Introduction to Mesoscale Meteorology: Part I

Jeremy A. Gibbs

University of Oklahoma

gibbz@ou.edu

January 13, 2015

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Overview

Administrative JunkWho the heck is Jeremy Gibbs?Course InformationPolicies

Introduction to MesoscaleDefinition of Mesoscale

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Hello. I’m Dr. Jeremy Gibbs.

I All too often, students are intimidated by instructors and aretimid about approaching them or asking questions.

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Don’t be intimidated.

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About me

I Completed my Ph.D. in Dec. 2012 from SoM

I My advisor was Evgeni Fedorovich

I I study boundary layer flows, turbulence, numerical modeling

I I currently work as a postdoc on a the PECAN project

I Now that you know my degree and job, save some timetalking and just call me Jeremy.

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Instructors

Jeremy

I Up until spring break

I gibbz@ou.edu, NWC 5648, TR 12:45 pm-1:30 pm

Dr. Ming Xue

I After spring break

I mxue@ou.edu, NWC 2502, TBA

Ms. Rachel Miller

I Grader

I rlmiller93@yahoo.com, NWC 5110

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Websites

I Desire2Learn

I http://twister.ou.edu/MM2015

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Textbooks

I Required: Markowski, P. and Y. Richardson: MesoscaleMeteorology in Midlatitudes. Wiley-Blackwell, 430pp.

I Optional: Ray, P. S. (Editor): Mesoscale Meteorology andForecasting. American Meteorological Soc., 793 pp.

I Optional: Lin, Y.-L.: Mesoscale Dynamics. CambridgeUniversity Press, 646 pp.

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Notes

I I will provide detailed notes.

I Are you okay if I simply post them online (that’s a lot ofpaper to print)?

I Do you prefer to have them prior to class?

I I’ll post them both D2L and the course website.

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Content

This course is designed for you to understand, by applyingatmospheric dynamics and physical analysis techniques, mesoscaleand convective-scale phenomena.

Topics include mesoscale convective systems, severethunderstorms, tornadoes, drylines, low-level jets, mountain wavesand orographic precipitation, land/sea breezes, boundary layerrolls, and hurricanes.

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Grading

I Homework sets (2 or 3): 10%

I In-class exams (2): 30%

I Term project: 30%

I Comprehensive final exam: 30%

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Homework

Homework is due by the end of class on the required submissiondate. You will be assessed a 20% penalty per day for late work.

Work will not be accepted 2 days beyond the announced due date.

You are encouraged to discuss homework problems with oneanother; however, solutions submitted under your name mustexpress your own descriptions and calculations and be written byyou - not copied in whole or in part from someone elses work orfrom a common work session on the chalkboard. Cheating isstrictly forbidden and could result in suspension or expulsion fromthe University. Dont even think about doing it! It is yourresponsibility to read and understand the Student Code andpolicies on Academic Integrity

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Exams

Two in-class examinations will be given during the regular lectureperiod (dates TBD). Make-up exams will be given only underexceptional circumstances.

The final examination is comprehensive and will be held in NWC5600 on Thursday May 7, 2015 from 10:30 am - 12:30 pm. Amake-up final exam will be given only under exceptionalcircumstances.

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How to succeed

You are encouraged to visit Dr. Gibbs, Dr. Xue during officehours, or to make special appointments. The best way to reach usis via e-mail. Please take advantage of office hours!

Do not wait until the final exam to begin studying. The best wayto ensure success is to keep up with the course material and to askquestions. Students who actively participate in lectures and availthemselves of office hours typically learn and retain the material ata much higher level.

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Attendance and etiquette

You are all adults paying to be here. I have no desire to takeattendance. It is up to you decide if you attend lectures. I thinkyou might benefit from attending regularly, and I highlyrecommend doing so.

I realize lectures can sometimes become boring. If you decide toplay Angry Birds or post embarrassing photos of your instructor toTwitter, make sure your sound is turned off.

This class time is awful. If you want to bring your lunch, feel free.Just don’t make someone else clean up after you.

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Have fun

Finally, HAVE FUN! In this course, we will try to understand manyof the mesoscale phenomena that we encounter in real life. Theyare very relevant to your career as a meteorologist! Most weatherthat we see occur on the mesoscales!

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What does Mesoscale cover?In this class, we will try to understand many of the mesoscalephenomena that we encounter in real life.

I mountain waves

I density currents

I gravity waves

I land/sea breezes

I heat island circulations

I clear air turbulence

I low-level jets

I fronts

I mesoscale convective complexes

I squall lines

I supercells

I tornadoes

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The goal

We will focus on the physical understanding of these phenomena,and use dynamic equations to explain their development andevolution.

First, we must define mesoscale.

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We tend to classify weather systems according to their intrinsic orcharacteristic time and space scales. Often, theoreticalconsiderations can determine the definition.

There are two commonly used approaches for defining the scales:dynamical and scale-analysis.

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Dynamical

The dynamical approach asks questions such as the following:

I What controls the time and space scales of certainatmospheric motion?

I Why are thunderstorms a particular size?

I Why is the planetary boundary layer (PBL) not 10 km deep?

I Why are raindrops not the size of baseball?

I Why most cyclones have diameters of a few thousandkilometers not a few hundred of km?

I Tornado and hurricanes are both rotating vortices, whatdetermine their vastly different sizes?

There are theoretical reasons for them! There are many differentscales in the atmospheric motion. Let’s look at a couple ofexamples.

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Dynamical

Figure: Hemispherical plot of 500mb height (contour) and vorticity(color) showing planetary scale waves.

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Dynamical

Figure: Sea level pressure (black contours) and temperature (redcontours) analysis at 0200 CST 25 June 1953. A squall line was inprogress at the time in northern Kansas, eastern Nebraska, and Iowa.[From Markowski]

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Atmospheric Energy Spectrum

I Atmospheric motions exist continually across space and timescales.

I Spatial scales range from ∼ 0.1 µm (mean free path ofmolecules) to ∼ 40, 000 km (circumference of Earth), whiletemporal scales range from sub-second (small-scaleturbulence) to multi-week (planetary-scale Rossby waves).

I The ratio of horizontal space and time scales is approximatelythe same order of magnitude (∼ 10 ms−1) for these features.

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Atmospheric Energy Spectrum

Figure: Average kinetic energy of west-eastwind component in the free atmosphere(solid line) and near the ground (dashedline). (After Vinnichenko, 1970; see alsoAtkinson, 1981)

I There are a fewdominant time scalesin the atmospherewhen looking at thekinetic energyspectrum plotted as afunction of time.

I The figure also showsthat the energyspectrum of theatmospheric motion isactually continuous!

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Atmospheric Energy Spectrum

There is a local peakaround one day(associated with thediurnal cycle of solarheating), and a large peaknear one year (associatedwith the annual cycle dueto the change in theearth’s rotation axisrelative to the sun). Thesetime scales are mainlydetermined by forcingexternal to theatmosphere.

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Atmospheric Energy Spectrum

There is also a peak in thea few days up to aboutone month range.Synoptic scale cyclones upto planetary-scale waves.There isnt really anyexternal forcing that isdominant at a period of afew days. Thus, this peakmust be related to thesomething internal to theatmosphere. It is actuallythe scale of the mostunstable atmosphericmotion.

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Atmospheric Energy Spectrum

There is another peakaround one minute. Thisappears to be associatedwith small-scale turbulentmotions, including thosefound in convective stormsand the PBL.

There appears to be a‘gap’ between severalhours to ∼ 30 minutes.This ‘gap’ actuallycorresponds to themesoscale, the subject ofour class.

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Atmospheric Energy Spectrum

We know that manyweather phenomena occuron the mesoscale,although they tend to beintermittent in both timeand space. Theintermittency may be thereason for the ‘gap’.

Mesoscale is believed toplay an important role intransferring energy fromlarge scales down to thesmall scales.

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Atmospheric Energy Spectrum

While intermittent, we cannot do without it.Otherwise, the heat andmoisture will accumulatenear the ground and wewill not be able live at thesurface of the earth!

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Energy Cascade

I As scales decrease, we see finer and finer structures.

I Many of these structures are due to certain types ofinstabilities that inherently limit the size and duration of thephenomena.

I Also, there exists the exchange of energy, heat, moisture, andmomentum among all scales.

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Energy Cascade

I Most of the energy transfer in the atmosphere is downscalestarting from differential heating with latitude and land-seacontrast on the planetary scales.

I Energy in the atmosphere can also transfer upscale, however.We call the energy transfer among scales energy cascade

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Dynamical

Figure: Energy cascade

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Energy Cascade

Example: A thunderstorm feeds off convective instabilities (asmeasured by CAPE) created by e.g., synoptic-scale cyclones. Athunderstorm can also derive part of its kinetic energy from themean flow. The thunderstorm in turn can produce tornadoes byconcentrating vorticity into small regions. Strong winds in thetornado creates turbulent eddies which then dissipate andeventually turn the kinetic energy into heat. Convective activitiescan also feed back into the large scale and enhancing synopticscale cyclones.

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Energy Cascade

What does this have to do with the mesoscale?

It turns out that the definition of the mesoscale is not easy.Historically, the mesoscale was first introduced by Ligda (1951) inan article reviewing the use of weather radar. It was described asthe scale between the visually observable convective storm scale (afew kilometers or less) and the limit of resolvability of a synopticobservation network i.e., it was a scale that could not beobserved. The mesoscale was, in early 1950’s, anticipated to beobserved by weather radars.

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The End

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