Last modified on Dec 08, 2022 14:43

Contributors: Cagnazzo, C. (ECMWF/C3S), Zanacchi, M. (ECMWF/C3S)

Issued 08/12/2022

Introduction

Directive (EU) 2016/2284 on the reduction of national emissions of certain atmospheric pollutants (“the NEC Directive” or NECD) states in Article 5(2) that:

 ‘If in a given year a Member State, due to an exceptionally cold winter or an exceptionally dry summer, cannot comply with its emission reduction commitments, it may comply with those commitments by averaging its national annual emissions for the year in question, the year preceding that year and the year following it, provided that this average does not exceed the national annual emission level determined by the Member State's reduction commitment.’ 

NECD Recital (17) explains that this comes from the revised Gothenburg Protocol. Indeed, the amended Gothenburg Protocol says: “5. If in a given year a Party finds that, due to a particularly cold winter, a particularly dry summer or unforeseen variations in economic activities, such as a loss of capacity in the power supply system domestically or in a neighbouring country, it cannot comply with its emission reduction commitments, it may fulfil those commitments by averaging its national annual emissions for the year in question, the year preceding that year and the year following it, provided that this average does not exceed its commitment.”

Purpose of the application

The current application helps to identify if a specific year for a Member State is considered an exceptionally cold winter or an exceptionally dry summer.

In this version of the application an exceptionally cold winter is identified:

• When the temporally averaged JFM (January, February, and March) temperature for a Member State is below the 10^th percentile, estimated over the reference period 1991-2020.

An exceptionally dry summer is identified:

• When the temporally summed JJA (June, July, and August) total precipitation for a Member State country is below the 10th percentile, estimated over the reference period 1991-2020.

Note that the temporal averaging/summation is done for all grid points within a Member State’s boundaries, after which the median value (50^th percentile) is taken to estimate the exceptional (below threshold) values described above.

The application automatically updates in the month following the end of the required season. For the JFM temperature, this is on day 30 of April. For the JJA precipitation it is day 30 of September.

Application interface

The application presents an interactive map of Europe displaying the representative temperature and representative total precipitation for each Member State (Figure 1, left panel). The variable to be displayed is selected via a dropdown menu situated above the map. The representative temperature is the temporally averaged and spatial median temperature described in the previous section. Likewise, the representative total precipitation is the temporal sum and spatial median precipitation value described above. A time slider positioned at the bottom of the map frame allows the user to manually select a required year or automatically scroll through each year in the period (1991-2021).

Figure 1: The application’s main interface consists of an interactive map (left panel) displaying either temperature or precipitation, and a detailed timeseries (right panel) displaying the exceptionally cold winters (temperature) or exceptionally dry summers (precipitation) that fall below the threshold. Here, the default variable temperature is selected (left panel). The exceptionally cold winters are identified by the red data points situated below the threshold line in the timeseries (right panel).

The map’s graphical display contains four visible layers, applicable to both variables:

• Mean temperature (Jan-Feb-Mar)/ Total precipitation (Jun-Jul-Aug) – the default map selection - displays the representative variable from the dropdown menu for the selected year in the time slider.
• Threshold (10^th percentile) – displays the 10^th percentile value for each Member State for the selected year in the time slider.
• Below threshold – a binary display showing the Member States falling below the threshold for the selected year in the time slider.
• Proximity to threshold  – shows for each year the difference between the variable and the threshold. This layer displays how far each Member State is from the threshold value for the selected year in the time slider.

The data value is presented upon hovering the mouse over the Member States.

Figure 2: Four layers are available to display on the interactive map. Here, the Member States that fall below the threshold for the selected year (2006) are displayed in red (left panel). The exceptionally cold winters for a selected Member State may also be discerned by the red data points in the timeseries (right panel).

Figure 3: The interactive map displaying the total precipitation variable for June, July, August 1991. Other years may be displayed on the time slider situated at the bottom of the map. The application’s detailed view displaying the exceptionally dry summers for the selected Member State (right panel). The years for which exceptionally dry summers occur are displayed in red.

Detailed view:

Clicking on a Member State in the interactive map will open a detailed view for that Member State and present a timeseries graph – line- or bar-graph depending on the variable selected (Figure 1, right panel). The graph presents the representative values calculated for each year together with the threshold identified by the 10th percentile (red line). The exceptionally cold winters are identified by the red data points and are situated below the threshold line.

Input data

The temperature and precipitation data are obtained from the monthly mean ERA5 catalogue entry* in the Copernicus Climate Data Store (CDS). All CDS data are freely available and accessible for everyone to use. The data used are:

• 2m-temperature: monthly mean surface air temperature for 1959-present.
• total precipitation: monthly mean total precipitation for 1959-present.

More information on the variables is available in the catalogue documentation available through the link to the catalogue provided below.

*(https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels-monthly-means)

Methodology

The application performs the following steps for calculating the below threshold years for temperature:

• Step 1: Retrieve the monthly mean temperature data for January, February, and March (JFM) over Europe (area:  34-72N; -25-45E) for each year between 1991 to present from the CDS catalogue entry.
• Step 2: Calculate the average temperature over the three months for each year, for each grid point (temporal dimension).
• Step 3: Calculate the Member State median JFM temperature for each year using the data from step 2 above (spatial dimension).
• Step 4: Estimate the 10th percentile over the reference period (1991-2020) for each year and for each Member State.
• Step 5: Identify the years that fall below the 10^th percentile estimated in step 4 and mark them as below the threshold years.

The choice of the reference period is based on the WMO definition of Climatological Standard Normals.

The application performs the following steps for calculating the below threshold years for precipitation:

• Step 1: Retrieve the monthly mean precipitation data for June, July, and August (JJA) over Europe (area:  34-72N; -25-45E) for each year between 1991-present from the CDS catalogue entry.
• Step 2: Calculate the sum of the precipitation over JJA for each year for each grid point (temporal dimension).
• Step 3: Calculate the Member State median JJA precipitation for each year using the data from step 2 above (spatial dimension).
• Step 4: Estimate the 10th percentile over the reference period (1991-2020) for each year and for each Member State.
• Step 5: Identify the years that fall below the 10^th percentile estimated in step 4 and mark them as below the threshold years.