Storyboard: Extreme precipitation and atmospheric rivers in West Antarctica

Why ice shelves matter

Ice shelves in Antarctica are important gatekeepers of sea level rise.

They act as a buffer for glaciers that flow off the vast ice sheet, slowing the rate at which ice is released into the ocean. 

In this study, we explore how much rain and snow falls over ice shelves in the Amundsen Sea embayment, including the critically important Thwaites and Pine Island glaciers.

"Rivers in the sky"

• Precipitation (snow and rain) is the only way ice losses from glaciers and ice shelves are counterbalanced.
• Lots of precipitation falls during extreme events, including during "atmospheric rivers" - long, thin "rivers in the sky" which bring lots of heat and moisture to Antarctica.
• You can see an example in the figure here from Wille et al. (2023)

Mini-ensemble

We use regional climate models (RCMs) to explore the patterns of extreme precipitation during atmospheric rivers.

Where?

Amundsen Sea Embayment, West Antarctica, focusing on the ‘gruesome twosome’ of ice loss: the Thwaites and Pine Island Glaciers (see map).

When?

We look at a case in February 2020 (summer).

How?

We use a ‘mini-ensemble’ of three RCMs: the MetUM, HCLIM and Polar-WRF (see schematic).

We use these models to downscale ERA5 reanalysis to very high-resolution (1 km grid spacing).

Snow & Rain

We see lots of snow falling over the Thwaites and Pine ice shelves (Figure)

We have snowfall observations at two locations on the Thwaites ice shelf, shown by the dashed and dotted lines in panel a.

The coloured lines show median simulated rain and snowfall amounts, while the shaded regions show the range of modelled values across the ice shelves.

ERA5 (yellow) has the lowest snowfall amounts and no rainfall, while the RCMs all show more snow and some rain. The RCMs therefore perform better compared to the observations than ERA5. However, none of the RCMs show as much snowfall as the observations.

Spatial patterns of precipitation

• Precipitation is concentrated around steep terrain in all RCMs and ERA5.
• The RCMs have more snow and rain than ERA5 across the whole region.
• While the ERA5 and RCM-simulated snowfall amounts are more comparable, the high-resolution RCMs have much more rain than ERA5.
• ERA5 is at approximately 30 km resolution, while the RCMs are at 1 km resolution.
• This shows the benefit of using high-resolution RCMs to explore the dynamics of extreme precipitation in West Antarctica

What's going on?

The first panel shows a map view of the model domain, and the next two show transects cutting through the Thwaites and Pine Island glaciers, respectively. The lines shown in panel a correspond to the transects in panels b and c.

In all plots the colours show air temperatures (blue colours are below 0°C, red colours above), the open contours show accumulated rainfall amounts (up to 20 mm w.e.) and the arrows in the first panel show 10 m wind speeds.

The colours show that temperatures begin to warm during this event as the atmospheric river flows over steep terrain, warming the surface over the ice shelves via the foehn effect.

Rainfall is highest over the slopes of the steep terrain – often falling at temperatures below 0 °C, making the rain 'supercooled'.

Precipitation processes

The amount of snow that falls is increased by the seeder-feeder effect, which is shown in the diagram. Ice and snow from a higher level “seeder” cloud falls into a lower level 'feeder' cloud, and cloud processes in the feeder cloud, multiplying the number of snow particles, and increasing the amount of snow that falls.

Meanwhile, sub-zero rainfall occurs, meaning the rain is falling as supercooled drizzle.

Video Summary

Watch the full video here

Terrain is important

Increasing precipitation over ice shelves and changing the type of precipitation that falls

Model resolution matters

Higher-resolution is essential for capturing the dynamics of rainfall and extreme snow during atmospheric rivers in this region

More observations are needed

Especially of rain!

Check out our Carbon Brief guest post for more details here