Explaining long-term trend patterns of precipitation over Europe

by Peter Hoffmann

Hydro-Climatic Risks

Changes in Atmosphere Dynamics at Midlatitudes

Strong Westerly Jet Weak Westerly Jet

How rainfall patterns are effected by dynamic changes beyond the temperature rise?

Arctic Amplifiction leads to a weakening of the Jet Stream

from West to East
from West to East

Total Annual Precipitation | PR

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Not all rain is the same

Every local rain event has a large-scale context








European Weather-Types

Expert Classification

Hess/Brezowsky: Großwetterlagen

European Weather-Type Sequences

Simplification of the large-scale Weather Variability by using Categorical Data


Example: Ahrtal Catastrophe
Year Month Day GWL CE
2021 7 10 TRW
2021 7 11 TRW
2021 7 12 TRW
2021 7 13 TRM
2021 7 14 TRM
2021 7 15 TRM
2021 7 16 NEZ
2021 7 17 NEZ
2021 7 18 NEZ

every day is allocated to one of these 30 synoptic patterns

Changes in European Weather-Type Sequences

New dominant Weather-Types explain the present weather variability

1961-1990 1991-2020
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TRM, TRW, SWZ are often associated with extreme weather events

Causal Linkage between large-scale Circulation and Rainfall Patterns

Composite Pattern for Low Pressure System over Central Europe (TM)

TM

Cause

  • large-scale transport of air masses
  • patterns are recurring and classifiable

Effect

  • local weather phenomena: wet | dry
  • location of instabilities (fronts)

Cause-Effect

  • linkage: weather-type and weather phenomena

dynamical link between the Mediterranean and Central Europe by moisture transport




Weather-Type Specific Total Precipitation

Long-Term Trend Pattern



Westerly Cyclonic (W)

Low-Pressure Systems from the North-Atlantic


Total Annual Precipitation | PRW

Frequency and Total Precipitation in most parts of Europe is decreasing.

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W



Changes in Weather-Type Frequency

W: (112|-40) BM: (46|-11) SW: (28|+8) NW: (28|+2) TRM: (25|+13) TRW: (16|+8)


New Dominant Weather-Types




Trough over Central Europe (TRM)

Low-Pressure Systems with Moisture Transport from the Mediterranean


Total Annual Precipitation | PRTRM

Frequency and Total Precipitation increasing especially over the Mediterranean.

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TRM



Is this only explained by the Frequency Change?

Extraction of the Dynamic Factor

Total Annual Precipitation | Total vs Dynamic | PRTRM

Total Total-Dynamic Dynamic
--- purple: intensified precipitation only frequency driven

Dynamic Factor?

observed daily precipitation is replaced by long-term monthly means per weather-type (here TRM)




Does the changing Weather-Types explain the Trend Patterns?

Lets remove the most dominate Weather-Type W

Total Annual Precipitation | PR-W

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-W



Recontructing Total Precipitation Patterns

only using the dominant Weather-Types: W | TRM | TRW | NW | NE | BM

Total Annual Precipitation | PRDOM

Only a few dominant Weather-Types explain most of the trend features in total precipitation.

DOM



A very high demand on Climate Models

What can we expect from climate scenarios?

Re-Identification of Weather-Types in Climate Models

from Atmosphere Fields to Synoptic Patterns by training a Decision Tree

objective classification of existing Weather-Types using Z500 atmospheric fields over Europe from ERA5 and CMIP6.


EC-Earth3: the best performing model

too much westerly winds
wet weather-types are overestimated
dry weather-types are underestimated

Final Remarks

  • Dynamic Changes explain Trend Features in Total Precipitation over Europe (new dominant meridional weather-types)

  • Decreasing Trends of Total Precipitation from the North-Atlantic with a westerly wind context

  • The lower the influence from the North-Atlantic the more extreme the resulting Precipitation Patterns

  • Climate Models overestimate the westerly wind context, underestimate dry and overestimate wet Weather-Types

  • A few signs of dynamic changes in Climate Model Scenarios

  • Recommedation:

    • not all biases can be adjusted (dynamical bias)
    • because the simulated weather variability follows slightly other rules
    • bias adjustments should only consider the respective large-scale context

Comparison of observed and simulated Trends

Annual Precipitation: 1981-2023

ERA5 Euro-Cordex

KNMI Climate Explorer

# <mark>Motivation</mark> <hr> <center> ![w:300 h:220](./img/rain.gif) ![w:300 h:220](./img/suny.gif) ![w:300 h:250](./img/low_cd.gif) ![w:300 h:250](./img/high_cd.gif) </center> <div style="left:0%; top:88%; position: absolute"><hr width="1300px" color="lightgray" size="3"></div>

![w:400 h:250](./img/TRM.gif)

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