A6 Extreme Precipitation and Flooding

Topic

Extreme precipitation encompasses events where rainfall rates or total water accumulations vastly exceed a region's local climatology, directly causing floods, landslides, and loss of life. Three distinct physical mechanisms can produce extreme precipitation: atmospheric rivers—narrow filaments of water vapor transport connecting the tropics to mid-latitudes; slow-moving or stationary fronts, which organize convection over a single area for hours or even days; and orographic amplification, which forces water vapor to rise and condense when moist air encounters a mountain range.

Of these three mechanisms, atmospheric rivers are responsible for the majority of extreme precipitation events on the western coasts of mid-latitude continents, such as those of California, Portugal, or Chile. An atmospheric river is identified by two simultaneous indicators: first, the total amount of water vapor accumulated in its air column exceeds twenty-five millimeters of precipitable water—far higher than levels in the surrounding air; second, the horizontal flux of that water vapor—the amount passing through a section of the atmosphere per second—exceeds two hundred and fifty kilograms per meter of width per second, concentrated in a band less than a thousand kilometers wide that transports, in total, several hundred kilograms of water vapor per meter of that band every second. The longer an atmospheric river remains stationary over a region, the greater the total rainfall accumulation it leaves behind; an atmospheric river persisting for one or two days can deposit between two hundred and four hundred millimeters of water on a region. In the Mediterranean Sea, another mechanism specific to the area—known as an isolated upper-level depression or "cold drop" (*gota fría*)—produces extreme precipitation by combining the advection of cold air at high altitudes with convection fueled by the warm Mediterranean Sea itself; Unlike the atmospheric river scenario, here the moisture is not supplied by a filament of water vapor, but rather by the warm Mediterranean Sea itself evaporating beneath cold air that has descended from high altitudes.

Orographic amplification—the third mechanism—operates differently from the previous two: instead of adding more water vapor, it forces moist air to rise up the windward slope of a mountain range; this forced ascent cools the air and causes its water vapor to condense into rain. The steeper the orographically forced ascent, the heavier the precipitation on the windward side; orographic amplification can multiply windward rainfall by two to five times compared to the amount falling on adjacent plains.

Regardless of the mechanism producing the extreme precipitation, whether that rain results in a flood depends on the condition of the soil receiving it. When the soil is already near field capacity—meaning it had high moisture levels before the rain began—virtually all the additional rainfall from the extreme event turns into runoff rather than infiltrating the ground, resulting in saturation flooding. Conversely, when the soil is impermeable, or when rain falls with very high intensity—even on soil that would otherwise absorb water—the water fails to infiltrate in time, resulting in rapid-runoff flooding.

Thus, saturation flooding and rapid-runoff flooding are the two pathways through which extreme precipitation—whether originating from an atmospheric river, an isolated upper-level low over the Mediterranean, slow-moving or stationary fronts, or orographic amplification—ultimately translates into actual damage: the floods, landslides, and loss of life that opened the description of this phenomenon. The atmospheric mechanism delivering the water determines how much rain falls and over which area, but it is the state of the ground that ultimately decides just how destructive that extreme precipitation proves to be.

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