Weather Events
Atmospheric Rivers (AR) provide the majority of water vapor transport from the tropics towards the mid-latitudes and are associated with episodes of heavy and prolonged orographic precipitation over land. While there is little precipitation above water, when the AR hits the coast and moves inland it can release massive amounts of rainfall, especially when the stream of wet and moist air is lifted over a mountain chain. These extreme precipitation events can lead to flash flooding, mudslides, and catastrophic damage to life and property.
ARs are responsible for many of the largest winter floods in the mid-latitudes as well as for many annual daily precipitation maxima in Western Europe. They can also trigger precipitation as far inland as Germany and Poland.
Figure 36: SEVIRI VIS 0.8 µm image form 29 May 2024 at
12:00 UTC. Streamlines
at 1000 hPa indicate the
advection of moist saturated air within an AR.
Green
symbols indicate precipitation over France.
Figure 37: Schematic showing where precipitation occurs
after an AR makes landfall.
The low-level winds that accompany ARs are generally stronger over the sea than over land where they are weakened by friction processes.
There is general agreement that ARs occur in Europe more frequently during the cold season when extratropical cyclones are located further south. Additionally, ARs cause more precipitation in winter because of the higher meridional temperature gradient over Europe. The precipitation intensity also depends on the duration of the event, the amount of transported water vapor and the strength of the low-level jet.
Table 1: Table of meteorological parameters affected by ARs.
| Parameter | Description |
|---|---|
| Precipitation | High precipitation amounts over land in case of orographic lifting. High precipitation amounts over land when the AR is stationary over a longer period. |
| Humidity | High relative humidity (up to around 700 hPa) mixed with clouds. |
| Wind | Low-level jet over the ocean, wind speed reduced by friction over land. |
| Cloudiness | Overcast, low-level clouds. |
Precipitation products such as the Precipitating Clouds product from the Nowcasting SAF or the Blended SEVIRI/LEO Precipitation Product from EUMETSAT show high likelihood of rainfall as soon as the AR reaches land.
Figure 38: SEVIRI VIS 0.8 µm image and Precipitating Clouds product [%] prob.] of the NWCSAF from 29 May 2024 at 12:00 UTC.
Figure 39: SEVIRI VIS 0.6 µm image and Blended SEVIRI / LEO Precipitation Product from 29 May 2024 at 10:00 UTC.