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To explore fog water harvesting potential in California, we conducted long-term measurements involving three types of mesh using standard fog collectors (SFC). Volumetric fog water measurements from SFCs and wind data were collected and recorded in 15-minute intervals over three summertime fog seasons (2014)(2015)(2016) at four California sites. SFCs were deployed with: standard 1.00 m 2 double-layer 35% shade coefficient Raschel; stainless steel mesh coated with the MIT-14 hydrophobic formulation; and FogHa-Tin, a German manufactured, 3-dimensional spacer fabric deployed in two orientations. Analysis of 3419 volumetric samples from all sites showed strong relationships between mesh efficiency and wind speed. Raschel mesh collected 160% more fog water than FogHa-Tin at wind speeds less than 1 m s -1 and 45% less for wind speeds greater than 5 m s -1 . MIT-14 coated stainless-steel mesh collected more fog water than Raschel mesh at all wind speeds. At low wind speeds of < 1 m s -1 the coated stainless steel mesh collected 3% more and at wind speeds of 4-5 m s -1 , it collected 41% more. FogHa-Tin collected 5% more fog water when the warp of the weave was oriented vertically, per manufacturer specification, than when the warp of the weave was oriented horizontally. Time series measurements of three distinct mesh across similar wind regimes revealed inconsistent lags in fog water collection and inconsistent performance. Since such differences occurred under similar wind-speed regimes, we conclude that other factors play important roles in mesh performance, including in-situ fog event and aerosol dynamics that affect droplet-size spectra and droplet-to-mesh surface interactions.

We present turbulence spectra and cospectra derived from long-term eddy-covariance measurements (nearly 40,000 hourly data over three to four years) and the transfer functions of closed-path infrared gas analyzers over two mixed hardwood forests in the mid-western U.S.A. The measurement heights ranged from 1.3 to 2.1 times the mean tree height, and peak vegetation area index (VAI) was 3.5 to 4.7; the topography at both sites deviates from ideal flat terrain. The analysis follows the approach of Kaimal et al. (Quart. J. Roy. Meteorol. Soc. 98, 563-589, 1972) whose results were based upon 15 hours of measurements at three heights in the Kansas experiment over flatter and smoother terrain. Both the spectral and cospectral constants and stability functions for normalizing and collapsing spectra and cospectra in the inertial subrange were found to be different from those of Kaimal et al. In unstable conditions, we found that an appropriate stability function for the non-dimensional dissipation of turbulent kinetic energy is of the form φ (ζ) = (1 − b − ζ) −1/4 − c − ζ , where ζ represents the non-dimensional stability parameter. In stable conditions, a non-linear function G xy (ζ) = 1 + b xy ζ c xy (c xy < 1) was found to be necessary to collapse cospectra in the inertial subrange. The empirical cospectral models of Kaimal et al. were modified to fit the somewhat more (neutral and unstable) or less (stable) sharply peaked scalar cospectra observed over forests using the appropriate cospectral constants and non-linear stability functions. The empirical coefficients in the stability functions and in the cospectral models vary with measurement height and seasonal changes in VAI. The seasonal differences are generally larger at the Morgan Monroe State Forest site (greater peak VAI) and closer to the canopy. The characteristics of transfer functions of the closed-path infrared gas analysers through longtubes for CO 2 and water vapour fluxes were studied empirically. This was done by fitting the ratio between normalized cospectra of CO 2 or water vapour fluxes and those of sensible heat to the transfer function of a first-order sensor. The characteristic time constant for CO 2 is much smaller than that for water vapour. The time constant for water vapour increases greatly with aging tubes. Three methods were used to estimate the flux attenuations and corrections; from June through August, the attenuations of CO 2 fluxes are about 3-4% during the daytime and 6-10% at night on average. For the daytime latent heat flux (Q E), the attenuations are found to vary from less than 10% for newer tubes to over 20% for aged tubes. Corrections to Q E led to increases in the ratio (Q H + Q E)/(Q * − Q G) by about 0.05 to 0.19 (Q H is sensible heat flux, Q * is net radiation and Q G is soil heat flux), and thus are expected to have an important impact on the assessment of energy balance closure.

Atmospheric aerosols scatter and potentially absorb incoming solar radiation, thereby reducing the total amount of radiation reaching the surface and increasing the fraction that is diffuse. The partitioning of incoming energy at the surface into sensible heat flux and latent heat flux is postulated to change with increasing aerosol concentrations, as an increase in diffuse light can reach greater portions of vegetated canopies. This can increase photosynthesis and transpiration rates in the lower canopy and potentially decrease the ratio of sensible to latent heat for the entire canopy. Here, half-hourly and hourly surface fluxes from six Flux Network (FLUXNET) sites in the coterminous United States are evaluated over the past decade (2000–08) in conjunction with satellite-derived aerosol optical depth (AOD) to determine if atmospheric aerosols systematically influence sensible and latent heat fluxes. Satellite-derived AOD is used to classify days as high or low AOD and establish the relationship between aerosol concentrations and the surface energy fluxes. High AOD reduces midday net radiation by 6%–65% coupled with a 9%–30% decrease in sensible and latent heat fluxes, although not all sites exhibit statistically significant changes. The partitioning between sensible and latent heat varies between ecosystems, with two sites showing a greater decrease in latent heat than sensible heat (Duke Forest and Walker Branch), two sites showing equivalent reductions (Harvard Forest and Bondville), and one site showing a greater decrease in sensible heat than latent heat (Morgan–Monroe). These results suggest that aerosols trigger an ecosystem-dependent response to surface flux partitioning, yet the environmental drivers for this response require further exploration.

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