WX BRIEF

Droughts and floods

Too much-or too little-rain can have a huge impact on aviation.


Polar view of the 300-hPa pressure level (about 33,000 ft) and the Northern Hemisphere polar jet. The large waves in the jet create troughs and ridges that can support drought and flood conditions over the middle latitudes.

Residents of India or Bangladesh, for example, routinely find themselves wading knee deep in water where once were roads. However, such floods tend to subside relatively quickly as the water drains toward the coast.

In addition, due to the regularity with which these monsoons affect these regions, transportation and communication networks are likely to be less affected for long periods than in locations with sporadic flooding.

Unfortunately, the flooding that causes the greatest adverse impacts often comes from a combination of jet stream configuration and river drainage basins. The jet stream lies more or less over the midlatitudes, and brings needed rainfall to the breadbaskets of America, Europe and Asia.

Australia, New Zealand, Argentina, Chile and South Africa benefit from the Southern Hemisphere jet. However, the jet stream meanders, and several long waves-spanning half a continent-result in a series of large trough-and-ridge patterns.

A look at a polar 300-hPa chart will reveal these undulations. The track of the jet stream is also the track of most midlatitude weather systems. The lows at their centers tend to move along beneath the jet. A favorite track of these rainmakers is to move up from the base of a trough into the backside of a ridge.

If there is warm, humid air downwind of the system-as is often the case beneath the downwind ridge, this air provides the heat and moisture needed to keep the storm churning and sending one thunderstorm after the next barreling across the landscape. Ordinarily, this means a few hours of rain as the system moves through.

By the time the next system has developed, the wave in the jet has moved the trough and ridge to new positions and a different area gets the rain. But, occasionally, the jet gets stuck in the same pattern for several weeks.

When this happens, storm systems take the same path every few days, bringing more rain to the region over a short time span than its river system is designed to remove. Drought can be even more complicated.

There is no one definitive definition of drought. Under that heading, we can have meteorological drought, agricultural drought, hydrological drought-even socioeconomic drought. In a general sense, drought can be seen as a lack of precipitation sufficiently out of the ordinary to cause an adverse effect on some system or another.

Dried lake bed in Woolly Hollow State Park in Arkansas. Though drought may not affect aviation directly, indirect costs can be high as prices for commodities increase.

Interestingly, the lack of rainfall that normally precedes and accompanies a drought is caused by the same pattern of a stationary jet stream. While the flooding rains track along the jet itself, the area both above a trough and beneath a ridge can suffer a long absence of rain-producing storms.

Neither the atmosphere at the surface nor that aloft is supportive of convection. In this situation, a drought beneath the ridge tends to be worse from the standpoint that this is where hot air tends to reside.

Temperatures in the 100s F (40s C) under clear skies can buckle pavement and cause electrical brownouts and blackouts as inhabitants try to stay cool with air conditioning. Drought is also a problem in places that rely on the heavy monsoon rains to provide sufficient water for the year.

These subtropical locations may be dry for much of the year, but the annual migration of the monsoon front can fill wells and irrigate fields to produce abundant rice yields. Occasionally, larger-scale factors at work, such as El NiƱo, cause changes in the monsoon patterns.

In such cases the front may be delayed from reaching an area, may not reach it at all or may provide insufficient rain. When this happens, subtropical communities that rely on the seasonal rainfall can be severely affected, and regions that had once been stable for business aviation may quickly become no-fly zones as desperate factions battle over scarce water and food supplies.

In addition, droughts present another indirect hazard to aviation. As the vegetation in a normally verdant region dries, the chance of a random spark igniting a wildfire increases dramatically.

Without help from dampening rain clouds, firefighters have a much tougher time battling these blazes, which may grow to consume hundreds of thousands of acres and cast a smoky pall over busy flight corridors and airports.

Droughts and floods in a changing climate

Consider that many things are not proofed against extremes because the cost was deemed to outweigh the benefit of such resilience. Now, consider that most climatologists are comfortable with saying that the climate is changing and this could mean more frequent floods and droughts in many places that experience them.

People are already wondering how two 500-year floods could strike the same region within a decade. But if a changing climate causes more frequent recurrences of extremes such as a 500-year flood level, that level will quickly become a 100-year or even a 50-year flood.

Despite flooding in the Midwest, parts of the southeast, south central and western US were in severe to extreme drought at the end of Jun 2008, as identified by the weekly US Drought Monitor, available from www.drought.unl.edu.

In addition, engineers continue to build higher levees and dykes to keep swollen rivers in their banks and protect surrounding communities built on the river's flood plain. And never mind that this means the river will be carrying much more water when it does top its banks, and that levee breaks may occur unexpectedly, hundreds of miles downstream from where the excess rain fell.

Likewise, many of the aquifers on which we rely to sustain irrigation when rainfall is insufficient are being depleted at rates faster than they are being replenished. When they are dry, those regions will have no fallback when drought comes.

Such a situation will only serve to worsen the impacts of any drought that nature has generated. Needless to say, if floods or droughts become more frequent and severe, the impact on aviation will also increase.

These extreme events make regions inaccessible, either due to natural obstacles, such as flooded airports or roads, or due to social instability. They also disrupt transportation, communication and energy networks, which can quickly conspire to hinder the movement of aircraft as a result of power losses, road and rail blockages by flood waters, or buckled rails and asphalt.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University and was a scientist with NASA's Global Change Master Directory. Shein holds a commercial license with instrument rating.

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