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

Miscellaneous New Homework Exam Results

Review of October 29 th : Clouds and the LCL

Mixing RatioAdiabatic Lapse Rate Temperature Moist Adiabatic Lapse Rate Temperature Dewpoint

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 (DALR) 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: y/search?id=level-of-free-convection1

An Example The red arrow indicates the LCL, while the yellow arrow indicates the LFC

Mountain Effects and Temperature The air ends up warmer due to the extra latent heat release

The same process that causes cyclones to strengthen leads to cloud formation!

On to severe weather!

Convective Available Potential Energy (CAPE) CAPE is a measurement of energy available to a rising air parcel Large values of CAPE (in the thousands) are often associated with severe weather The value of CAPE is proportional to the area between the actual environmental temperature profile and the temperature profile of an air parcel Remember that air parcel warmer than its environment will rise on its own, so CAPE just basically measures for how much of the atmosphere the parcel will be warmer (and by how much) Therefore, values of CAPE mean stronger updrafts

From:

Thunderstorms Thunderstorms (rain, lightning, gusty winds, etc.) are most commonly not severe We call a thunderstorm “severe” when it has very strong winds, large hail, or tornadoes Severe thunderstorms require certain atmospheric conditions Non-severe thunderstorms are commonly called “air mass” thunderstorms

Forcing All of the things we learned about last class (the LCL, LFC, CAPE) only matter if there is something to force an air parcel to rise There are many different types of forcing Some commonly associated with thunderstorms are: - Strong surface heating - An approaching cold front - A seabreeze front - And many more … some of which we’ll learn about later

Air Mass Thunderstorms The cold air from the rainfall cuts off the updraft, causing the thunderstorm to die

Commonly Found Characteristics of Severe Weather (1) Large values of CAPE (as mentioned earlier) contribute to the strength of an updraft (2) Wind shear is the change in wind speed or direction with height - Speed shear often causes severe weather (often called a “squall line”) with strong straight-line winds - Directional shear often causes supercells and tornadoes by causing the updrafts to rotate (3) Large amounts of moisture often allow for the LFC to be reached at a lower level in the atmosphere, and thus often result in more CAPE In order for severe weather to occur, generally all of these need to be present in some form

Directional Wind Shear We would say that the wind is “backing” if the wind direction rotates counterclockwise with height We would say that the wind is “veering” if the wind direction rotates clockwise with height Generally, severe weather will only be found if the wind is veering with height

Speed Shear

Directional Shear

Severe Thunderstorms For severe thunderstorms, the change in the wind with height forces the downdraft to be separate from the updraft, which allows the storm to live longer and become severe

Detecting Severe Weather For meteorological considerations, there are two primary types of remotely sensed observations: (1)RADAR (2)Satellite

RADAR RADAR is an acronym for “RAdio Detection And Ranging” RADAR uses radiowaves or microwaves to detect objects RADAR dishes send out a pulse of electromagnetic radiation, which can be reflected back by objects The length of time it takes for the pulse to return, as well as the strength of the return pulse indicate how near/far and how big the object(s) are - From:

From: ay/Dictionary/radar/DI90G1.jpg Pictures

Thunderstorms over Michigan

Doppler RADAR Doppler RADAR makes use of the “Doppler Effect” to determine whether objects are traveling towards or away from the radar site Doppler RADAR measures the change in wavelength of the incoming signal (compared with the signal that was sent out)

Velocity (from the Doppler RADAR) This feature is extremely useful for detecting tornadoes – many tornadoes are first “detected” using this method

Types of Severe Weather For our purposes, we can consider two types of severe weather: (1)A squall line is mostly linear continuous line of thunderstorms associated with speed wind shear, and they often produce strong straight line winds (2)Supercells are smaller storms associated with directional wind shear, and often cause tornados and large hail

A Squall Line

A Supercell Reflectivity: Velocity: From: We call this feature a “velocity couplet”, and it often signifies a tornado