Transcript lecture17

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,
Winds at upper levels
and centripetal (or centrifugal) 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
Mathematically:
p
  g
z
Solution to above equation:
P  PMSL X e
Vertical pressure
+ gravity = 0
gradient force
 Z 


 8 km 
An exponentially decaying
solution of pressure with height
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 left
High pressure to your right.
Crude approximation, assuming the winds are in geostrophic
balance.
Scales of Atmospheric Motion vs. Lifespan
The general circulation
Atmospheric oscillations
Topics for today’s
discussion
We’ve already started
to investigate some
of the synoptic-scale
features…
Basically here’s our roadmap for the rest of the course.
The smallest type of atmospheric
motion is turbulence.
Just a few
bumps, folks!
Nothing to be
worried about!
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
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 friction
Obstructing objects or terrain features.
Changes in wind speed above the ground.
Billow Clouds
A visual indicator of wind shear
STRONG WINDS ABOVE CLOUD
TURBULENT EDDIES
WEAK WINDS BELOW CLOUDS
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, 1966
Crashed 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…
Just a few
bumps, folks!
Nothing to be
worried about!
Why is he
likely to have a
bumpy ride?
Percentage of positive reports of
turbulence on U.S. commercial jet flights
WINTER
COLORADO
SUMMER
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
PGF
NIGHTIME
PGF
Water is relatively cool
Water is relatively warm
Land is relatively warm
Land is relatively cool
Onshore flow at the surface
Offshore flow at the surface
Offshore flow aloft.
Onshore flow aloft.
Occurs because water has a high heat capacity, so it heats and
cools more slowly than the land.
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 AFTERNOON
MID MORNING
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
DAYTIME
NIGHTTIME
Air is heated along mountain
slopes
Air is cooled along mountain
slopes
Warmed air is locally less dense
than the surrounding air above
the valley.
Cooled air is locally more dense
than the surrounding air above
the valley.
Upslope flow.
Downslope flow.
We’re surrounded by
mountains on three
sides, so mountain
valley circulations
play a BIG role in our
weather—especially
during the monsoon!
X
University
of AZ
RINCON MTNS.
Local
Topography
of Tucson, AZ
Summary of Lecture 17
Hydrostatic 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 shear
Thermal: 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