Extreme Winds in New Zealand: Homogenisation, Climatology and Design Wind Speed Estimates

Abstract

Improvement of the design wind load estimations is a critical feature of the optimisation of structural designs, particularly in a county like New Zealand that experiences strong winds and has mountainous and complex terrain, which influence the airflow passing above them. In addition, in any design project, safety considerations must be balanced against the additional cost of over-design. Therefore, accurate estimations of design wind speeds are highly beneficial for safety as well as for optimising the design and minimising the costs. This study aims to estimate design wind speeds and associated directional multipliers, also lee-zone multipliers for New Zealand through the analysis of historical wind data recorded at meteorological stations, utilising a high-resolution convection-resolving numerical weather prediction model (New Zealand Convective-Scale Model (NZCSM)), and lastly analysing two global reanalysis products. New Zealand’s historical wind data have not been analysed in the past two decades for design wind-load purposes. In addition, no attempt has been made to thoroughly homogenise the mean and gust wind speed data recorded prior to the 1990s and to convert them to equivalent Automatic Weather Stations (AWS) records. Furthermore, lee zones, areas affected by the wind speed-up due to the presence of mountains, can significantly influence the design wind loads, thus, it is crucial to estimate the spatial extent and magnitude of the lee multiplier accurately. In this study, the wind data were initially subjected to a robust homogenisation algorithm, which accounts for changes in both instrumentations and signal processing procedure, and also eliminates the effects of local topography. Extensive wind-tunnel and theoretical investigations were conducted to study the response characteristics of anemometers as well as the effects of various gust durations on the maximum wind speeds. Correction factors were proposed to convert the wind gust measurements with a certain gust duration to equivalent measurements of other gust durations of interest. The influence of hills and local topography on airflow was studied through wind-tunnel experiments and numerical simulations of 2D and 3D single isolated hills and also consecutive multiple hills. Then, the homogenised wind data were separated into synoptic and non-synoptic events to gain a better understanding of New Zealand’s gust climatology and sources of extreme events. It was demonstrated that synoptic events dominate the design wind speeds at most locations in New Zealand. For extreme value analysis, three different extreme value distributions were used, namely Type I (using Gumbel, Gringorten and BLUE fitting methods), Type III (using maximum likelihood and probability weighted moments methods), and Peaks-Over-Threshold (POT) approach. In addition, the predictions of NZCSM along with historical wind speeds were used to identify the lee zones, which confirms the existing zones and provides evidence to support introducing new zones, and obtain estimates of the lee-multipliers. Substantial changes have been proposed for the next version of the Australian/New Zealand wind-loading standard (AS/NZS 1170.2) based on the results of this study. The changes include adding a new wind region to New Zealand, refinements of wind zone boundaries, revising all regional wind speeds and directional multipliers, and modifying the lee-zone regions and multipliers. In addition, in order to assess the possible effects of climate change on the proposed design wind speeds, long-term trends in the magnitude and frequency of annual and seasonal maximum gust wind speeds were investigated. The results demonstrate that the annual and seasonal trends in both magnitudes and frequencies of extreme winds are generally either negative or not statistically significant over the considered period. Therefore, based on the derived gust trends, at this stage, it seems that the long-term gust wind speed trends are not likely to have a significant effect on New Zealand’s design wind speeds. Lastly, an attempt was made to increase and improve the spatial resolution of the New Zealand’s design wind map utilising gust wind speed data from two global reanalysis products, namely ERA5 and ERA-Interim with spatial resolutions of 31 km and 80 km, respectively. Initially, the reanalysis data were evaluated against observation data recorded at 52 meteorological stations across New Zealand. Then, extreme value analysis using three methods based on maximum annual maximum gust speeds, namely the standard Gumbel, Gringorten and BLUE, and one approach based on daily gusts over a threshold, namely the method of independent storms, were employed to estimate the design wind speeds. The performances of the reanalyses in representing gust wind climate over both relatively flat and also complex and mountainous regions were evaluated in detail. The validation results revealed the dependency of bias on the wind speed intensity, such that the positive and negative biases increase at low and high wind speeds, respectively. A simple method was proposed to correct the reanalysis gust data based on the computed biases and wind speed intensity. High-resolution wind maps, which also contain the lee-zones and wind speed-up effects due to the presence of mountains, are proposed for New Zealand. As opposed to the design wind speeds proposed for AS/NZS 1170.2:2021, which needed to be sufficiently conservative for most parts of the wind regions, the high-resolution wind maps show the variation of design wind speeds within each region. Overall, it was demonstrated that reanalyses can be used as a complementary method to observation data to estimate the design wind speeds with a higher-spatial resolution. Also, there are often gaps in historical time series, which can be filled in by the reanalysis data.