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The New Zealand Reanalysis (NZRA) is the first high-resolution convection-permitting atmospheric regional reanalysis model over Aotearoa New Zealand (NZ). NZRA has a spatial horizontal grid spacing of 1.5km and is forced using data from the Bureau of Meteorology Atmospheric high-resolution Regional Reanalysis for Australia (BARRA-R) with approximately 12km horizontal grid spacing. This paper outlines the development of NZRA, including the numerical weather forecast model and driving model. In addition, the performance of NZRA is compared against BARRA-R and global reanalysis products (ERAInterim and ERA5) and validated against observational data collected during June-July 2014 as part of the Deep Propagating Gravity Wave Experiment (DEEPWAVE) field campaign. The results demonstrate that dynamically downscaling BARRA-R to NZRA considerably enhances the wind and temperature predictions, particularly capturing well the diurnal temperature cycle, wind speeds above 95th percentile, and gust wind speeds over the high elevation regions of NZ. In addition, NZRA provides better estimates of total precipitation over the North Island, Canterbury region, West Coast and the southern part of the South Island compared to the Virtual Climate Station Network (VCSN) gridded observation-based product, other reanalyses and NIWA's operational convective-scale forecast model, NZCSM, as was run in 2014. [ABSTRACT FROM AUTHOR]

Featured Application: Coupling of very high-resolution NWP and CFD models, as outlined here, could be applied to the study of bias correction in NWP models, natural hazard studies, risk assessment under different extreme weather conditions, and urban and environmental design studies. In this study, the resilience of large cities and their built infrastructure in New Zealand to extreme winds, is investigated by coupling the outputs of a very high-resolution, 333-m resolution, numerical weather prediction (NWP) model with computational fluid dynamics (CFD) simulations. Following an extreme wind event on 18 September 2020 in Auckland, in which two trucks travelling over the Auckland Harbour bridge tipped over and damaged the bridge structure, a CFD simulation of airflow over the bridge using the Reynolds-averaged Navier–Stokes (RANS) method and NWP wind speed forecasts as the inlet profile is conducted. The 333 m NWP forecasts were validated against four nearby observation sites, showing generally high correlations of greater than 0.8 and low mean bias ( ± 3 m s−1) and RMSE ( < 3 m s−1) values. The CFD-based estimates of the mean wind speed-up over the bridge showed that the mean wind speed could increase by a factor of 1.15–1.20 in the vicinity of the road where the toppled vehicles were travelling. Additionally, NWP forecasts and CFD estimates of wind gusts at the maximum bridge height, within the area not affected by the bridge structure, agreed well with a value of about 25 m s−1. An anemometer mounted at the top of the bridge arch measured a higher gust wind speed of about 35 m s−1 that could have been influenced by the gust speed-up resulting from the flow separation from the bridge arch, which is demonstrated in the CFD results. The results demonstrate the importance of understanding localised wind speed-up effects and distinguishing them from the approaching undisturbed airflow. [ABSTRACT FROM AUTHOR]

This paper describes how a combination of high-resolution numerical modeling, a robust homogenization algorithm, and local pressure observations have been used to understand and reconcile time series of daily, seasonal, and annual peak wind gusts recorded at observing sites in the Cook Strait region of New Zealand. The homogenization algorithm consists of corrections for the relocation of masts, changes in instrumentations, data acquisition and signal processing, surrounding surface roughness, and measurement heights. In addition, a statistical method, penalized maximal F test (PMFT), was used to assess the homogeneity of the wind speed time series and detect and eliminate all remaining, undocumented, artificial (i.e., nonclimatic) breakpoints. A three-dimensional time-dependent computational fluid dynamics (CFD) simulation was carried out using the Gerris model to characterize the turbulence environment at the mast sites and to estimate topographic speedup effects. Trends in magnitudes and frequencies of the homogenized seasonal and annual peak wind gusts are evaluated and presented for the Cook Strait region. The pressure gradients between pairs of stations were used to study the correlation between the gust wind speeds and the pressure field. A high-resolution convection-resolving numerical weather prediction model [the New Zealand Convective-Scale Model (NZCSM)] was employed to aid the interpretation of results and analyze wind trends. The trend in gust speeds is also shown to be consistent with larger-scale NCEP–NCAR reanalysis pressure trends. The homogenization algorithm showed promising results in eliminating the artificial breakpoints and trends. Overall, strong correlations were found between the homogenized gust speeds, the pressure field across the region, and NZCSM predictions. [ABSTRACT FROM AUTHOR]