Chapter 9: Small-scale and Local Systems

Small- and local-scale Flows In the previous chapter, we discussed larger-scale winds. We found that these winds are driven by the pressure gradient force. Aloft, the Coriolis force is in balance with the PGF, creating geostrophic winds. At the surface, friction opposes the flow, forcing air to move toward low pressure and away from high pressure.

Small- and local-scale Flows These larger-scale flows occur at what is called the synoptic scale. There are other scales of flow in the atmosphere: Microscale flows occur over a few minutes and are typically short-lived (e.g., smoke from a chimney) Mesoscale flows occur over a few kilometers to about 100 kilometers, including things like sea breezes, thunderstorms, and small tropical systems Planetary scale flows cover the entire globe

Small- and local-scale Flows There are other scales of flow in the atmosphere: Microscale flows occur over a few minutes and are typically short-lived (e.g., smoke from a chimney) Mesoscale flows occur over a few kilometers to about 100 kilometers, including things like sea breezes, thunderstorms, and small tropical systems Planetary scale flows cover the entire globe Sometimes, the plantery and synoptic scales are combined into what is called the macroscale.

Small- and local-scale Flows

Turbulence Turbulence is a disturbed flow of air that can produce wind gusts and eddies. There are two common ways in which turbulence can be induced, both of which are related to friction in a fluid flow, which we call viscosity.

Viscosity Molecular viscosity is due to the random motion of the gas molecules in a fluid. Consider two layers of air on top of each other, one moving faster than the other, what happens?

Viscosity Molecular viscosity is due to the random motion of the gas molecules in a fluid. Consider two layers of air on top of each other, one moving faster than the other, what happens? Eddy viscosity is related to the internal friction induced by whirling eddies. Consider air moving over a landscaped dotted with trees and/or buildings; it breaks into several irregular, twisting eddies. What do these eddies do?

Turbulence In eddies, the wind speed and direction fluctuate very rapidly, producing irregular air motions that we call wind gusts. When eddies are formed via obstructions at the surface, we call this mechanical turbulence. NOTE: The effects of mechanical turbulence on air flows far exceeds the effects of molecular viscosity.

Turbulence Surface heating and instability cause turbulence to extend to greater altitudes. Thermals and convective cells that are formed when the Earth’s surface warms form vertical motions that create thermal turbulence. Thermal turbulence tends to be minimal in the early morning (no surface heating) and largest during the hottest part of the day.

Example of mechanical turbulence combined with thermal turbulence.

Turbulence This turbulence changes the wind profile in the planetary boundary layer (which is the region of the atmosphere influenced by surface friction)

Turbulence It is this same effect that causes the temperature at the surface to be higher on windy nights than on calm nights

Turbulence The depth of this mixing (and the frictional influence) depend on three main factors: Surface heating – produces a steep lapse rate and strong thermal turbulence Strong winds – produces strong mechanical turbulence Rough/hilly landscape – produces strong mechanical turbulence Combined, these effects can cause strong a gusty winds at the surface (i.e., Laramie)

Eddies Not all eddies are alike . . .

Eddies Not all eddies are alike . . . ≠

Eddies Eddies can be big or small. The size of the obstacle and the wind speed are the two primary factors that control the size of eddies. For example, air moving over a shrub produces a small eddy, while air moving over a mountain produces large eddies with much stronger motions.

Eddies

Eddies Not all eddies are formed at the surface… Turbulent eddies can also be created when the wind abruptly changes speed or direction, which we call wind shear. The shearing creates eddies along a mixing zone; in clear air, this is called clear air turbulence.

Eddies The shearing creates eddies along a mixing zone; in clear air, this is called clear air turbulence.

Eddies This can happen in the presence of moisture… Kelvin-Helmholtz waves

Local Wind Systems Eddies can result in local wind systems (e.g., sea breezes). Typically, these local wind systems are caused by differential surface heating (i.e., adjacent regions being warmed/cooled at different rates).

Local Wind Systems Initially, the isobaric (constant pressure) surfaces are all parallel to the surface. What is the wind like?

Local Wind Systems Initially, the isobaric (constant pressure) surfaces are all parallel to the surface. What is the wind like? Differential heating (warm to the south, cool to the north) lifts the isobars to the south, creating a PGF aloft. Now what is the wind like?

Local Wind Systems Initially, the isobaric (constant pressure) surfaces are all parallel to the surface. What is the wind like? Differential heating (warm to the south, cool to the north) lifts the isobars to the south, creating a PGF aloft. Now what is the wind like? The surface pressure decreases to the south as air is transferred northward aloft, creating a surface low. Now what is the wind like?

Local Wind Systems These circulations are called thermal circulations. The local regions of high and low pressure are called thermal highs and thermal lows. There a several common examples: sea breezes land breezes lake breezes mountain breezes valley breezes

Sea/Land Breezes During the day, the land warms faster than the adjacent water, creating high pressure aloft over the land. At the surface, air moves onshore. This is called a sea breeze.

Sea/Land Breezes During the day, the land warms faster than the adjacent water, creating high pressure aloft over the land. At the surface, air moves onshore. This is called a sea breeze. At night, the land cools faster than the adjacent water, creating a high pressure aloft over the water. At the surface, air moves offshore. This is called a land breeze.

Sea/Land Breezes Over a peninsula, e.g., Florida, sea breezes can form on both coasts, moving inland as the day progresses. The breezes converge, creating uplift to help promote thunderstorm development.

Lake Breezes The convergence of coastal breezes is not solely confined to regions next to oceans. Large lakes (e.g., the Great Lakes) are also capable of producing local wind systems, which are called lake breezes. Similar to Florida, small land areas that are surrounding by large lakes can also exhibit converging breezes, e.g., upper peninsula of Michigan.

Lake Breezes Think of these as small-scale cold fronts. Recall that frontal lifting is a mechanism to trigger convection.

Valley/Mountain Breezes During the day, valley walls warm faster than the air at the same altitude, creating a thermal low along the valley wall and rising air. We call this a valley breeze.

Valley/Mountain Breezes At night, the valley walls cool faster than the air at the same altitude, creating a thermal high along the valley wall and sinking air (cold air drainage). We call this a mountain breeze.

Valley/Mountain Breezes If the rising air due to daytime warming along mountain slopes is strong enough, cumulus clouds can form!

Katabatic Winds Katabatic winds typically originate over high plateaus that become snow-covered in winter. The snow keeps the surface temperature low, creating a shallow dome of high pressure. The PGF acts to push air toward low altitudes. As the air moves downhill, it warms via compressional warming.

Katabatic Winds Katabatic winds typically originate over high plateaus that become snow-covered in winter. The snow keeps the surface temperature low, creating a shallow dome of high pressure. The PGF acts to push air toward low altitudes. As the air moves downhill, it warms via compressional warming.

Katabatic Winds

Chinook Winds Chinook winds are warm, dry, downslope winds that descend the eastern slope of the Rocky Mountains, warming at the dry adiabatic lapse rate, i.e., 10 °C/km. (note, similar situations occur in other regions of the world; however, we use different names) When these winds move through, temperatures rise and relative humidities decrease very rapidly (RH < 5% is not uncommon). Chinook winds are enhanced when the air first rises over the mountains, condensing to form clouds, and then subsiding down the eastern slope – why does this enhance the warming?

Chinook Winds Air moving up the mountains cools; condensation begins, which limits the cooling. The air sinks down the eastern side, warming at the dry adiabatic lapse rate.

Santa Ana Winds Santa Ana winds are warm, dry winds that blows downhill from the deserts to the east and northeast of southern California and into the Los Angeles basin. Air is typically funneled through the narrow mountain valleys and can be very strong (~100 mph). They air is very dry because the air originates over the high desert. These winds are synonymous with high pressure forming over the Great Basin.

Santa Ana Winds Recall that this high pressure over the Great Basin creates a PGF toward southern California, which drives the strong winds. These winds are often associated with fast-moving forest fires in southern California.

Santa Ana Winds

Desert Winds Winds of all sizes form over desert regions. These winds often lift dust into the air, creating dust storms. In desert areas with loose sand, sandstorms are common. If dust/sand is lifted into the air from cold downdrafts along the leading edge of thunderstorms, a large, dark dust cloud forms, which is called a haboob.

Desert Winds Large haboob moving through Phoenix, AZ, on July 5, 2011.

Seasonal Winds Some thermal circulations extend over much larger areas than local sea and land breezes, e.g., the monsoon. Monsoon wind systems change direction on seasonal time scales. Such a system is well developed in eastern and southern Asia.

Seasonal Winds In winter, the air over the continent becomes colder than the air over the open water. A large, shallow high-pressure area forms over the continent. Air subsides and blows offshore.

Seasonal Winds In summer, the air over the continent becomes warmer than the air over the open water. A large, shallow low-pressure area forms over the continent. Air rises and blows onshore.

Seasonal Winds The rising onshore flow in summer is enhanced by the orographic boundary, allowing deep, precipitating clouds to form. Therefore, the summer is the wet season.

Seasonal Winds A similar seasonal change in circulation occurs of the southwestern US. Recall our conversations earlier in the semester regarding the abundant moisture being transported into the mountain west.