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G. G. Lala
and
J. E. Jiusto

Abstract

An automatic instrument for the measurement of cloud condensation nucleus (CCN) concentrations utilizing a thermal gradient diffusion chamber and light scattering has been developed. The concentration of droplets (activated CCN) in an illuminated volume is determined by the measurement of the peak light intensity scattered at 45°. The CCN concentration is linearly related to the scattered light signal at a fixed supersaturation S, but the sensitivity exhibits an S 0.55 dependence over a range of supersaturations. Calibration of the system against the photographic method verifies the linear dependence of scattered light on number concentration. The main features of the system are the automatic sampling, measurement and recording of CCN droplet concentration by means of scattered light with the capability of direct calibration by means of the photographic method. Results from four days of continuous hourly measurements of the CCN spectra show the influence of meteorological events on CCN. Included in the data are a frontal passage, fog formation and dissipation, and the occurrence of a nocturnal peak.

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David R. Fitzjarrald
and
G. Garland Lala

Abstract

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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J. Zamurs
,
G. Lala
, and
J. Jiusto

Abstract

This paper presents some findings and recommendations for obtaining more reliable information from ice nucleus concentration measurements with the membrane filter technique commonly in use. Variables related to chamber design, filter preparation and crystal counting are discussed.

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James E. Jiusto
and
G. Garland Lala

Abstract

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G. Garland Lala
and
James E. Jiusto

Abstract

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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Michael B. Meyer
and
G. Garland Lala

Abstract

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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J. D. McTaggart-Cowan
,
G. G. Lala
, and
B. Vonnegut

Abstract

An aircraft instrument is described that gives a real-time measurement of the number of ice crystal particles per unit volume in cirriform clouds. Its method of detection is based on the mechanisms of contact electrification, as applied to the collision between a stainless steel wire and an ice crystal. The signal conditioner, which employs a series of integrated circuits, converts the frequency of crystal collisions into a voltage. Several examples of actual flights are shown.

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Edward E. Hindman II
and
G. Garland Lala

Abstract

No abstract available.

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G. Chimonas
,
F. Einaudi
, and
D. P. Lalas

Abstract

Pre-storm conditions are often characterized by an atmosphere in the presence of rather strong wind shears and a temperature inversion. The latter acts as a lid for moisture in the boundary layer. In this paper we discuss the possibility that a gravity wave generated by wind shear can reach sufficiently large amplitude to induce condensation. We show that under certain circumstances the ensuing heat release takes place in such a phase with respect to the initial gravity wave so as to reinforce it, substantially increasing its rate of growth. Thus, the lifting due to the wave will grow and so will the condensation. By showing that in the early stages after the first condensation occurs, we have a positive feedback between the gravity wave and the induced condensation, we strengthen the case for gravity waves as possible lifting agents leading to condensation and eventually to convection. The present calculations are not meant to describe the system after the onset of convection and as such they differ from existing CISK theories. The results also appear to indicate that the presence of a critical level in the region of large relative humidity may be a prerequisite for a strong feedback between the gravity wave and the induced condensation.

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D. N. Asimakopoulos
,
D. G. Deligiorgi
, and
D. P. Lalas

Abstract

Preliminary results from a series of experiments utilizing an acoustic sounder located at the top of a 1026 m mountain are presented which show some interesting features of the evolution of the atmospheric boundary layer.

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