Atmospheric rivers over California’s wildfire burn scars raise fears of deadly mudslides

Subject: Geography

Context:

Rivers of muddy water from heavy rainfall raced through city streets as thousands of people evacuated homes downhill from California’s wildfire burn scars amid atmospheric river storms drenching the state in early January 2023.

Atmospheric river storms:

Atmospheric rivers are long, narrow bands of moisture in the atmosphere that extend from the tropics to higher latitudes. These rivers in the sky can transport 15 times the volume of the Mississippi River.
- Atmospheric rivers are typically located within the low-level jet, an area of strong winds in the lower levels of the atmosphere, ahead of the cold front in an extratropical cyclone.
When that moisture reaches the coast and moves inland, it rises over the mountains, generating rain and snowfall. Many fire-weary westerners welcome these deluges, but atmospheric rivers can trigger other disasters, such as extreme flooding and debris flows.
Atmospheric rivers occur globally, affecting the west coasts of the world’s major land masses, including Portugal, Western Europe, Chile and South Africa.

Also called “Pineapple Express” storms that carry moisture from Hawaii to the United States West Coast are just one of their many flavours.
- In the 1960s meteorologists coined the phrase “Pineapple Express” to describe storm tracks that originated near Hawaii and carried warm water vapour to the coast of North America.
- By the late 1990s atmospheric scientists had found that over 90 per cent of the world’s moisture from the tropics and subtropics was transported to higher latitudes by similar systems, which they named “atmospheric rivers.”
Wildfire burn scars are particularly risky because wildfires strip away vegetation and make the soil hydrophobic – meaning it is less able to absorb water.
A downpour on these vulnerable landscapes can quickly erode the ground, and fast-moving water can carry debris, rocks and mud with it.

Classification:

Atmospheric river classification scale ranks the storms from 1 to 5, similar to systems for categorizing hurricanes and tornadoes.
- Atmospheric River category 1 (AR1) and AR2 storms caused estimated damages under $1 million.
- AR4 and AR5 storms caused median damages in the 10s and 100s of millions of dollars
- The most damaging AR4s and AR5s generated impacts of over $1 billion per storm. These billion-dollar storms occurred every three to four years.
The most significant finding was an exponential relationship between the intensity of atmospheric rivers and the flood damages they caused. Each increase in the scale from 1 to 5 was associated with a 10-fold increase in damages.

Climate change intensifies the risk:

Soot and ash deposits on snowpack can increase snowmelt, change the timing of runoff and cause snow-driven flooding.
Atmospheric rivers are predicted to grow longer, wetter and wider in a warming climate.
- Greenhouse gases trap heat in the atmosphere, warming the planet. This causes more water to evaporate from oceans and lakes, and increased moisture in the air makes storm systems grow stronger.
- In dry conditions, atmospheric rivers can replenish water supplies and quench dangerous wildfires. In wet conditions, they can cause damaging floods and debris flows, wreaking havoc on local economies.