Abstract
Abstract. Solar radiation is one of the main factors which introduce significant deviations between thermometers reading and true air temperature value. Techniques to protect the sensors from direct radiative influence have been adopted almost since the beginning of meteorological observations. Reflected radiation from a snow-covered surface can also cause extra warming to thermometers hosted in solar shields, which are not always optimised to protect the sensors from this further radiative heat transfer. This phenomenon can cause errors in near-surface temperature measurements results, with a relevant impact on the quality of data records and series. This study experimentally evaluates the effect of reflected radiation from a snow-covered surface on the accuracy of air temperature measurements. The investigation is based on the evaluation of temperature differences between pairs of identical instruments, positioned above ground covered by natural vegetation, with one instrument in snow-free conditions and the other above a snow-covered surface, at the same time and at the same site. The work involved a representative number of sensors and shields, in terms of different typologies, technologies and engineering solutions, from different manufacturers. A mountain site with acceptable field conditions, offering long-lasting snow presence to maximise data availability, was selected to perform the experiment. Quantities of influence, such as relative humidity, wind speed and direction and solar radiation (global and reflected), were constantly measured. The main findings of this work show that none of the involved instruments were immune to the extra heating due to the snow-reflected radiation. Excluding nighttimes and days of high wind or low incident radiation, the differences among sensors positioned above natural soil and identical ones exposed to snow albedo ranged up to more than 3 ∘C. Solar screens with forced ventilation showed a partially reduced effect compared to most of the naturally ventilated ones. A full data analysis is reported here, together with complete results and uncertainties.
Highlights
The World Meteorological Organization (WMO), Commission for Climatology and the Global Climate Observing System (GCOS) recommend the study and definition of measurement methods for reference-grade networks and installations to generate top quality data for meteorology and climate studies (GCOS, 2019)
This study experimentally evaluates the effect of reflected radiation from a snow-covered surface on the accuracy of air temperature measurements
The investigation is based on the evaluation of temperature differences between pairs of identical instruments, positioned above ground covered by natural vegetation, with one instrument in snow-free conditions and the other above a snow-covered surface, at the same time and at the same site
Summary
The World Meteorological Organization (WMO), Commission for Climatology and the Global Climate Observing System (GCOS) recommend the study and definition of measurement methods for reference-grade networks and installations to generate top quality data for meteorology and climate studies (GCOS, 2019). While the practical/technical features offered by these shields are optimised and prescribed (WMO, 2012), their capability to protect the thermometers from radiation reflected by the ground is rarely evaluated or taken into account in measurements or documented in data sheets This is dependent on the so called albedo, indicated with α, which is the ratio of reflected radiation with respect to the global radiation received by the ground that, in the case of snow cover, is increased up to 95 % (Barry and Blanken, 2016). This reflected component can cause extra warming of instruments, introducing errors in near-surface temperature data series, with a relevant impact on detected maximum values and anomalies Such instrumental errors have different magnitudes, depending on the equipment and the technical solutions adopted in manufacturing thermometers and shields. This phenomenon is relevant in monitoring mountain climate, where the duration of snow cover is high (Nigrelli et al, 2018)
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