8.4: Cyclogenesis

Storm Tracks

The development of a mid-latitude cyclone is known as cyclogenesis. Certain regions in North America are more favorable for cyclogenesis, including the eastern slopes of mountain ranges such as the Rockies and the Sierra Nevada, the Atlantic Ocean off the Carolina Coast, and the Gulf of Mexico. When air flows westward across a north-south extending mountain range, the air on the leeward (downwind) side tends to have cyclonic curvature, which adds to the development of a cyclone. This is called lee cyclogenesis, and cyclones that are a result of this are often called lee-side lows/cyclones.

Cyclones may also develop near Cape Hatteras, North Carolina, where warm, moist air from the Gulf Stream can increase the north-south contrast in air-mass temperature and moisture to the point that cyclogenesis might occur. These cyclones are called northeasters (or nor’easters) and normally move northeast along the Atlantic Coast. These storms can bring heavy rain or snow, as well as high winds, to areas along the East Coast. Typical cyclone storm tracks are named after the region in which they form, like the Hatteras low, Alberta Clipper, or Colorado low. Alberta clippers and Colorado lows form or redevelop on the leeward side of the Rockies. Mid-latitude cyclones always move toward the east due to the prevailing westerlies.

What influences the strength of a mid-latitude cyclone and determines how long it will persist? Some surface conditions influence cyclogenesis, but the primary driver of mid-latitude cyclone development lies in the winds aloft. How does upper-level flow influence mid-latitude cyclones?

Mid-latitude Cyclone in Three Dimensions

Developing surface lows are usually more intense with height and appear on upper-level charts as a trough or a closed low. However, the low in the upper levels usually lies to the west of the surface low in the Northern Hemisphere. This is a necessary condition for a low-pressure system to continue developing and intensifying. If the upper-level low were directly over the surface low, the surface low would quickly dissipate. This is because winds converge inward toward the low, but only at the surface. This convergence at the surface causes the air mass to “pile up” and air density to increase just above the surface low. The increase in air mass causes surface pressures to rise, and the low fills in and dissipates. How then do cyclones intensify and develop? The air that accumulates at the surface must have an “exit path” out of the column, allowing the surface air pressure to continue decreasing and the cyclone to strengthen. The vertical structure of the atmosphere must allow air to rise from a low-pressure system at the surface.

Figure \(\PageIndex{1}\) shows an idealized model of the vertical structure of a cyclone and anticyclone in the Northern Hemisphere. A surface low and a surface high are accompanied by an upper-level trough and ridge, respectively.

Figure \(\PageIndex{1}\): This diagram shows how the jet stream influences surface weather, with upper-level convergence over a ridge linked to surface high pressure, and upper-level divergence over a trough linked to rising air, low pressure, and warm and cold fronts. Air converges on the upstream side of an upper-level trough and diverges on the downstream side. A ridge is associated with a high-pressure system on the surface, and a trough is associated with a low-pressure system. (CC BY-SA 4.0; Britt Seifert via Atmospheric Processes and Phenomena).

On the right-hand side is a Northern Hemisphere frontal cyclone exhibiting both a warm and a cold front. The cold air behind the cold front at the surface also extends upward into the air aloft. Recall that the layer between two pressure surfaces is thinner when the air temperature of the layer is cold (more dense), and thicker when the layer is warm (less dense). When pressure levels are more closely spaced, pressure decreases more rapidly with height in a column of cold air. Because of this, low pressure is found in a body of cold air aloft, just as you find behind a cold front, because the constant pressure surfaces are squeezed closer to the Earth’s surface. Thus, an upper low is often found in the cold air aloft to the west of, or behind, the surface low. The surface low tilts toward the northwest as it moves upward. Directly above the surface low, the airflow spreads out and diverges. This allows the converging surface air to rise and flow out of the air column at the tropopause, reinforcing vertical motion. When divergence in the upper levels exceeds convergence at the surface, surface pressures will be lower farther out, and the low will intensify and deepen. Put another way: when air exits the column more rapidly aloft than it enters the column at the surface, then the amount of air in the column will reduce, the surface pressure will lower, and the cyclone will intensify.

Note the convergence directly aloft of the high-pressure system. Here, the air mass increases aloft and piles up, while air flows clockwise (in the Northern Hemisphere) and out of the anticyclone at the surface. If air were able to flow freely out of the anticyclone, the air pressure would rapidly drop, and the anticyclone would dissipate. Therefore, to maintain or strengthen the high-pressure system, air must be continually added to the anticyclone. This happens when there is convergence above a surface high. The air that accumulates aloft sinks within the column, increasing surface pressure. If the convergence aloft is stronger than the divergence at the surface (more air is added than is removed), then the surface pressure will increase.

Winds at the 500-mb pressure level tend to steer surface low and high-pressure systems. Generally speaking, surface storm systems travel at approximately 16 knots in summer and roughly 27 knots in winter. This is due to stronger jets and upper-level flow in winter, resulting from greater north-south temperature contrasts.

Redistribution of Heat

Mid-latitude frontal cyclones are both a vital part of the global circulation and a result of it. They’re also an important pattern in the climatology of mid-latitude regions. The temperature gradients that cause frontal cyclones form as a result of the colliding surface air from the polar and Ferrel cells. The strong temperature gradient, characterized by cold air from the polar region and warm air from the tropics, is the energy source that drives frontal storms. On the other hand, frontal cyclones are a significant contributor to the global redistribution of heat. Warm air moving poleward in a warm front and cold air moving equatorward in a cold front are later stabilized after the temperature gradient balances itself by forcing the cold air aloft (occluded front). Frontal cyclones are thus both a result of and a contributor to global circulation and the redistribution of heat from the equator to the poles.

The chapter began with images of these storm systems extending across scales comparable to continents and landmasses. It also described how these storm systems last from days to over a week. Instabilities along the polar front are always growing and dying as they pass over fixed points on Earth. The point is that if you live somewhere along the storm track in the Northern or Southern Hemisphere during wintertime, these storm systems dictate your weather. For example, in Boston, Massachusetts, winter weather may look something like this: a few days of warm, clear weather, followed by a sudden change (a passing cold front) that results in a significant drop in temperatures and some heavy rain, then cold, dry weather for a few days. This cold weather gradually transitions to warmer conditions, accompanied by light rain and rising temperatures (warm front). Although variable, this pattern recurs week after week. Depending on the stage of the frontal storm as it passes over you, it may be more or less severe, and you may receive more or less rain, snow, or other wintry weather. Precipitation and temperature variations associated with frontal cyclones are a crucial component of the climatology of mid-latitude weather.