AOS101 Lecture 10

A severe thunderstorm is defined as a thunderstorm that produces - Hail of 1 inch diameter (in central US) or larger and/or wind gusts 58 mph or greater and/or a tornado Occurs most frequently during the spring and summer when there are the following atmospheric conditions: – Conditionally unstable atmosphere – Moisture – Upward vertical motion (“Lifting”) – Wind shear

 A tornado is defined as “a violently rotating column of air descending from a thunderstorm and IN CONTACT with the ground.” –NWS Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation

Several favorable environmental conditions must be in place before a tropical cyclone can form: – Some initial disturbance such as a thunderstorm complex, which may slowly develop – Warm ocean waters (at least 80°F) – Potentially unstable atmosphere favorable to convection – Moist air near the middle of the troposphere – Low values of vertical wind shear between the surface and upper troposphere If these conditions persist for several days, a tropical cyclone may form

When these disturbances first appear, they are called tropical depressions – Not named yet Once the disturbance has developed with surface wind speeds stronger than 39 mph, the storm is classified as a tropical storm After further strengthening and surface wind speeds greater than 74 mph, the system is upgraded to a hurricane Atlantic Hurricane season is June 1 – November 30

EYE EYEWALL RAINBAND HURRICANE KATRINA

 What makes the wind blow?  We need to think about Newton's Laws  1 st Law ◦ An object at rest will remain at rest; an object in motion will remain in motion as long as no force is exerted on the object.  2 nd Law ◦ The total force exerted on an object is equal to the acceleration of the object times its mass.

 Pressure Gradient Force (PGF)  Coriolis force (CF)  Centripetal force  Frictional Force

 Our atmosphere is full of forces that become balanced  As a result, we can say something about it’s motion  Balanced forces tell us many things. ◦ For example, the wind direction is a balance between the Coriolis force, PGF, and frictional force

 The pressure gradient is a change in pressure over a given distance. Pressure gradient force compels fluids to move from high pressure to lower pressure. The PGF acts to increase lower pressure and decrease higher pressure

 Direction of PGF – always points from HIGH pressure toward LOW pressure, directly perpendicular to an isobar  Magnitude of PGF- strength is directly related to the strength of the pressure gradient  The PGF is the force that causes the wind to blow! Pressure gradient = 4 mb per 100 km

◦ When isobars are very close together, the numerator in the PGF equation is large (a very large change in pressure) ◦ So the pressure gradient is large, and thus, the PGF is very strong.

 The Coriolis force is an apparent force that results from the constant rotation of the Earth.  In N. Hemisphere, acts at a 90° angle to the right of the object in motion (such as the wind)  This means that a wind from the south would have a CF acting toward the east

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba…

Missile starts at Dallas, which is at a latitude of N, rotates with the Earth at a speed of m/s.

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of N, rotates with the Earth at a speed of m/s. Missile travels toward Winnepeg which, at a latitude of N, rotates with the Earth at a speed of m/s. The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX

Imagine Dallas, TX fires a nuclear missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of N, rotates with the Earth at a speed of m/s. Missile travels toward Winnepeg which, at a latitude of N, rotates with the Earth at a speed of m/s. The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX Since the Earth rotates slower the farther north you go, the missile appears to deflect to the right of its intended target

Imagine Dallas, TX fires a missile at Winnipeg, Manitoba… Missile starts at Dallas, which is at a latitude of N, rotates with the Earth at a speed of m/s. Missile travels toward Winnepeg which, at a latitude of N, rotates with the Earth at a speed of m/s. The missile will conserve its angular momentum as it travels north, meaning it will travel around the Earth at the speed of the Earth’s rotation at Dallas, TX Since the Earth rotates slower the farther north you go, the missile appears to deflect to the right of its intended target Missile lands north of Ottawa.

 We cannot see the planet rotating, so when something is moving, we perceive it as being deflected to the right of its intended path in the N. Hemisphere  Deflection is dependent on latitude ◦ 0 at equator and maximum at the poles  Deflection intensity is directly related to wind speeds ◦ Acts only as the wind starts to blow

 QOYU QOYU

 A balance between the ◦ Pressure gradient force ◦ Coriolis force  Balance allows PGF to be equal and opposite the CF. This balance will tell use the magnitude of the geostrophic wind  The geostrophic wind blows parallel to lines of constant pressure, with low pressure on the left  Movie: What happens if its not in balance Movie: What happens if its not in balance

Geostrophic Balance 996 mb 1000 mb 1004 mb L H x

Geostrophic Balance 996 mb 1000 mb 1004 mb L H Coriolis Force Pressure Gradient Force

Where the pressure gradient is small, the PGF is also small, resulting in a weak wind. 996 mb 1000 mb 1004 mb L H Coriolis Force Pressure Gradient Force Geostrophic Wind Where the pressure gradient is large, the PGF is also large, resulting in a strong wind.

 The wind can be approximated as nearly geostrophic in the upper levels of the troposphere. PGF CF How are there different heights at 500 mb?

PGF/ CF/ Centripetal

 Friction affects geostrophic balance by putting a drag-force on the air: friction always acts in the direction opposite the direction of the wind wind FR

 This throws the wind out of geostrophic balance – there is now a net force acting on the wind in the direction opposite its motion windFR PGF CF

The Frictional Force Since friction acts in the opposite direction of the wind, it slows the wind Change in speed  change in magnitude of the Coriolis force Friction + Coriolis force ~ PGF  no longer geostrophic balance and winds can cross the isobars How does friction affect geostrophic balance?

Friction 996 mb 1000 mb 1004 mb L H Upper Level Wind Balance: PGF/ CF Lower Level Wind Balance: PGF/ CF/ Friction Causes wind to cross isobars at ~30° angle at surface

Height above the surface –The further away from the surface, the less friction –For instance, the winds at 300 mb experience less friction than the winds at the surface Wind speed –The stronger the wind, the more friction will oppose the motion –Therefore, slower winds experience less friction than fast winds Surface Type –The rougher the surface, the greater the friction –For example, the friction over an open body of water is weaker than that over a mountainous terrain More factors that affect the frictional force:

In regions (upper levels) where friction is negligible, the winds are approximately geostrophic