Alex Gallagher and Dr. Robert Fovell Understanding Differences in New York State Mesonet (NYSM) and ASOS Wind Observations Alex Gallagher and Dr. Robert Fovell SUNY Albany DAES NROW XIX 11/8/2018

ASOS vs. NYSM Comparison Specs ASOS 1min NYSM Anemometer Type Sonic Mounting Height 10m Averaging Interval 2min 5min Sampling Interval 3s for gust, 5s for sustained 3s Number of Stations 899 CONUS, 36 NY 125 Typical Location Airports Varies

CONUS ASOS Locations: 800+ Stations

NY ASOS Locations: 36 Stations

NYSM Locations: 125 Stations

Big Weather Web (BWW) Ensemble CONUS+ at 20 km WRF v. 3.7.1 Initialized with 00Z GFS (or GEFS) 47 ensemble members (13 from UAlbany) Since January 2016 Control: MYJ PBL Noah LSM Kain-Fritsch cumulus RRTMG radiation Thompson microphysics Model evaluation with MET on 3-hourly outputs

ASOS Network-Averaged 10m Winds: BWW Control Run ASOS 1min-data, 821 stations 7 months of 84hr forecasts, February-August 2018 Avg Total Bias +0.27 m/s, 00Z Bias +0.07 m/s, 12Z Bias +0.52m/s

36 NYS ASOS Stations: 7 Month Avg ASOS NY, 36 stations Forecast bias grows gradually over time but comparable to CONUS.

124 NYSM Stations: 7 Month Avg NYSM winds weaker than ASOS winds for all forecast hours. NYSM Average Total Bias +1.49 m/s, 00Z Bias +1.49 m/s, 12Z Bias +1.47 m/s Forecasts are comparable.

Hypothesis 1: Siting/Obstruction NYSM stations are sited in locations where obstructions are more prominent, causing wind speeds to be systematically lower than at ASOS. These obstructions have a greater impact on mean wind speeds than gusts. Obstructions in NYS have a seasonal cycle to them and as such so will the degree to which wind speeds are slowed. Site obstruction is revealed by network averaged gust factors (GFs). 𝐺𝐹= 𝐺𝑢𝑠𝑡 𝑀𝑒𝑎𝑛 𝑊𝑖𝑛𝑑

Sustained Wind Speed Comparison July 2017 Sustained Wind Speed Comparison Gust Speed Comparison Slope=0.701 𝑅 2 =0.751 Slope=0.931 𝑅 2 =0.764 NYSM 30% Slower! NYSM sustained wind speeds are ≈30% slower than ASOS. NYSM and ASOS gusts are nearly one to one.

Sustained Wind Speed Comparison January 2018 Sustained Wind Speed Comparison Gust Speed Comparison Slope=0.788 𝑅 2 =0.797 Slope=1.034 𝑅 2 =0.823 NYSM 21% Slower! Obstructions are less effective in winter (no leaves) and wind speeds are ≈21% slower than ASOS. Gusts are still highly comparable.

Creating a Direct Comparison Specs ASOS 1min NYSM Anemometer Type Sonic Mounting Height 10m Averaging Interval 2min 5min Sampling Interval 3s for gust, 5s for sustained 3s Number of Stations 899 CONUS, 36 NY 125 Typical Location Airports Varies The difference in averaging interval can be addressed by creating observations in the style of ASOS-onemin using the raw 3sec samples from mesonet stations.

Sustained Wind Speed Comparison July 2017 (ASOS Style) Sustained Wind Speed Comparison Gust Speed Comparison Slope=0.704 𝑅 2 =0.855 Slope=0.836 𝑅 2 =0.871 Still 30% Slower! Sustained wind speeds are largely unaffected by modifying averaging interval. NYSM Gust speeds are slightly decreased.

Sustained Wind Speed Comparison January 2018 (ASOS Style) Sustained Wind Speed Comparison Gust Speed Comparison Slope=0.789 𝑅 2 =0.826 Slope=0.936 𝑅 2 =0.846 Still 21% Slower! Sustained Wind: decreasing number of samples has no implicit tendency to increase/decrease the mean. Gust: shrinking number of samples reduces the variance of a collection of samples and likelihood of a larger maximum.

Seasonal Comparison Aug-17 Oct-17 Dec-17 Feb-18 Apr-18 The NYSM wind speeds are more comparable to ASOS between Oct-Feb, when major obstructions (trees) are expected to be less effective. Although gust and mean wind ratios follow the same pattern, gust ratios remain closer to unity throughout.

Gust Factor Comparison Month ASOS NYSM (ASOS Style) NYSM July 2017 1.3 1.54 1.72 January 2018 1.29 1.53 1.69 Altering the averaging interval to match ASOS eliminates all significant differences between ASOS and NYSM except station siting. NYSM GFs are smaller and closer to that of ASOS after changing the averaging interval but remain significantly larger. This helps reaffirm that GF contains information about site exposure.

ASOS KBTV GF=1.27 200m 100m

ASOS COPA GF=1.72 200m 100m

Sustained Wind Speed Comparison July 2017 (Best NYSM) Sustained Wind Speed Comparison Gust Speed Comparison Slope=0.765 𝑅 2 =0.824 Slope=0.883 𝑅 2 =0.839 24% Slower Only the best exposed NYSM stations, according to WMO siting standards, are used for comparison. NYSM winds are slightly faster, sustained 30%24%, but significant differences remain.

Sustained Wind Speed Comparison January 2018 (Best NYSM) Sustained Wind Speed Comparison Gust Speed Comparison Slope=0.845 𝑅 2 =0.713 Slope=0.977 𝑅 2 =0.741 15% Slower NYSM sustained winds go from 21%15% slower than ASOS. Even the best exposed NYSM stations are systematically slower than ASOS.

Correcting for Exposure We can correct NYSM observations for higher exposure by leveraging network average GFs. Gusts are less effected by obstructions than mean wind. Gusts between the two networks are largely comparable, especially the unmodified NYSM observations. 𝑁𝑌𝑆𝑀 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝑁𝑌𝑆𝑀 𝑜𝑟𝑖𝑔 × 𝐺𝐹 𝑁𝑌𝑆𝑀 𝐺𝐹 𝐴𝑆𝑂𝑆 𝐺𝐹 𝐴𝑆𝑂𝑆 𝐺𝐹 𝑁𝑌𝑆𝑀 = 1.3 1.7 =0.765≈0.701−0.788

July 2017 (Adj. NYSM) Original Sustained Wind Comparison Adjusted Sustained Wind Comparison After correction NYSM are much closer to ASOS, 30%8% slower. Remaining differences are likely due to the difference in wind climatology or larger scale influences (topography).

January 2018 (Adj. NYSM) Original Sustained Wind Comparison Adjusted Sustained Wind Comparison The correction makes winds comparable across seasons. This correction has adjusted NYSM winds to what they would theoretically have been if they were as well exposed to winds as ASOS.

January 2018 (Adj. NYSM) The GF correction method does a decent job of raising NYSM observed wind speeds. Avg. bias +1.49 m/s+0.87 m/s Remaining difference between forecast and observations is likely due to model short coming and requires further investigation.

Concluding Remarks Forecasts shown to be skillful for ASOS are largely positively biased when verified against NYSM. Station siting is the most significant difference between the two networks. Network averaged sustained wind speeds are 25-30% slower in NYSM than ASOS, while gusts are mostly similar. Differences between the two networks remain substantial even after changing the NYSM averaging interval to match ASOS. Even the best sited/exposed NYSM stations are systematically under-reporting wind compared to ASOS. Using the ratio of the network average GFs NYSM mean wind observations can be corrected for exposure and more closely resemble those of ASOS.