Airmasses, Fronts and Frontal Analysis: Understanding Atmospheric Processes, Slides of Topography

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AIRMASSES, FRONTS

and

FRONTAL ANALYSIS

Airmasses

• An airmass is a large body of air with relatively uniform

thermal and moisture characteristics.

• Airmasses cover large regions of the earth, typically several

hundred thousand square kilometers.

• Airmasses can be as deep as the depth of the troposphere

or as shallow as 1 to 2 km.

• Airmasses form when air remains over a relatively flat region

of the earth* with homogeneous surface characteristics for an

extended period of time.

(* Canadian and Siberian plains, cool oceanic regions such as the North Atlantic and Pacific, deserts, such as the Sahara and the American southwest, and tropical oceanic regions including the equatorial Atlantic and Pacific, and smaller water bodies such as the Caribbean Sea and the Gulf of Mexico).

Key features of an airmass on weather maps:

The centers of cold airmasses are associated with high pressure

on surface weather maps. High pressure develops in response to

cooling.

Fig. 13.4 SHW

Fig. 13H SHW

In winter, high-pressure centers form and are the dominant feature

over the northern parts of the continents of Asia and North

America.

In summer, when the oceans

are cooler than the

landmasses, large high-

pressure centers are the

dominant feature of the

atmosphere over the North

Atlantic and Pacific Oceans.

The high-pressure center over

the Atlantic is called the

“Bermuda High” because it is

centered near Bermuda,

while its Pacific counterpart is

called the “Pacific High”

Fig. 25.1 SHW

The centers of very warm airmasses appear as semi-permanent

regions of low surface pressure.

Examples: 1) Monsoon Low in summer over S.E. Asia

2) North American Monsoon Low over

Deserts of the U.S. southwest

3) Equatorial low pressure belt

4) Icelandic Low over the N. Atlantic

5) Aleutian Low over the N. Pacific in winter.

Note: Semi-permanent low-pressure centers differ substantially

from migrating tropical and extratropical low-pressure centers

associated with cyclones and hurricanes.

Not all airmasses are in contact

with the ground

Airmasses exist aloft in the troposphere, residing on top of other airmasses. Airmasses are three-dimensional, and the boundaries between airmasses are often quite sharp and distinct Stratosphere: a large airmass that covers the entire globe

Fronts: Boundaries between airmasses

Meteorologists classify fronts based on:

a) the thermal and moisture characteristics of the

airmasses

b) the direction of movement of the airmasses

c) whether the boundary between the airmasses is

in contact with the ground (a surface front), or

can only be found aloft (an “upper level front”).

Identifying a cold front on a surface weather map:

1. Surface front is located at the leading edge of the strong temperature gradient.
2. Surface front is located at the leading edge of accompanying strong dewpoint gradient.
3. Surface front coincides with pressure trough – pressure dropping as the front approaches and rising after it passes.

Identifying a cold front on a surface weather map:

4. Cold air is advancing with time, replacing warm air.
5. A sharp wind shift, generally from winds with southerly component to winds with westerly or northwesterly component, will occur at front.
6. Precipitation may be behind, along, or ahead of front (or may not be present) but when present, it will be organized along a line.

Leading edge of a cold front on radar often appears as a very

Long narrow line of high reflectivity

Cold Fronts : The leading edge of a cold front often appears

like a density current, with a head, waves, and accompanying

sharp discontinuity in temperature, wind, and moisture content.

Shapiro et al. 1985, MWR, Fig. 2.6 Bluestein Vol. II

Data from 600 m high tower called the Boulder Atmospheric Observatory. A cold Front passed the tower On 19 Sep 83. Potential Temperature Wind Barbs Temperature Wind Speed

Deep Tropospheric Cold

Front

Boundary between airmasses is marked by a sharp gradient in temperature Temperature (C) Potential Temperature (K) Wallace and Hobbs 1977 Fig. 3.19/3.

“Back-Door” cold front:

A cold front that approaches an area from the east Back door cold fronts are common in New England and along the east side of the Rocky Mountains From Bluestein Vol II, Fig. 2.