ATMOSPHERIC RIVERS

COMET MODULE SUMMARY

What Are Atmospheric Rivers?

• Anomalous areas of large/strong moisture transport– Long and narrow (like a real river)• Generally around 2000km long and 400-600km across

• Cause some of the extreme precipitation events and flooding

Identifying AR’s

• IR and WV satellite images aren’t the best for identifying an AR– You can see where there

may be precipitation but you can’t know for sure

• Better products are Integrated Water Vapor (IWV), Integrated Vapor Transport (IVT), and Precipitable Water (PW) satellite images

Identifying AR’s

• IWV images show the amount of water vapor present in a vertical column of the atmosphere– In the PNW, a system must have IWV values of at least 20mm to qualify as an AR

• In areas closer to the tropics this value will need to be higher because there is more moisture and heat readily available (it’s not a set value)

Identifying AR’s

• IVT images show water vapor transport within a column of the atmosphere– Systems must have high transport values in addition to water vapor content

• It’s like an actual river so if it’s not actually moving anything it’s not really a ‘river’• AR’s have values greater than 300 km m-1 s-1

Identifying AR’s• PW images are very similar

to IWV images except they show the depth of water that would be present if all the moisture in a column of atmosphere were condensed into liquid form– Also measured in mm so

typical AR values are at least 20mm (in the PNW)

– NWP also makes PW anomaly images that are helpful in diagnosing AR’s

Identifying AR’s

• A few of the AR’s in the image are circled– Notice the AR does not have to travel over land to be considered an AR

Precipitation Requirements

• Obviously no matter how much moisture is transported by an AR there needs to be lifting mechanisms for precipitation to occur

• These include:– Orographic lifting– Frontal boundaries– Instability– Isentropic ascent– Upper level disturbances

Precipitation Requirements

• A factor in how severe a precipitation event is, is the movement of the AR axis– If the axis stays relatively stationary, there is more

likely going to be an extreme event or flooding in that area

– If the axis moves (propagates) then the amount of precip in a given area will be much less and the risk of flooding much smaller

Extreme Precipitation Events

• Obviously in the PNW atmospheric lift readily occurs in AR’s due to the mountain ranges like the Sierra Nevada and Cascade ranges

• In places without large mountain ranges to create lift, other mechanisms must be present (like frontal boundaries, etc.)

May 2010 Tennessee Flood Event

• In May 2010, much of Central/Western Tennessee and Eastern Kentucky flooded due to an AR event

• The land there is relatively flat so the flooding was due to other factors

May 2010 Tennessee Flood Event

• Flooding occurred over two days (May 1-2, 2010)

• The event caused flooding in many regions within the Western Tennessee/Eastern Kentucky area

• Caused flooding in the Cumberland River area

May 2010 Tennessee Flood Event

May 2010 Tennessee Flood Event• The area that was affected

was relatively flat, so other mechanisms had to be present for the flooding to occur– In this case, there was

instability and outflow from MSCs in the area

– Image shows 250mb heights (grey lines), 850 streamlines (red arrows), IWV values greater than 45mm (light green areas), surface fronts (black lines), outflow boundaries (dashed lines), and radar echoes (dark green, yellow, and orange areas)