# The HD(CP)\({}^{2}\) Data Archive for Atmospheric Measurement Data
## 3 The HD(CP)\({}^{2}\) Observation Data Product Standard  
As a final outcome of the discussions, the HD(CP)\({}^{2}\) observation data product standard (HOPS) was developed for the archive, including a metadata scheme. Data files are required to be created in the NetCDF format, which is a self-describing, machine-independent data format that supports the specification of array-oriented scientific data. Programming libraries allow creation, access, and modification of NetCDF-formatted data. NetCDF [15] is a well-known format and is often used in the climate community, which has adopted it as a primary standard. For the data contained in the NetCDF files, the principles given in the Climate and Forecast (CF) metadata conventions have to be followed as far as possible [16]. In particular the HD(CP)\({}^{2}\) definition of the coordinate and data arrays is based on these conventions. The next-to-last NetCDF version 3 and the current version 4 (also with the compression option) are supported. In consideration of the \"easy-to-use\" approach, data are stored on a daily file basis, with only a few exceptions, e.g., for airborne measurements or data of satellite overpasses, due to the large size of the respective daily data. An HD(CP)\({}^{2}\) dataset refers to a whole set of (daily) data files from one measurement series, sharing the same metadata. One set might contain many thousands of files, depending on the time period of the respective (continuing) measurement. The datasets are version-controlled by assigning a unique version number to different stages. Each change in the data, e.g., a fixed bug in the processing or a new position of the instrument, implies a new version number. In general, each daily data file should contain only one variable and its estimated error, if available. There are exceptions for level 1 data, which may also contain more than one variable per file, and measurements for which scalar or single variables do not make sense, e.g., for the variable group wind (consisting of the three components plus horizontal wind speed and direction). Each variable is described by a variable name, and in the case of a CF variable, by a standard_name attribute, and its unit. However, since the CF metadata conventions were developed for model data, not every kind of observation data is covered. Therefore, we cannot provide a CF standard_name attribute for all variables obtained by HD(CP)\({}^{2}\)-related instruments. Variables, which do not have a CF standard name yet have been provided with a long_name attribute. The variables' units comply with the International System of Units (SI Units). An important variable for all datasets is the time variable of the measurement. The convention, therefore, is to specify time in seconds since 1 January 1970 00:00:00, as commonly used in UNIX-like operating systems. Time must be related to the Coordinated Universal Time (UTC, Universal Time, Coordinated). Particularly, a dataset file must have a unique name, which reflects the categorization system, consisting of seven parts as follows:  
\[<\text{kkk}>\_\textless\text{sss}>\_\textless\text{instnn}>\_\textless\text{ln}> \_\textless\text{var}>\_\textless\text{vnn}>\_\textless\text{VYYYMMDDhhmmss.nc.}\]  
The description of the respective parts is given in Table 1.  
The construction of the file name is described in detail in the HOPS document. Using standardized datasets means that no adaptation to an individual instrument or a publisher-specific variable name is necessary. As an example, the air temperature has the variable name ta, with the standard name 'air_temperature' and the unit \(K\), as given in the CF conventions. According to the HD(CP)\({}^{2}\) taxonomy, the air temperature (ta) in a level 2 product (l2) of the first microwave radiometer (mwr00) from the supersite (sups) JOYCE (joy), first version (v00) from 1 January 2014, should be named as sups_joy_mwr00_l2_ta_v00_20140101000000.nc. Well-documented data should enable other researchers to understand, use and reuse the data correctly.