Why do we get a Subtropical Jet in the Upper Troposphere?

 

• The Coriolis Force (CF) is a force relative to the inertial reference frame of the rotating Earth. As a consequence moving parcels of air are deflected to the right in the northern hemisphere and the left in the southern hemisphere. Acceleration due to CF is given by fV where f is the Coriolis parameter. This is given by the following equation:

 

• The Pressure Gradient Force (PGF) is due to the flow of air from high to low pressure. This is given by the expression:

 

• Balancing: CF=PGF leads to:

 

 

• P=rRT; dP/dZ=-rg; leads to;

• Then combing equations 2 & 4 leads to:

 

 

• This means that the poleward gradient in temperature, with decreasing temperature from equator toward the pole (highest gradient at subtropics) leads to a vertical increase in wind speed with height. This is known as the thermal wind balance. The result is an upper level jet at the subtropical upper troposphere.
• If air is stationary at the pole and it moves toward the pole it gains angular momentum, since it is moves towards the axis of rotation. This leads to a spin up of momentum and is directed toward the east, also accounting for the increase in wind speed towards the pole.

 

 

Leading to:

 

• This results in an acceleration at 30^oN of the equator with a magnitude of 130m.s-1, neglecting friction.