Small-Scale Variability of Solar Radiation
5 June 2023 - 1 September 2023
Lead Scientist: Hartwig Deneke
Within the scope of the Small-Scale Variability of Solar Radiation campaign (S2VSR), a unique sensor network consisting of 40-60 autonomous pyranometer stations developed by the Leibniz Institute for Tropospheric Research in Germany will be deployed for a 12-week period at the ARM Southern Great Plains observatory. Each station records global radiation together with air temperature and relative humidity with a sampling frequency of 1-Hz. The network will provide a data set with an unprecedented level of detail of the small-scale variability of global solar radiation at the surface, by resolving fluctuations down to the second- and decameter-scale. Stations will be distributed across a 4x4 to 6x6 km2 domain surrounding the Central Facility.
The contribution of these scales to the overall variability in solar radiation is large, in particular for shallow convective clouds, and is not sufficiently resolved by current satellite observations and atmospheric models. In addition, at these scales, the effects of 3-dimensional radiative transfer cause significant deviations from the commonly used independent column approximation.
Several previous field campaigns have been conducted in Germany with this pyranometer network. Thus, S2VSR will benefit from the experiences gained, in particular with respect to the selection of an optimal station layout and quality control of these data. S2VSR will offer several unique opportunities by comparison and combination with routine measurements carried out at the ARM SGP site. This includes the surface radiation measurement sites, cloud profiling observations by cloud radar and lidar, as well as the Clouds Optically Gridded by Stereo (COGS) 4D cloud product. Of particular interest is also a comparison of ground-based observations with satellite observations by the geostationary GOES-R ABI instrument (with up to 1min/500m resolution) as well as the Sentinel-2 satellite mission (with up to 10m resolution).
The observations of the S2VSR field campaign will be subsequently used towards the following scientific goals:
- Detailed characterization of the spatiotemporal variability of the global radiation field at the surface, by means of the spatiotemporal semivariogram
- Assessment of the representativity of conventional point-like global radiation observations at the surface for extended spatial domains such as satellite pixels or model grid boxes
- Connection of macro- and microphysical cloud properties with the observed spatiotemporal variability in solar radiation, including the effects of 3D radiative transfer
- Consistency of surface radiation observations with satellite imagery and products including the GOES-R ABI geostationary instrument and the Sentinel-2 MSI imager.
- Feasibility of a radiative closure experiment considering observed and modeled radiative fluxes using both multi-station observations at the surface and satellite observations at the top of atmosphere, in particular for the upcoming EarthCARE satellite mission.
Data Management Plan
Data recorded within the scope of the S2VSR campaign are collected by up to 60 pyranometer stations operating autonomously on battery power without network connectivity during the campaign period. These stations record and store data from an onboard analog-digital converter of a microprocessor on an SD card as raw ASCII data files. Each station records global solar irradiance based on a silicon-cell pyranometer based on a voltage reading from an instrument amplifier, together with readings from a temperature and humidity sensor and logger status information including battery voltage, at 10Hz frequency. Details of the instruments are described in Madhavan et al. (2016). Data are stored as single-line-based data records. These data records are interleaved with 1-Hz GPS messages for accurate timekeeping and precise position information. Data files are subsequently copied to a laptop during weekly maintenance, and complementary log book entries are recorded manually on potential data quality issues such as soiling and leveling of the pyranometer sensors.
After each weekly maintenance cycle, raw data files will be transferred manually together with digital log book entries (stored in an Excel table template) to a TROPOS server, where they are archived and analyzed, subjecting the observations to a variety of quality checks. The raw data are then converted to NetCDF files, adding quality flag information including flags based on the log book entries, averaging of the 10-Hz data to 1-Hz resolution, and applying predetermined calibration coefficients for the conversion of digital counts to physical units. The current file format follows the Climate and Forecast conventions and is based on a standard established for the HOPE field campaign described by Macke et al. (2017). Several changes have, however, been made to quality control procedures and data format since the first campaign. Ongoing quality control will be carried out during the campaign to identify potential data issues as early as possible, possibly adapting quality control and trying to resolve issues. After the campaign, a final quality control check will be conducted, taking into account observations from ARM’s radiation stations if available.
Raw data files contain data for about a week, and have a file size of 150-200MB uncompressed, or about 20MB compressed per station (with GZIP compression). Thus, a total of about 15GB of raw data are expected (20MB x 60 stations per 12 weeks). Based on the current conversion software, daily NetCDF data files containing all station records are expected to have a file size of about 200MB per file, resulting in a total data size in the order of 10-20GB (7 days/week x 12weeks x 200MB). A further reduction using NetCDF4 compression is achievable and planned.
Both raw and processed data files will be archived at TROPOS following institutional rules on research data management and taking into account data management requirements by ARM, which ensures long-term archival of these data for at least 10 years. Data management will be guided and overseen by a TROPOS data manager. Processed data files will be uploaded after conversion and quality control to the ARM Data Center within six months after the end of the campaign. It is also planned to make the campaign data freely available for internet access through an opendap server run by TROPOS.
Outside the direct scope of the campaign, it is also planned to reprocess observations from past pyranometer network campaigns, including those of S2VSR with an improved and homogeneous quality control procedure and an improved calibration strategy using a consistent data format, and describe and publish this data set in a data publication. This will make data easily accessible via a digital object identifier (DOI) to outside users. This effort is independent of the S2VSR campaign.
Madhavan, BL, J Kalisch, and A Macke. 2016. “Shortwave surface radiation network for observing small-scale cloud inhomogeneity fields.” Atmospheric Measurement Techniques 9(3): 1153–1166, https://doi.org/10.5194/amt-9-1153-2016
Macke, A, P Seifert, H Baars, C Barthlott, C Beekmans, A Behrendt, B Bohn, M Brueck, J Buhl, S Crewell, T Damian, H Deneke, S Dusing, A Foth, P Di Girolamo, E Hammann, R Heinze, A Hirsikko, J Kalisch, N Katthoff, S Kinne, M Kohler, U Lohnert, BL Madhavan, V Maurer, SK Muppa, J Schween, I Serikov, H Siebert, C Simmer, F Spath, S Steinke, K Traumner, S Tromel, B Wehner, A Wieser, V Wulfmeyer, and X Xie. 2017. “The HD(CP)2 Observational Prototype Experiment (HOPE) – an overview.” Atmospheric Chemistry and Physics 17: 4887–4914, https://doi.org/10.5194/acp-17-4887-2017