Forecasting Peak Wind Gusts Using Meteorologically Stratified Gust
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JUNE 2020 K A H L 1129 Forecasting Peak Wind Gusts Using Meteorologically Stratified Gust Factors and MOS Guidance JONATHAN D. W. KAHL Atmospheric Science Group, Department of Mathematical Sciences, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin (Manuscript received 16 March 2020, in final form 16 April 2020) ABSTRACT Gust prediction is an important element of weather forecasting services, yet reliable methods remain elusive. Peak wind gusts estimated by the meteorologically stratified gust factor (MSGF) model were eval- uated at 15 locations across the United States during 2010–17. This model couples gust factors, site-specific climatological measures of ‘‘gustiness,’’ with wind speed and direction forecast guidance. The model was assessed using two forms of model output statistics (MOS) guidance at forecast projections ranging from 1 to 72 h. At 11 of 15 sites the MSGF model showed skill (improvement over climatology) in predicting peak gusts out to projections of 72 h. This has important implications for operational wind forecasting because the method can be utilized at any location for which the meteorologically stratified gust factors have been de- termined. During particularly windy conditions the MSGF model exhibited skill in predicting peak gusts at forecast projections ranging from 6 to 72 h at roughly half of the sites analyzed. Site characteristics and local wind climatologies were shown to exert impacts on gust factor model performance. The MSGF method represents a viable option for the operational prediction of peak wind gusts, although model performance will be sensitive to the quality of the necessary wind speed and direction forecasts. 1. Introduction (Adame et al. 2018). Gusts associated with nonconvective high winds are of special concern—they have the po- Wind gusts, short-lived extremes within the spectrum tential to cause more injuries and fatalities than thun- of wind variation, occur when high momentum air is derstorm or hurricane winds, especially since people are brought to the surface. They accompany both convec- less likely to take shelter in a nonconvective high wind tive and nonconvective phenomena, and are sensitive to scenario as compared to a tornadic situation (Ashley various physical factors including wind speed, boundary and Black 2008). layer turbulence, surface roughness, stability, and to- Reliable gust forecasts can reduce the risk of human pographic flows (Letson et al. 2018; Harris and Kahl danger, cost drawbacks, and structural damage. For 2017). Wind gusts are often associated with serious wind energy, reliable forecasts can prevent cut-out events hazards and structural damage (Sheridan 2018) and thus in which wind turbines abruptly stop, causing fatigue in are an important consideration for a wide range of ap- turbine components, transitory changes in power trans- plications, activities and research areas. Principal among mission, and losses in energy yield (Gutiérrez and Fovell these are structural design for wind loads and water- 2018; Suomi and Vihma 2018; Jung et al. 2017). tightness (Van Den Bossche et al. 2013), wind energy Gust prediction is an important element of weather generation (Sinden 2007), insurance, timber, and avia- forecasting services. Despite its importance, however, tion industries (Klawa and Ulbrich 2003; Usbeck et al. reliable gust prediction remains elusive. This is primarily 2009; Manasseh and Middleton 1999), and fire danger because (i) energy-containing turbulent eddies in which gusts are imbedded are usually far too small for ob- Supplemental information related to this paper is available at serving networks and mesoscale numerical models to the Journals Online website: https://doi.org/10.1175/WAF-D-20- resolve, creating the need for parameterizations and 0045.s1. downscaling of model output (Fovell and Cao 2014); and (ii) observational wind data contain reporting artifacts Corresponding author: Jonathan D. W. Kahl, [email protected] that hinder the development and verification of gust DOI: 10.1175/WAF-D-20-0045.1 Ó 2020 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/28/21 10:50 PM UTC 1130 WEATHER AND FORECASTING VOLUME 35 forecast models (Harris and Kahl 2017). Current gust (Fig. 1; Table 1), were utilized in the present study to forecasting techniques include physical approaches avoid the averaging and reporting artifacts present in the involving features of boundary layer turbulence, and considerably less voluminous hourly dataset (HK17). statistical models employing extreme value statis- Winds at the ASOS stations are measured by Vaisala, tics or underlying assumptions about wind distribu- Inc. model 425 sonic anemometers at an altitude of 10 m tion as a function of predictor variables (Sheridan AGL (www.weather.gov/asos). Wind speed and direc- 2011, 2018). tion reported in the ASOS database represent 2-min One statistical approach, the gust factor model, em- averages of 3-s averages, reported every minute, while ploys a combination of climatological measures of local the gust represents the maximum 3-s average occurring gustiness along with wind speed forecasts. With this each minute. Wind speed and gust are reported in integer 2 model, peak wind gusts are predicted by multiplying the values of knots (kt; 1 kt 5 0.514 m s 1) and rounded up to gust factor, the ratio of the peak wind gust to the average the nearest knot (Letson et al. 2018). wind speed, by a predicted wind speed. The gust factor is Since the raw 1-min wind and gust records did not based on observed wind and gusts and thus is a clima- undergo extensive quality control measures (EPA 2015), tological measure of gustiness. Harris and Kahl (2017), several additional quality checks were performed to hereafter HK17, showed that the gust factor model is a ensure the fidelity of the observational dataset. First, viable method of forecasting wind gusts when the gust all data files were first filtered for records that were factors are stratified by wind speed and wind direction. repeated, out of chronological order, or ‘‘garbled’’. HK17 demonstrated the potential of the meteorologi- Second, gross error checks were performed to exclude cally stratified gust factor (MSGF) model by utilizing a records with missing data fields, negative wind speeds, or ‘‘perfect prog’’ evaluation procedure in which wind wind directions outside of 08–3608. Finally, the quality forecast errors were ignored. Their evaluation can be control measures of Letson et al. (2018) were applied to made more practical by considering actual wind speed eliminate erroneously large winds and gusts. The re- forecasts, a crucial component of any gust forecasting sulting 1-min data were used to construct an hourly technique (Fovell and Cao 2014). dataset containing mean wind speed, mean wind direction, In this paper we extend the work of HK17 by evalu- peak wind gust, and gust factor (GF 5 peak gust/mean ating the MSGF model when coupled with wind fore- wind speed). A minimum of 54 (total) 1-min measure- casts provided by two model output statistics (MOS) ments were required to produce an hourly record. This products. The evaluation is performed over 2010–17 at process yielded between 60 000 and 67 000 hourly records 15 diverse locations across the United States. Specifically, at all but one site, representing 86%–96% of all possible we address the following research questions. Does the reporting hours (Table 1). The exception was Lander, MSGF model demonstrate skill in predicting peak Wyoming, with 58 536 hourly records (83.5% of possible gusts when coupled with MOS wind forecasts? How is reporting hours). model performance affected by MOS type, forecast b. Meteorologically stratified gust factors projection, and during strong wind events? Our anal- ysis offers evidence that the MSGF model represents a Average hourly gust factors for each site were strati- viable option for the operational prediction of peak fied by both wind speed (0–5, 5–10, 10–15, and .15 kt) wind gusts. and wind direction (308 bins). In the present work, ‘‘meteorologically stratified’’ refers to this stratification, which was shown by HK17 to give the best perfor- 2. Data and methods mance in gust factor models. An example in the form of a ‘‘gust web’’ diagram (Fig. 2) illustrates features of a. Wind and gust data the wind and gust climatology of the Providence, Rhode Wind and gust measurements over the period 2010–17 Island, site (KPVD). Gust factors at KPVD exhibit from the U.S. Automated Surface Observing System the typical decrease with increasing wind speed (e.g., (ASOS) were obtained for 15 stations from online archives Letson et al. 2018). At this site, stronger winds (mean provided by the National Centers for Environmental speeds . 10 kt) occur most frequently with wind direc- Information (dataset 6405, available at ftp.ncdc.noaa.gov/ tions in a broad sector from south-southeast (1508) pub/data/asos-onemin). Stations were selected at di- clockwise to north-northeast (308). Gust factors associ- verse locations across the United States to represent ated with winds . 15 kt are largest in the northwest different wind climatologies, altitudes, climate types, sector (GF 5 1.81) and smallest in the north-northeast and surface characteristics. Wind data with 1-min reso- sector (GF 5 1.58), with differences likely due to horizontal lution, roughly 4 000 000 observations at each station inhomogeneity in surface roughness (Pryor et al. 2014; Unauthenticated | Downloaded 09/28/21 10:50 PM UTC JUNE 2020 K A H L 1131 FIG. 1. Locations of ASOS sites at which the meteorologically stratified gust factor model was evaluated. Suomi et al. 2013). Meteorologically stratified gust speed and direction forecasts were obtained from factors for the other sites are provided in the online MOS guidance (Glahn and Lowry 1972), a widely supplemental material. used product with a demonstrated improvement upon raw forecast model output (e.g., Ma et al.