ABSOLUTE VORTICITY ADVECTION

(TERM B )

Term B (pictured above), or the first term to the right of the equal sign (on the homepage), is the differential geostrophic absolute vorticity advection term. This term is proportional to the rate of increase of geostrophic absolute vorticity advection with increasing height. A few important details should be recalled in order to fully understand Term B:

  1. Absolute vorticity advection usually increases with increasing height.

  2. At the surface (1000-hPa) low and high, the vorticity advection is zero.

  3. Aloft, the magnitude of the vorticity advection tends to maximize for short wave systems.

Term B can most easily be understood while looking at a chart of an idealized shortwave trough. We will later be looking at actual data for a shortwave trough using IDV; however for explanation purposes, we will use a textbook example.

In this image the broken contour lines are 1000-hPa isoheights indicating a surface high pressure (H) and low pressure (L). The solid contour lines represent geopotential heights of the 500-hPa isosurface. The R and T refer to the ridge and trough positions of the 500-hPa wave.

We will now assign Term B a sign depending on the position of the point of interest.

This implies sinking motion above the surface high. This is because:

  1. Negative vorticity advection (NVA) above the surface high corresponds to increasing geopotential (via the Q-G Height Tendency Equation).
  2. Thus, geopotential thickness is increasing above the surface high.
  3. Horizontal temperature advection is very small above the surface high.
  4. So, the only way to warm the air (increase the geopotential thickness) is via sinking motion and corresponding adiabatic warming.

This implies upward motion above the surface low. This is because:

  1. Positive vorticity advection (PVA) above the surface low corresponds to decreasing geopotential.
  2. Thus, geopotential thickness is decreasing above the surface low.
  3. Horizontal temperature advection is very small above the surface low.
  4. So, the only way to cool the air (decrease the geopotential thickness) is via rising motion and corresponding adiabatic cooling .

 

Q1 - The bundle should load showing the height field pictured in the previous “Loading the IDV Bundle” section. Based on the labeled contours (geopotential meters), what traditional isobaric surface do you think this height field represents?

 

Obtain an image of the 500-hPa absolute vorticity maximum (as pictured below) in IDV by following these steps

1) Locate the 'Displays' menu on the right side of the screen.

2) Under the heading titled 'VORTICITY' check the box next to "500-hPa Absolute Vorticity".

3) After checking this box, click on the actual title "500-hPa Absolute Vorticity Contours".

 

Q2 - The window that appears is titled “500 hPa Absolute Vorticity Contours; Level: 500 hectopascals.” What is the unit of relative vorticity according to the contour interval listing?

 

4) Close the window titled "500 hPa Absolute Vorticity Contours; Level: 500 hectopascals".

 

You should recall from previous lessons that vorticity advection tends to propagate the trough rather than amplify the trough. Now use the VCR controls just to the left of the word “ Displays ” to move the image forward one frame at a time.

Q3 – What is the maximum absolute vorticity at 500 hPa in the trough for the entire loop? Over what state is this located? At what time and date does it occur? (You may use the magnifying glass to zoom in on the vorticity maximum to decipher the values.)

Once you have answered question 3, continue on to the next steps to visualize the 850-hPa geopotential height contours.

5) Uncheck the box next to "500-hPa Absolute Vorticity Contours".
6) Use the VCR settings to move the image to "2005-11-01 18:00:00Z".
7) Now check the box next to "850-hPa Geopotential Height Contours".

 

Q4 – Does it appear that the 850-hPa low and the 500-hPa low are vertically stacked? If they are, what typically will happen to the reconstituted system? (Hint: Will the system begin to move quicker or slower, become intense or weaken?)


Now uncheck the box next to “850-hPa Geopotential Height Contours” so that you are left with only the 500-hPa height contours. Your IDV screen should look very similar to the one pictured below.

Use the VCR controls at the top to move the image forward or backwards.

 

Q5 – Do the geopotential heights at 500-hPa form a closed low? If so, where?

You should notice that where the 500-hPa low closes off there is also an area of maximum vorticity. Determining the vertical motion and geopotential height field is of extreme importance to forecasters. Geopotential height field changes are sometimes directly related to vorticity advection.

Recall the equation:

Geopotential height rises and falls are associated with corresponding decreasing and increasing geostrophic relative vorticity (hereafter referred to as relative vorticity).

Recall that negative relative vorticity corresponds to anticyclonic circulation, and positive relative vorticity corresponds to cyclonic motion (in the NH).

Q6 – Fill in the blanks: If relative vorticity is _________ (becoming more cyclonic), then geopotential heights are _________ (all else being equal).

Conversely, if the relative vorticity is decreasing (becoming more anticyclonic), then geopotential heights are rising (all else being equal). This would be in the case of a high pressure system building into a region.

NOTE: From here on we will refer to decreasing relative vorticity as Negative Geostrophic Absolute Vorticity Advection (NVA) and increasing relative vorticity as Positive Geostrophic Absolute Vorticity Advection (PVA).

Next, follow these steps:

8) Check the box next to "500-hPa Absolute Vorticity Contours".

9) Check the box next to "500-hPa Geopotential Height Contours".
10) Now using the VCR controls at the top of the screen slowly cycle through the animations.

 

Q7 – Is there PVA or NVA ahead of the 500-hPa low? What is this doing to the 500-hPa height field ahead of the low?

 

If the geostrophic vorticity is increasing and is concentrated aloft, geopotential heights are falling (all else being equal). This is because in areas of PVA there is divergence aloft. According to Dine's compensation principle divergence aloft must lead to upward vertical motion.

The geopotential height falls are associated with the cooling of the atmospheric column. This cooling can be due to adiabatics and corresponding upward motion. Within IDV:

12) Check the box next to "Downward Vertical Motion".
13) Check the box next to "Upward Vertical Motion".

 

 


Q8 - Do you see upward vertical motion downstream or upstream of the region of maximum absolute vorticity? How about downward vertical motion? Why is this?

 

A good way to think about Dine's compensation principle is by studying the image below:

 

14) Uncheck the box next to "OMEGA SURFACES".

15) Check the box next to "500-hPa Geopotential Height Contours".

 

Now use the IDV animation to answer the next question.

Q9 – What is the sign of Term B ,  ahead of the low as it progresses eastward? What about behind the low? (Hint: remember the qualities of a Laplacian).

 

Looking at the animation and applying what you now know about vorticity advection:

 

Q10 – Does vorticity advection act to amplify or propagate the trough?

ANSWERS TO QUESTIONS

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