### An overview of the DART 'preprocess' program

First and foremost, check out the DART/preprocess/preprocess.html document for detailed information.

The preprocess program actually builds source code to be used by all the remaining modules. It is imperative to actually run preprocess before building any executables. This is how the same code can assimilate state vector 'observations' for the Lorenz_63 model and real radar reflectivities for WRF without needing to specify a set of radar operators for the Lorenz_63 model!

preprocess combines multiple 'obs_def' modules into one obs_def_mod.f90 that is then used by the rest of DART. Additionally, a new obs_kind_mod.f90 is built that will provide support for associating the specific observation TYPES with corresponding (generic) observation KINDS. More on that later. The list of source codes is contained in the &preprocess_nml namelist and they ultimately determine what observations and operators are supported. If you want to add another 'obs_def' module, you must rerun preprocess and recompile the rest of your project. preprocess is designed to abort if the files it is supposed to build already exist. For this reason, it is necessary to remove a couple files (if they exist) before you run the preprocessor. It is just a good habit to develop.

\rm -f ../../../obs_def/obs_def_mod.f90
\rm -f ../../../obs_kind/obs_kind_mod.f90
./preprocess
ls -l ../../../obs_def/obs_def_mod.f90
ls -l ../../../obs_kind/obs_kind_mod.f90

For example, with a namelist that looks like:

&preprocess_nml
input_obs_kind_mod_file = '../../../obs_kind/DEFAULT_obs_kind_mod.F90',
output_obs_kind_mod_file = '../../../obs_kind/obs_kind_mod.f90',
input_obs_def_mod_file = '../../../obs_def/DEFAULT_obs_def_mod.F90',
output_obs_def_mod_file = '../../../obs_def/obs_def_mod.f90',
input_files              = '../../../obs_def/obs_def_gps_mod.f90',
'../../../obs_def/obs_def_QuikSCAT_mod.f90',
'../../../obs_def/obs_def_GWD_mod.f90',
'../../../obs_def/obs_def_altimeter_mod.f90',
'../../../obs_def/obs_def_reanalysis_bufr_mod.f90'
/


preprocess will combine DEFAULT_obs_def_mod.F90, obs_def_gps_mod.f90, obs_def_QuikSCAT_mod.f90, obs_def_GWD_mod.f90, obs_def_altimeter_mod.f90, and obs_def_reanalysis_bufr_mod.f90, into obs_def_mod.f90 - which can be used by the rest of the project.

### Building and Running 'preprocess'

preprocess is an executable, so it should come as no surprise that it must be built in the normal DART fashion. The DART/mkmf/mkmf.template must be correct for your environment, and the input.nml must have your desired preprocess_nml set correctly. Given that ...

csh mkmf_preprocess
make
./preprocess

will build and run preprocess.

The first command generates an appropriate Makefile and the input.nml.preprocess_default file. The second command results in the compilation of a series of Fortran90 modules which ultimately produces an executable file: preprocess. The third command actually runs preprocess - which builds the new obs_kind_mod.f90 and obs_def_mod.f90 source code files. The rest of DART may now be built.

### A little background for 'preprocess'

IMPORTANT: Since each 'observation kind' may require different amounts of metadata to be read or written; any routine to read or write an observation sequence must be compiled with support for those particular observations. The supported observations are listed in the input.nml&obs_kind_nml block. This is the whole point of the 'preprocess' process ...

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### An overview of the observation sequence

Observation sequences are complicated, there's just no better way to describe it. Trying to automatically accomodate a myriad of observation file formats, structure, and metadata is simply not an easy task. For this reason, DART has its own format for observations and a set of programs to convert observations from their original formats to DART's format. There are definitely some things to know ...

An obs_seq.in file actually contains no observation quantities. It may be best thought of as a perfectly-laid-out notebook - just waiting for an observer to fill in the actual observation quantities. All the rows and columns are ready, labelled, and repeated for every observation time and platform. This file is generally the start of a "perfect model" experiment. Essentially, one instance of the model is run through perfect_model_obs - which applies the appropriate forward operators to the model state and 'writes them down' in our notebook. The completed notebook is renamed obs_seq.out.

An obs_seq.out file contains a linked list of observations - potentially (and usually) observations from different platforms and of different quantities - each with their own error characteristics and metadata. These files arise from running perfect_model_obs OR from any number of converter programs. The creation of observation sequences from real observations is not automatic and an email to the DART team asking for advice for your specific types of observations is perfectly within reason.

There is something called an obs_seq.final file - which contains everything in the obs_seq.out file as well as a few additional 'copies' of the observation. Remember, DART is an ensemble algorithm. Each ensemble member must compute its own estimate of the observation for the algorithm. The obs_seq.final file may contain each of these estimates (namelist controlled). Minimally, the mean and spread of the ensemble estimates is recorded in the obs_seq.final file. The best method of determining the performance of your 'real world' experiment is to compare in observation-space since we can never know the model state that perfectly represents the real world.

IMPORTANT: Since each 'observation kind' may require different amounts of metadata to be read or written; any routine to read or write an observation sequence must be compiled with support for those particular observations. The supported observations are listed in the input.nml&obs_kind_nml block. This is the whole point of the 'preprocess' process ...

#### observation sequence file structure

explanationobs_seq.outobs_seq.final
There are extensible parts of the observation sequence file; for example, the number of observation kinds contained in the file, whether the locations have 1 or more components, how many quality control values are available for each observation, where those quality control values come from, how many 'copies' of each observation there are ... et cetera. The images to the right are links to full-size images. They are from entirely separate experiments. They are just meant to show the flexibility of the file format.

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## Creating observations and sequences.

It is strongly encouraged that you use a single observation to test a new model implementation.

Experience has shown that starting 'simple' is the fastest way to good results. Starting with a single observation will exercise a sufficient portion of the procedure and provide insight into where to spend more effort. Starting with a single synthetic observation will allow you to focus on the more interesting parts of the DART scheme without getting bogged down in the world of observation data formats.

### Creating a synthetic observation sequence.

There are several steps to create an observation sequence file, which follows directly from the modular nature of the DART programming philosophy.

1. Decide what observations you want to investigate and edit the input.nml&obs_kind_nml block.
2. Build and run preprocess to create code that supports the observations you want.
3. Build and run create_obs_sequence to define the specifics about the observation you want.
4. Build and run create_fixed_network_sequence to replicate those specifics through time.
5. Build and run perfect_model_obs to create an observation consistent with the model state and specified error distribution at the requested times and locations.

#### Example: generating observations for the Lorenz '63 model.

1) There are no 'real' observations for the Lorenz '63 model, so the appropriate namelist settings are:

&obs_kind_nml
assimilate_these_obs_types = 'RAW_STATE_VARIABLE' /

&preprocess_nml
input_obs_def_mod_file = '../../../obs_def/DEFAULT_obs_def_mod.F90',
output_obs_def_mod_file = '../../../obs_def/obs_def_mod.f90',
input_obs_kind_mod_file = '../../../obs_kind/DEFAULT_obs_kind_mod.F90',
output_obs_kind_mod_file = '../../../obs_kind/obs_kind_mod.f90',
input_files = '../../../obs_def/obs_def_1d_state_mod.f90' /

2) Run preprocess in the normal fashion.

3) create_obs_sequence creates an observation set definition (typically named set_def.out), the time-independent part of an observation sequence. It may help to think of it as trying to define what sorts of observations will be taken at one 'reading' ... you walk out to the box and take temperature, humidity, and wind observations all at the same time and place, for example. You can think of it as one page in an observer's notebook, and only contains the location, type, and observational error characteristics (normally just the diagonal observational error variance) for a related set of observations. There are no actual observation values, nor are there any times associated with the definition. The program is interactive and queries the user for the information it needs. Begin by creating a minimal observation set definition in which each of the 3 state variables of L63 is directly observed with an observational error variance of 1.0 for each observation. To do this, use the following input sequence (the text including and after # is a comment and does not need to be entered):

The following is a screenshot (much of the verbose logging has been left off for clarity), the user input looks like this.

   [unixprompt]$./create_obs_sequence Starting program create_obs_sequence Initializing the utilities module. Trying to log to unit 10 Trying to open file dart_log.out Registering module :$url: http://squish/DART/trunk/utilities/utilities_mod.f90 ${ ... } Input upper bound on number of observations in sequence 4 Input number of copies of data (0 for just a definition) 0 Input number of quality control values per field (0 or greater) 0 input a -1 if there are no more obs 0 Registering module :$url: http://squish/DART/trunk/obs_def/DEFAULT_obs_def_mod.F90 \$
{ ... }
initialize_module obs_kind_nml values are
-------------- ASSIMILATE_THESE_OBS_TYPES --------------
RAW_STATE_VARIABLE
-------------- EVALUATE_THESE_OBS_TYPES --------------
------------------------------------------------------
Input -1 * state variable index for identity observations
OR input the name of the observation kind from table below:
OR input the integer index, BUT see documentation...
1 RAW_STATE_VARIABLE
-1
input time in days and seconds
0 0
Input error variance for this observation definition
1.0
input a -1 if there are no more obs
0

{ this gets repeated ... until you tell it to stop ... }

input a -1 if there are no more obs
-1
Input filename for sequence (  set_def.out   usually works well)
set_def.out
write_obs_seq  opening formatted file set_def.out
write_obs_seq  closed file set_def.out


Rest assured that if you requested to assimilate more realistic observation types, you will be queried for appropriate information by create_obs_sequence. Below is a table that explains all of the input you should need to supply for observations of the L63 model state.

 4 # upper bound on num of observations in sequence 0 # number of copies of data (0 for just a definition) 0 # number of quality control values per field (0 or greater) 0 # -1 to exit/end observation definitions -1 # observe state variable 1 0   0 # time -- days, seconds 1.0 # observational variance 0 # -1 to exit/end observation definitions -2 # observe state variable 2 0   0 # time -- days, seconds 1.0 # observational variance 0 # -1 to exit/end observation definitions -3 # observe state variable 3 0   0 # time -- days, seconds 1.0 # observational variance -1 # -1 to exit/end observation definitions set_def.out # Output file name

4) create_fixed_network_sequence takes the observation set definition and repeats it in time, essentially making multiple pages in our notebook. Again, the program is interactive and queries the user for information. You should be able to simply follow the prompts. The table below represents the input needed for the L63 example:

 set_def.out # Input observation set definition file 1 # Regular spaced observation interval in time 1000 # 1000 observation times 0, 43200 # First observation after 12 hours (0 days, 12 * 3600 seconds) 0, 43200 # Observations every 12 hours obs_seq.in # Output file for observation sequence definition

5) perfect_model_obs advances the model from the state defined by the initial conditions file specified in the input.nml and 'applies the forward operator' to harvest observations to fill in the observation sequence specified in obs_seq.in. The observation sequence finally has values for the observations and is saved in a file generally named obs_seq.out. perfect_model_obs is namelist-driven, as opposed to the previous two (whose input is a lot harder to specify in a namelist). Take a look at (and modify if you like) the input.nml&perfect_model_obs_nml section of the namelist.

The End. Not only should you have an observation sequence file (usually obs_seq.out) , you also have a file containing the exact evolution of the model consistent with those observations - the True_State.nc.

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### Real Observations - Converting to a DART-compatible format.

Real observations come in a mind-boggling diversity of formats. We have converters for some formats in the DART/observations directory. Many of the formats require their own libraries (like HDF), and require intimate knowledge of the data format to extract the portions required for the DART observation sequence file. Please feel free to browse the converters and their companion documentation. Feel free to donate converters for formats we don't already support! We like that kind of stuff.

The DART framework enforces a clean separation between observations and the models used for assimilation. The same observations can be used in any model which understands how to generate a value for the requested type of observation from the models' state-space values (i.e. the forward observation operator must exist - DART provides many for the most common state variables).

In many cases, the original datasets are in a standard scientific format like netCDF, HDF, or BUFR, and library routines for those formats can be used to read in the original observation data. The DART software distribution includes Fortran subroutines and functions to help create a sequence of observations in memory, and then a call to the DART observation sequence write routine will create an entire obs_seq file in the correct format.

In many cases, a single, self-contained program can convert directly from the observation location, time, value, and error into the DART format. In other cases, especially those linking with a complicated external library (e.g. BUFR), there is a two-step process with two programs and an ASCII intermediate file. We are currently leaning towards single-step conversions but either approach can be used for new programs.

The DART system comes with several types of location modules for computing distances appropriately. The two most commonly used are for data in a 1D system and for data in a 3D spherical coordinate system. All the programs in the DART/observations directory assume the location/threed_sphere/location_mod.f90 3D sphere location module is being used.

With the myriad of observation file formats, HDF, Grib, BUFR, netCDF, ... we simply have not had the time nor need to support all of them. The converters are a work in progress. There are currently about 10 other observation sources and types which we are in the process of collecting information and conversion programs for and which will eventually be added to this directory. In the meantime, if you have converters for data or interest in something that is not in the repository, please email the DART group. Your best bet is to contact our group at dart@ucar.edu with a specific request and we can steer you to the most similar process.

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### Manipulating observation sequences.

First and foremost, check out the DART/obs_sequence/obs_sequence_tool.html document for detailed information and examples.

obs_sequence_tool is the primary tool for manipulating observation sequence files. Observations sequence files are linked lists of observations organized by time. That is to say, the observations may appear in any order in the file, but traversing the linked list will result in observations ordered by time. There are tools for querying the linked list to extract the 'keys' to the list items for a particular date range, for example. obs_sequence_tool can be used to combine observation sequences, convert from ASCII to binary or vice-versa, extract a subset of observations, etc.

For testing, it is terribly useful to extract a small number of observations (like ONE) from an existing observation sequence file.

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### The difference between observation TYPE and observation KIND.

Broadly speaking, observation TYPES are specific instances of a generic observation KIND. The distinction is useful for several reasons, not the least of which is to evaluate observation platforms. Zonal wind observations from QuikSCAT vs. radiosondes, for example. They are both observations of zonal winds (what we call KIND_U_WIND_COMPONENT), but they are different observation TYPES; QKSWND_U_WIND_COMPONENT, and RADIOSONDE_U_WIND_COMPONENT, respectively. The forward observation operators are implemented based on observation KIND. When requested, the model generates a KIND_U_WIND_COMPONENT, it doesn't need to know that it will be compared to a QuikSCAT value or a radiosonde value.

However, it is usually scientifically very interesting to be able to compare the assimilations one TYPE of observation vs. another. One observation sequence file can have lots of types of observations; DART has the capability to assimilate (or evaluate) any combination of observation types without getting bogged down in dataset management. The same observation sequence can be used for experiments that include/exclude certain observation types - ensuring that you are performing the experiment you THINK you are performing ...