Earth Sciences 80A - Lecture 20

Weather: Winds and Rainstorms

The major cause of property loss and death from natural disasters is associated with floods resulting from severe weather.

Having considered the long-term properties of the climate system, and the gradual changes that have affected the Earth, we will now focus on the short-term processes associated with weather. Driven by temperature-induced air pressure variations, winds blow on the surface of the Earth, with a great variation in force. These winds transport both dust particles, providing a source of both damaging dust storms and beneficial windblown sedimentary deposits, and water, resulting in storms and hurricanes as well as beneficial rainfall. In a sense, we must take the bad with the good, as weather circulation plays a key role in the critical water cycle at the surface, without which we could not exist.

First we begin by considering what is wind? The working definition for us is horizontal motions of the atmosphere driven by atmospheric pressure differences. Winds blow from high pressure regions to low pressure regions. The strength of the pressure differences determines the wind speeds, which have a variety of categories:

The familiar names reflect the fact that wind is highly variable, or gusty, with turbulent flow. This is very common experience for all of us. Some of the types of winds include daily sea breezes, driven by differential pressures over land and sea associated with daily heating and evening cooling. There are storm updrafts and downdrafts, notably associated with thunder showers. Tornadoes are intense localized wind cylinders with winds of 150-300 mph. There are drainage winds, or catabatic winds, which involve heavy cool air driven down off of mountain sides or glaciers. In the Alps these are called Foehn, while in the Rockies these are Chinook winds. Winds driven in off of desert regions are called Santa Annas, haboobs and harmatans. The diversity of wind forms reflects the complex factors influencing wind pressures such as soil types, geography, diurnal heating patterns etc.

Wind varies continuously at any one location, as the complex dynamic system of the atmosphere is rich in all scales of motion from very local to global patterns. The variability of winds is recorded by anemometers, or wind meters in various locations. Prevailing wind patterns associated with large-scale atmospheric motions interact with local conditions to drive the specific wind history at any point, such as the fluctuating sea breezes along the Monterey Bay. This is also true of regions with recurring weather patterns, such as tornadoes.

Tornadoes arise from east ward moving high winds overriding northwestward moving lower winds, which sets up strong spinning circulations that can spin-up and establish stable cyclonic motions. The central midwest experiences many tornadoes in a tornado alley associated with the jet stream interactions with winds off of the Gulf of Mexico. While individual tornadoes are not uncommon, large outbreaks of hoards of tornadoes occur about every 50 years in the mid-west to eastern U.S., strung out along trajectories that track the jet stream. While the conditions favoring the origin of tornadoes are well understood, the specific behavior of localized wind conditions is difficult to predict accurately, and great emphasis is on rapid detection by Doppler radar of the strong wind shears that accompany tornado conditions.

Winds and pressure differences can be directly destructive as in the case of tornadoes, but the power of wind is greatly enhanced by the presence of dust and water suspended by the atmospheric turbulence. Sediment is transported by wind with a weight dependence due to the resisting effects of gravity. Wind drags along sediments, either by saltation or bouncing along the surface or by suspension. Big particles, such as sand grains tend to stay low in the atmosphere, driven by drag of the passing wind. Turbulence of the wind is not very important, and the sand grains transport only moderate distances of 1 cm to 1 m, mainly by saltation. When there are large concentrations of sand, wind can sweep into formations called dunes, with a elongate side in the upwind direction called the stoss, and a steep downwind side called the lee. Sand particles move up the stoss by saltation, reaching the brink, and then avalanching down the lee face. The angle of repose is about 31 degrees for typical sands, and dipping layers build outward on the lee side. Effectively, the dune moves downwind, with the cross sets parallel to the lee face defining rock layers if the dune is buried and cemented. Dunes are often important in the rock record because they indicate past wind directions and climatic conditions, they often are sources of water or oil, and modern dunes are great places to play on. Small particles, such as silt and clay are able to transport much longer distances, effectively being suspended in the air by turbulence, which overcomes the resisting gravity. When the winds die, the fine materials settle out and leave deposits called Loess, which is often very rich agriculturally.

The processes of moving sediment by wind have thresholds, where wind speeds must exceed certain levels in order to get particles of a given size to move. This places great importance on rare large wind events, as these are the cases where nonlinear thresholds trigger substantial events. An example is the December 20, 1977 San Joaquin valley dust storm, which was prompted by a very high pressure system sitting over Idaho and Utah, driving winds westward toward and off-shore low pressure region. The winds blew down valleys trending westward from the Sierra Nevada, and achieved speeds of 200 mph. This sufficed to entrain huge amounts of sand and dust into the wind, causing loss of visibility on Interstate 5 and huge car accidents and fatalities. Massive erosion of top soil resulted as well, due to the lack of ground cover resulting from agricultural practices.

While wind-borne dust constitutes an important and sometimes deadly geological agent, far more important is the water carried by wind. Transport through the atmosphere is one of the most important stages of the water cycle. If we consider where water is at any given time in the water cycle, some 97.6% is in the oceans, 1.9% is ephemerally locked up in ice caps and glaciers, 0.5% is in ground and soil water, 0.02% is in rivers and lakes, and 0.0001% is in the atmosphere. But large quantities cycle through the atmosphere, feeding into each of the other reservoirs. The water transported by the atmosphere is of great importance to humans, and when there are excess rainfalls that cause floods, this water becomes one of the deadliest of natural catastrophes. The conditions at the surface of the Earth are such that water can exist in all three phases (gas vapor, liquid and frozen solid), and the fluid is quite dense and chemically reactive, which makes it an effective erosional agent both mechanically and chemically. Water has a low viscosity, which enables its rapid transport on the surface as well, which enhances its role in erosion, sediment deposition, and ground penetration.

The water cycle involves evaporation, precipitation, runoff, ice storage, groundwater, and transpiration from plants. Long-term climatic conditions can greatly modify the water cycle acting at any given time, as do global atmospheric patterns. For example, the latitudinal variation in annual precipitation shows an equatorial peak of about 80 inches/yr, minima of 40 inches/yr at 30 degrees north and south (the limits of the Hadley cells at low latitudes) and little rainfall at the poles. Within the latitudinal structure there is great variability of weather and rainfall, strongly influenced by geography and wind patterns. In the U.S., some of the most common weather patterns include thunderstorms, which pose hazards due to lightning (responsible for 100-150 deaths/yr, about the same as tornadoes), hail and flashfloods. An example of the latter is the Big Thompson Flood in the Colorado Rockies, which took place on July 31, 1976. Beginning at 8:30 PM, there was 10 inches of rain in 90 minutes, which lead to a swollen river that flooded highway 134, drowning 139 people. Flashfloods tend to be most intense in arid regions where runoff quickly gathers into canyons and other drainages. The primary region of U.S. thunderstorms is in the southeast, where 50-100 occur each year, while fewer than 5 occur annually in California. Hailstorms are concentrated in the Rock Mountains, where strong upwelling currents interact with cold air to produce large frozen aggregates. 4-8 occur annually in Colorado and nearby states.

Even larger storm systems tend to spawn in low latitudes where warm tropical oceans provide large water evaporation. These include hurricanes, also known as cyclones, typhoons, and willywillies. Hurricanes originate where surface ocean temperatures are more than 27 degrees C, which tends to limit them to the late summer season. These storms are large enough that corriolis forces play a significant role, so they 'spin' counterclockwise in the northern hemisphere (tracking a squall line in toward the low pressure center) and clockwise in the southern hemisphere. Winds in excess of 70 mph define a hurricane, with speeds of 150 mph characterizing the largest storms. Hurricanes are notable for their intense, damaging winds, the high rate and duration of rainfall that accompanies them, and the intense storm surge that is caused by the low pressure region at the eye. Yearly storms generate near the equatorial Atlantic, feeding into the Caribbean and southeastern U.S., in the northern Indian Ocean feeding into southern Asia, and in the far western Pacific, feeding into southeast Asia, Philippines, China and Japan.

Massive U.S. losses have resulted from hurricanes. Hurricane Andrew caused about $30 billion in damage, while Hugo caused over $10 billion. This is comparable to the damage from large earthquakes such as the 1989 Loma Prieta event ($9 billion), and the 1994 Northridge event ($30 billion). The 1900 hurricane which hit Galveston caused 6000 deaths, but warning systems have kept loss of like down to several hundred for each of the more recent events this century.

The large storm 'footprint' associated with a hurricane, involves the large area over which rain is delivered to the surface. Hurricanes can deposit huge volumes of water in short times, which quickly overload the intrinsic drainage capabilities of the landscape, resulting in flooding. Basically, there is a balance between rainfall input, infiltration as groundwater and runoff. Runoff occurs whenever rainfall rate exceeds infiltration rate, which is common for hurricanes. For example, hurricane Agnes in 1972 dumped 28 cm of rain in 18 hours over 93,000 square kilometers. The water dumped on a landscape transports through a network of streams, feeding from first order streams to higher and higher order streams connecting throughout a given drainage basin. The distance and character of each hillslope length, leading to the nearest stream channel influences how quickly runoff gets into the stream, where it can transport quite efficiently. For example, a paved slope causes very rapid transport of water over the surface, while a vegetated slope allows water to move only slowly through a tortuous path. The response of a drainage system is characterized by a flood hydrograph, which is a plot of time history over which a given input from a storm drains out of a system. This will give a spread out curve for the entire drainage basin, the shape of which determines whether flooding takes place.

For flooding, there are important characteristics of the storm as well as of the landscape: Storm Characteristics

1. rainfall intensity
2. rainfall duration
3. footprint of the storm (how much/many drainage basins are covered)

Landscape Characteristics

1. vegetation
2. infiltration rate (soil type)
3. pore space available; soil thickness, antecedent rain
4. hillslope lengths (distance for overland transport to stream)

Every channel experiences fluctuations in water level, which results in some characteristic stream morphologies. The active river channel is the low region in a river bed, usually banked by course levees, and surrounded by a river floodplain. If the region is uplifting, or has previously had higher water levels there may be fluvial terraces of higher level floodplains that are now abandoned. It is important to recognize that the floodplain is intrinsically part of the river during floods that overbank the levees, which may happen as often as every 2-3 years. Sediment is transported in the river, with analogous behavior to sediment in the atmosphere. Finer silts are suspended and travel in the turbulent flow for long distances, while coarser materials travel as bedload, over short distances with threshold behavior. When floods occur, coarse materials are dropped out near the levees, while silt and clay are deposited out on the floodplain, which makes these rich agricultural areas.

We can try to characterize river flood behavior based on the time between different levels of peak water discharge. This reflects the regional storm statistics, and gives general probabilities upon which to base mitigation efforts. If we plot peak annual discharge versus the logarithm of time between recurring levels of comparable height, a linear curve is found for most streams, with the bankfull stage (maximum channel capacity) typically being achieved every 2-3 years. Higher levels occur less frequently, and we can extrapolate the curve to estimate how large the flow will be for rare events like the 100 year flow. Regions with different flood hydrographs display different behavior for the recurrence times.

Some regions, such as Bangladesh are particularly prone to flooding disasters. This country is very low elevation, and encompasses the Ganghes-Brahmaputra delta. It is densely populated, with about 100 million people, and there are massive cyclones every other year (60 killer events over the past 120 years). The lack of high ground, high population density, and lack of warning systems add to the human toll. On November 13, 1970 1 million people died in a massive flood from monsoon rains. This recurs time and again. In the U.S. flooding disasters have been far less fatal, with the highest toll being 600 lives lost in the 1938 floods in New England. Modern warning systems, more favorable geography, and less common massive storm occurrence reduce the risk in the U.S..

Return to the Earth Sciences 80A Lectures Page

Page maintained by Thorne Lay.
This page was last reviewed on 3/09/01.