Meteorology 553- Synoptic Meteorology I

Assignment 2 Due: November 4

Purpose: (1) become familiar with interactive access to gempak including ezscripts; (2) evaluate the terms of the barotropic vorticity equation; (3) contrast the forecasts made by the barotropic model to the more complete dynamics of the MRF model.

Becoming familiar with gempak and scripts

Using gempak interactively is the preferred method to access the data bases of weather information available here. However, gempak uses extensive computer resources and the potential for affecting the performance of the workstations is large. Please follow the directions below carefully.

Create a directory in your root directory, called baro, by using the following command: mkdir baro . As a general rule, it is best to run gempak from a specific directory where you know that the defaults are set up properly.

Move to that directory, by using the command: cd baro

Copy 1 file to your directory by using the following commands: (1) cp ~gempak/met553/gemglb.nts . Use the command `more' to look at this file to see that it contains a long list of parameters that will be set when any gempak routine is called. Look at the Gempak manual to see what a few of these parameters are. This file is updated after each time a gempak routine successfully ends; if you abort a gempak program, then this file will not be updated and you may have to reenter any modifications to parameters that you had made.

The first part of this exercise uses scripts developed originally by Ron Miller, UCAR, and Brad Coleman, Seattle WSFO. They are located in $GEMPAKHOME/scripts/ezscripts. You should look in that directory and `more' each of the scripts that you use to understand what they are doing.

Type `ezset'. Enter the time as 94102800; enter the model as baro; enter the device as xw. This script sets the file based on the time and model type and sets the device to be used as the interactive xwindow device.

Type `ezarea'. This script defines the plotting area. Be patient and wait until the pointer changes to a cross-hair. Then, click near the upper left corner of the figure and drag over the entire domain. Later you can look at a sub-domain by choosing a smaller domain.

Type `ezvadv'. This script plots 4 panels for the forecast times from F000 to F036 at intervals of 12 hours. This will take a while. (1) Upper left: 500 mb height (white lines; contour interval 120m) and absolute vorticity (yellow lines; contour interval 2x10-5/sec); (2) Upper right: 500 mb height (same as 1) and absolute vorticity advection (red lines; positive vorticity advection at intervals of 2x10-10/s*s; green lines; negative vorticity advection at the same intervals); (3) Lower left: 500 mb height and relative vorticity advection (same contouring as in (2)); (4) Lower right: 500 mb height and planetary vorticity advection (same contouring as in (2)).

After the script completes, and the prompt is evident, type `gdcntr'. Type `;l' to loop the frames and `;s' to step through them one at a time. Answer the following questions based on the gempak output. After you finish be sure to type `gpend' to terminate the gempak device driver!!!

Evaluating the Contributions of Relative and Planetary Vorticity Advection

How significant is the planetary vorticity advection to the instantaneous changes in absolute vorticity? Discuss the relative contributions of planetary and relative vorticity advectionto the movement of a single trough/ridge complex in this instance.

How do the instantaneous absolute vorticity tendencies as a result of absolute vorticity advection relate to the 12-hour changes in the absolute vorticity and height fields? How representative are the vorticity tendencies in describing how the ridge/trough complex chosen by you moves?

Comparison of the Barotropic and MRF models

Repeat the steps preceded by a yellow ball for the mrf model; that is, using the same graphical area (you need not run ezarea) and model type `mrf', loop through the mrf forecast for the same period.

Compare the forecasts based on the barotropic and MRF models to the analyses over the subsequent forecast period (I leave it to you to figure out how to do this; there are many ways). How good is the barotropic model forecast for the trough/ridge complex of interest to you? How good is the MRF forecast?

What factor(s) might explain any differences between the MRF and barotropic model forecasts?

Send your written response to this assignment to me by e-mail to jhorel@atmos.met.utah.edu.

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