Weather Systems, Hurricanes, etc…

Nili HarnikDEES, Lamont-Doherty Earth Observatory

nili@ldeo.columbia.edu

Coriolis Force

The Coriolis Force On Earth

Coriolis force

Cloud types, as a function of height

Tropical cyclones (or hurricanes or typhoons)

Tropical cyclone, also called hurricane and typhoon, is the names given to an intense low pressure region that forms and migrates in the tropical ocean regions and is associated with intense winds and a very strong convection activity which brings thunderstorms andlarge amounts of rainfall

Tropical cyclones (or hurricanes or typhoons)

Hurricane Vertical Cross Section•The massive disturbances that sometimes grow in a time frame of a week or so, need specific and favorable conditions to occur, such as high sea surface temperatures (at lease 26°C) and weak vertical wind shears. Once they do, they spreads over a radius of a few hundred kilometers. •Hurricanes are surrounded by rings of towering thunder cloudsspiraling up to a small circle at the center of the storm, with a radius of 30-40 km. Here the winds can reach a speed of 100 km/hour and more and the most intense rainfall occurs. Inside this ring lies the eyeof the storm, where the air is still and the convection is suppressed by slow downward motion (subsidence).

Regions of Hurricane Activity•Hurricanes are active in the "trade wind" belts - the regions just north or south of the equator where the winds blow quite steadily from east to west (easterlies).

•Here tropical disturbancesgenerally form, initiated by weak pressure perturbations that exist all the time in the tropics. They move west with the trade winds in a steady, relatively slow motion (10-20 km/hour). During this phase they intensify mainly through the release of latent heat in the surrounding clouds and a small percentage reach full hurricane intensity.

•Hurricanes tracks curve eastward and they speed up north of ~30°N

Hurricane watch

• http://www.hwn.org/• Satellite loop:

http://www.hwn.org/atlantic/ir-loop.htm

• http://hurricane.terrapin.com/

Mid Latitudes

The rule of the Coriolis force….

Geostrophic balance

Geostrophic Balance: f u = - (∆ p / ∆ y) / ρPressure Gradient Force (PGF) = Coriolis Force (CF)

Geostrophic flow and friction

This means that a new balance is achieved in which friction and Coriolis forces together counter the pressure gradient force.

Geostrophic flow and friction

That is why low pressure systems generally bring stormy weather, while high pressure systems bring sun.

Isobars

strong gradient

weak gradientstrong gradient

Midlatitude weather systems /Fronts.

In the middle latitudes the flow is broken into sequences of low and high pressure cells moving from west to east. As we discussed above, these lows and highs affect the local vertical motionthrough convergence and divergence induced by surface friction. They also sweep air masses of different temperature from north and south bringing alternations in temperature to the regions they pass by in their eastward procession. The moving air masses collide and at their boundaries fronts are created in the low pressure centers.

A passage of a front indicates an imminent change of temperature and also a change in weather. There are two kinds of fronts: Warm fronts and cold fronts.

Synoptic Map

•This segment taken from a synoptic (weather) map of surface pressure shows isobars (contours of equal pressure in mb) and small flags, depicting the wind direction (the flags fly in the direction of the wind) and speed (each full flag bar is 10 knots and half a bar is 5 knots with 1 knot = 1/2 m/s). •The flow is very close to geostrophic balance everywhere with a small tendency to flow across isobars towards the low pressure center - a result of the friction effect close to the surface.

Midlatitude weather systems /Fronts.

Warm fronts and cold fronts.

Midlatitude Weather Systems

Midlatitude weather systems /Fronts.

• Cold fronts.

Life Cycle of Midlatitude Cyclones

Dynamic MeteorologyThe Coriolis Force / geostrophic flow