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Introduction
European winter windstorms are a major cause of losses to the insurance sector. To help the sector better understand this risk the Operational Windstorm Service for the Insurance Sector has been developed. The development of the service follows from a Proof of Concept for a Sectorial Information Service to provide information about European windstorms, which is the Windstorm Information Service (WISC), part of the Copernicus Climate Change Service (C3S). This document describes the Tier 1 indicators and how they are produced by the University of Reading. The Tier 1 indicators provide summary statistics for wind storm activity per year and per European country.
Executive Summary
The summary Tier 1 indicators, available from the Climate Data Store (CDS), are derived from ERA5 reanalysis data for the period 1979 to 2021 using the following methodology. Extratropical cyclones are first identified and tracked in the ERA5 reanalysis data for the October to March periods of 1979- 2021 using an automated cyclone tracking algorithm based on the method described by Roberts et al, (2014) and described in the Product User Guide for the Operational Storm Tracks. The Tier 1 indicators use the cyclone tracks and ERA5 maximum land 10m winds to produce summary statistics of the Storm Severity Index (SSI) by country.
Scope of Documentation
This document describes the C3S Tier 1 dataset using the standard C3S format for product descriptions, i.e., in terms of product target requirements, product overview, input data and methodology as well as use of the dataset. It is the primary document for users.
Version History
Version 1.0. The underlying methodology used to derive the Tier 1 indicators has been consistent within this project and the previous project, WISC. The earlier WISC Tier 1 indicators were derived from ERA-Interim and ERA-20C since these were produced before ERA5 became available.
Product Description
Product Target Requirements
The storm tracks were sourced from the ERA5 reanalysis with storms identified using an automated cyclone-tracking algorithm applied to the 850hPa relative vorticity, based on (Hodges, 1995, 1999), i.e., the same approach as used in the eXtreme WindStorms (XWS; Roberts et al, 2014) and C3S Windstorm Information Service Copernicus (WISC) activity. This used the 3-hourly analyses from ERA5 as opposed to the mixture of analyses and forecasts from ERA-Interim to allow 3 hourly data as used in XWS and WISC. The vorticity threshold used for the extended winter storm tracks was 1x10-5s-1 and tracks were required to exist for at least 1 day and to have travelled at least 1,000km. This provided tracks of the 3-hourly locations of the following six variables, the last 5 at full resolution, for each storm:
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Where A is the area over land in km2 and 𝑢10𝑚 is 10m wind speed from the reanalysis data. This is equivalent to the SSI used in Dawkins et al., (2016).
Product Overview
Tier 1 indicators are computed per Oct-Mar season and then converted to decadal averages for the final product.
Data Description
Table 1: Overview of key characteristics of the Tier 1 indicator dataset.Anchor table1 table1
Data Description | |
Dataset title | Tier 1 Indicators |
Data type | Summary statistics of European windstorms. |
Topic category | Natural risk zones, Atmospheric conditions |
Sector | Insurance |
Keyword | Number of cyclones, intensities. |
Dataset language | English |
Domain | Europe defined as follows:
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Horizontal resolution | Statistics derived from ERA5, ~30 km. |
Temporal coverage | 1979-10-01/to/2021-03-31 |
Temporal resolution | Annual, Decadal |
Vertical coverage | 10-m near surface |
Update frequency | No updates expected |
Version | 1.1 |
Model | ERA5 |
Experiment | N/A |
Provider | University of Reading (UREAD) |
Terms of Usehttps://cds.climate.copernicus.eu/api/v2/terms/static/licence-to-use-copernicus-products.pdf | Licence |
Variable Description
The Tier 1 indicators, which are provided in Excel format, include the following fields:
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Variables | |||
Long Name | Short Name | Unit | Description |
Number of windstorms (vorticity) | No. storms | Number | Based on the storm tracks. A storm is said to pass through a region if a storm's track vorticity center passes through the region. These are provided per year and per decade. |
Number of windstorms (wind) | No. storms | Number | A storm is said to pass through the region if the land only 10m wind speed maximum (within 3° of the vorticity center) passes through the region. These are provided per year and per decade. |
Mean maximum 10m wind speed (u10m land only) (>0.0 m s-1) | Mean Wind | m s-1 | Mean maximum wind speed at 10m over land calculated for each study region directly from the reanalysis data. The threshold used here was 0.0 m s-1. These are provided per year. |
Number of extreme windstorms (u10m land only) (>15.6 m s-1) | No. storms (land > 15.6 m s-1) | Number | A storm is said to pass through the region if the land only 10m wind speed maximum (within 3° of the vorticity center) is greater than 15.6 m s-1 and passes through the region. This is the same threshold used for ERA-Interim in WISC. These are provided per year and per decade. |
Mean maximum wind speed (u10m land only) (>15.6 m s-1) | Mean Wind (land > 15.6 m s-1) | m s-1 | Calculated for each study region directly from the reanalysis data. The threshold used here was 15.6 m s-1. Maximum averaged over all storms. These are provided per year and per decade. |
Number of extreme windstorms, (u10m land only) (>20.0 m s-1) | No. storms (land > 20.0 m s-1) | Number | A storm is said to pass through the region if the land only 10m wind speed maximum (within 3° of the vorticity center) is greater than 20.0 m s-1 and passes through the region. These are provided per year and per decade. |
Mean maximum wind speed (u10m land only) (>20.0 m s-1) | Mean Wind (land > 20.0 m s-1) | m s-1 | Calculated for each study region directly from the reanalysis data. The threshold used here was 20.0 m s-1. Maximum averaged over all storms. These are provided per year and per decade. |
Number of extreme windstorms (u10m land only) (>25.0 m s-1) | No. storms (land > 25.0 m s-1) | Number | A storm is said to pass through the region if the land only 10m wind speed maximum (within 3° of the vorticity center) is greater than 25.0 m s-1 and passes through the region. These are provided per year and per decade. |
Mean maximum wind speed (u10m land only) (>25.0 m s-1) | Mean Wind (land > 25.0 m s-1) | m s-1 | Calculated for each study region directly from the reanalysis data. The threshold used here was 25.0 m s-1. Maximum averaged over all storms. These are provided per year and per decade. |
Mean severity of storms, per year and per decade, 20.0 m s-1 threshold. | SSI | None (Unitless SSI) | The severity of each storm was assessed using a Storm Severity Index (SSI). This metric was also calculated from the land masked wind speed of the re-analysis data. Wind threshold > 20.0 m s-1. These are provided per year and per decade. |
Mean severity of storms, per year and per decade, 25.0 m s-1 threshold. | SSI | None (Unitless SSI) | The severity of each storm was assessed using a Storm Severity Index (SSI). This metric was also calculated from the land masked wind speed of the re-analysis data. Wind threshold > 25.0 m s-1. These are provided per year and per decade. |
Input Data
The input data to the cyclone track and Tier 1 production is summarised in Table 3 and described below.
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Input Data | ||||
Model name | Model centre | Scenario | Period | Resolution |
ERA5 | ECMWF | Reanalysis | 1979-2020 | ~30km |
Input Data 1
ERA5 Reanalysis
Data from the ECMWF ERA5 reanalysis (Hersbach et al, 2020) extracted directly from the mars archive for the period 1979-2020 at 3 hourly intervals for the seasonal period of October to March when the majority of European windstorms occur. The 850hPa relative vorticity is spectrally filtered prior to applying the tracking algorithm. Full resolution MSLP, 925hPa winds and 10m winds are also added to the cyclone tracks.
Method
Background
The same tracking methodology has been applied to the ECMWF ERA5 data set as was used with ERA- Interim (Dee et al, 2011) in the Windstorm Information Service Copernicus (WISC1) and eXtreme Wind Storms' (XWS2) catalogue (Roberts et al, 2014) projects. However, since ERA5 has 3-hourly analyses there is no need to use the forecast splicing to get 3-hourly data as was used for WISC and XWS. The ERA5 3 hourly analysis fields are used to be consistent with WISC and XWS. The Tier 1 indicators make use of the cyclone tracks and ERA5 10m wind speeds.
Model / Algorithm
The cyclone tracking and cyclone track data is described in the Operational Storm Track product user guide.
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Where A is the area over land in km2 and 𝑢10𝑚 is 10m wind speed calculated from the re-analysis data. This is equivalent to a definition of SSI used in Dawkins et al., (2016), with the only difference being the threshold of 10m wind speed used. This metric is also calculated from the land masked, and the ERA5 masked wind speeds.
Validation
As with the cyclone tracks there is effectively no "truth" to contrast with, so the comparison is made with the Tier1 data produced for WISC using ERA-Interim to gauge the differences between the two data sets. The same procedures as used in WISC have been used to compute the Tier 1 indicators for ERA5 based on the 10m wind intensity over land. The indicators include values for the number of storms passing through a European country, the number of extreme storms defined by three threshold values (15.6 m s-1 as used in WISC and 20 and 25 m s-1), the mean wind intensity for the extreme storms and the Storm Severity Indexes (SSI). Some significant differences have been found between the ERA5 and ERA-Interim Tier 1 data which is the result of the differences in the location of the wind maxima, the intensity of the 10m winds and possibly the use of different land-sea masks, the latter two being related to the differences in resolution of the two reanalysis data sets.
Concluding Remarks
The operational storm tracks are derived from ERA5 reanalysis data for the period 1979 to 2021 using the following methodology. Extratropical cyclones are identified and tracked in the ERA5 reanalysis data for the October to March periods of 1979-2021 using an automated cyclone tracking algorithm based on the method described by Hodges (1995, 1999) and previously used for WISC and XWS (Roberts et al, 2014). The storm tracks were used as the basis for the statistical downscaling of storm footprints. The Tier 1 indicators are computed from the cyclone tracks and the ERA5 10m wind speeds and provide a set of summary statistics presented as decadal averages.
References
Dawkins L. C., Stephenson, D. B., Lockwood, J. F. and Maisey P. E. (2016) The 21st century decline in damaging European windstorms, Natural Hazards and Earth System Science, 16, 1999-2007, doi:10.5194/nhess-16-1999-2016.
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This document has been produced in the context of the Copernicus Climate Change Service (C3S). The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |
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