Extreme Wind Model
The Climate Risk Engines extreme wind model draw from two types of extreme wind; (1) synoptic wind and, (2) convective wind. Both are modelled under extreme wind, though they have different drivers for baseline hazard data and climate projection data. Tropical Cyclone wind damage is modelled and reported as a separate hazard.
Synoptic Wind
Overview
Synoptic winds are driven by pressure gradients between large-scale atmospheric pressure systems (on the order of hundreds of kilometres, e.g. low pressure systems) that are the result of temperature differences across the Earth. Air travelling at high speeds, even in short gusts, can directly damage buildings and infrastructure. High winds can also cause indirect disruption when trees drop limbs onto power lines or debris puts life and property at risk.
Baseline Hazard Data
Data on wind speeds is extracted from local weather stations in its native form. Such data may include short duration gusts, through to 10-minute sustained wind speeds. There are a range of conversion tables available to convert between these parameters.
Climate Change Projections
GCM and RCM models are executed at scales that capture changes in synoptic wind patterns. Climate projections are used to establish baseline wind gust speed for threshold return frequencies and then used to project how this speed changes over time.
Convective Wind
Overview
Convection, the processes that produce thunderstorms, poses numerous threats to individuals and property including damaging straight-line winds, large hail, heavy rainfall, lightning and tornadoes. Convective Available Potential Energy (CAPE) and wind shear (bulk wind difference) between the Surface and 6 km (S06) are considered the most influential environmental attributes for the formation and development of severe convection that is capable of producing damage. Severe convective winds is generally considered as wind gusts >= 25 m/s (90 km/hr). These wind gusts have the same impact as synoptic wind.
Local Context Data Use
At a given location, the fraction of severe convective environment and non-severe convective environments affect how often severe convective wind occurs, this is called Severe Convective Fraction.
Baseline Hazard Data
The convective wind model applies distributional regression to take a combination of CAPE and S06 (Convective Wind Metrics) and estimate the convective wind distribution (Wind Speed Distributional Regression). Convective Wind Metrics Convection is a means whereby the atmosphere resolves vertical instability. There are numerous measures of instability that are considered by storm scientists and forecasters, with CAPE being the most frequently used. CAPE alone is insufficient in determining the organisation of convection and its likelihood of producing severe weather. Wind shear is an additional environmental factor that influences the organisation of convection through numerous processes. There are two components of wind shear - speed shear (the difference in wind speeds between two layers of the atmosphere), and directional shear (the change in wind direction with height). In severe weather forecasting, S06 is commonly used to anticipate the organisation of convection. Specifically, there is a clear relationship between the scale parameter of a Weibull distribution and the sum of normalised CAPE and S06. Wind Speed Distributional Regression The future wind speed distribution is determined from relating CAPE+S06 to historical wind speed distributions.
Climate Change Projections
The input data to the convective wind model is calculated from CMIP6 data.
