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.edu/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

COLD AIR WARM AIR

1000 mb, 0 meters

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 WARM AIR

1000 mb, 0 meters

500 mb

600 mb

500 mb

600 mb

Constant Height

Low Thicknesses High Thicknesses

COLD AIR WARM AIR

1000 mb, 0 meters

500 mb

600 mb

500 mb

600 mb

Constant Height

This is a region of a strong horizontal pressure gradient

COLD AIR WARM AIR

1000 mb, 0 meters

500 mb

600 mb

500 mb

600 mb

Constant Height

Therefore, we would expect a strong geostrophic wind here (the wind blows into the slide)

COLD AIR WARM AIR

1000 mb, 0 meters

500 mb

600 mb

500 mb

600 mb

Constant Height

Jet Stream

This is a region of strong temperature contrast

COLD AIR WARM AIR

1000 mb, 0 meters

500 mb

600 mb

500 mb

600 mb

Constant Height

Jet Stream

A FRONT is present here!

Upper Jet Streams are frequently found above areas of strong temperature gradients in the lower atmosphere (aka, above fronts)

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)

1000 mb geostrophic wind

500 mb geostrophic wind

Thermal 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