Case 23: 8 - 11 November 1998

Case 23: 8 - 11 November 1998

Q-G Forcing of Vertical Motion in an Extratropical Cyclone

Note: I plan to enhance these labs with example imagery pertaining to each set of questions

Objectives

Compare the quasi-geostrophic forcing of vertical motion using the Q-vector formulation at different levels
Compare the q-g forcing of vertical motion to model calculated vertical motion
Compare the q-g forcing of vertical motion to observed cloudiness and precipitation

Background

The quasi-geostrophic omega equation using the Q-vector formulation is given in Chapter 6 of Holton (1992). The forcing of vertical motion is given by the divergence of Q. In this lab we will compare the quasi-geostrophic forcing to observed precipitation and cloudiness in an effort to see to what degree these observed quantities were influenced by quasi-geostrophic forcing.

Please remember that in this lab we will be looking at the forcing of vertical motion (i.e. the right hand side of the first equation) and we are not determining w itself. Recall that forcing at one level can influence vertical motion at another through the 3 - D Laplacian on the left hand side of the equation. Therefore, use caution when comparing the pattern of forcing to clouds and precipitation.

Alternatively, the Sutcliff-Trenberth method of determining vertical motion by the advection of vorticity by the thermal wind is also a useful way to determine quasi-geostrophic forcing of vertical motion. Since the two formulations are slightly different, it can be instructive for the student to examine the differences between these two forcing distributions in this case.

Lab Exercise
Garp Instructions-- Vertical motion at 500 hPa

Open up garp for the November 1998 case.
Select the model plan projection icon and select the AVN model
Select the initialization time (F000) for 12 UTC 10 November 1998 run.
You will have to change level 1 (and level 2) quite often and you should always check that the correct level is indicated before you display your plot.
Sometimes you will need to change General to HAS in the middle of the pop-up window. HAS has several other options not available under General.
Plot the 500 mb height, then switch to the parameters under the HAS option.
Plot qvectors, Qvec_geo, and make sure level 1 is 300 mb and level 2 is 700 mb.
Plot Q-vector divergence. The way itÕs set up, it only plots the regions of convergence of Q (forcing of upward motion). To get the divergence of Q, go to more and set the minimum to be 3 and maximum 39 and turn OFF the color fill option.

Questions

1. Where (geographically) are the major regions of the forcing of upward and downward motion according to the forcing by q-vectors at 500 hPa? Where are these regions of upward and downward forcing with respect to the 500 hPa height pattern (i.e. upward forcing in a trough, in a ridge, etc.)?

Garp Instructions

Clear screen and plot the 12 UTC 10 Nov GOES-8 4 km IR satellite image. Zoom in on the area of interest.
Overlay the surface observations and change the station model so that it only plots the wind barb, observed weather, and 6-hour precipitation.
After you look at this and attempt to answer the question below, you could be brave and try to overlay q-vector divergence onto the satellite image. It's not very pretty.

Questions

2. Do regions of upward motion forcing correspond to cloudiness and reported rainfall and do regions of downward motion forcing correspond to clear areas? If there are regions where the corresondence between quasi-geostrophic forcing and observed cloudiness and precipitation does not meet expectations, speculate on reasons why.

Garp Instructions -- 700 mb forcing

Clear screen.
Plot the 700 mb height
Plot q-vector divergence for the layer 500 mb to 850 mb.
Clear screen and bring up the 12 UTC IR satellite image again.

Questions

3. Does the vertical motion forcing at 700 mb better represent the cloudiness/precipitation patterns than the vertical motion forcing at 500 mb?

Overlay the model derived vertical motion onto the satellite image. Choose the time: 1998111000F012, go to the GENERAL parameters and display Omega_mbs-1.

4. How well does the model forecast of vertical motion correspond to the observed cloudiness?

5. Based on all the plots created above, comment on whether the cloud field and observed precipitation was primarily forced by q-g synoptic-scale upward motion. If not, speculate on other reasons for the observed cloud distribution (i.e. convection?).

Main Points

Forcing at one level does not give the full picture of vertical motion
Q-G forcing of vertical motion does not explain all observed vertical motion, such as frontal lifting and convective motions.

Back to Nov 98 Overview Page