Near-surface fluxes of cloud water evolve vertically
Faculty of Sciences. Biology
Quarterly journal of the Royal Meteorological Society. - London
, p. 2663-2684
University of Antwerp
Occult deposition to vegetation, via mechanical interception of wind-blown cloud water, can be a significant fraction of total ionic chemical deposition for some forests. Applying micrometeorological methods to the estimation of cloud water deposition requires particular consideration since cloud droplets are not conservative but are subject to material change (phase change); sedimentation also affects fluxes of droplets. The budget equation for liquid water (LW) in orographic cloud predicts that LW fluxes will diverge due to condensation during mean ascent. For hilltop measurements such as these, other factors can contribute to changes in the vertical flux with height above the surface. Fluxes measured concurrently at two heights exhibit a persistent and significant divergence, while laterally separated measurements are found to agree. A LW budget equation is presented and simplified by scale analysis. Surface uptake is estimated by extrapolation of the measured fluxes. Estimated surface deposition is found to be substantially different from the flux measured at a reference height (10 m), often by a factor of two and occasionally with a different sign. This difference in estimated surface uptake extends to estimates of chemical as well as water deposition. The turbulent flux of LW is shown to be dependent on two criteria for describing 'steady-cloud' conditions, thus presumably minimizing the effects of entrainment. An often used model relating droplet deposition to the 'deposition velocity' for momentum is found to be inappropriate for application in complex terrain. Deposition of LW is estimated to range from 5 to 50 mg m(-2) s(-1) during the third field campaign of the Cloud and Aerosol CHemistry Experiment (CACHE-3), with an average of 19 mg m(-2) s(-1) (nearly 2 mm per in-cloud day) in late summer at this Pacific coastal site in North America.