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Edward E. Hindman II and G. Garland Lala

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G. Garland Lala and James E. Jiusto

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A one-dimensional model was developed to examine humidity fields within a conditioning chamber for measuring ice nucleus concentrations on millipore filters. Representative concentrations of ice and cloud condensation nuclei were assumed, and the interplay among these growing particles (vapor sinks), the supply flux of vapor, and the resultant relative humidity at and above the filter surface investigated.

The model suggests that water saturation is not achieved under typical operating conditions of such chambers. Maximum humidifies reached decrease with increasing numbers of either condensation or ice nuclei, thereby offering another possible explanation of the filter volume effect. Most favorable operating conditions for achieving highest chamber humidities are delineated. The results suggest that this technique is capable of detecting mixed condensation-freezing nuclei, deposition nuclei and some contact nuclei, with the former perhaps being most common not only in filter measurements but also in the atmosphere.

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David R. Fitzjarrald and G. Garland Lala

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Observations of 14 cases of radiation fog in the Hudson River valley in New York State are presented. Our emphasis is to connect the fog prediction problem to mechanisms in the nocturnal boundary layer that influence heat and moisture balances. Surface layer and boundary layer fogs are distinguished by the difference in dominant terms in the saturation specific humidity deficit budget. Fogs that persist longer than approximately 30 minutes are most frequently thicker than 50 m. The ultimate depth to which the fog grows is shown to be determined by initial conditions at sunset and by subsequent evolution of winds in the nocturnal boundary layer, as well as by surface transports and radiative cooling. Estimates of the surface and boundary layer heat budget are presented. Two new phenomena are identified: 1) A jump in specific humidity occurring during the early evening transition that shortens the time required to reach surface layer saturation; and 2) along-valley jetlike winds with maxima near 100 m altitude are shown to be frequent and their occurrence is associated with a threshold value of the along-valley surface pressure gradient. Such jets appear to have an important influence on deep fog, increasing or decreasing its likelihood depending on the sign of heat and moisture advection they associate with.

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James E. Jiusto and G. Garland Lala

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Michael B. Meyer and G. Garland Lala

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We present a detailed investigation of the local radiation fog climatology, carried out in support of our ongoing field program to study radiation fog mechanisms at Albany, New York. At Albany, a distinct ”radiation fog season” is observed during September and October. We show that this late-summer/early-autumn maximum in radiation fog observations is primarily due to a sufficient period of nocturnal cooling coupled with an adequate moisture supply. Five critical surface synoptic patterns are responsible for initiating the radiation fog process. In addition, radiation fog life cycles are generally confined to a modest time window centered on sunrise. Key parameters necessary for forecasting the onset time of fog are shown to be the initial relative humidity and nocturnal cooling rate.

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G. Garland Lala, Eric Mandel, and James E. Jiusto

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A numerical model of radiation fog was developed in order to test the sensitivity of variables comprising the model, and evaluate its capability for forecasting the onset of fog from standard radiosonde weather data. Four case studies were considered that included both fog and no-fog occurrences. The variables examined–initial surface temperature and moisture conditions, eddy exchange profiles, radiative flux divergence, and dew formation–were all found to influence critically the model's performance. Prediction of fog occurrence and temperature were reasonably encouraging provided a judicious (though somewhat arbitrary) choice of eddy mixing values was made.

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Michael B. Meyer, G. Garland Lala, and James E. Jiusto

The Cloud Physics Section of the Atmospheric Sciences Research Center-State University of New York at Albany conducted a cooperative field study (FOG-82) during the autumn of 1982 as part of an ongoing radiation-fog research program. A computer-controlled data-acquisition system consisting of sophisticated soil, surface, and boundary-layer sensors, as well as contemporary aerosol and droplet probes was developed. These data are being used to address a variety of critical problems related to radiation-fog evolution.

Scientists from 10 universities and research laboratories participated in portions of FOG-82. Research objectives included studies of fog mesoscale meteorology, radiation studies, low-level water budget, vertical fog structure, fog supersaturation, condensation nuclei, and fog-water chemistry, as well as radiation-fog life cycles. A comprehensive description of the FOG-82 program and objectives is presented.

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Michael B. Meyer, James E. Jiusto, and G. Garland Lala

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An extensive boundary-layer field program was conducted which included simultaneous measurements of visibility and particle size distributions during fog and haze. Several empirical expressions relating changes in visibility to characteristics of the aerosol (droplet) size spectrum and relative humidity are presented and evaluated. Detailed analysis of one evolving dense fog revealed several points of interest regarding the behavior of drop size spectra, including a scheme for approximating fog supersaturation.

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Ramon J. Cipriano, Duncan C. Blanchard, Austin W. Hogan, and G. Garland Lala

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A laboratory model of a breaking wave or whitecap was constructed, and the aerosol produced by it was investigated intensively. Submicron- and even Aitken-sized particles were produced: the presence of salt particles of mass <10−17 g(r<0.01 μm) could be inferred. The evidence strongly suggests that the submicron fraction is composed of film drops, derived primarily from bubbles larger than 1 mm in diameter. The shape of the CCN spectrum and overall mass distribution of the model-produced aerosol were similar to what is observed in clean marine air.

Whether or not the production rate of such small particles is globally significant when the model results are applied to the oceans depends to a large extent on the set of assumptions one makes concerning aerosol residence time and fraction of sea surface covered by whitecaps. However, there are realistic choices of these parameters which suggest that appreciable fractions of both CCN and CN in the lower marine atmosphere are produced directly by the sea.

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