Dew frequency across the US from a network of in situ radiometers
2018
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 30 min 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 – SD). Dew mostly forms at the top of the canopy for the
grasslands due to more effective radiative cooling and within the canopy for the
forests because of higher within the canopy RH. The high temporal resolution of
our data showed that dew duration reaches maximum values
( ∼6 –15 h) for RH∼96 % and for a
wind speed of ∼ 0.5 m s - 1 , independent of the ecosystem type.
While dew duration can be inferred from the observations, dew yield needs to
be estimated based on the Monin–Obukhov similarity theory. We find yields of
0.14 ± 0.12 mm night - 1 (1 SD 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 ( ∼ 10 W m - 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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