Over most of Europe June 2021 already had 1.5°C higher temperatures than the long term 1991 - 2020 average. Northeastern countries were especially affected by high temperatures in the second part of June. The heatwave also extended to Moscow and Siberia (read more here).

Globally, August 2021 was the third warmest month on record. In Europe as a whole, the temperatures in this period were close to the 1991-2020 average. However, there were significant temperature variations across the continent, with southern and easternmost Europe experiencing warmer than average conditions, while northern Europe had colder than average conditions (read more here).

LSA SAF Derived Land Surface Temperature (DLST; LSA-003) products provide 10 daily maximum (median) surface temperatures, which can be of great value when estimating the impact of heatwaves on an area. DLST provide maximum and median value composites of Land Surface Temperature (LST) for each time-slot (96 max values) as well as sets of Thermal Surface Parameters (TSP), which are obtained by fitting a diurnal temperature cycle model to sequences of the 10 day LST composites (Göttsche and Olesen, 2009). This way we summarize the information contained in the composites and also further reduce the impact of outliers and cloudy days. Here we use TSP for demonstrating the impact of the heatwave on LST in June and August 2021.

By summing up the two TSP ‘Minimum temperature’ and ‘Temperature amplitude’ we get an excellent coverage of Europe's 10 day maximum LST, as shown by the animation in Fig. 1. Compared to 2m air temperature, LST usually has a time shift and higher values, but the temperature anomalies of these two different quantities carry the same information.

 

Figure 1: Animation of LSA SAF derived 10 day maximum land surface temperatures in June 2021 (left) and August 2021 (right). Missing data, e.g. due to persistent clouds, are shown in white.

A break of a week after the end of July's heatwave an anticyclonic blocking occurred in mid-August 2021 over the Mediterranean. The same mechanism of jet stream blocking was also the reason for June’s heatwave, which prevented cold air intrusion. Advection of dry and warm air from Africa in combination with an anticyclone raised temperatures above 40°C over the Balkan, Italy and parts of Spain: at the same time, these conditions caused many wildfires (read more here).

On the 11th of August 2021, i.e. in the middle of the heatwave, a temperature of 48.8 °C was recorded on Sicily, Italy: if verified by the WMO, this temperature will be the highest temperature ever recorded in Europe (read more here).

The animations in Fig. 2 clearly show where regions with high positive anomalies are located: these occur over areas affected by the heatwaves, while negative anomalies indicate regions that are cooler than the reference period (2016 - 2020). The right animation in Fig. 2 (August) shows that the positive anomalies in north and northeast Europe are also collocated with the first heatwave in June. High surface temperatures occurred over eastern Europe as well as over Spain. From the first and the second decade (10-day interval) in August 2021 we can confirm that the highest temperatures occurred over the Balkan, Italy and parts of Spain (Fig. 1, right animation), where maximum LSTs rose up to 60 °C. Prolonged situations with such conditions are commonly associated with droughts. While we can also observe positive anomalies over most Mediterranean regions, over northern Europe we see negative or close to zero anomalies (Fig. 2). In the last 10 days of August there are also some positive surface temperature anomalies over Ireland and parts of Scandinavia, both of which were caused by the advection of warm air.

 

Figure 2: Animation of temperature anomalies for June 2021 (left) and August 2021 (right) with respect to the average values during the reference years 2016 – 2020. Missing data, e.g. due to persistent clouds, are shown in white.

Conclusion

With global warming we will experience more frequent warm or cold air intrusions, which is a consequence of the need to balance pole to equator energy differences. Especially over large and remote areas, such situations are best monitored and analyzed with the aid of satellite-retrieved LST products. Here we showed some examples of such analyses for June and August 2021, which were based on LSA SAF’s Derived Land Surface Temperature (DLST; LSA-003) product over Europe. Observing extreme temperature and anomaly patterns based on DLST products allows us to identify the regions where the land surface is warming the most, compared to previous eras.