B. Geerts and E. Linacre

6/'98

Heavy precipitation causes flash floods, especially in regions where rainfall is low overall. Such floods kill about 140 people annually in the USA. In the US they are most common between 6 pm and 3 am.

They result from slow-moving, locally developing thunderstorms, or else from a line of thunderstorms with little line-normal movement. The first case happens when low-level wind and upper-level winds oppose and cancel each other, so that the storm moves very little, yet it receives a continuous supply of low-level warm, moist air. It then deposits all its rain onto the same watershed. In the second case, the area is sequentially the recipient of rain from a series of storms. This process, called ‘training’, usually happens along a stationary front. Along coasts flash floods can also result from land-falling tropical cyclones, subtropical lows, or frontal lows, in particular when these lows are slow-moving and produce a strong low-level onshore wind.

(photo courtesy US National Weather Service)

Fig 1: A Doppler radar dome

In the USA a Doppler radar network, dense enough to cover the entire country, is operational to monitor flash-flood producing rainfall accumulations in watersheds. The radars (Fig 1) scan the watersheds continuously and the radar reflectivity is a measure of the instantaneous rainrate. The spatial resolution of these rainrate estimates, about 1x1km, is much higher than that of the conventional network rain gauges. Integrating these rainrates over time (Fig 2) and over the area of the watershed gives an instantaneous estimate of the rain volume that will run off into a river. Once a critical rain volume is reached, alerts will be disseminated to communities close to the river or creek in question.

Fig 2: An example of radar-estimated storm-total precipitation. The brighter the colour, the more precipitation accumulated. 5 to 7.5 cm fell in the yellow areas, the maxima (in orange) indicate precipitation totals over 7.5 cm.