Occult deposition is the direct uptake of cloudwater by vegetation that comes into
physical contact with wind-driven cloud droplets. This can be a significant pathway for
hydrological and chemical fluxes from the atmosphere to some forests. Methods for
estimating cloudwater fluxes to forests are reviewed. Previous studies have neglected
the fact that cloudwater is not a conservative atmospheric quantity. This invalidates
traditional micrometeorological approaches for estimation of cloudwater fluxes to forests.
A theory is developed to predict the change in the cloudwater flux with height due to
condensation in the updrafts of orographic cloud, allowing estimation of surface uptake
via eddy correlation measurements while accounting for condensation.
The performances of three microphysical instruments are examined. From collocated
measurements, errors in cloud liquid water content are determined for a Particulate
Volume Monitor (PVM) and a Forward Scattering Spectrometer Probe (FSSP) to be less
than 0.01 g m⁻³ and 0.035 g m⁻³ respectively. Similarly, the error bounds for surface-normal
cloud liquid water fluxes are found to be 2 mg m⁻² s⁻¹ for the PVM and 3.5 mg
m⁻² s⁻¹ for the FSSP. Smaller errors are found to be associated with the uncertainty in the
direction of the flux relevant to surface uptake. The FSSP is seen to have larger errors
when droplet concentrations exceed 600 cm⁻³.
A vertical divergence is detected in the cloudwater flux; the downward flux
decreases with increasing distance from the surface, usually changing sign by 15 m above
ground. Five candidate processes are identified as possible explanations for this measured
flux divergence. A scale analysis shows that the liquid water source (condensation due
to pseudoadiabatic ascent) is largely responsible for the change in flux with height.
Accounting for the change in flux with height results in a near doubling of the estimated
surface flux relative to the flux measured at a height of 10 m in the surface layer for
this silver fir forest. This factor applies to chemical as well as liquid water fluxes. This
source of liquid water also is seen to be important in developing a simple model for
cloudwater deposition. / Graduation date: 1997
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28698 |
| Date | 04 October 1996 |
| Creators | Kowalski, Andrew S. |
| Contributors | Vong, Richard J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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