Part 2—Calculate Summer Air Temperature Averages 1980-1999

In any step, click the Show me link to reveal extra information. If you prefer a printout of the full set of instructions for this part, choose Print from the File menu.

Step 1 - Add Data to ArcMap

  1. Open ArcGIS ArcCatalog and browse to the "NCAR Climate Change" folder created in Part 1. The following shapefiles should be present in the folder.
    AcrCatalog shp list before rename

  2. Begin by renaming each of the shapefiles by removing the "-180.0_180.0_-90.0_90.0" from each file name.

  3. Open ArcMap and add the three 1980-1999 20th Century Experiment shapefiles to the Layer list.
    4. The following three files should now be in the Layer list.
    layer list with 1980-99



  4. Right-click on one of the layers in the Table of Contents and select Open Attribute Table. Examine the attribute table. As downloaded from the portal, the table of attributes associated with each shapefile represents air temperature for each month (e.g., August) over the 20 year time frame.


Step 2 - Calculate Summer Averages for 1980-1999

The steps in the next two parts are used to analyze and compare the 20th Century Experiment ensemble averages for the northern hemisphere summer months (June-July-August) to the projected A2 Scenario ensemble averages for the same months. Any model run and/or set of years available from the NCAR GIS Climate Change Portal can be similarly analyzed.

The first step is to combine the months that were downloaded as separate files into one file in order to calculate a June, July, and August average for each of the 20 years. This is done separately for each model run. For example, from the 6 separate shapefiles, you will derive the June, July, August (JJA) average for each year and for each model.

First, (in Part 2, Step 2) for the 20th Century Experiment Model Run 1980 - 1999
  1. 1980 JJA average
  2. 1981 JJA average
  3. 1982 JJA average
    4. ...
  4. 1999 JJA average
And then again Part 3 for Scenario A2 Model Run 2021 – 2040
  1. 2021 JJA average
  2. 2022 JJA average
  3. 2023 JJA average
    4. ...
  4. 2040 JJA average
Finally, in order to calculate the JJA 20 year average in ArcMap, you will join the attribute tables of the three separate months of each 20 year time period. This allows the JJA yearly weighted average to be calculated.
  1. Return to ArcCatalog to make a copy of tas_June_1980_1999_20th_Century_Experiment. Rename the copy "tas_JJA_1980-1999_20th_Century_Experiment". You will use this file for your averages.
  2. Drag tas_JJA_1980-1999_20th_Century_Experiment file from ArcCatalog to the top of the ArcMap Layer list, above the three monthly layers.
  3. Join each of the July (07) and August (08) attribute tables to the tas_JJA_1980-1999_20th_Century_Experiment attribute table using the FID field.
  • Choose FID for the field in table to base the join on:
  • Choose "tas_July_1980-1999_20th_Century_Experiment" for the table to join to tas_JJA_1980-1999_20th_Century_Experiment
  • Choose FID as the filed in the table to base the join on:
  • Click OK
    join data table


  • Repeat the join again for "tas_August_1980-1999_20th_Century_Experiment"
    join data table august.



  • Open and examine the tas_JJA_1980-1999_20th_Century_Experiment attribute table. Verify that the joins were successful. You should now have temperature data for all three months for the years 1980-1999.


  • Add new fields to this table to accommodate the calculation of the yearly JJA weighted averages for 1980-1999. Name the new fields JJA1980avg, JJA1981avg, JJA1982avg...JJA1999avg for a total of 20 new fields.
  • Calculate JJA weighted average for each year using the Field Calculator in the attribute table
    • Weighted Average: From this step, for each unique cell from the CCSM model output, a JJA weighted average for each year is calculated for each model run. The weighted average is simply considering the unequal number of days within each calendar month, and weighting appropriately by multiplying each monthly average by the number of days within each month and dividing by the total. From this, for each unique cell from the CCSM model output, a JJA weighted average for each year is calculated for each model run. For example, the equation below demonstrates the calculation for the 1980 JJA average:number of days.
    formula



    To prevent syntax errors copy and paste the following equation into the Field Calculator.

    ((30 *[tas_JJA_1980_1999_20th_Century_Experiment.198006]) + (31 *[tas_July_1980_1999_20th_Century_Experiment.198007]) + (31 *[tas_August_1980_1999_20th_Century_Experiment.198008]))/92

  • Repeat for each of the 20 fields JJA1981avg-JJA1999avg

    • If you had difficulty completing the steps listed above, download this completed file and place it in your NCAR Climate Change Folder.
    Calculated Summer Averages for 1980-1999
    tas_JJA_1980_1999_20th_Century_Experiment_completed (Zip Archive 8.1MB Jan10 09)

    Step 3 - Calculate 20-year Average for 1980-1999

    1. Add a new field to the attribute table to accommodate the 20-year JJA Average. Name this field JJA_20_avg.
    2. Calculate 20-year mean using the Field Calculator in the Attribute table.
      To prevent syntax errors copy and paste the following equation into the field calculator.

      ([tas_JJA_1980_1999_20th_Century_Experiment.JJA1980avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1981avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1982avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1983avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1984avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1985avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1986avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1987avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1988avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1989avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1990avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1991avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1992avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1993avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1994avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1995avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1996avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1997avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1998avg] + [tas_JJA_1980_1999_20th_Century_Experiment.JJA1999avg])/20

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