ECMWF directors discuss European heat waves

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Florian Pappenberger, the director of forecasts at the European Centre for Medium-Range Weather Forecasts (ECMWF), discusses the European heat waves with Samantha Burgess, the deputy director of the Copernicus Climate Change Service (C3S), which is run by ECMWF

Europe has seen several heat waves since June 2022, during which several records were broken. The ECMWF directors point out that these were the result of different meteorological conditions, and that climate change has played a role in their impacts.

Different types of heat waves

Pappenberger comments, “Heat waves lasting around one to three days can be due to the transport of warm air from lower latitudes, in the case of Europe often from the Sahara, together with local heating from solar radiation.”

The heat that affected parts of western Europe in mid-July 2022 was partially caused by this phenomenon. A new record temperature of 40.3°C was reached in the UK, exceeding the previous record by 1.5°C. Temperatures also exceeded 40°C in France, 45°C in Spain and 46°C in Portugal.

“Alternatively, heat waves can be the result of a stationary high-pressure system with clear skies and weak winds,” Pappenberger adds. “These conditions can create longer heat waves, such as the recent one in mid-August. The effect on near-surface temperature depends on how much energy is used to evaporate water from the ground and plants, and how much is heating the air. If the soil is already dry or the surface is just concrete and tarmac, there is little cooling of the near-surface temperature due to evaporation. Instead, most of the energy will heat the air and thus increase the magnitude of the heat wave.”

There is no standardized definition of a heat wave. However, the World Meteorological Organization (WMO) has recently said that a heat wave is ‘a period of statistically unusual hot weather persisting for a number of days and nights’.

The influence of climate change

The heat in the summer of 2022 raised the question of whether climate change is responsible.

Burgess says, “The series of European heat waves this summer were caused by particular weather patterns but the temperatures experienced were hotter than they would have been, because of climate change. As the climate warms, Europe will experience more frequent and more intense heat waves.”

The chart below of southwestern Europe temperatures highlights the three years when maximum average temperatures were reached: 2003, 2021 and 2022.

Daily maximum and minimum temperatures (March-August) averaged over southwestern Europe. Source: C3S/ECMWF

“Climate change makes very high temperatures much more likely, as has been shown specifically for the UK,” Burgess says. For example, without climate change, temperatures above 40°C in the UK would have been extremely unlikely, the WMO said with reference to a World Weather Attribution analysis.

Daily surface air temperature anomaly for July 2022 relative to the daily average for the period 1991-2020. Data source: ERA5. Credit: Copernicus Climate Change Service/ECMWF

The link with droughts

Heat waves can cause droughts but droughts also occur without a heat wave. “A long heat wave will result in a drought due to the lack of rain and enhanced evaporation due to the high temperatures,” Pappenberger says. “Droughts will also increase the temperature during heat waves due to a lack of moisture in the soil, causing less evaporation from plants. However, there can be droughts without a heat wave and a heat wave without a drought.”

The range at which ECMWF predicts heat waves

ECMWF specializes in global medium-range forecasts, in other words from three to 10 days.

This chart aims to provide pointers to areas where anomalous values of 2 m temperature are likely to occur based on the ECMWF ensemble forecast (ENS) system. It shows the Extreme Forecast Index (shading) for August 11-14, 2022, in a forecast from August 5, 2022

Pappenberger continues, “Especially in the first few days of a forecast, there is a challenge in capturing the magnitude of temperatures. For example, our integrated forecasting system (IFS) does not yet represent built-up areas. We should thus expect the model to underestimate temperatures for large airports and cities.” ECMWF also covers the extended range, up to 42 days ahead, and the long-range, up to 13 months ahead. “In the extended range we have seen some early signals for warmer temperatures than normal,” he said. “Future improvements of the land surface model, including cities, are expected to provide a more accurate coupling between the atmosphere and the surface in terms of energy exchange. They should help to better capture the magnitude of heat waves.”

Burgess points out, “The C3S services show, for example, that the hottest summer in Europe was in 2021, yet 2020 was a warmer year overall.” The Intergovernmental Panel on Climate Change (IPCC) says in its sixth assessment report that the ‘frequency and intensity of hot extremes will continue to increase’ even if global warming is stabilized at 1.5°C.

To view the original article from the ECMWF, click here.

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