Quantifying uncertainties in climate data: measurement limitations of naturally ventilated thermometer screens
2021
Harrison, R. Giles | Burt, Stephen D.
Climate science depends on accurate air temperature measurements. To achieve this, well ventilated thermometers protected from direct sunlight and precipitation are needed, commonly through using louvred enclosures known as screens or shields. Maintaining good natural ventilation is critical for accurate measurements. Ventilation effects on air temperature uncertainties are quantified here using an aspirated thermometer as a reference, for air temperatures spanning -2.8°C to 35.5°C. Of 81462 5 min mean temperature values obtained, 50% were within ±0.07°C of the reference and only 2% lay beyond -0.66°C to 0.47°C, where negative values represent the naturally ventilated screen thermometer underestimating air temperature. Larger absolute differences arose from a combination of radiation exchange and time response effects, which are separated here. Firstly, using 20s data, the exponential time response of the naturally ventilated thermometer is shown to vary with wind speed u as u-0.5, and exceeds the conventional 1 min averaging time for wind speeds up to 5 ms-1 (at 2 m height), increasing to at least 15 mins when calm. Secondly, radiation exchange effects (both by day and by night) dominated at low wind speeds (<1 ms-1 at 2 m height). Insufficient time response damps recording of temperature extremes, potentially also influencing climatological means. A new method to reduce the uncertainties is presented. This reduces negative skew in the temperature bias from -1.05 to -0.26, and, for almost 90% of the data, also reduces the spread.
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