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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-450
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Sep 2018

Research article | 06 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Dew frequency across the US from a network of in situ radiometers

François Ritter1, Max Berkelhammer1, and Daniel Beysens2 François Ritter et al.
  • 1Department of Earth and Environmental Sciences, University of Illinois at Chicago, Illinois, USA
  • 2Physique et Mécanique des Milieux Hétérogènes, CNRS, ESPCI, PSL Research University, Sorbonne Université, Sorbonne Paris Cité, Paris, France and OPUR, Paris, France

Abstract. Dew formation is a ubiquitous process but its importance to energy budgets or ecosystem health is difficult to constrain. This uncertainty arises largely because of a lack of continuous quantitative measurements on dew across ecosystems with varying climate states and surface characteristics. This study analyzes dew frequency from the National Ecological Observatory Network (NEON), which includes 11 grasslands and 19 forest sites from 2015 to 2017. Dew formation is determined at 30min intervals using in situ radiometric surface temperatures from multiple heights within the canopy along with meteorological measurements. Dew frequency in the grasslands ranges from 15% to 95% of the nights with a strong linear dependency on the nighttime relative humidity (RH), while dew frequency in the forests is less frequent and more homogeneous (25±14%, 1 Standard Deviation). Dew mostly forms at the top of the canopy for the grasslands due to more effective radiative cooling and within canopy for the forests because of higher within canopy RH. The high temporal resolution of our data showed that dew duration reaches maximum values (~6–15h) for RH~96% and for a wind speed of ~0.5ms−1, independent of the ecosystem type. While dew duration can be directly assessed from the observations, dew yield needs to be estimated based on the Monin-Obukov Similarity Theory. We find yields of 0.14±0.12mmnight−1 (1SD from nine grasslands) similar to previous studies, and dew yield and duration are related by a quadratic relationship. The latent heat flux released by dew formation is estimated to be non-negligible (~10Wm−2), associated with a Bowen ratio of ~3. The radiometers used here provide canopy-averaged surface temperatures, which may underestimate dew frequency because of localized cold points in the canopy that fall below the dew point. A statistical model is used to test this effect and shows that dew frequency can increase by an additional ~5% for both ecosystems by considering a reasonable distribution around the mean canopy temperature. The mean dew duration is almost unaffected by this sensitivity analysis. In-situ radiometric surface temperatures provide a continuous, non-invasive and robust tool for studying dew frequency and duration on a fine temporal scale.

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Short summary
There currently is no standardized approach to measure dew formation, making it difficult to compare its frequency and importance across ecosystems. Recently, canopy surface temperature data from 30 sites in the US was measured continuously using in situ infrared-radiometers. The analysis presented in this manuscript provides the first continental-scale standardized synthesis of dew formation. This work provides a basis to consider how changing climate and land-use will influence dew formation.
There currently is no standardized approach to measure dew formation, making it difficult to...
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