue: Model Forecast (NAQC) Red: Post-Processed Model (POST) Green: Persistence (PERS)

Summary of Baltimore Results Pro: Removal of bias and reduction of false alarm rate. Hit rate also reduced but, at 75%, approximately equal to, or better than, historical expert forecast accuracy. Small (~5%) improvement in skill scores. Half of the misses and false alarms occur in range [74, 78] ppbv (close calls).

Application to Connecticut Ozone The majority of high O 3 events occur at coastal monitors (62%). Only 5 of 24 observed Code Orange days did not have a coastal monitor above the threshold (76 ppbv, 8-hour average).

Background on Model Performance in CT The NOAA-EPA model, on a statewide domain basis, did a reasonable job in Over- prediction bias of 5 ppbv (~10%). Bias correction, on the seasonal or synoptic scale, slightly improves forecasts but has little impact at the Code Orange (Air Quality Alert) threshold. Hit rate of 84% but high false alarm rate (34%). High bias for warning forecasts. Expert forecast, typical of most locations along I- 95, has fewer false alarms but many more “misses”. Low bias for warning forecasts.

Behavior of Model Forecasts in Long Island Sound Similar to Chesapeake Bay/Baltimore Used NOAA-EPA model forecasts for locations of all CT monitors. As in Baltimore, exploratory analysis with CART suggests domain wide average peak O 3 ([65-70 ppbv])and/or land-sea gradient ([15-20 ppbv]) are useful thresholds for analysis.

Results of Post Processed Forecasts (Statewide) Model forecasts (NAQC) have a high hit and false alarm rate (high bias) while the expert forecast (Forecast) is the reverse. The two post-processed forecasts Mean and Mean plus Gradient straddle the bias threshold but provide better skill scores (Threat Score, or CSI shown above in green.

Summary and Conclusions In populated NE US Corridor, numerical model O 3 forecast guidance is useful but has high false alarm rate due to modeled steep O 3 gradients along land-sea boundaries. A variety of simple post-processing methods, available within operational time constraints, can limit false alarm frequency. Efficacy of methods vary from location to location and include: – Removal of coastal locations – Bias correction (of various lengths) – Domain mean peak concentrations and/or strength of gradient

Acknowledgements This research is supported by ongoing grants from the Delaware Valley Regional Planning Commission (including the State of Pennsylvania Department of Environmental Protection) and the State of Delaware Department of Natural Resources and Environmental Control. Assistance for this presentation also provided by Sonoma Technology (Dianne Miller and Jessica Johnson) and Michael Geigert of the Connecticut Department of Environmental Protection.

Additional Slides

Preliminary Comments This presentation is from the operational forecaster’s perspective. Air quality forecasting in the US is a state/local government responsibility. The air quality forecasting community in the US is therefore non-centralized and diverse in terms of expertise, experience and resources.

Sea Breeze Fronts Can Lead to High O 3 NOAA-EPA Model O 3 forecast for August 11, 2010 (1200 UTC run) HPC Surface Analysis, 1800 UTC, August 11, 2010

Hi-Res NAM Forecast: Sea Breeze Front Forming Behind Frontal Boundary

Sea Breeze is Observed Sea Breeze Verified (left) 3-hour temperature change and winds, 1900 UTC, August 11 Figure Courtesy: SPC Mesoscale Analysis

Example of Mean Domain Model O 3 and Observations Scatter plot of observed peak O 3 and mean model Air Quality Index (AQI) for in CT.

CT Domain Wide Code Orange Threshold Forecasts: Model and Expert

Connecticut Miss and False Alarm Cases Missed cases (4) were marginal Code Orange cases (76-78 ppbv) with only one hour exceedance in each case. Three of seven False Alarms were “close calls” (≥72 ppbv) and remaining four were False Alarms under any of the post-processing methods. False Alarms: 6/20 (lingering clouds), 7/20 (clouds/frontal boundary), 8/1 (SE winds), 8/31(?).