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Robert S. Schemenauer and Pilar Cereceda

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Robert S. Schemenauer and Pilar Cereceda

Abstract

One exciting new application of meteorology is the prospect of using high-elevation fogs as an and land's water resource. This has now become reality in northern Chile where a pilot project has used 50 fog collectors to generate an average of 7200 1 of water per day during three drought years. The chemical composition of the fog water is of primary importance and is examined in this paper.

A small, carefully cleaned fog-water collector was used at the site (elevation 780 m) to study the incoming fog (cloud). The ion and trace-element concentrations met Chilean and the World Health Organization's (WHO) drinking-water standards. The pH values, however, were at times extremely low. Samples from 1987 and 1988 were consistent with those from the larger dataset in 1989. The lowest observed pH was 3.46. The acidity was associated with high concentrations (89%) of excess sulfate in the 15 fog-water samples (based on Cl as the seawater tracer element). The NO3 /SO4 equivalents ratio for the fog samples was 0.18, showing the dominance of SO4 in determining the acidity of the fog samples. The relative abundances of ions and trace elements in the dry deposition are very similar to those in the fog water, suggesting that the aerosols originate primarily from evaporated cloud droplets over the ocean. Based on enrichment-factor calculations (with Cl as the indicator element for seawater and A1 for the earth's crust), sea salts were the main source of Na&plus, Mg++, and Cl in the fog water; soil dust was the main source of Fe, Al and Ti; and other sources provided Ca++, K+, NH4 +, Br SO4 NO3 As,Cd,Pb,V,Mn,Ni,Cu,SrSb,and Ba in the fog water.The use of enrichment factors based on the relative abundances in soil extracts suggests that As, V, Cu, and Sr may be available from wetted soil dust.

The output from the large (48 m2) fog collectors was also acceptable, except for several of the 24 trace elements, which exceeded the maximum allowable values in the first flush of water after a dry period of a few days. The pH values were again near 4 and would have to undergo a simple treatment to raise them to a value of 6 or more to meet the drinking-water standard. The output from a 2000-1 fog-water storage tank was completely acceptable and that from a 25 000-1 storage tank completely acceptable, except for a low pH. In contrast, both the water presently being used in a nearby village and local spring water were unacceptable. It is concluded that fog water is an attractive alternative as a water supply even after collection on the large meshes at this site.

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Roland List and Robert S. Schemenauer

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Drag coefficients and Best numbers of models of six planar snow crystals, two conical graupel and two conical small-hail particles were determined experimentally in glycerin-water mixtures and salt solutions. The Reynolds number (Re) range covered for the crystals was 0.1 to 200 and for the conical models 10 to 2000. It was found that the drag coefficients of dendritic shapes differed by factors of up to 4 from that of a disc of equal thickness and at an equal Reynolds number. The drag ratio is roughly constant with Re and linearly related to the ratio of the respective surface areas. The drag coefficients of the conical models assumed values between 0.5 and 2.0. During steady fall they decreased with increasing Re; however, as soon as oscillations started this trend reversed. Since tumbling does occur for larger graupel and small hail at Re > 300–1000 their main characteristic motions and frequencies are also discussed. Values for oscillation frequencies are given and the motions are described in detail.

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Robert S. Schemenauer and Pilar Cereceda

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The collection of fog droplets by vegetation is an important wet deposition process. It can, in fact, dominate the chemical and hydrological input to certain high elevation watersheds. However, measurements of fog deposition are rarely made and, where they do exist, comparisons of deposition rates in different locations have been hampered by the use of innumerable types of collection devices. A simple, inexpensive, 1-m2 fog collector that can produce measurements of the deposition of fog water to a vertical surface is described here. The collector has been used successfully in five countries to investigate the variation of fog deposition in complex terrain and to estimate the deposition to trees and to much larger fog collectors. It is proposed that it be employed widely as a standard to quantify the importance of fog deposition to forested high elevation areas and to measure the potential collection rates in denuded or desert mountain ranges.

The standard fog collector costs about the same as a rain gauge ($100 U.S.) to construct and can be used with a variety of recording devices. It is a flat panel made of a durable polypropylene mesh and mounted with its base 2 m above ground. Fog collection rates are typically 1–10 L m−2 of vertical collecting surface per day but can reach values of 30–40 L m−2 day−1. The presence of drizzle or light rain with the fog, coupled with 10 m s−1 winds, has produced collection rates as high as 300 L m−2 day−1. If a standard fog collector is installed at a site with wind speed measurements and a conventional rain gauge, a reasonable estimate can be made of the proportions of fog and rain being deposited on the vertical mesh panel. This information is fundamental to the understanding of acidic wet deposition at higher elevations and to comprehensive hydrological calculations in watersheds.

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Pilar Cereceda and Robert S. Schemenauer

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The topography in Chile is extremely complex and many types of fog are found; both factors complicate the presentation of the data. Despite this, measurements from standard meteorological stations suggest a latitudinal maximum in fog frequency between 35° and 40°S for coastal stations. This is supported by data from inland stations in Chile and the available observations from Argentina on the Atlantic coast of South America. Along the Chilean coast the average number of days with fog ranges from 3 to 59 per year. The variation in fog frequencies is related to persistent synoptic-scale circulation patterns and to ocean currents.

Specialized fog observations wore made at three remote locations in northern Chile to determine fog frequencies on the coastal mountains. The sites were in a very add region (26°–28°S) near a large-scale fog-water collection project. Fog frequencies as high as 189 days per year with another 84 days of patchy fog were reported at an altitude of 860 m. These are 3–15 times higher than at low-elevation coastal locations at similar latitudes. Clearly, observations from standard meteorological stations are not suitable for estimating higher-elevation fog frequencies.

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Robert S. Schemenauer and G. A. Isaac

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The microphysical and dynamical characteristics of 156 natural summer cumulus clouds have been documented for three locations in North America: Yellowknife, Northwest Territories; Thunder Bay, Ontario; and Miles City, Montana. The measurements (469 aircraft penetrations) were made in six consecutive years from 1975 to 1980 using state-of-the-art cloud physics instrumentation. All measurements discussed were obtained near −7°C. Yellowknife clouds had low liquid water contents (0.3 g m−3) and high large (>70 μm) particle concentrations (0.9 L−1). Thunder Bay clouds had higher liquid water contents (1 g m−3) and low large particle concentrations (0.04 L−1). Miles City clouds, which were similar in dimensions to those near Yellowknife, had low liquid water contents (0.3 g m−3) and low large particle concentrations (0.1 L−1). Yellowknife and Thunder Bay clouds produced precipitation through the warm and cold rain processes but the observed Miles City clouds did not precipitate naturally. Measurements of cloud top lifetime appear to be useful in explaining the differences between locations. Cloud top lifetime is defined in this paper in terms of the persistence of cloud liquid water at the penetration altitude near −7°C. Lifetime was found to increase with cloud width in each location but did not appear closely related to initial LWC, cloud depth, cloud base temperature, inside-outside cloud temperature difference, environmental humidity, turbulent energy dissipation rate, energy flux, heat flux nor wind shear.

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Robert S. Schemenauer, Pilar Cereceda, and Nazareno Carvajal

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Robert S. Schemenauer and A. A. Tsonis

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The aircraft measurements from the HIPLEX-1 weather modification experiment have been examined to determine if the nature of the change of liquid water content (LWC) in the supercooled portion of the clouds can be simply described, Three different data sets were created from the −8 and −5°C aircraft data base. Neither a simple linear nor a simple polynomial fit to the data are suitable for reasons discussed in the text. Two different forms of an exponential model were fit to two of the data sets. When a model for the decay of the maximum 1-km liquid water content (χ) of the form χ=χ0 ebt was fit to data set number two, this yielded a cloud liquid water decay constant (τ) of 560 s (9.5 min), with a correlation coefficient r=0.47 and r 2=0.22. This reduce the mean first pass χ value of 1.05 g m−3 to e−1 or 0.39 g m−3 in 9.5 min and to (2e)−1 or 0.14 g m−3 in 19 min. The best fit to the observations, however, comes from a calculation of an average rate of change of LWC at a constant altitude (−8°C) in the clouds. This is of the form χ=χ0+bt and gives a lifetime of 15 min for the maximum 1-km average LWC in the 20 HIPLEX-1 clouds. That is, the highest LWC regions in the upper part of the clouds would be expected to completely disappear in about 15 Min. Regions of lower LWC would disappear more quickly. This is a major limitation on both natural and artificial rain forming processes.

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Robert S. Schemenauer and Michael J. Curry

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The capability of a Mee Industries Model 120 ice particle counter (IPC) to differentiate between ice particles and water drops was investigated in laboratory and field studies. The threshold voltage setting as well as the particle size were found to be critical in determining counting efficiency. The results show that ice crystals are counted with an efficiency more than ten times as high as are water drops of the same average size. Increasing the threshold voltage setting of the instrument increases the discrimination factor but also results in a decrease in the absolute number of particles counted. The availability of concurrent information on particle sizes and concentrations from other probes allows the Mee IPC phase determinations to be made with much greater confidence. Methods for utilizing data from the Mee IPC as well as the limitations of the instrument are discussed.

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Michael J. Curry and Robert S. Schemenauer

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The response characteristics of an optical array precipitation spectrometer probe (PMS-OAP-200Y) have been studied in the laboratory using precision glass beads ejected from a specially constructed air gun. The low-end behavior of the probe is described in terms of a high-pass filter characteristic, which can be used to explain the response of the instrument to a particle population having a wide distribution of sizes.

It is shown that the particle concentrations measured in channel 1 of the OAP-200Y require correction by a factor which is a function of particle size distribution. For typical experimental situations in rain, the correction factor is approximately 1.8. The remaining size channels do not require correction, provided that the probe sample area and channel width have been properly determined.

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