ASOS Background Observation based 1st order stations NWS/FAA measurements replaced by ASOS in mid-1990s NWS/FAA Use of METAR reporting system began July 1996 Anemometers Equipment: Observation-based – Belfort F-420 (2 kts start-up threshold Initial ASOS – Belfort 2000 digital (1.51 kts start-up threshold) Newer ASOS – Vaisala Sonic (~ 0 kts start-up threshold)

Problems with the Standard ASOS Wind Measurements 2-min avg. winds measured 10 min. before hour Reported wind speed value thru 2009 is truncated (ex. 3.8 knots ►3 knots) Winds below 3 kts reported as calms (METAR) Wind speeds between 2-6 kts with 60 deg. or more change in wind direction in two minute average reported as variable winds (METAR) As a result: No hourly averages, no low wind speeds, high incidence of missing data (calms or variable)

Advantages of 1-Minute ASOS Wind Measurement Data 2-min. average wind speed and direction are available every min. True 1-hour averages can be calculated. Does not ignore wind speeds lower than 3 kts, uses wind speeds down to anemometer threshold Reduced calm/variable winds hours, account for truncation Available since 2000 at 1st order NWS stations, since 2005 at other sites AERMOD was validated with low wind speeds similar to 1-min ASOS, lack of low wind speeds in std. ASOS may (will) result in under-prediction of impacts

Point Sources – Ratio of H1H (RURAL)

Point Sources - Ratio of H1H (URBAN)

Volume/Area Sources - Ratio of H1H (RURAL)

Current and Future Work Develop computer program to read 1-min. ASOS wind data and generate 1-hour averages, document in users guide (James Thurman) Release guidance/clarification memo on the use of the 1 min. ASOS data (OAQPS) Conduct study comparing 1 min. ASOS vs. Std. ASOS at 7 NWS stations This work is described in the ASOS/Met. Data Subgroup’s document: Proposal to Study the Use of One Minute (Two-Min. Avg.) ASOS Wind Speed and Direction Data in Permit Modeling