Assignemnt: Introduction to Atmospheric Science

Lab Exercise: Mid-Latitude Cyclones and Weather Forecasting

Introduction

In this exercise we will investigate the structure and evolution of middle latitude cyclones with different tracks and intensities and see how these storms affect the weather at a particular location. We will also examine various subjective forecast techniques for predicting the weather at these locations.

The basic job of mid-latitude cyclones is to move cold air downward and toward the equator and warm air upward and toward the poles. The forced movement of different air masses can be seen through the presence of fronts surrounding the cyclone. The vertical motions also result in precipitation where the air is rising.

Forecasting the weather at a given location largely involves predicting the movement and strength of a mid-latitude cyclone and applying the typical conditions you would observe at various locations around the storm. Forecasts can be made using computer models which solve equations governing the flow in the atmosphere or through a variety of more traditional techniques, such as persistence, trend, analogue, or climatological forecasts. Each of these later methods can be useful in certain situations, but have drawbacks that need to be considered.

Getting started:

1. Go to any on-campus computer lab, such as the Miller center or the lab in WSB. Use one of the PC's in any of these labs. The program we're going to use should be available in most of the on-campus labs.

2. To start the "Cyclones" program, click on the "Start" button, go to "All Programs", then "Riverside Scientific" (or "RSI Programs"), then "Cyclones" to start the program.

3. Take a few minutes to familiarize yourself with the program.

a. Under the "Change Cyclone" menu you can vary the starting location of the cyclone, the track it will follow, and the intensity of the storm.

b. Using the "Map Views" on the left side you can overlay isobars, isotherms, wind barbs, precipitation type, and cloud amount and type.

c. You can observe conditions at a single station by either dragging the instrument shelters at the lower left onto the map or by selecting a city from the drop-down menus at the upper right.

d. After you've watched a few animations, you can go to the "Make a forecast" menu and try to predict how the weather will change at various cities for different cyclones.

4. When you're comfortable using the program you can try the following exercises.

Exercises:

1. From the "Change Cyclone" Menu, pick "Plains" cyclone. Run the simulation with the intensity set at 10, 50, and 90. In Map Views, select "Temperature" to overlay isotherms.

What happens to the isotherm spacing for the three different intensities? Which cyclone has the strongest temperature gradient?

2. For the "50 Intensity" simulation:

Where, relative to the low and the fronts, are the temperature gradients the strongest? Where are the warmest and coldest air?

3. Turn off the "temperature" and turn on the isobars "Pressure" map view. Run the simulation for the Plains cyclone at "50 Intensity." Check the "show isoline labels" at the top to see the value of the isobars.

a. When and where does the minimum pressure occur along the cyclone's track?

b. Go to the "Change Cyclone" menu. Run the cyclone simulation again for the Alberta and Gulf simulation. Does the position of the lowest pressure isobar change from where it was for the Plains simulation?

4. Go back to the "Plains" cyclone and set the map view to "precipitation"

a. Where does most of the precipitation occur relative to the low pressure center and fronts? Make sure to describe note where there is precipitation in a large area versus a narrow band.

b. Change the cyclone type to Alberta. What type of precipitation is most common in an Alberta cyclone? (check the "circled i" next to the precipitation to see what symbols the program uses)

If there are different types of precipitation, where relative to the low pressure area and the fronts is rain or snow more likely?

c. Change the cyclone type to Gulf and run the simulation again. What type of precipitation is most common in a Gulf cyclone? If precipitation is mixed, where relative to the low and its fronts are the rain and snow located?

5. Overlay "Cloud Type" and click the information icon to show the type of clouds associated with each code. What types of cloud are associated with precipitation? Does this agree with the precipitation overlay?

6. Create a cyclone which results in a pressure of 992 mb (rounded) at Boston, MA. Region: Track: Intensity: _ _ °N °W What is the path of the low pressure system that gets the pressure that low?

7. Create a cyclone which results in a dewpoint temperature of at least 55°F at Boston, MA. Region: Track: Intensity: _ °N °W Where relative to the low and fronts is Boston when it reaches that dew point?

8. Create a cyclone which results in a temperature of less than 10°F at Winnipeg, MB for 18 consecutive hours.

Region: Track: Intensity: _ °N °W Where relative to the pressure centers is Winnipeg when it stays that cold for that long?

9. Create a cyclone which results in rainfall at Winnipeg, MB.

Region: Track: Intensity: _ °N °W To get this rainfall, where does the cyclone track relative to Winnipeg?

For the next problems, use a Pacific cyclone with a track of 50 and intensity of 50 centered at 43°N, 160°W.

10. There are very few surface observations over the Pacific Ocean, so forecasters must rely on satellite data to identify cyclones. One feature they look for is the so-called comma cloud (looks like a comma).

Identify a time period where the cyclone exhibits a comma cloud: Day Hour

At what stage of development is the cyclone during this time?

11. Go to Day 5, Hour 6 and examine the conditions at Boise, ID and Havre, MT. Is the atmosphere at these two locations stable or unstable?

Boise, ID Havre, MT

(Hint: check the cloud type to help you).