Announcements

Midterm exam #1 will be handed back at the end of class. Will go over written questions and give distribution of scores.

Homework assignment #1 due next Monday.

Groups 3A and 4A pick up labs on Monday.

Athlete grade forms will be filled out today and turned in by me. Apologize for lateness on this, but needed to have exam grades for this.

Summary of Force Balances:Why the wind blows

Force Balance Forces Involved Where it happens

Geostrophic Pressure gradient and Coriolis

Winds at upper levels (with no curvature)

Gradient Pressure gradient, Coriolis, and centripetal (or centrifugal)

Winds at upper levels with curvature.

Cyclostrophic Pressure gradient and centrifugal

Smaller-scale, tight rotations like tornadoes and hurricanes

Gradient + Friction

Pressure gradient, Coriolis, centripetal, and friction

Surface winds

NATS 101 Section 4: Lecture 17

Turbulence and Local Winds

Couple of important loose ends from Chapter 8…

Hydrostatic Balance:(One more) force balance in the vertical

Vertical pressure gradient force

gz

p

Mathematically:

Solution to above equation:

km e X 8Z

MSLPP

An exponentially decaying solution of pressure with height

+ gravity = 0

Flashback:Change in density and pressure with height

Density and pressure decrease exponentially with height. For each 16 km in altitude, the pressure decreases by a factor of 10. Explained by hydrostatic balance.

Buys-Ballot’s Law

If you stand with your back to the wind (in Northern Hemisphere):

Low pressure on your leftHigh pressure to your right.

Crude approximation, assuming the winds are in geostrophic balance.

Scales of Atmospheric Motion vs. Lifespan

We’ve already started to investigate some of the synoptic-scale features…

Topics for today’s discussion

Basically here’s our roadmap for the rest of the course.

The general circulationAtmospheric oscillations

The smallest type of atmospheric motion is turbulence.

It’s what the airline pilot cheeringly calls “bumpy air” or “choppy air” on your flight.

It seems like to me they never really seem to know exactly when it’s going to happen!

Jay Jay the Happy Jet Plane

Just a few bumps, folks! Nothing to be worried about!

Turbulence: Irregular atmospheric motion characterized by currents up and down, or eddies. A departure from smooth (or laminar) flow.

Mechanical Turbulence (Forced Convection)

Mechanical turbulence is ultimately due to changes in wind speed over a distance, or wind shear.

The change can be due to (for example):Surface frictionObstructing objects or terrain features.Changes in wind speed above the ground.

Billow CloudsA visual indicator of wind shear

STRONG WINDS ABOVE CLOUDSTRONG WINDS ABOVE CLOUD

WEAK WINDS BELOW CLOUDSWEAK WINDS BELOW CLOUDS

TURBULENT EDDIESTURBULENT EDDIES

Terrain-Induced Turbulence

Where Jay Jay the Happy Jet Plane SHOULD NEVER FLY: In the vicinity of a mountain wave induced rotor!!

Lenticular Cloud on Mt. Fuji:Indicative of mountain lee waves

BOAC Flight 911: March 5, 1966Crashed on Mt. Fuji due to lee wave turbulence

Thermal Turbulence(Free Convection)

Formed by rising thermals due to surface heating during the day. These may form convective clouds by afternoon.

So if you want the smoothest ride, take the morning flight!

So Jay Jay the Happy Jet Plane decides to go from Tucson to Denver…

Why is he likely to have a bumpy ride?

Just a few bumps, folks! Nothing to be worried about!

Percentage of positive reports of turbulence on U.S. commercial jet flights

WINTER SUMMER

COLORADO COLORADO

(UCAR RAP)

Mostly mechanical turbulence due to mountain waves

Mostly thermal turbulence due to summer thunderstorms.

COLORADO HAS THE MOST TURBULENT AIR AROUND!

Flight Simulators

Today’s pilots are well trained how to deal with turbulence, as well as other adverse weather phenomena, in sophisticated flight simulators.

Modern jet aircraft also have instrumentation to indicate pilots of severe wind shear, largely through the efforts of research labs like National Center for Atmospheric Research.

Local Winds:Sea-Land Breeze

Mountain-Valley Circulation

Basically the same physical idea for each one:

1. A direct thermal circulation which occurs a small enough scale that the pressure gradient is the dominant force which drives the wind.

2. Driven by the diurnal cycle of solar heating.

Sea-Land Breeze

DAYTIME NIGHTIME

Water is relatively cool

Land is relatively warm

Onshore flow at the surface

Offshore flow aloft.

Water is relatively warm

Land is relatively cool

Offshore flow at the surface

Onshore flow aloft.

Occurs because water has a high heat capacity, so it heats and cools more slowly than the land.

PGF PGF

Sea Breeze Circulation and Sea Breeze Front

Sea breeze forces air to converge over land, leading to upward motion and formation of clouds and storms.

Sea Breeze Front: North Carolina Example

EARLY MORNING

MID MORNING

MID AFTERNOON

LATE AFTERNOON

(University of Wisconsin)

Florida Peninsula

Sea breeze is blowing the smoke from a forest file away from the Gulf of Mexico side (FL Dept. of Forestry).

Mountain-Valley Winds

Air is heated along mountain slopes

Warmed air is locally less dense than the surrounding air above the valley.

Upslope flow.

Air is cooled along mountain slopes

Cooled air is locally more dense than the surrounding air above the valley.

Downslope flow.

DAYTIME NIGHTTIME

Local Topography

of Tucson, AZ

XXUniversity University

of AZof AZ

CATALINA MTNS.

CATALINA MTNS.

RIN

CO

N M

TN

S.

RIN

CO

N M

TN

S.We’re surrounded by

mountains on three sides, so mountain valley circulations play a BIG role in our weather—especially during the monsoon!

TU

CS

ON

MT

NS

.T

UC

SO

N M

TN

S.

Summary of Lecture 17Hydrostatic balance is the force balance in the vertical. Gravity balances the upward pressure gradient force. This explains the exponential decrease in pressure with height.

Buy’s ballot law gives a crude approximation to where high and low pressure are relative to your position.

Various scales of atmospheric motion are used to classify weather and climate phenomena. Be familiar with them.

Turbulence is irregular atmospheric motion characterized by eddies.Mechanical: Due to wind shearThermal: Due to differential heating leading to thermals.

The local circulations mentioned today are direct thermal circulations driven by the diurnal cycle of solar heating.

Sea-land breezes: Occur because of the difference in heat capacity between water and land.Mountain- valley winds: Occur because of the heating and cooling of elevated terrain. Important for weather in the western U.S.,especially in summer.

Reading Assignment and Review Questions

Reading: Remainder of Ch. 9

Chapter 9 Questions

Questions for Review (8th ed.): 1,3,4,5,6,8,20,21 (9th ed.): 1,3,4,5,6,7,9,21,22

Questions for Thought: 1,5,8,10,11