Transcript AOS 100: Weather and Climate

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Homework Update
Review of September 20th:
Contouring and Thickness
• Some things to remember about contour
analysis:
- Lines never cross, nor do they ever intersect
(i.e. you’ll never have a 3-line intersection)
- Every contour separates values that are
greater than that contour, and values that are
less than that value
- Remember to use pencil, for erasing!
- Once contour analysis is completed, it makes
looking at the data much easier (and quicker)
Review Continued
Review Continued
• Atmospheric thickness is simply a measure of
the vertical distance between two different
pressure levels
• If that layer is warm, the value of thickness will
be greater than a layer that is cold
• Also, a difference in temperature at the surface
can lead to a pressure gradient at upper levels
(which therefore means there will be a stronger
wind at these locations at upper levels)
Upper Jet Streams are frequently found above areas of strong
temperature gradients in the lower atmosphere (aka, above
fronts)
500 mb
Jet
Stream
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
A FRONT
is present
here!
600 mb
Constant
Height
WARM AIR
Strong 850 mb
temperature
gradients
Strong 300 mb
wind speeds
The Thermal Wind
• Based on what we’ve learned, we can say that
the change in strength of the geostrophic wind
with height is directly proportional to the
horizontal temperature gradient
• This relationship is known as the Thermal Wind
• The direction and strength of the thermal wind
tells us about the temperature structure of the
atmosphere
• A strong thermal wind means a stronger
temperature gradient in the atmosphere (and
therefore there is a strong geostrophic wind
shear with height)
Thermal Wind
• It is easy to calculate, if you know the
geostrophic wind at different levels
• Say we’re trying to calculate the thermal
wind for the 1000-500 mb layer:
– Simply subtract the upper geostrophic wind
(500 mb) vector from the lower geostrophic
wind vector (1000 mb)
Thermal Wind
1000 mb
geostrophic
wind
500 mb geostrophic
wind
It’s pretty easy!
A Useful Feature
• The Thermal Wind always blows with cold
thickness to the left (and blows parallel to
the constant lines of thickness)
Thermal Wind Continued
• The thermal wind isn’t an actual, observable
wind
• However, it does tell us useful things about the
atmosphere, such as:
- the strength of the temperature gradient in a
layer
- and therefore the strength of the geostrophic
wind shear
- and most importantly, the direction it points is
roughly the direction you would predict a surface
cyclone to move
Precipitation
• Obviously, clouds need to form first in order for
precipitation to form
• Clouds will only form when the air reaches
saturation (so where relative humidity = 100%)
• This can occur either by adding moisture to the
air, or by cooling the air
• Of these two options, the second is a much
more common method of forming clouds
• One of the easiest ways to make the air cool is
by forcing it to rise
Lapse Rate(s)
• A “Lapse Rate” is merely the rate at which temperature
decreases with height
• The Environmental lapse rate is therefore simply the rate
at which the temperature of the atmosphere decreases
with height
• Sometimes, a “parcel” of air is considered:
- The dry adiabatic lapse rate (DALR) is for a parcel with
0% RH, and is about 9.8ºC/km
- The moist adiabatic lapse rate (MALR) is for a parcel
that is saturated, and is close to 6.5ºC/km in the lower
atmosphere
- This difference is caused by the release of latent heat as
water changes phase (like from a gas to a liquid), which
warms the air
Surface dewpoint = 20ºC
Adiabatic Lapse Rate Mixing Ratio
Moist Adiabatic
Lapse Rate
Temperature
Dewpoint
Temperature
Cloud Formation
• Soundings can be used to figure out not
only where clouds currently are, but where
they would form
• The Lifted Condensation Level (LCL) tells
you where clouds would form if you forced
a parcel of air to rise
• To find this, you need to know both the
temperature and mixing ratio
Continued
• In order to find the LCL, use the dry adiabatic
lapse rate to find how far up it would take for the
parcel to cool to the dewpoint
• This is done by following the DALR up from the
temperature, and the mixing ratio up from the
dewpoint, and figuring out where they cross
• Once this happens, clouds should form
• If the parcel is forced to keep rising, it will now
cool by the moist adiabatic lapse rate
MALR
Level of Free Convection (LFC)
• The LFC is defined as “The level at which
a parcel of air lifted dry-adiabatically until
saturated and saturation-adiabatically
thereafter would first become warmer than
its surroundings in a conditionally unstable
atmosphere”
From:
http://amsglossary.allenpress.com/glossar
y/search?id=level-of-free-convection1
An Example
The red arrow
indicates the
LCL, while the
yellow arrow
indicates the
LFC