Tuesday Weather Topic in Greater Depth

From: Subject: Tuesday Weather Topic in Greater Depth Date: Thu, 8 Oct 2009 10:02:10 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/online/archive/course/09_fall/f09w04t_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Tuesday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Tuesday, 29 = September 2009=20

ESTIMATING WARM or COLD AIR ADVECTION

The term "advection" is a generic term used to describe the = horizontal=20 transport of any atmospheric property by the wind. While the most common = examples are warm air or cold air advection, the term can be used to = describe=20 other horizontal transport processes such as moisture advection, where=20 atmospheric water vapor is transported by wind. Occasionally, warm or = cold air=20 advection is described simply as "temperature advection".

How can we tell if warm or cold air advection were occurring? In = theory, in=20 order for advection of air of different thermal characteristics to occur = several=20 factors must work in concert. One consideration is that the atmosphere = must be=20 in horizontal motion, with stronger winds increasing the chances for = advection=20 to occur. Calm conditions do not produce thermal advection because no = horizontal=20 transport takes place. Secondly, some horizontal differences in air = temperature=20 must exist over the region. A large air mass in its source region will = have=20 horizontally homogeneous thermal properties. Consequently, not much = temperature=20 advection will be exhibited within the center of the air mass itself. = Finally,=20 the winds must flow fairly directly across the zone of strongest = horizontal=20 temperature contrast in order for the horizontal transport of = atmospheric=20 thermal properties to occur.

Practically, one way to identify the type of thermal advection is to = look at=20 how the air temperature at a given location changes over a given time = interval=20 and look at the wind direction. Consider the cold air that moved = southward on=20 northerly winds (winds from the north) across the northern Plains at the = end of=20 last week. Another example would involve cold air following a cold = front. Over=20 the course of the day, strong cold northwesterly winds after frontal = passage=20 would cause air temperatures at individual stations to decrease as the = cold air=20 moved southward. Some stations may report falling temperatures = throughout the=20 daylight hours. Much of this cooling at individual stations occurred = over a=20 24-hour interval and could be attributed to cold advection. However, one = must be=20 careful with selecting the time interval for temperature comparison. A = cooling=20 in the late afternoon and early evening may be as much the part of the = usual=20 diurnal heating cycle associated with nighttime radiational cooling = rather than=20 with cold advection.

How can we tell where temperature advection patterns are occurring = from a=20 surface weather map? You will have to look at both the air temperature = and wind=20 flow patterns on the map. To determine the temperature pattern, an = isotherm=20 analysis of the map will be needed. Remember that when isotherms are = drawn=20 connecting those stations having the same temperatures, the isotherm = patterns=20 will describe the overall temperature patterns observed across a = geographic=20 region.

Next, you will need to estimate the wind speed and wind direction for = the=20 region in question. If you have a map with plotted wind information = (with the=20 traditional wind barbs and wind arrows on the standard surface station = model),=20 you should lightly draw some arrows on your chart that parallel the = general wind=20 direction over the region so as to represent the typical wind flow for = that=20 region. The wind speeds can be read from the number of wind barbs.=20 Alternatively, if you had a map with isobars that analyzed the pressure = pattern,=20 you can estimate the winds from an extension of the "hand-twist" model. = The=20 winds would spiral inward in a counterclockwise fashion around a Low in = the=20 Northern Hemisphere. At any location application of this model means the = surface=20 wind direction would be along the isobar lines but at a slight angle = across the=20 line toward lower pressure values. (Low pressure is on the left.) The = wind=20 speeds are related to the isobar spacing. Higher wind speeds are = associated with=20 a closer spacing of the isobars and conversely, low wind speeds occur = with when=20 the isobars are far apart.

Now look at how the isotherms and the general wind flow appear = together. The=20 map and description of the temperature advection appearing on page 117 = of AMS=20 Weather Studies text, shows that you will be able to identify the = occurrence=20 of warm air advection when the wind blows from a warm region with = isotherms=20 having larger numbers toward a colder region where the isotherms have = lower=20 numbers. Conversely, cold air advection occurs where the wind flow is = from=20 colder regions delineated by lower-valued isotherms toward warmer = regions with=20 higher-valued isotherms.

Several simple rules of thumb may help in the location of temperature = advection from a typical surface weather map:

Little temperature advection occurs on either side of a stationary = front.=20 As its name suggests, the front and the two air masses that the front=20 separates do not move horizontally in any quick fashion and are = essentially=20 stationary. Typically, the isotherms and winds on either side of the = front=20 parallel the frontal zone because the wind does not carry air of a = different=20 temperature across the isotherms.=20
Little temperature advection occurs near the center of a surface=20 high-pressure cell because of the weak winds typically found there.=20 Furthermore, the centers of high-pressure systems tend to have weak = horizontal=20 temperature gradients because the air sinks and spreads out from a = common=20 region aloft, leading to relative homogeneity. As a result, large, = slowly=20 moving high-pressure systems are often described in terms of an air = mass,=20 whose properties have been defined by the nature of the airmass source = region.=20 On the other hand, mid latitude low pressure systems experience many = different=20 thermal advection patterns because of the converging flow of winds = carrying=20 air of quite different thermal properties on different sides of the = low.=20
Warm advection occurs either:=20
- In the vicinity of a warm front. A warm front marks the leading = edge of=20 a warm air mass that is replacing a colder air mass. If the warm = front were=20 oriented in an east-west fashion and if it were moving toward the = northeast,=20 then the region of warm advection near the surface would appear to = the south=20 of (or "behind") the advancing front. This warm advection would be = caused by=20 warm southerly winds. As the warm air is carried aloft over the = cooler air,=20 warm advection is carried aloft in what is often called = "overrunning"; or,=20
- Along the western flank of many high-pressure cells, where = southerly=20 winds flow northward and carry warm air from the south toward the = typically=20 colder north. High pressure systems that are of tropical origin or = carry=20 winds from off the oceans (or Gulf of Mexico) often provide warm = advection=20 to interior regions of the U.S.
Cold advection occurs either:=20
- In the vicinity of a cold front. Since a cold front marks the = leading=20 edge of a cold air mass, the cold advection would occur after the = front has=20 passed, with the cold westerly or northwesterly wind flow; or,=20
- Along the eastern flank of many high-pressure cells. The = clockwise=20 circulation around the high will bring colder air southward on = northwesterly=20 and northerly winds on the east side of the high. A cold high = pressure=20 system, often described as a polar or arctic air mass, that moves = southward=20 from Canada produces cold advection within regions in the U.S.=20

Return to the Tuesday=20 Daily Weather Summary

Prepared by Edward J. Hopkins, Ph.D., email mailto:hopkins@meteor.wisc.edu
=A9=20 Copyright, 2009, The American Meteorological Society. =