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 BalanceForces InvolvedWhere it happens GeostrophicPressure gradient and Coriolis Winds at upper levels (with no curvature) GradientPressure gradient, Coriolis, and centripetal (or centrifugal) Winds at upper levels with curvature. CyclostrophicPressure 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 Mathematically: Solution to above equation: 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 left High 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 circulation Atmospheric 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 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 WEAK WINDS BELOW CLOUDS TURBULENT 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, 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… 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 (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 DAYTIMENIGHTIME 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

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 X University of AZ CATALINA MTNS. RINCON MTNS. We’re surrounded by mountains on three sides, so mountain valley circulations play a BIG role in our weather—especially during the monsoon! TUCSON MTNS.

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 (8 th ed.): 1,3,4,5,6,8,20,21 (9 th ed.): 1,3,4,5,6,7,9,21,22 Questions for Thought: 1,5,8,10,11