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Paul M. Tag

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Paul M. Tag

Abstract

A two-dimensional model is developed to simulate dissipation of fog using passive burner lines under either cross-wind or no-wind conditions. The vorticity model developed by Murray (1970) forms the basis for the development. Among the additions to the model are a stretched vertical grid, provision for an ambient wind field and variable eddy exchange coefficients.

The model is tested by comparing results to empirical temperature distribution data resulting from burner lines, located both outdoors and in a wind tunnel, positioned in a cross wind. Equally good comparisons are achieved by running the model at these two different physical scales. It is determined that the parameterization of the eddy coefficients most influences the resulting temperature profiles, and that a form in which deformation and buoyancy are summed gives the best results. A coefficient based solely on the deformation or vorticity gradients is found to be inadequate. Several additional experiments which utilize a soil heat flux parameterization support empirical estimates of a 5% heat loss to the soil.

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Paul M. Tag

Abstract

A two-dimensional model (Tag, 1979) is used to perform sensitivity experiments simulating fog dissipation using passive burner lines under both cross-wind and no-wind conditions. For a cross-wind experiment, heat output and cross-wind speed are found to be the two overriding factors which control the height of a clearing. It is determined that the extent of the fog clearing is inversely linked to the fog liquid water content, and that the fog thermal structure as well as the moisture emitted by the hydrocarbon combustion in the line have minimal effect on the size of the resulting clearing. Under no-wind conditions, where two lines of equal heat output are positioned on either side of a runway, heat output and line separation are the key controls to a surface clearing. Depending on the strength of the line heat release, either a downdraft or updraft can form between the lines, with the latter producing the quicker surface clearing. The no-wind experiments suggest that the use of a blower system in conjunction with two burner lines would be more efficient than relying on heat-induced circulations alone.

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Paul M. Tag

Abstract

A numerical study of the use of highly charged water drops to clear warm fog has been conducted. The mechanism studied is the polarization of neutral fog droplets and their capture by the charged drops. A multi-level microphysical model is used to investigate the degree of visibility improvement resulting from variations in seeding drop size and charge, the concentration of seeding material and the fog being seeded. It is determined that visibility improvement decreases with decreasing fog droplet size and increases with increasing seeding rate and seeding drop charge. For the same amount of seeding water, a treatment spectrum with an average radius between 10 and 15 μm is ideal. In contrast to the findings of Part I (an applied electric field), visibility improvement here results both from a removal of fog water (to the ground) and from a transfer of water from the fog spectrum to the larger treatment drops.

Field tests of this technique have proven inconclusive. A further evaluation is made by comparing model results to comparable numerical experiments of hygroscopic seeding, a technique that has been field tested on several occasions. It is concluded that the charges and treatment concentrations simulated in this study would not be adequate for clearing fog; unless charges and seeding concentrations can be greatly increased, charged drop seeding is probably not a viable fog dissipation technique.

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Paul M. Tag

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The diagnosis and conservation of energy during condensation is examined. It is shown that the latent enthalpy, when defined in conjunction with the individual enthalpies of water vapor and liquid water, cannot be a function of the latent heat of condensation L but a modified value (L′) which is ∼30% larger than L. The additional energy represented in L′ can be thought of as a necessary absorption by the liquid water to bring the post-condensation air-vapor-liquid system into thermal equilibrium. The difference between L′ and L is a function of the difference in specific heats of water vaper and liquid water. If we assume that (C pd C pr is constant, as is required in our energy conservation derivation, L′ is shown to vary by only 0.59% when computed over the range −50 to +60°C; a representative value for L' is 3.142×106 J Kg−1

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Paul M. Tag

Abstract

It has been suggested that the use of charged particles or electric fields be considered as a technique for dissipating warm fogs. The study presented here attempts to determine the degree of improvement one could expect as the result of one aspect of electrically enhanced coalescence—enhanced coalescence due to an externally applied electric field on neutral drops. For this purpose, a numerical simulation with a one-dimensional microphysical fog model which incorporates the process of collision-coalescence was conducted. Collision efficiencies appropriate to two extreme electric fields were utilized for the numerical experiments. It was determined that a noticeable improvement in visibility can be achieved only under extremely large field strengths, and then only for certain fog spectra.

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Thomas F. Lee
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Paul M. Tag

The 3.7-μm channel on-board the National Oceanic and Atmospheric Administration's (NOAA) Advanced Very High Resolution Radiometer (AVHRR) provides the unique capability to detect small, but hot, surface features. We present an image-processing technique based on a pixel-by-pixel subtraction of 10.8 μm from 3.7 μm brightness temperatures. We also develop an automated technique which classifies hotspots based on: 1) the brightness temperatures at 3.7 and 10.8 μm at a given pixel, and 2) a background temperature based on the immediately surrounding pixels.

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Paul M. Tag
and
Thomas E. Rosmound

Abstract

Accuracy and energy conservation are examined in a three-dimensional (3D) anelastic model. For both dry and moist (noncondensing) atmospheres, we prescribe analytic solutions of momentum, potential temperature and mixing ratio for both periodic and closed boundaries. Accuracy is assessed by comparing amplitudes and phase speeds from both the numerical and analytic solutions. Kinetic and potential energies and enthalpy (including air, vapor, liquid and latent) are calculated for both the mean

and perturbation states. To assess the energetics involved in phase changes, we examine a separate cloud simulation. Two-dimensional (2D) and hydrostatic experiments are also conducted using the cloud simulation.

For the linear analytic wave solutions, phase speeds as a function of time step for our semi-implicit model are compared to both implicit and explicit linear stability generated speeds. We show that an explicit scheme enhances the phase speed up to the CFL cutoff while an implicit scheme retards the phase speed. For the quasi-Lagrangian method of moisture advection, we find that a water conservation algorithm is necessary to maintain conservation of total perturbation energy. Similarly. the correct inclusion of moisture in the computation of density is most critical to energy conservation. In comparing a 2D forced cloud to the 3D simulation, only 17% of the perturbation energy which changes form in the 3D case does so in the 2D experiment-in direct relation to the larger cloud in the 3D simulation. And finally, comparing experiments both with and without the hydrostatic assumption, we verify earlier 2D findings that the magnitude of the vertical motion is larger in a hydrostatic model.

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Paul M. Tag
and
Steven W. Payne

Abstract

Cloud top entrainment instability, as a mechanism for the breakup of marine stratus, is examined with a three-dimensional, planetary boundary layer (PBL) model. Specifically, we examine the criterion developed by Randall and Deardorff; this criterion states that stratus will break up if the equivalent potential temperature gradient at cloud top becomes less than a critical value. To examine this hypothesis, we simulate a horizontally uniform stratus layer which is excited from above by small random temperature perturbations. The buoyancy instability ratio (BIR), defined as Δθe(Δθe)crit and computed at cloud top, is calculated locally across the domain and also averaged to define a mean value. Six cases, involving different wind speeds and above-cloud soundings, produce different initial BIRs and different breakup sequences. In general, we find that a mean BIR greater that one is a necessary condition for stratus breakup; however, we also find that the timing of breakup following achievement of the critical ratio is different from run to run. The low wind speed cases, initially most stable at cloud top, are the first to break up, while the higher wind speed (most unstable) cases require longer time to break up. We conclude that an additional mechanism is necessary to stimulate vertical motion in order to take advantage of the cloud-top entrainment instability. In our simulations, that additional stimulation comes from vertical motions generated by Rayleigh-type instability in the PBL.

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James E. Peak
and
Paul M. Tag

The U.S. Navy has plans to develop an automated system to analyze satellite imagery aboard its ships at sea. Lack of time for training, in combination with frequent personnel rotations, precludes the building of extensive imagery interpretation expertise by shipboard personnel. A preliminary design starts from pixel data from which clouds are classified. An image segmentation is performed to assemble and isolate cloud groups, which are then identified (e.g., as a cold front) using neural networks. A combination of neural networks and expert systems is subsequently used to transform key information about the identified cloud patterns as inputs to an expert system that provides sensible weather information, the ultimate objective of the imagery analysis.

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