European heatwaves lead to droughts

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Recurring heatwaves caused droughts in parts of Central Europe in Summer 2015.

Date & Time
July, August and September 2015
Satellites
Meteosat-10
Instruments
SEVIRI
Channels/Products
Normalised Difference Vegetation Index, Fraction of Vegetation Cover (FVC)

More information and detailed analysis of the feature can be found in the In Depth section.

 

In Depth

by Izabela Zablocka and Jakub Walawender (IMGW), Andreas Wirth (ZAMG), Isabel Monteiro (IPMA)

This year the summer was very hot and dry in many European countries. Air temperature records were broken in several locations over Central and Eastern Europe.

The cumulative effect of persistent heatwaves, with long episodes without rain, caused serious problems for people, agriculture, and industry.

Defining heatwaves and droughts

Summer 2015 heatwaves

The Terra/Aqua MODIS 8-day Land Surface Temperature anomaly product was used to show the spatial extent and intensity of the Summer 2015 heatwaves in Europe.

Maps for three distinct heatwave events, created using NASA data, show where the daytime Earth's surface temperature was higher or lower than the average land surface temperature for the same week in the period 2001–2010 (Figures 1, 2 and 5 below).

July heatwave

Figure 1
 
Figure 1: Land surface temperature anomaly 4–11 July, 2015. Credit: NASA. Full resolution

The first heatwave occurred in the beginning of July and covered almost the whole Europe. Central Europe was the most affected area.

  • Germany's temperature record was broken on 5 July when 40.3 °C was recorded in Kitzingen in Bavaria.
  • From 3–8 July, temperatures in Poland exceeded 30 °C, with a maximum of 36.4 °C recorded in Slubice, western Poland.
  • On 6/7 July Vienna, Austria had the hottest night ever with 26.9 °C
  • Innsbruck, Austria recorded daily maximum of 38.2 °C on 7 July
  • Seibersdorf, Austria recorded 30.4 °C at 08:00 UTC on 7 July.

August heatwave

Figure 2
 
Figure 2: Land surface temperature anomaly 5–12 August, 2015. Credit: NASA. Full resolution

A month later parts of Europe were hit by another heatwave:

  • Germany tied its all-time record high on 7 August, when temperatures in Kitzingen again hit 40.3 °C.
  • 7 August was also the hottest day ever recorded in Berlin, 38.9 °C, beating the old record of 38.6°C.
  • On 8 August a number of all-time records were broken:
    Minsk, Belarus — 35.8 °C
    Wroclaw, Poland — 38.9 °C
    Genoa, Italy 38.5 °C
    Kaunas, Lithuania — 35.3 °C

This record-breaking heat was accompanied by a dominant high pressure which helped maintain the sustained dry period.

Even rainfall episodes which came with passing fronts, sometimes bringing intense rainfall, did not improve the situation. Many areas, in particular Poland, were affected by a serious drought.

Figure 3
 
Figure 3: Weekly LSA SAF Fraction of Vegetation Cover (FVC) change from March to August

The animated gif of the LSA SAF Fraction of Vegetation Cover (FVC) (Figure 3) shows the weekly change in vegetation cover of Europe, from March to August.

River water levels in Poland at the end of August were very low, reaching 42 cm on the Vistula river in Warsaw. That was the lowest level ever recorded, the previous record was 52 cm.

Due to high temperatures and low level water in rivers, for the first time in almost three decades Poland’s national suppliers cut electricity to big factories.

Although there was no risk of power cuts to hospitals and private users, residents were asked to save water and limit their use of electricity, especially during the day.

 
Figure 4
 
Figure 4: Mean and anomaly of FVC in Europe.

The Land SAF Fraction of Vegetation Cover (FVC) image (Figure 4) shows the amount of vegetation during August distributed in a horizontal perspective.

 

September heatwave

Figure 2
 
Figure 5: Land surface temperature anomaly 29 Aug–5 Sept, 2015. Credit: NASA. Full resolution

The final extreme temperatures of the summer appeared in last days of August and first day of September. 48 stations in Germany alone set new all-time September heat records. Numerous European cities broke all-time September heat records, including:

  • Pottschach, Austria: 36.0 °C
  • Javornik, Czech Republic: 37.4 °C
  • Druskininkai, Lithuania: 35.1 °C
  • Tarnow, Poland: 36.8 °C
  • Michalovce, Slovakia: 36.4 °C
  • Voznesens'k, Ukraine: 38.8 °C

The heat continued on 2 September, with Falesti, Moldava reaching 38.6 °C, beating the all-time record set the previous day.

Impacts seen by satellite data

Comparing the 10 days aggregation of the NDVI (Normalised Difference Vegetation Index) product for 31 August 2014 and 31 August 2015 (Figure 6), we can see huge difference for most of Europe.

This product estimates the land surface characteristics derived from satellite data. It uses reflectances from the SEVIRI Level 1.5 image data for the VIS0.6 µm and the VIS0.8 µm channels.

The NDVI is widely used to characterise the density and vitality of vegetation cover. It can also be useful to identify drought. Blue indicates dense and healthy vegetation cover, while red means no vegetation or very dry vegetation. In can be seen in the picture that the dark blue coloured areas in August 2014 reached far more southwards than this year. That means that at the end of this very hot and dry summer, vegetation cover over the large part of Europe was in much worse condition than at the same time last year.

It should be noted the comparison shows two extreme cases — Summer 2014, especially August, was particularly cold and wet in Central Europe and Summer 2015 was very hot and dry.

Comparison of 2014 and 2015
Image from 31 August 2014 Image from 31 August 2015
Figure 6: Comparison of NDVI (Normalised Difference Vegetation Index) product for 31 August 2014 and 31 August 2015.

See also:

All-Time Record Heat in Germany (The Weather Channel)
Rare September Heat Wave Scorches Europe (Jeff Masters' Blog, Wunderground)
Summer crop forecasts revised further downwards (EC Joint Research Centre)
Europe and Pacific Northwest face record heat (NASA Global Climate Change website)
Europe is parched, in a sign of times to come (The Guardian)
Land Surface Temperature Anomaly [Day] (1 month) (NASA Earth Observations)
Global Analysis — July 2015 (NOAA's National Centers for Environmental Information)
Briefing 20 August 2015 by Andreas Wirth on the EUMeTrain website.

Defining heatvwaves and droughts

There isn't a universal definition of what are heatwaves and droughts. Both of these phenomena cannot be defined in absolute values and so depend on the region where they occur.

Heatwaves and droughts are strong deviations from average air temperature and rainfall over particular region which continue for several days.

The World Meteorological Organization's (WMO) definition says a heatwave occurs when the daily maximum temperature for more than five consecutive days exceeds the average maximum temperature by 5 °C in the standard 30-year period (the current WMO standard period is 1 January 1961 to 31 December 1990).

For many parts of Central Europe a heatwave is usually declared when maximum daily air temperatures exceed 30 °C, then continue to be above 25 °C for at least three consecutive days.

In the UK the average threshold temperature is 30 °C by day and 15 °C overnight for at least two consecutive days.

Heatwaves and droughts do not necessarily happen at the same time. The more extreme the heatwave, the greater the chance that it will cause a drought. In recent years the number of extreme heatwaves coinciding with a drought has increased.

 
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