Transcript AOS 100: Weather and Climate

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Exam Update
Review of September 8th: Cyclone
Growth and Decay
• We’ve learned that the locations of positive
vorticity advection (PVA) and negative vorticity
advection (NVA) determine where areas of
divergence and convergence exist in the
atmosphere
• These locations determine the areas that are
favorable for surface cyclones (or anticyclones)
to intensify or weaken
• Since the locations of favorable convergence
and divergence change over time with respect to
a surface cyclone (or anticyclone), cyclones
conventionally strengthen or weaken depending
on their location
Review Continued
• What this means is
that cyclones that are
intensifying will tilt
with height
• The intensifying
cyclone will often be
downstream of the
upper level cyclone
• A weakening cyclone
will often be directly
below, or upstream of
the upper level
cyclone
Review Continued
• Due to friction,
air is always
converging near
surface cyclones
• This means they
will begin to
weaken as soon
as the location
of upper level
divergence is no
longer above it
An Example
From
Tuesday
Morning:
http://www.ral.ucar.e
du/weather/surface/
Observations about Observations
• Conventionally, only temperature, dewpoint,
wind speed and direction, cloud cover, pressure,
current weather, and visibility (if less than 10
miles) are shown
• However, much of the planet goes unobserved
• Large differences in temperature, dewpoint, etc.
can exist from location to location
• This is why we must do contour analysis in order
to “fill in” the missing data
How Do We Do Contour Analysis?
• You can think of it like a glorified version of
“connect the dots”
• However, for contour analysis, we have to “fill in”
some of the missing data
• The goal of contour analysis is to allow for easier
interpretation of the current weather
• Some things to remember:
- Lines never cross
- Always use a pencil so you can erase lines
• Now for an example …
Thickness
• Recall that warm air is less dense than cold air
• Therefore, a certain mass of warm air will take
up more space than the same mass of cold air
• Atmospheric thickness is simply a measure of
the vertical distance between two different
pressure levels
• Based on the above, large thickness values
correspond to a higher average air temperature
than small thickness values
www.nco.ncep.noaa.gov/pmb/nwprod/analysis/namer/gfs/00/model_m.shtml
A Conceptualization
The Horizontal
surfaces are “heights”
above sea level
The wavy surface is
the 500 mb level
Cold
Warm
The Big Picture
The pressure
surfaces are close
together at the pole
… and further apart
near the equator
This means that
along a horizontal
surface, a pressure
gradient exists
Consider an Example
So where would you expect lower
thicknesses?
Or, to ask it another way, where would
you expect to find lower pressures along
a line of constant height?
COLD AIR
1000 mb,
0 meters
WARM AIR
Low Thicknesses
High Thicknesses
500 mb
600 mb
Constant
Height
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
WARM AIR
This is a region of a
strong horizontal
pressure gradient
500 mb
600 mb
Constant
Height
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
WARM AIR
Therefore, we would expect a
strong geostrophic wind here
(the wind blows into the slide)
500 mb
600 mb
Constant
Height
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
WARM AIR
500 mb
Jet
Stream
500 mb
600 mb
COLD AIR
1000 mb,
0 meters
This is a region of
strong temperature
contrast
600 mb
Constant
Height
WARM AIR
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