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Introduction

This document attempts to provide a Quick Start Guide for EVA. Our goal is to have you using the software in less than 10 minutes. This document assumes you have already downloaded and installed the software. If you would like to do more than visualization of pre-prepared data, you will need to run your own simulations. You can do so with EdGCM, and the help of the EdGCM Quick Start guide.

Sample Data

You may use some pre-prepared data to get started visualizing results quickly. These files can be duplicated by running the Modern_SpecifiedSST and Global_Warming01 run in EdGCM, and then post-processing the results. To learn how to do this you can read the EdGCM Quick Start Guide

Download the following data files and save them to your Desktop for this tutorial.

Data Browser

  1. Launch EVA
    • EVA can be launched by clicking on the EVA icon, or more commonly, by selecting the View button from the EdGCM Analyze Output window. If you launch EVA manually, press the Open button, navigate to where you have some EdGCM data and open the files.
  2. Select the file of interest in the top right panel
    • A list of variables available in this file is shown in the top middle panel
  3. Select one variable of interest in the top middle panel
    • A list of times available for this variable is shown in the top right panel
  4. Select one time of interest in the top right panel
    • Your choice: File, Variable, Time is show in the bottom panel
  5. Select your choice in the bottom panel

Notes:

Producing Maps

  1. Press the Plot button at the bottom right of the Data Browser to image the data in map form
  2. Use the Tool Bar to customize the map.

Differencing Data

EVA allows you to compare multiple datas (variables, times, files, whatever), either by displaying the images side-by-side or by differencing two datas to create a new data set and image.

Why Difference Data?

Differences of data are created for multiple reasons. One example of why differencing data is so useful is this:

For a Global Warming experiment, the global average temperature increases by 4 to 5 degrees, less near the equator, but up to 10 or 12 degrees at the poles. The dynamic range of the annual temperature can easily be seen via EVA. It is roughly 80 degrees C (-50 to +30). If you plot a modern climate next to a global warming climate annual average surface air temperature image, the 80 degree change across each image is much larger than the few degree difference between the images, so you eye cannot detect the 4 to 5 degree global average change.

If you difference the two images, then the change between them is highlighted.

Different reasons apply for differencing Precipitation minus Evaporation, or January minus July values of a variable to learn about Summer/Winter differences.

How to Difference Data in EVA

There are 3 ways to difference data with EVA. All methods produce a new file, similar to the data file produced by Analyze Output, except the file contains only 1 variable/time (the difference requested). The new file is opened in the EVA Data Browser automatically.

Data Browser

  1. Use the Data Browser to select your files/variables/times so that two rows exist in the bottom pane of the data browser, or if more than two rows exist, select two of the multiple rows.
  2. Select the Differencing... menu and choose Data 1 - Data 2 or Data 2 - Data 1
    • Data 1 refers to the top item selected in the bottom pane, and Data 2 refers to the lower item selected

Contextual Menu

  1. Image the two files/variables/times you want to difference
  2. Right or Ctrl (not Cmd) + click on one of the images.
  3. Select Difference... from the popup menu
  4. Select Me - X or X - Me depending on the direction of the math operation you would like.
    • Me refers to the image where you brought up the menu
    • X refers to the other data, not yet selected
  5. The next sub-level menu lists all other open map images. Select the other data set you would like differenced
  6. The new data set is opened in the Data Browser

Drag and Drop (OS X Only)

  1. Image the two files/variables/times you want to difference
  2. Click and hold on one of the images
  3. Drag it over the other image
  4. Let go of the mouse button
  5. The new data set is opened in the Data Browser

Visualization Tips

  • Select a colorbar with white in the middle, so that 'no difference' has no color
  • Set the range of the data so that it is symmetric around 0. The Center on 0 button provides a shortcut for this often used feature

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