Assignment 1: chapter 4 fronts and jets

Question 1

How the Rossby number varies along and across the front or jet?

Answer 1

If the length of a front or jet is on the order of 1000 km, then the Rossby number is usually reasonably small for flow along the front or jet so that geostrophic balance is approximately maintained across but not along the front or jet.

Question 2

Giving the necessary diagrams, describe how the winds change at the surface and in the upper atmosphere across the frontal zones.

Answer 2

• A front is associated with wind shifts in a cyclonic direction (backing) at the surface. This is true with the passage of all frontal types
• The wind direction also shifts in the vertical with height in two ways:
  
  • Veering - Veering is a change in wind direction, clockwise (anticyclonic) in the Northern Hemisphere. Ex: wind changes from southwest to northwest.
    Veering generally occurs through a warm front.
  • Backing - If the wind direction changes in anticlockwise (cyclonic) direction, it is called backing. Ex: wind changes from northwest to southwest.
    Backing occurs with height through a cold front.

Question 3

With the help of a diagram, describe how a cross-isobaric flow occurs from both sides towards the front.

Answer 3

Friction causes the wind near the ground to drift across the isobars toward lower pressure. This causes a cross-isobaric flow towards the front from both sides, resulting in convergence and rising motion in the region of a front.

Question 4

Mention the factors favoring a steep frontal slope. What type of weather is associated with steeper slope fronts?

Answer 4

• Factors favoring a steep slope are:
  
  • a large wind velocity difference between air masses
  • large temperature difference and
  • high latitude
• Because cold air tends to run under warm air, the steeper the slope, the more intense the lifting and vertical motion of the warm air and, therefore the more intense the weather.

Question 5

Describe how jet streams are associated with frontal zones

Answer 5

• Across the frontal zone, there is a stronger horizontal temperature gradient,
• which is strongest near the surface, and weakens with increasing altitude. According to the thermal-wind relationship, the geostrophic wind will increase with height in strong horizontal temperature gradients.
• If the frontal zone extends vertically over a large portion of the troposphere, then the wind speed will continue to increase with height, reaching a maximum near the tropopause. Thus, jet streams are associated with frontal zones.

Question 6

Briefly mention how the meridional and zonal temperature gradients cause changes in zonal and meridional winds respectively.

Answer 6

Question 7

Suppose the air temperature increases northwards from 15^oC to 20^oC over a distance of 600 km, how does the geostrophic wind change with altitude? Assume f=10^-4s^-1

Answer 7

Question 8

With the help of diagrams, describe how the winds in the entrance and exit regions of a jet streak, become ageostrophic.

Answer 8

• As this air enters into a region of stronger pressure gradient, it finds itself slower than the new, faster geostrophic wind speed. Thus, it becomes ageostrophic (not geostrophic) for a short time while it accelerates toward the faster geostrophic wind speed.
• When the air parcel is too slow, its Coriolis force is smaller than the new larger pressure gradient force. This temporary imbalance turns the air at a small angle toward lower pressure. This is what happens as air flows into a jet streak.
• The opposite happens as air exits a jet streak and flows into a region of weaker pressure gradient. The wind is temporarily too fast because of its inertia, so the Coriolis force (larger than pressure-gradient force) turns the wind at a small angle toward higher pressure.

Question 9

A west wind of 30ms^-1 in the entrance region of a jet streak increases to 50ms^-1 in the center 400 km to the east. Show the resultant wind in a neat sketch and find the ageostrophic wind component at the center of the jet streak.

Answer 9

Question 10

Giving a neat sketch, describe the regions of divergence and convergence around the jet streaks. What are the regions of cyclogenesis and cyclolysis?

Answer 10