Researchers at ESSL, Munich Re, the Ludwig-Maximilians University Munich (LMU) and the German Aerospace Centre DLR have developed a new statistical method to investigate trends in thunderstorm and severe weather activity. Anja Rädler, the main developer of this method, called AR-CHaMo, will defend her Ph.D. thesis on this topic at LMU.

Anja Rädler says: “the strength of our approach is that we are now able to assess changes in severe weather frequency using climate models that are themselves too coarse to simulate each and every thunderstorm”. To do this, the probability of lightning and severe weather is determined as a function of parameters such as instability, moisture and wind shear, using severe weather observations from ESSL’s European Severe Weather Database.

Annual number of 6-hourly periods with hail of 2 cm or larger in Europe (1979 – 2016), modelled using AR-CHaMo. From: Rädler et al, 2018.

In a new article in the Journal of Applied Meteorology and Climatology, the development of AR-CHaMo is explained in detail, and trends of lightning and severe weather frequency since 1979 are presented. The basis of these projections are a numerical model (ERA-Interim), that simulates the past weather since 1979. The annual average predicted number of large hail events (see map below) conforms quite well to what we know: the Alpine forelands are the Europe’s hailfall hotbed.

The number of simulated hail events was not constant during the last three decades. When considering hail activity over Central Europe, significant increases are detected.

Simulated number of 6-hourly periods with hail > 2 cm across Germany and the Alps (blue) and Western and Central Europe (green), 1979-2016. From: Rädler et al, 2018.

Of course, it is of interest to know what the reason for the increase is. Anja Rädler: “we found that the driving factor is the increasing humidity close to the earth’s surface. This is something we expect to happen when temperatures increase, because warmer air can contain more moisture.” Indeed, temperature has increased since 1979, at least partly, because of the rising CO2 content of the atmosphere.

The map below shows which areas have seen the strongest increase, according to ERA-Interim and the AR-CHaMo model.

Trend of number of 6-hourly periods with severe hail of 2cm or larger (1979-2016). Dots denote where a ignificant trend was found. From: Rädler et al, 2018.

The next question Anja and her team will address is what will happen in the future. First results that were published by the ESSL team in 2017 suggest that the moisture increase will continue and create more situations favorable for severe thunderstorms during the 21st century. Using the new AR-CHaMo method, the team will soon be able to give an estimate of how much more hail, tornadoes, severe winds and lightning we should count on in different climate change scenarios.

As promised, we would like to bring you an overview of the major convective windstorms in Europe last year.

The first event we are going to cover is the case of 29 May 2017 in Russia. This event did not feature one of the longest-lived convective windstorms of that year in Europe, nor did it involve the highest measured wind gusts (which reached up to 30 m/s). Yet, its societal impact was the largest of all the cases. As the windstorm hit the metropolitan area of Moscow, it resulted in 18 fatalities and 168 injuries (economic loss estimated at 25 million rubles). The first reports of wind damage came in from 11:15 UTC. Between 12 and 13 UTC, the convective system reached its maturity as it passed over Moscow and it decayed shortly after 14 UTC (Fig. 1).

Fig 1. Chronological progression of severe wind reports in a convective windstorm of 29 May 2017 in the Moscow area.

Radar imagery shows that at 12:30, a linearly oriented convective system was moving into the Moscow area. The convective system was not particularly large, without very high reflectivity values and did not show a classic “bow-echo” structure, which would typically be associated with damaging wind gusts (Fig. 2). The apparent lack of strong updrafts was also confirmed by virtually no lightning activity in the southern part of the system (Fig. 3).

Fig 2. Precipitation intensity (mm/h) s in Moscow region for 12:30 UTC 29.05.2017 (data from Web-GIS ‘Meteorad’ of the Central aerological observatory based on Roshydromet radar network). Arrow points to the direction of the storm movement.

Fig. 3 Combined information on weather phenomena (based on Roshydromet radar network) and lighting detection networks (WWLN, ALVES, Vaisala LS-8000) in Moscow region for 12:30 UTC 29.05.2017 (data from Web-GIS ‘Meteorad’ of the Central aerological observatory). Arrow points to the direction of the storm movement.

The reason for this untypical behaviour were the background environmental conditions. The convective storm formed ahead of an advancing mid-tropospheric trough (Fig. 4 left). Buoyancy was rather low with CAPE values around 400 J/kg according to the Era-Interim. At the same time, vertical wind shear was moderate, with 0-6 km bulk shear values around 15 m/s (Fig. 4 right). Overall, the environment did not seem to be too favourable for an extremely severe convective event, which would be typically anticipated in high CAPE and high shear regime. However, a combination of very strong flow in the lower troposphere and a dry boundary layer (Fig. 5) created favourable conditions for powerful downdrafts transporting high momentum air down from above.

Fig. 4 (Left) 500 hPa geopotential height (black contours), temperature (colour scale) and wind barbs, (Right) CAPE (colour scale) and 0 – 6 km bulk vertical wind shear (wind barbs) for 29 May 2017 12 UTC according to ERA-Interim reanalysis. Blue dot represents location of Moscow.

Fig. 5 Moscow Dolgoprudnyj 12 UTC sounding. Courtesy of University of Wyoming.

The convective system weakened soon after leaving the Moscow area, probably suffering from the lack of ideal environmental conditions. Nevertheless, this case  illustrates that high-impact convective windstorms are possible in a wide variety of conditions.

ESSL would like to thank Alexander Chernolusky from the A. M. Obukhov Institute of Atmospheric Physics for his contribution to this case study.

The Bulletin of the American Meteorology Society has published an article that summarized the accomplishments of ESSL in it first 10 years of existence, such as the ESSL Testbed, Climate research and, of course, the European Severe Weather Database. Read the article on-line at:

https://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-16-0067.1

The last 2017 edition of our ESSL Newsletter is out. Read more about the successful ECSS2017, our new researcher Chris Castellano and the upcoming activities in 2018.

Previous ESSL Newsletters can be found here.

ESSL Research Associate Christopher Castellano

Online registration for the ESSL Testbed 2018,

for the seminars “Forecasting Severe Convection I”
and “Forecasting Severe Convection II” (this time with Prof. Paul Markowski from PSU),

for the new seminar “Forecasting Convective Precipitation and Flash Floods” (by Prof. Russ Schumacher from CSU)

as well as for the third “Workshop on Tornado and Windstorms Damage Assessment” is now open.

All events are listed here with links to the event descriptions and the registration pages. In the past year some activities were booked out already in early spring. It is therefore advisable to reserve your place as soon as possible.

During the closing session of the ECSS2017 in Pula, Croatia, the following ECSS awards were presented by ESSL:

• Best Oral Presentation Jury Award
• Best Poster Jury Award
• Best Oral Presentation Audience Award
• Best Poster Audience Award

The awardees are listed here.

Best Oral Presentation Jury Award: Chernokulsky, Alexander; Kurgansky, Michael; Mokhov, Igor; Selezneva, Evgeniya; Shikhov, Andrey; Azhigov, Igor; Zakharchenko, Denis; Antonescu, Bogdan; Kühne, Thilo. The modern climatology of Northern Eurasia tornadoes and waterspouts

At the ECSS2017 the Heino Tooming Award (about) was presented to Gatzen, Christoph (Germany); Kreitz, Michaël (France); Leprince, Sébastien (France); Schielicke, Lisa (Germany); Rabrenović, Maja (Serbia); and Enno, Sven-Eric, for their work entitled “Combined analysis of severe convective wind gusts in European data sets”.

Tooming Awardees 2017

At the 9th European Conference on Severe Storms in Pula, Croatia, Dr. Joshua Michael Aaron Ryder Wurman was awarded the fourth Nikolai Dotzek Award by the ESSL Executive Board. Dr. Wurman has been given the award for

• his work on radar techniques including bistatic radar and mobile radars,
• his groundbreaking work in developing the Doppler-on-Wheels radars that were first operated in the large field program VORTEX in the mid 1990’s,
• the outstanding research he has done with the Doppler-on-Wheels and the work which supervised:

This research has revealed the structure of flow in tornadoes in extreme detail; it has also contributed to important new insight into the immediate environment of tornadoes, and into other weather phenomena that the DoWs have scanned around the world.
The scientific community owes a lot to Dr. Josh Wurman for creating an abundance of research opportunities with the Doppler-on-Wheels data, and for having inspired many future researchers around the world.

Nikolai Dotzek Awardee 2017: Dr. Joshua Wurman

Dr. Wurman is affiliated with the Center for Severe Weather Research in Boulder, Colorado, USA.

Read more: about the Nikolai Dotzek Award.

These are the official ECSS conference photos:

(c) ESSL, Thomas Schreiner

(c) ESSL, Katja Dorninger

Full resolution:  Photo 1    Photo 2

Our latest ESSL Newsletter 2017/3 contains an interview with our ESWD quality control manager Thilo Kühne. There you can also find news on ESSL’s climate research and about the successful Ph.D. dissertation defence of ESSL Senior Trainer, now Dr. Tomáš Púčik.

Thilo Kühne

Dr. Tomáš Púčik