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Giovanni Online Users Manual: 18. MERRA

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 All of the questions below are linked to descriptive answers in this chapter of the Giovanni Online Users Manual regarding the Giovanni MERRA instance. Click the question of interest. 

 

  1. What is MERRA?
  2. Where can MERRA product documentation be found?
  3. How are the MERRA data grouped in Giovanniω What types of data are available?
  4. How is the spatial area of interest selected?
  5. How is the vertical profile selected for 3-dimensional products?
  6. How are data parameters selected for analysis?
  7. How is the time range of interest selected?
  8. What analysis functions are available?
  9. What plotting preference options are provided for data visualization?
  10. How can the results of an analysis be downloaded?
  11. How many output data formats are available?
  12. Are there any limits on the spatial or time range that can be selected?
  13. Are there any limitations on the number of parameters that can be selected?
  14. Are there any hands-on practice cases?

 

What is MERRA?

MERRA, the Modern Era Retrospective-analysis for Research and Application, is an atmospheric observation reanalysis using the Goddard Earth Observing System Data Assimilation System Version 5 (GEOS-5) performed by NASA’s Global Modeling and Assimilation Office at Goddard Space Flight Center. This project has generated a long-term (1979-present) synthesis that places satellite observations, including those of the NASA EOS suite, in a climate context, focusing on improved estimates of the hydrological cycle over a broad range of weather and climate time scales. Detailed information about this project can be found at MERRA project web page http://gmao.gsfc.nasa.gov/research/merra/.

 

Where can MERRA product documentation be found?

The draft MERRA File Specification Document is currently available. It describes the output variables and files that will be available from the MERRA reanalysis.  The current version is dated September 2008.  

File Specification for MERRA Products, Version 2.1, Release Date 9/2/2008

(PDF document)

How are the MERRA data grouped in Giovanniω What types of data are available?

Currently, the parameters in Giovanni are simply grouped by product collection.  The products collections available are:

 

Instance Name

Group/Product Name

Resolution (degrees)

Collection Name

 

MERRA_MONTH_2D

(MERRA Monthly 2D Data Collections)

Land surface diagnostics

2/3 x 1/2 

MATMNXLND.5.2.0

Atmospheric single-level diagnostics

2/3 x 1/2

MATMNXSLV.5.2.0

Surface turbulent flux diagnostics

2/3 x 1/2

MATMNXFLX.5.2.0

Surface and TOA* radiation fluxes

2/3 x 1/2

MATMNXRAD.5.2.0

Vertical integrals of tendencies

2/3 x 1/2

MATMNXINT.5.2.0

Vertical integrals of quantities

2/3 x 1/2

MAIMNXINT.5.2.0

 

 

 

MERRA_MONTH_3D

(MERRA Monthly 3D Data Collections)

Upper-air ozone tendencies by process

1.25 x 1.25

42 levels

MATMCPODT.5.2.0

Upper-air humidity tendencies by process

1.25 x 1.25

42 levels

MATMCPQDT.5.2.0

Upper-air wind tendencies by process

1.25 x 1.25

42 levels

MATMCPUDT.5.2.0

Upper-air temperature tendencies by process

1.25 x 1.25

42 levels

MATMCPTDT.5.2.0

Upper-air diagnostics from turbulence

1.25 x 1.25

42 levels

MATMCPTRB.5.2.0

Upper-air cloud related diagnostics

1.25 x 1.25

42 levels

MATMCPCLD.5.2.0

Upper-air diagnostics from moist processes

1.25 x 1.25

42 levels

MATMCPMST.5.2.0

Upper-air diagnostics from radiation

1.25 x 1.25

42 levels

MATMCPRAD.5.2.0

Basic assimilated fields from IAU** corrector

1.25 x 1.25

42 levels

MAIMCPASM.5.2.0

* TOA:  Top Of Atmosphere

** IAU:  Incremental Analysis Update

How is the spatial area of interest selected?

The spatial area of interest can be selected by either of the two methods described below

Java applet map: The Java applet map can be utilized to "click and drag" a selection box to designate the spatial area of interest.

Geographic coordinates: The geographic coordinates of the corners of the spatial area box can be input individually. This method can be used if Java is not enabled. Note that decimal and not degree (seconds and minutes) coordinates must be entered.

 

How is the vertical profile selected for 3-dimensional products?

Below the spatial selection area map is a box labeled "Vertical Profile".  If a 3-dimensional data product is selected (see below), the Upper Level and Lower Level atmospheric pressure levels (in units of hectopascals, hPa) must also be selected.  The selection boxes will be gray if a 3-dimensional data product is not selected.  If the levels are not selected for a 3-dimensional parameter, an error message will appear. 

To select the Upper Level and Lower Level, click on the arrow box, and a menu of levels will appear.   1000 is sea level, and the lowest values correspond to stratospheric levels.  Note that not all visualization options support the display of vertical profile data. 

How are data parameters selected for analysis?

To select data parameters for analysis, clickthe box next to the parameter of interest.   Every parameter selected will generate an individual visualization.

How is the time range of interest selected?

The starting date (year/month/day), and the ending date (year/month/day) for the analysis are selected using the calendrical drop-down menus on the Giovanni interface page, which are located below the Parameter Selection interface. Error messages will be generated if the beginning month or the end month are out of the data set time range, or if the end month is before the beginning month.   The begining dates and end dates for the data in Giovanni MERRA are shown next to the menus.

What analysis functions are available?

There are currently 5 data analysis and visualization options in the Giovanni MERRA 2D instance, and 9 analysis and visualization options in the Giovanni MERRA 3D instance.

Giovanni MERRA 2D instance:

Giovanni MERRA 3D instance:

Note, however, that the MERRA 3D instance has a few 2D data sets.  Several of the visualization and analysis options will only work with the 2D data sets;  the options which work with 3D data are the three Cross Map options, and Vertical Profile.

 

What plotting preference options are provided for data visualization?

For data plots with 3-dimensional data, the Image X and Image Y dimensions can be adjusted, and the Number of Ticks (corresponding to latitude or longitude increments) can be changed.

2D data provides several plot preference options, which are described in the Plot Preferences chapter of the Giovanni Online Users Manual.    Note that the chapter describes the Image X/Y option, which correspond to the Image Width and Image Height adjustment options, respectively.

How can the results of an analysis be downloaded?

To download the results of an analysis, click on the "Download Data" link above the visualizations on the Results page.    "Initial Data Retrieval"  results are the data files acquired to perform the analysis.   The results are listed under the name of the operation which generated the visualization.  

To download an individual results data file, click on the icon for the desired data format (see below).   To download more than one data file in Batch mode, click the desired data files and then click "Download in Batch".    To acquire the actual visualization image, click the link to the GIF image.

How many output data formats are available?

Three numerical and two image data output formats are available.  Numerical data is available in HDF, netCDF, and ASCII.   Image data output formats are GIF and KMZ.    If Google Earth is installed on a user's home computer, clicking the KMZ icon will allow immediate load and display of the visualizationin Google Earth.

 

Are there any limits on the spatial or time range that can be selected?

No. Longer time ranges will take considerably more time for initial data acquisition from the archive (fetching) and may require a longer period of elapsed time for processing. Also, the time range for which data can be requested is limited by the time range of the data set itself (see question 7).     MERRA data is global and there are no limitations on the spatial area that can be selected.

Are there any limitations on the number of parameters that can be selected?

No.   However, note that if 2D and 3D data are selected in the Giovanni MERRA 3D instance, plotting options which work for 2D data will not function for 3D data, and vice versa.  Therefore, combining 2D and 3D data parameters is not possible.  

For the multiple data display of time-series data in the Giovanni MERRA 2D instance, the maximum number of parameters that can be selected is five.

Are there any hands-on practice cases?

The document below was prepared as a supplement to Giovanni demonstration cases featuring MERRA for a short course presented at the January 2009 annual meeting of the American Meteorological Society.

Giovanni MERRA Practice Cases

 

 


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