The behavior of MTD is controlled by the contents of the configuration file passed to it on the command line. The default MTD configuration file may be found in the $MET_BASE/config/MTDConfig_default file. Prior to modifying the configuration file, users are advised to make a copy of the default:

The configuration items for MTD are used to specify how the space-time-object-based verification approach is to be performed. Just as MODE may be used to compare any two fields, the same is true of MTD. When necessary, the items in the configuration file are specified separately for the forecast and observation fields. In most cases though, users will be comparing the same forecast and observation fields. The configurable items include specifications for the following:

• The verification domain.
• The forecast and observation fields and vertical levels or accumulation intervals to be compared.
• The forecast and observation object definition parameters.
• Options to filter out objects that don't meet a minimum volume.
• Matching/merging weights and interest functions.
• Total interest threshold for matching/merging.
• Flags to control output files.

For this tutorial, we'll configure MTD to process the same series of data we ran through the Series-Analysis tool. Just like MODE, MTD compares a single forecast field to a single observation field in each run.

Open up the $MET_TUTORIAL_DATA/config/MTDConfig_tutorial file for editing with the text editor of your choice and edit it as follows:

• Set the fcst dictionary as follows:
  
  fcst = {
     field = {
        name  = "APCP";
        level = "A03";
     }
     conv_radius = 2;
     conv_thresh = >=0.4;
  }
• Set the obs dictionary as follows:
  
  obs = {
     field = {
        name  = "APCP_03";
        level = "(*,*)";
     }
     conv_radius = 2;
     conv_thresh = >=0.4;
  }
• Retain all objects with minimum volume of 500:
  
  min_volume = 500;

Save and close this file.

Next, we'll run MTD on the command line using the following command:

Just as with MODE, MTD applies a convolution operation to smooth the data. However, there are two important differences. In MODE, the convolution shape is a circle (radius = conv_radius). In MTD, the convolution shape is a square (width = 2*conv_radius+1) and for time t, the values in that square are averaged for times t-1, t, and t+1. Convolving in space plus time enables MTD to identify more continuous space-time objects.

If your data has high enough time frequency that the features at one timestep overlap those at the next timestep, it may be well-suited for MTD.

The MTD output typically consists of six files: five ASCII statistics files and one NetCDF object file. MTD does not create any graphical output. In this example, the output is written to the $MET_TUTORIAL_DATA/output/mtd directory as we requested on the command line.

The MTD output file naming convention begins with mtd_ followed by the last valid time it encountered. The output file names may be modified using the output_prefix option in the configuration file which should be used to prevent the output of one run from over-writing the output of a previous run. The six MTD output files are described briefly below:

• The NetCDF object file ends in .nc and contains gridded fields of the raw data, simple object indices, and cluster object indices for each forecast and observed timestep.
• The ASCII file ending with _2d.txt contains many columns similar to the output of MODE. This data summarizes the 2-dimensional object attributes for each individual time slice of the 3D forecast and observation objects.
• The ASCII files ending with _3d_single_simple.txt and _3d_simple_cluster.txt contain 3D space-time attributes for simple and cluster objects, respectively.
• The ASCII files ending with 3d_pair_simple.txt and _3d_pair_cluster.txt contain 3D space-time attributes for pairs of simple and cluster objects, respectively.

Use the ncview utility (if available on your machine) to view the NetCDF object output of MTD:

Select the variable named fcst_raw and click the time index to advance through the timesteps. Now, do the same for the fcst_object_id variable. Notice that the objects are defined in the active areas in the raw fields. Also notice some features merging (i.e. combining) as time passes while other features split (i.e. break apart). While they may be disconnected at a particular timestep, they remain part of the same space-time object.

Next, explore the ASCII output files and pay close attention to the header columns. Notice the generalization of the 2D MODE object attributes to 3 dimensions. Area meausure become volume. MTD measure the object speed. Each object has a beginning and ending time.