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Introduction
Executive summary
European winter windstorms, or extra-tropical cyclones (ETC), are a major cause of losses to the insurance sector. To help the sector better understand this risk, the Enhanced Windstorm Service (EWS) for the insurance sector has been developed. The development of the service follows user feedback from previous phases (operational and proof of concept activities), implemented as part of the Copernicus Climate Change Service (C3S).
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Datasets will be updated through an automatised procedure running codes underlying data production over the periodic ERA5 dataset new releases.
Scope of the documentation
Windstorm data documentation describes the production, underlying methodological phases, and publication of the “enhanced operational windstorm service” (EWS) developed in the framework of the ECMWF-implemented C3S2_413 contract.
Dataset Description
Dataset Target Requirements
The present dataset aims to promote a knowledge-based assessment of the nature and temporal evolution of windstorms associated with ETCs. This contract presents a continuation, a temporal extension, and an enhancement of the original C3S Windstorm Service. The original service data content will be extended to the detection and tracking of Pan-European potentially harmful windstorms, associated with extratropical cyclones, for the whole available period provided by ECMWF's ERA5 reanalysis dataset (1940 - present) and to a second tracking algorithm, TempestExtremes (TE, Ullrich et al., 2021), in addition to the previously used TRACK / Hodges algorithm (Hodges 1995, 1999, Hoskins and Hodges 2022) hereafter called Hodges. This documentation traces all the steps made in the production of two main types of windstorm datasets: (i) Extratropical cyclones (ETC) tracks based on the use of two detecting/tracking algorithms and (ii) the associated footprints mapping the maximum value of 10m-height wind gust during a specific event.
Dataset Overview
Data description
Windstorm track. This product contains the Pan-European potentially harmful windstorm tracks, associated with extratropical cyclones, for the whole available period provided by ECMWF's ERA5 reanalysis dataset (Hersbach et al., 2020) spanning from 1940 to present. A storm track is defined as a sequence of longitude–latitude points which track an extra-tropical windstorm over time and is defined by the tracking algorithm. Two tracking algorithms are used: (i) Hodges algorithm (Hodges 1995, 1999, Hoskins and Hodges 2002), already in use in the previous Windstorm service and (ii) TempestExtremes (TE, Ullrich et al., 2021). The two tracking algorithms rely on an automated object identification leveraging two different variables, i.e., relative vorticity and mean sea level pressure, respectively (see "Methods" in section 3 for more details). Spatial and temporal filtering is applied to these variables to isolate key characteristics of low-pressure systems, identifying potential candidates suitable for track definition.
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Table 1: Overview of the dataset.
Data Description | |
Dataset title | Windstorm tracks and footprints derived from ERA5 reanalysis over Europe from 1940 to present |
Data type | Windstorm tracks: Vector (.csv) Windstorm footprints: Gridded Storm summary indicators: Vector (.csv) |
Projection | Regular latitude-longitude grid |
Horizontal coverage | Windstorm tracks: 80°W-35°E; 5°N-70°N Windstorm footprints: 25°W-35°E; 30°N-70°N Storm summary indicators: Europe (NUTS (Nomenclature of Territorial Units for Statistics) 0, 1 and 2) |
Horizontal resolution | Windstorm tracks: 0.25° x 0.25° Windstorm footprints: 0.25° x 0.25° & 0.016° x 0.016° (downscaled) Storm summary indicators: 0.25° x 0.25° |
Vertical coverage | Surface |
Vertical resolution | Single level |
Temporal coverage | 1940 to present |
Temporal resolution | Windstorm tracks: 6 hourly time steps for a single storm event Windstorm footprints: Single storm event Storm summary indicators: Yearly |
File format | NetCDF-4 & CSV |
Conventions | Climate and Forecast (CF) Metadata Convention v1.6, Attribute Convention for Dataset Discovery (ACDD) v1.3 |
Versions | 1.0 |
Update frequency | Monthly |
Provider | CMCC |
Terms of Use | |
Product description
Windstorm track
The windstorm tracks contain the 6-hourly evolution of a storm event; they are provided in text format (.csv) and include the fields listed in Table 2.
Table 2: Overview and definition of the fields included in the windstorm tracks product.
Name | CSV column name | Unit | Definition |
Time | time | YYYYMMDDHH | Timestamp of the tracked storm center. |
Latitude | latitude | degrees | Latitude of tracked storm centre. |
Longitude | longitude | degrees | Longitude of tracked storm centre. |
10m Wind Gust Speed | fg10 | m s-1 | 10m wind gust speed at the tracked storm centre. This parameter represents the maximum 3-second wind at 10 m height computed every time step and the maximum is kept since the last post-processing (6-hour interval). It corresponds to the: “10m_wind_gust_since_previous_post_processing” of the ERA5 single-level dataset at the corresponding coordinates and time. |
Land-Sea Mask | lsm | % | Reports the percentage of land area in the cell corresponding to the tracked storm centre. |
Mean Sea Level Pressure | msl | hPa | Mean Sea Level Pressure at the tracked storm centre. This parameter is the pressure (force per unit area) of the atmosphere at the surface of the Earth, adjusted to the height of the mean sea level. The value reported along the storm track corresponds to the ERA5 single-level Mean Sea Level Pressure variable at the corresponding coordinates and time. TempestExtremes algorithm: The tracked storm centre is defined by the Mean Sea Level Pressure minimum. Hodges algorithm: The tracked storm centre is defined by the Mean Sea Level Pressure minimum within a 5 radius from the 850 hPa relative vorticity maximum. |
Tracking algorithm | algorithm | - | Name of the tracking algorithm used to identify the event. |
Windstorm footprint
The windstorm footprints, which are provided in NetCDF format (.nc), include for each storm event the maximum 3-second 10-m wind gust (m s-1) over the 72 hours centred around the footprint central time. The NetCDF file of a footprint contains a single variable which depends on three principal coordinates (latitude, longitude and track_id). The track_id coordinate is included to easily merge several footprints during an analysis. On top of the principal coordinates three secondary coordinates (track_start_time, footprint_central_time and track_end_time) characterise respectively the start, central and end time of the event corresponding to the footprint. All the coordinates of a footprint file are listed in Table 3.
Table 3: Overview and definition of the coordinates of the windstorm footprints product.
Coordinate | Format | Definition |
track_id | int64 | Unique ID number of the windstorm event. |
latitude | float64 | Latitude of the footprint grid point. |
longitude | float64 | Longitude of the footprint the grid point. |
track_start_time | datetime64[ns] | Time stamp of the first point of the corresponding storm track. |
track_end_time | datetime64[ns] | Time stamp of the last point of the corresponding storm track. |
footprint_central_time | datetime64[ns] | Reference time used to center the footprint. It is defined as time which the tracking algorithm identified as having the maximum 925 hPa wind speed over land within a 3 degree radius of the track centre. |
Each footprint NeCDF contains a set of attributes characterising the footprint and summarised in Table 4.
Table 4: Overview and definition of the attributes characterising the windstorm footprints product.
Attribute | Definition | Attribute values | Download form values |
product | Product contained in the NetCDF. | "footprint" | Product: "Windstorm footprint" |
algorithm | Tracking algorithm used to identify the event. | "hodges" or "tempest extreme" | Tracking algorithm: "Hodges" or "TempestExtremes". |
screening | This attribute is designed to document a potential sector specific screening. No specific screening has been implemented to date so only the "raw" events are available. | "raw" | Not implemented in the form since only a single value is available. |
track_start_date | Start date of the event. | "YYYYMMDD" | Year / Month / Day selection |
track_id | Unique ID number of the track. | integer | Footprints are not selectable by ID in the form only by start date. |
resolution | Characterises the spatial resolution of the footprint. It can be either "original" when the footprint has the resolution of the dataset it derives from (0.25° x 0.25°) or "downscaled" when a downscaling has been applied to the footprint (0.016° x 0.016°). | "original" or "downscaled" | windstorm footprint resolution: "Downscaled (0.016° x 0.016°)" or "Original (0.25° x 0.25°)". |
field | Characterises the spatial extent of the footprint. It can be either "full" when the footprint covers the full European domain or "decontaminated" when the footprint only covers the 1000km radius around the corresponding storm track center. | "full" or "decontaminated" | Spatial extent: "Full domain" or "Storm footprint area". |
version | Version of the dataset. | "v1.0" | Not implemented in the form since only a single version is available. |
date | Date at which the file was produced. | "YYYY-MM-DD HH:MM:SS" | Not relevant in the form. |
Windstorm summary indicators
The windstorm summary indicators product is provided in text format (.csv) and include the fields listed in Table 5.
Table 5: Overview and description of the fields included in the storm summary indicators product.
Name | CSV column name | Unit | Description |
Year | year | dimensionless | Year for which the indicator has been aggregated. |
Region | region | - | NUTS code of the region over which the indicator has been aggregated. |
Threshold | threshold | m s-1 | The wind gust threshold considered in the indicator evaluation. |
Yearly storm count | storm_number | dimensionless | Number of occurrences of storms exceeding a given threshold in a given region in a year. A storm event is considered to affect a region if its decontaminated footprint intersects the region. For each storm event, the decontaminated footprint of wind gust exceeding a given threshold (0, 15.6, 20, and 25 m s-1) is considered to count storm events. |
Mean wind gust | mean_wind_gust | m s-1 | Average wind gust speed for storms exceeding a given threshold (0, 15.6, 20, and 25 m s-1) over a given region (NUTS0, 1 and 2). |
Storm severity index | ssi | m5 s-3 | Index quantifying the severity of the storm affecting a region (NUTS0, 1 and 2). The severity is calculated by multiplying the total area of a region affected by a storm by the cube of the mean wind speed gust speed exceeding a threshold (0, 15.6, 20, and 25 m s-1). The Storm Severity Index is defined in section 3.1.5.3. |
Normalised storm severity index | normalised_ssi | dimensionless | The Normalised Storm Severity Index is based on the SSI but uses the total area of the region and the 98th wind gust speed percentile. These two quantities are used to normalise the SSI and represent the spatial extent of the area affected and climatological wind gust extremes respectively. The Normalised Storm Severity Index is defined in section 3.1.5.4. |
Area ratio | area_ratio | dimensionless | Ratio between the area of a region affected by wind gusts exceeding a given threshold and the total area of the region. |
Wind gust ratio | wind_gust_ratio | dimensionless | Ratio between the average wind gust speed over the region of interest for storms exceeding a given threshold and the 98th percentile (P98) of the wind gusts probability distribution function considering all the decontaminated footprints over the period 1991 – 2020 in that region. |
Tracking algorithm | algorithm | - | Name of the tracking algorithm used to identify the events. |
Input data
Table 5: Overview of data for input to the Enhanced Windstorm Service.
| Input Data | ||||
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| Model name | Model center | Data type | Period | Resolution |
| ERA5 | ECMWF | Reanalysis | 1940-present (monthly updates linked to new ERA5 release) | 31 km |
Methods
Tracking algorithms
Hodges and TempestExtremes tracking algorithms use different atmospheric variables for detecting and tracking ETC tracks, 850 hPa relative vorticity and mean sea level pressure, respectively. This determines differences in synoptic-scale dynamics influencing tracks’ detection. In general terms, relative vorticity is better at capturing smaller spatial-scale processes compared to mean sea level pressure based tracking (Hodges et al., 2003). In the context of the windstorm dataset within EWS, this translates into a substantially larger number of events produced by Hodges compared to TempestExtremes. However, not all the events tracked by TempestExtremes are included within the Hodges dataset. This indicates that considering a different tracking variable determines not only a different number of detected events but also events having different dynamical features. In terms of historical events reproducibility, a comparison with a series of 50 windstorms reported in the Extreme Windstorm catalogue (XWS, Roberts et al., 2014) shows larger matching events from Hodges. Hodges reproduces 39 out of 50 XWS events against 17 reproduced by TempestExtremes. However, the comparison between the two algorithms and the reference XWS is suboptimal since this latter leverages the same Hodges algorithm (though a previous release), to detect and track historical events. In this regard, spatial and temporal parameters defining shared events (i.e., at least half of event points with a temporal and spatial discrepancy lower than one day and two degrees respectively), can modulate the number of matching events found in the two tracking algorithms and reference XWS.
TempestExtremes Algorithm
TempestExtremes algorithm (TE, Ullrich et al., 2021) is run considering ERA5 reanalysis (Hersbach et al., 2020) 6-hourly mean sea level pressure. In the present configuration, TempestExtremes algorithm requires:
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- ID;
- Longitude and latitude;
- Mean sea level pressure at each step;
- Date;
- Wind gust value at 10m height;
- Land-sea-mask reporting the percentage of land area of ETC track grid nodes.
Hodges algorithm
The tracking algorithm which has already been used to produce the original Windstorm Service is the Hodges algorithm (Hodges 1995, 1999, Hoskins and Hodges 2022). The application of this algorithm has been extended to the entire ERA5 reanalysis period, so the two algorithms are operationally available for the entire ERA5 period (1940-present).
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Footprints decontamination
European windstorm can cluster in time. In fact, maximum gust footprints could likely be derived from two or more events. To minimise this “contamination”, instead of taking the maximum gust over the whole domain, only gusts inside a 1000km radius of the track position at that time are assumed to be part of the event.
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Figure 2: Original and decontaminated footprints for one representative windstorm
Footprint statistical downscaling
Original footprints are statistically downscaled through a multiple linear regression-based model (van den Brink, H. W. (2020)).
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Table 6: Overview of the windstorm tracks and footprints datasets input variables.
Dataset | Input Data | Period | Resolution |
Windstorm tracks (tracking algorithms) | |||
TempestExtremes | 6-hr Mean sea level pressure | 1940-present | 31km |
Hodges | 3-hourly frequency 850 hPa relative vorticity | 1940-present | 31km |
Windstorm footprints | |||
Original resolution | 10m height wind gust | 1940-present | 31km |
Statistically downscaled | · 10m height wind gust; · wgSLh (wind gust estimated from wind shear between two height levels, van den Brink, 2020); · ELEV (elevation derived from the 1 km resolution elevation file). | 1940-present | 1km |
Windstorm summary indicators | |||
Storm count | Original resolution decontaminated footprints. | 1940-present | 31km |
Mean wind gust | Original resolution decontaminated footprints. | 1940-present | 31km |
Storm Severity Index | Original resolution decontaminated footprints. | 1940-present | 31km |
Normalised Storm Severity Index | Original resolution decontaminated footprints. | 1940-present | 31km |
Windstorm summary indicators
The catalogue entry contains summary indicators which are aggregated annually over NUTS 0, 1, and 2 regions. All indicators are contained in a single csv file covering the whole period from 1940 to the present. The available indicators are defined in the sections below and are:
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All four indicators have been computed considering four wind gust thresholds 0, 15.6, 20 and 25 m/s.
Yearly storm count
The yearly storm count looks at the occurrence of storms exceeding a given threshold in a given region. A storm event is considered to affect a region if its decontaminated footprint intersects the region. For each storm event, the decontaminated footprint of wind gust exceeding a threshold is considered to count storm events exceeding a given threshold.
Mean wind gust
The mean wind gust indicator looks at the average wind gust speed for storms exceeding a given threshold in a given region. For a given year all the decontaminated footprints are filtered for each threshold to keep the footprint area exceeding the threshold and averaged over time. The resulting average mean wind gust speed is then aggregated spatially in the predefined NUTS regions. Areas of a region that are not affected by a storm are not considered as “no values” and are ignored when performing the spatial average.
Storm Severity Index (SSI)
The Storm Severity Index (SSI, Dawkins et al., 2016) aims at quantifying the severity of the storm affecting a region. To do so the indicator combines both the total area of a region affected by a storm in a year and the mean wind speed gust speed exceeding a threshold. The SSI can be summarised by the following formula:
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The complete time series of any region can be consulted by clicking on the regions even if it shows no values on the map.
Normalised Storm Severity Index (NSSI)
The Normalised Storm Severity Index (NSSI) aims to quantify the severity of the storm affecting a country/region with a dimensionless number. The NSSI is based on the SSI but uses two extra quantities in order to normalise the SSI in terms of spatial extent and wind gust climatology.
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Mathinline \overline{P_{98}}is the 98th percentile of the wind gusts PDF considering all the decontaminated footprints over the period 1991 – 2020. The overline stands for the spatial average.
Concluding Remarks
EWS represents an enhancement and temporal extension of the original C3S windstorm service. It is operational meaning that it automatically updates windstorm datasets based on ERA5 period new releases on a monthly basis.
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-EWS datasets can serve as reference products to evaluate the capability of global and regional climate models to reproduce windstorm features and temporal evolution.
References
van den Brink, H. W. (2020). An effective parametrisation of gust profiles during severe wind conditions. Environmental Research Communications. Institute of Physics. https://doi.org/10.1088/2515-7620/ab5777
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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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