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EAS 308, Meeting 1
Course objectives:
I will teach you enough elementary meteorology, focusing on convection, so you will
understand a state-of-the-art atmospheric numerical model which predicts convection.
Then, you will learn how to adapt and run that model to predict convection for the NE
USA. Using data from the Internet for the NE USA, you will be able to validate the
predictions of convection.
Learning will be accomplished through lectures, readings and in-and out-of-class
assignments. Your competence will be evaluated with a mid-term and end-term exam.
This education and training will enable you to complete an individual project. You will
use an online data base to identify a significant glider flight in the NE USA. You will use
the atmospheric model to predict the convection for the flight. Then, you will use the
actual flight data and other meteorological data to determine how well the model
predicted the actual convection.
Successful completion of this course will enable you understand atmospheric numerical
models, give you experience in operating and interpreting predictions from a model and
teach you to use on-line data bases to validate predictions you make using a model.
Air in motion (Bradbury, Ch. 1)
Our atmosphere is constantly in motion. How do we detect that motion? Of course, the
wind, movement of clouds……others? I’m keenly aware of the vertical motions as I fly
my glider (an aerodynamically nearly-perfect flying machine). I must locate rising air in
order to counteract the constant pull of gravity and avoid or minimize the time I spend in
sinking air.
The atmospheric motion is driven on the global scale by the N-S temperature gradient
between the cold Polar Regions and the warm Equatorial Regions. On the storm-scale,
the atmosphere is driven by differences in temperature between, for example, the cold,
dry northern latitudes and the warm, moist sub-tropical latitudes. Finally, on the local
scale the atmosphere is driven by difference in temperature between, for example, the
mountains and nearby plains.
Pressure:
A basic characteristic of our atmosphere is that pressure (force/area) decreases with
increasing height. Gravity pulls the air molecules and, thus, they are most densely
packed nearest the surface. The change in pressure (Δp) for a change in height (Δh) is
given by the hydrostatic equation
Δp = - ρ g Δh
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where ρ is density and g is the force due to gravity. Pressure, density and temperature are
related in the Ideal gas equation:
p = ρ RT
where R is the universal gas constant. Now, combining the hydrostatic equation and the
ideal gas equation leads to
Δp/p = - g Δh/RT
Keeping temperature constant, we can integrate resulting in
h ~ - const ln p
or as height increases pressure decreases logarithmically as seen in Fig. 1. So, in this
case and in others as well, the atmosphere obeys the basic laws of physics.
Temperature:
But, it’s a different story with temperature. Temperature initially decreases with
increasing altitude (Fig. 2) as the Earth’s surface is the major source of atmospheric heat.
Notice how the temperature in the vertical varies systematically defining the troposphere,
the stratosphere, the mesosphere and the thermosphere. The troposphere ends when the
temperature ceases to decrease with increasing height, at the so-called tropopause. The
temperature ceases to decrease in the stratosphere because of the heating caused by
absorption of solar UV energy by the ozone layer. There is no heat source above this
layer leading to another region of decreasing temperature (the mesosphere) and finally
the high-energy particles from the Sun interact with the few atmospheric molecules
leading to the thermosphere (encompasses the ionosphere, the layer that allows radio
wave to reflect producing long-range communications).
Isobars:
Lines that connect points of equal sea-level pressure are called isobars (Figs. 3 and 4).
Sea-level pressure is determined for stations above sea-level by using the hydrostatic
equation to calculate the pressure at that station had it been at sea-level; eg. the higher the
elevation of the station the lower the actual pressure and hence the greater the correction
to sea level.
Notice in Fig 4, that the air at the surface in this Northern Hemisphere location spirals
counter-clockwise and inward in the region of low pressure and vice versa for the region
of high pressure.
Let’s inspect a surface weather chart at www.weather.unisys.com, an excellent Internet
weather site, to locate the isobars.
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Height contours:
Above the surface the height of a constant pressure surface is used to define the regions
of high and low pressure (Fig. 5). The height of the 300 mb surface is depicted (standard
sea-level pressure being 1013 mb (Fig. 1)). Notice in Fig. 5, the cold dense polar air is
shallower than the warm less dense subtropical air leading the a steep “slope” and, hence,
a strong pressure gradient that gives rise to the Polar-Front Jet Stream (Fig. 6). Let’s
compare the 300 mb chart that corresponds to the surface chart we inspected earlier at the
Unisys site. Can we locate the Jet-Stream?
The geostrophic wind:
The geostropic (earth-turning) wind is caused by a balance between the pressure gradient
force and the Coriolis force (Fig. 11) in the absence of friction (friction affects winds
only in contact with the ground (Fig. 14). Thus, the winds flow along the height contours
aloft and deviate from the isobars at the surface. This deviation from the isobars leads to
the inward spiraling winds in a low pressure region and vice versa for a high pressure
region as seen in Fig. 4. The inward spiraling air leads to converging air and rising
motions in regions of low pressure and vice versa for regions of high pressure. Rising air
cools and can cool to the dew point temperature (temperature at which condensation
occurs at constant pressure) forming clouds and ultimately precipitation. In contrast,
sinking air warms and can warm to above the dew point temperature causing clouds and
precipitation to evaporate.
The Coriolis force is an apparent force caused by the Earth’s rotation, deflecting objects
moving over the surface to the right in the NH and to the left in the SH (Fig. 10).
Now, let’s reinforce this learning by completing Assignment 1.
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