Formation of the Extratropical Cyclone (Cyclogenesis)

Cyclogenesis Extratropical cyclones often form when an upper level trough (e.g. a shortwave) approaches a front that is stationary or moving relatively slowly. In order for a low pressure system to form at the surface, there must be net divergence in the column of air and the air must start to rotate counterclockwise.

Cyclogenesis (Cont.) During the development of the extratropical cyclone the system typically tilts to the west with height. This means that the upper level trough is usually found to the west of the surface trough or low.

Cyclogenesis (Cont.) N E Upper Level Shortwave at 500 mb Upper Level Divergence Positive Vorticity

Cyclogenesis (Cont.) H H Colder air mass N E Upper Level Shortwave at 500 mb H Warmer air mass

Cyclogenesis (Cont.) Once the stationary front begins to rotate cyclonically the eastern section becomes a warm front as warmer air begins to move north and to push out colder air. The western section of the front begins to move south and becomes a cold front as the colder air begins to push out warmer air.

Cyclogenesis (Cont.) Colder air is denser than warmer air and the colder air is more affected by the gravitational force. The colder air tends to stay near the surface, while the warmer air tends to rise over denser colder air.

Cyclogenesis (Cont.) The different densities mean that the warmer air often has a hard time pushing out the denser colder air. By contrast the denser colder air has much less trouble pushing out the less dense warmer air.

Cyclogenesis (Cont.) The net result is that cold front moves faster than the warm front and a bend or kink develops along the front. The bend in the front is called a wave on the front (or frontal wave).

Wave on the Front H H H N Retreating cold air mass E 500 mb flow Warm air mass H Developing new cold air mass

Extratropical Cyclone H E H L H

Changes at 500 mb As the colder air moves southward behind the cold front, it causes the 500 mb heights to decrease As the warmer air moves northward with the warm front, it causes the 500 mb heights to increase ahead of the developing surface low.

Changes at 500 mb (Cont.) The effects of the advection of the warmer air is to build the downstream ridge. The effects of the advection of the colder air is to deepen the trough. Thus, the thermal advection increases the amplitude of the shortwave.

L Changes at 500 mb (Cont.) H H H N 500 mb flow E Warm air advection builds the downstream ridge. L Cold air advection deepens the trough at 500 mb. H

Clouds and Precipitation The movement of the fronts tends to produce favored regions of rising and sinking air that may produce clouds and precipitation.

Extratropical Cyclone .. .. 2 N H 500 mb flow .. .. .. E .. H L 3 H 1 4

Cross-Section of a Warm Front Clouds Warmer air rises over denser colder air Warm front aloft Warmer air mass Colder air mass 1 2 Precipitation Warm front at surface

Warm Front (Cont.) Along the warm front the warmer air is forced to rise over the retreating colder air in a process called overrunning. If there is sufficient water vapor in the air, then the process will produce clouds and may produce precipitation.

Warm Front (Cont.) Far ahead of the warm front the first clouds are the high clouds made up of ice crystals called cirrus clouds. The clouds gradually become lower and thicker and the more gradual slope of the warm front tends to produce the layered stratus clouds.

Warm Front (Cont.) Precipitation is typically found ahead (i.e. to the north) of the warm front. During the colder seasons the precipitation may be rain, sleet, freezing rain or snow. During the warmer seasons the precipitation is typically rain.

Warm Front (Cont.) If enough precipitation evaporates back into the air, then fog or mist may occur.

Warm Front (Cont.) North of a warm front: Temperatures are colder; The winds are from the east; It is often cloudy; The dew point temperature is lower; Precipitation may be falling.

Warm Front (Cont.) South of a warm front: Temperatures are warmer; The winds are from the south or southeast; The sky may be partly cloudy or clear; The dew point temperature is higher; The precipitation may end.

Cold Front Cold front aloft Colder air mass Warmer air mass 4 3 Cold front at surface

Cold Front (Cont.) The slope of the cold front is typically much steeper than the slope of the warm front. The steeper slope usually produces more vertical clouds like the cumulus and cumulonimbus clouds.

Cold Front (Cont.) If the air ahead of the cold front is sufficiently warm and moist, then the lifting by the cold front may produce thunderstorms and heavy rainfall.

Cold Front (Cont.) East of a cold front: The temperatures are warmer; The winds are from the south or southwest; It may be cloudy or partly cloudy; Thunderstorms and heavy precipitation may be occurring; The dew point temperature is higher.

Cold Front (Cont.) West of a cold front: The temperatures are colder; The winds are from the north or northwest; It may be cloudy or partly cloudy; Showers may occur if the air is unstable; The dew point temperature is lower.

Warm Sector The region between the advancing cold front and the warm front is called the warm sector. The weather in the warm sector can be highly variable depending on the source region of the air mass and its temperature, humidity and stability.

.. .. .. .. .. L The Occluded Front H H H N E Cut off Low at 500 mb 500 mb flow .. H H .. 6 5 .. H

Occluded front aloft warm air aloft Remainder of warm front aloft Advancing colder air mass Retreating colder air mass 5 6 Occluded front at surface

Occluded Front (Cont.) By the time an occluded front forms the upper level trough is located almost directly above the surface low and the system is said to be vertically stacked. If the pool of cold air aloft becomes separated from the circumpolar vortex, then a cut off low may form.

Occluded Front (Cont.) The development of an occluded front usually indicates that the extratropical cyclone has peaked in intensity. Sometimes a new surface low forms at the triple point where the occluded front, the warm front, and the cold front meet.