# AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell.

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AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell

Miscellaneous Homework Update

Review of September 20 th : 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

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)

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 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

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

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

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