Gempak Script for the Garcia Method

Download the scripts to run the Garcia Method

Step 1: Pick a time and model
Step 2: Choose a forecast hour
Step 3: Specify the area of concern
Step 4: Choose an isentropic surface
Step 5: Analysis

From any office Linux machine, begin the program by typing


or for the verbose version with comments and explanations, type


when you are done, exit by typing


Step 1: Pick a time and model

A window will open up with the following choices,

0zETA   12zETZ   0zNGM   12zNGM   0zUWNMS

A list of available files will be displayed in the window from which you ran the script. (include example).

You will be asked if you want to create new isentropic data files. Generally, you will not have to. John Eise creates these and includes them into the model data. However, if this has not been done, or if those created do not cover the range of theta that you need, you should create new isentropic surfaces. This data will not be included with the model data, instead it will be put into a new file in the directory: /home/gempak/swetzel/thetadata. The rest of this program will use the new data if you choose to create it. Otherwise, it will expect isentropic data with the model.

Step 2: Choose a forecast hour

f00  f06  f12  f18  f24  f30  f36  f42  f48 

The Garcia method forecasts for a 12-hour period. The time you choose is the start of this period. For example, if you are using the 12Z model and you pick f06, you will make a forecast for the 12-hour period from 18Z to 0Z.

It is not recommended to use the f00 data.

Step 3: Specify the area of concern

The Garcia method suggests you decide on the placement of this cross-section using traditional rules of thumb (
see list). It is also useful to consider the Q-vector forcing shown in Figures 1 and 2 (click on thumbnail image for an example).

In both plots, layer averaged Q-vector convergence is shaded, layer average mixing ratio is in green, and sea level pressure is dotted white.

Figure 1 shows the initial time of your forecast period, chosen in Step 2. Each quadrant shows the quantities computed at a different level.

Low level (850-700mb) Mid level (700-500mb)
Upper level (500-300mb) Low-Mid level (850-500mb)

Figure 2 shows 6 and 12 hours after the initial time for mid and low level layers as shown below. Note that this Figure is overwritten in the next step due to a Gempak limit of 5 plot windows.

+6 hours,
Low level (850-700mb)
+6 hours,
Mid level (700-500mb)
+12 hours,
Low level (850-700mb)
+12 hours,
Mid level (700-500mb)

It is important to look at the Q-vector convergence throughout the 12-hour period to decide if there will be forcing for ascent present for the whole forecast period.

Also note that if the forcing is in the mid or upper levels, it might not be appropriate to apply the Garcia technique which assumes the majority of the vertical motion occurs at 700mb and below.

After you choose which quadrant of Figure 1 you want to draw the cross section in, place the cursor in that quadrant and wait until it becomes a white cross. Then click on one end point of the cross section with the left mouse button and hold it down as you move the cursor to the other end point.

Step 4: Choose an Isentropic Surface

The left side of Figure 2 (disregard the right side for now) shows the cross section you chose in Step 3. Mixing ratio (g/kg) is shaded, isentropes (K) are white, Q-vector convergence is purple, and the 700mb- 750mb layer is outlined in black.

Figure 2 (click to enlarge)

From this figure, choose a theta (K) corresponding to the isentropic surface which intersects the 700-750mb layer over your area of concern.

288  290  292  294  296  298  300  302  304  306  308  310  312 

Step 5: Analysis

Note: actual images in this step (Figures 3-5) also have arrows denoting windspeed and direction on the isentropic surface, these arrows do not appear on the images shown here.

The conventional Garcia method ends with Figure 3 (click to enlarge):
This Figure shows mixing ratio in g/kg (shaded) on the isentropic surface you chose earlier. Pressure is also plotted in white, with the 700 and 750mb isobars thicker than the rest. Arrows representing winds on the isentropic surface as well as latitude/longitude lines are also shown.

The Garcia method recommends you use the quantities in Figure 3 to estimate the maximum mixing ratio that could be advected into your area of concern in the next 12 hours. Then, compute the average between that quantity and the mixing ratio in Figure 3. Two inches of snow are then predicted for each g/kg of this final result.

Figures 4 and 5, and the right side of Figure 2 give you an alternative way of computing the advection of mixing ratio. These plots are described below.

Figure 2 (see image above):
Right Side top: Advection of mixing ratio by the wind at the initial time, multiplied by 12 hours. This results in the expected change in mixing ratio if the current advection remained the same for 12 hours, as recommended by the Garcia method. Blue values indicate the mixing ratio will rise and red values predict a drop in mixing ratio at that location.

Bottom Right: This is the change in the mixing ratio according to the model from your initial time to time+12 hours. Notice this is not the same as computing this by the advection method, since the system moves and evolves over 12 hours and the advection patterns change.

Figure 4 (click to enlarge):

Similar to Figure 3, except the shaded quantity is the average of the mixing ratio at the initial time with the mixing ratio plus changes predicted by 12 hour advection (changes shown in Figure 2 top right).

Figure 5 (click to enlarge):

Similar to Figure 3, except the shaded quantity is the average of the mixing ratio at the initial time with the mixing ratio 12 hours later (changes shown in Figure 2 bottom right).