# The HD(CP)\({}^{2}\) Data Archive for Atmospheric Measurement Data
## 5 Value-Added Services
### The HD(CP)\({}^{2}\) Portal  
The HD(CP)\({}^{2}\) web portal, located at the Integrated Climate Data Center (ICDC) of the University of Hamburg [26], was created as the central entry point to the data archive. The main task of the portal is the clearly arranged presentation of all available datasets, the respective metadata, and the respective links to the archive. For each dataset, the web portal provides a standardized data sheet, shown in Figure 4, automatically generated from its metadata. Included is information about the measuring instrument, the main measurement variables, and some global information such as the start and end of measurements and the version number. The data sheet also contains a link to the dataset and the associated metadata. The metadata files are periodically harvested from the distributed data servers using the OAI metadata harvesting protocol [27]. The user download of a dataset itself is performed by the distributed data servers, whose infrastructure is based on virtual hardware allowing an easy extension of hardware resources, e.g., the number of processors, memory or disk and backup storage (see Section 4). The extendible architecture may also face an increasing number of users and a growing volume of datasets in the future. All users have free online access to the metadata, but during the first project phase they have to authenticate themselves as project members to download the data themselves. With the start of the second phase, all data shall be freely accessible for the whole scientific climate community.  
Figure 4: Example of a data sheet provided by the HD(CP)\({}^{2}\) web portal. The drop-down selection menu, based on the search tree (see Figure 5), appears directly below the project logo.  
Beyond that, the portal offers general information about the HD(CP)\({}^{2}\) project, descriptions of the different measurement classifications, the measurement campaigns, and the HOPS document. Furthermore, the portal presents an overview of all forward operators used in HD(CP)\({}^{2}\). Forward operators translate the state of the atmosphere within the model into virtual measurements, which can be compared directly to real measurements. The inventory of all forward operators allows a comparative assessment among themselves, due to the standardized metadata for each operator.  
Following the \"easy-to-use\" approach, the web portal offers the possibility to search for specific datasets by using the selection menu based on the HD(CP)\({}^{2}\) search taxonomy. The search tree, shown in Figure 5, is designed to facilitate the user's search for a specific dataset. The datasets are classified according to the measurement type in long- or short-term observations. The long-term observations are divided into local observations, equivalent to the measurement at supersites, and full domain observations, which include satellite and network data. The short-term observations are sorted according to the different campaigns, e.g., the HOPE campaign or Intensive Observation Periods (IOPs). At the next level, datasets are divided into specific groups, such as instrument groups, variable groups or satellites.  
A user without any experience in meteorological observations might look for a specific variable, while an observation expert might look for an instrument group at a certain location (supersite).  
For this reason, the data producers have the opportunity to create more than one keyword list for one dataset, for example ceilometer datasets from the supersite JOYCE should be classified at the first level as Long-Term Observations and at the second level as Local Observations. At the third level, there are more possibilities: the dataset could be associated with Supersites \(\rightarrow\) JOYCE, Instrument groups \(\rightarrow\) Lidar, and Variable groups \(\rightarrow\) Aerosol. Consequently, one dataset is sorted into three different categories, which increases the probability of being found and used.  
Figure 5: Search tree, following the metadata taxonomy for search keywords.