Lavinia Laiti1*, Lorenzo Giovannini1, Luca Panziera2, Daniele Andreis3, Fabio Zottele3, Gianni Toller3, Dino Zardi1
1 Atmospheric Physics Group, Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento - Italy
2 MeteoSwiss, Locarno Monti - Switzerland
3 Geographic Information System Unit, Technology Transfer Center, Edmund Mach Foundation, San Michele all’Adige - Italy
*lavinia.laiti@unitn.it
Abstract
The accurate measurement of the solar radiation available at the Earth’s surface is very important for agricultural applications. It is particularly challenging in mountainous areas, where orographic effects increase the spatial and temporal variability of radiation. State-of-the-art algorithms for the quality control of hourly observations of solar radiation are presented. The criteria are based on physical limits, temporal variability/persistence and spatial consistency of the data. The outcomes of the application of the quality control procedure to global irradiation observations collected at the radiometric stations of the Trentino region (Italian Alps) during the years 1987-2014 are reported, together with some examples of common instrumental, installation- and/or maintenance-related errors.
Keywords
Solar radiation, ground observations, quality control, complex terrain, Alpine region. Parole chiave
Radiazione solare, osservazioni al suolo, controllo di qualità, terreno complesso, regione Alpina. Introduction
The accurate measurement of the solar radiation available at the ground is essential to a wide number of agricultural, hydrological and energy applications. In particular, the temporal and spatial variability of solar radiation in mountainous regions is higher than over flat terrain, as an effect of the complex orography and typical weather phenomena. In general, ground-based observations of solar radiation are prone to multiple errors. They may be caused by sensor failures, wrong installation or calibration, or external factors, like soiling of the pyranometer’s dome by dust, or its shadowing (e.g. by trees or buildings). Indeed, the quality control (QC) of radiometric observations is mandatory to exclude erroneous and/or suspicious data (WMO, 2008), and even more important in mountainous regions, where operational conditions are typically rather harsh. Commonly applied QCs include tests based on lower/upper threshold values, inferred from solar radiation models or from the data themselves (i.e. based on long-term statistics or on data from neighboring stations). A set of QC criteria applied to the hourly radiation data collected at the radiometric stations of Trentino (in the Italian Alps) is presented (see Laiti et al., 2014). The quality tests are chosen among those proposed in the literature, e.g. by Younes et al. (2005) and Journée and Bertrand (2011), and partially modified to meet the specific characteristics of the present dataset (i.e. complex terrain related features). Materials and Methods
The study area is the Trentino region (Italian Alps), which is almost entirely mountainous. It has a very dense network
of radiometers (104 stations active in 2012), preferentially installed at the floor of the largest valleys. The stations are managed by the meteorological office of the Autonomous Province of Trento (Meteotrentino; 26 stations), the E. Mach Foundation (77 stations) and the University of Trento (one station). First-class Kipp&Zonen CM6B thermopile pyranometers and second-class Davis, MTX and Metex silicon-cell pyranometers, are installed. Hourly data of global horizontal irradiation (G) are available for the years 1987-2014.
Physical threshold tests. For daytime observations it is
checked that 0 G E and 0 G 1.1GC. The European
Solar Radiation Atlas model (Remund and Page, 2002) was used for the computation of hourly values of E (extraterrestrial radiation), GC (clear-sky global radiation)
and sunrise/sunset times, while the times of start/cessation of direct radiation (determined by local orography) were retrieved thanks to the r.horizon module of GRASS GIS (GRASS, 2015). For observations collected between astronomical and orographic sunrise (sunset) times, when only diffuse radiation D is measured, specific limits were also set, i.e. 0 G DMAX (DMAX572h ,where h is solar
elevation in radians). Moreover,
iGi0.03
iEi mustalso hold on daily basis (i = 1 to 24).
Step test. Unphysical jumps between observations taken at
the ith and the (i-1)th hour are identified as follows:
1 1 0.75 i i i i G G E E
Persistence test. The test checks the temporal variability of
For the daily mean μ and standard deviation σ of diurnal observations this expression must be satisfied:
1 0.35 8 G G E E .
Spatial consistency test. For each station the daily totals of
G are estimated by inverse-distance interpolation of
observations from the nearest stations with similar elevation and/or exposure. Then upper limits are applied to the difference between measured and estimated values, based on fixed proportions (50, 100 or 200%) of the mean bias over the stations involved in the interpolation.
Associated with the above tests, specific error codes were assigned to the single hourly records, together with final quality codes, namely 0 = valid data, 1 = suspicious data, 2 = erroneous data.
Results and Discussion
For some stations the number of available data is strongly reduced after the QCs, causing a few time series to be entirely rejected. This reveals that difficulties and costs associated with the maintenance of a dense radiometric network, combined with the harsh operational conditions typical of mountainous sites, may lead to measurements of inadequate quality. Two examples of issues commonly detected in the analysed time series are provided in Figs. 1 and 2. Volano station suffers from shadowing by a recently grown tree or a recently erected building, which causes a recurrent dip in the data during the late morning hours (between 10:00 and 12:00 LST). This error, particularly evident on clear-sky days, is detected thanks to the step test (see Fig. 1). On the other hand, many winter days of the time series recorded at Monte Bondone present anomalously small (i.e. almost null) and steady radiation data (Fig. 2). This problem is associated with unremoved snow presence over the dome of the pyranometer, and is identified thanks to the temporal persistence test, as well as to the application of a lower threshold to the daily totals of
G (equal to the 3% of the daily extraterrestrial irradiation).
Finally, concerning the spatial consistency test, the morphological complexity of Trentino sometimes hinders the possibility of finding neighbouring stations characterized by elevation and exposure conditions comparable to those of the station of interest. On the other
Fig.1 – Volano station: empty dots are valid observations, black dots are erroneous data according to the step test. Fig.1 – Stazione di Volano: i cerchi vuoti sono osservazioni valide, i cerchi neri sono dati errati secondo lo step test.
Fig.2 – Monte Bondone station: as in Fig.1, but for tests based on physical limits and persistence.
Fig.2 – Stazione di Monte Bondone: come in Fig. 1, ma per i test basati sui limiti fisici e sulla persistenza temporale.
hand, this test often also facilitate the identification of inhomogeneities in the time series.
Conclusions
This contribution describes the tests implemented for the QC of hourly global radiation data, collected in the mountainous region of Trentino (Italian Alps). The outcomes of the QC procedure highlight that part of the data does not have a sufficiently good quality, often due to the additional difficulties and costs associated with the maintenance of meteorological stations in mountainous areas. According to the QC results shown, the application of quality tests appears an essential preliminary step to ensure the quality of the radiation measurements and of eventual further analyses.
Notice that the validated solar radiation dataset presented here forms the basis for a high-resolution solar atlas of Trentino, produced by the geostatistical interpolation of ground-based observations.
Acknowledgments
The authors acknowledge Meteotrentino and the E. Mach Foundation for kindly providing the data.
References
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Laiti L., Andreis D., Zottele F., Giovannini L., Panziera L., Toller G., Zardi D., 2014. A Solar Atlas for the Trentino Region in the Alps: Quality Control of Surface Radiation Data. Energy Procedia, 59: 336-343.
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