Stuart G. Gathman

Optical Properties Of The Marine Aerosol As Predicted By The Navy Aerosol Model

A model is presented which predicts optical and infrared extinction and absorption from marine aerosol for wavelengths of 0.2 to 40 micrometers. The model utilizes standard meteorological measurements as inputs but has default values built into it so that input values are not absolutely necessary, but when they are available they will increase the accuracy of the predictions. The model is based on aerosol size distribution measurement data taken by several investigators from platforms close to the sea surface. The optical predictions of the model are derived from precalculated terms stored in the program which were originally obtained from lengthy Mie calculations on special aerosol size distributions.

Electricity in Volcanic Clouds Investigations show that lightning can result from charge-separation processes in a volcanic crater

In November of 1963 an oceanic volcano produced an island, Surtsey, just off the southern coast of Iceland. The volcanic crater was often flooded with sea water. Vigorous eruptions of steam and tephra were accompanied by an enhancement of the normal fine-weather potential gradient, and lightning was often observed. Measurements of atmospheric electricity and visual and photographic observations lead us to believe that the electrical activity is caused by the ejection from the volcano into the atmosphere of material carrying a large positive charge. The concentration of charge in the eruption plume as it issued from the orifice of the volcano is estimated to be of the order of 105 or 106 elementary charges per cubic centimeter.

Improved Field Meter for Electrostatic Measurements

An instrument system for the measurement of electrostatic fields is described. The use of more than 40 dB of negative feedback in the amplifier results in a sensitivity stable to within 1%. Careful attention to grounding and to surface preparation in the transducer head results in a zero level stable to within 1 V/m in ideal conditions. The effect of atmospheric variations in limiting the usable sensitivity is demonstrated.

Characterizing large aerosols in the lowest level of the marine atmosphere

This paper discusses techniques to describe the aerosol and the electro optical properties of the marine atmosphere from 15 meters down to the tops of the highest wave. Emphasis is placed on the experimental Rotorod technique to measure the concentrations of giant sea salt droplets. Data from these devices are parameterized using a lognormal function that is in turn related statistically to parameters such as wind speed, atmospheric stability and height above the surface. The new lognormal function is combined with the Navy Aerosol Model (NAM) to develop a first version of the Advanced Navy Aerosol Model (ANAM). Thus, ANAM allows the construction of an aerosol size distribution at any level from the wave tops to 15 meters an the assessment of electro optical parameters from this distribution using Mie theory. The first results of the ANAM model are compared to experimental data.

Electro-optical propagation assessment in coastal environments (EOPACE): summary and accomplishments

EOPACE (electro-optical propagation assessment in coastal environments) was a 5-yr multinational and interdisciplinary effort to improve the performance assessment for electro-optical (EO) systems operating in coastal environments. The initial results of the EOPACE program include: (1) the parameterization of the surf-zone generated aerosol-size distribution as a function of swell height; (2) the characterization of aerosol plume structures and the transport of surf generated aerosols; (3) the development of a quantitative surf aerosol source function; (4) the description of the contribution and impact of surf-zone generated aerosols on coastal infrared (IR) transmission; (5) the measurement and modeling of the near surface transmission effects (aerosol and molecular extinction, refraction, scintillation, and wave shadowing); (6) an analysis of the contribution of anthropogenic and land derived aerosols to the air mass characteristics in the coastal zone; (7) the application of direct and remote sensing techniques to develop the scaling parameters for aerosols in the prevailing air mass; (8) an analysis of near ocean surface bulk meteorological scaling which works well for unstable conditions but is less reliable for neutral and stable conditions; and (9) the incorporation of the improved sea radiance models into TAWS (target acquisition weather software) which improved the error analysis by a factor of 3. These initial accomplishments are described in this overview of the EOPACE effort. ©

Modeling Of Aerosols In The Marine Mixed-Layer

A model is developed to calculate the vertical variation of aerosol extinction coefficients throughout the marine atmospheric boundary layer. It is a mixture of empirical and physical models, formulated to describe the often observed non-uniform, but also non-logarithmic, profiles. The physical model is based on the dynamical processes affecting the production, mixing, deposition and size of the aerosol within the marine atmosphere. A status report is presented including a critical evaluation.

A Field Mill for Tethered Balloons

An instrument is described for obtaining time averages of the resultant of the vertical and one horizontal component of electric field in the lower atmosphere. This instrument is designed to be suspended from a tethered balloon and to telemeter the desired data to an operator by means of an audio pulse width modulation technique which requires the operator to be equipped only with a hand‐held AM receiver and a stopwatch. The net self‐charge on the field mill is maintained at or near zero by means of a 210Po alpha source. If we assume that the horizontal component of the electric field is small compared with the vertical, the field mill system can be used to observe the average vertical atmospheric electric field profile produced by the electrode effect near the ocean surface.

Guarded Double Field Meter

An instrument is described for absolute measurements of components of electrostatic fields in the free atmosphere near the earths surface. This apparatus provides a guarding system to minimize field distortion at the measurement site. A method utilizing models in an alternating electric field is used to evaluate the augmentation factor of the instrument as well as the effectiveness of the guarding system. The instrument has been used to measure horizontal electric field and to make vertical field profiles. It is also used simultaneously with a field meter flush with the earth to make an analog record of space charge density.

Nature of surf-generated aerosol and its effect on electro-optical systems

One of the unique sources of aerosol found in the coastal environment is the action of the breaking waves near the shore. This paper utilizes the data obtained from a series of experiments from the Electro Qptical propagation Assessment in coastal nvironments program, EOPACE, which has as one of its objectives the determination of the impact of surf produced aerosols in the propagation of IR radiation. These experiments were carried out in both San Diego and Monterey Bay. This paper discusses some of the observed dynamic aerosol production processes as well as longer term effects involved with the breaking surf and the cloud of aerosol it produces. Several techniques used to investigate these include: a) profiles of aerosol size distributions above the surf b) spectral analysis of observed size selected aerosol concentration time series c) transmission paths across the surf and d) simultaneous aerosol measurements from boat and beach.

EOPACE (electro-optical propagation assessment in coastal environments): overview and initial accomplishments

EOPACE is a five year multi-national effort to improve performance assessment for electrooptical systems operating in coastal environments. Existing propagation codes such as LOWTRAN/MODTRAN incorporate models that were developed for open ocean conditions and work quite well for this scenario. However, there are processes that are unique to near coastal regions which are not adequately accounted for in LOWTRAN/MODTRAN. Coastal environments may differ significantly from open ocean conditions, and need to be fully characterized. The objectives of EOPACE are threefold: (1) to investigate coastal aerosols by studying surfproduction, coastal air mass characterization, and near ocean surface transmission characteristics; (2) to develop mesoscale and data assimilation models; and (3) to evaluate EO systems performance by studying targets and backgrounds, polarization techniques, performance of forward looking infrared (FUR) and infrared search and track (IRST) systems, and tactical decision aids. Six EOPACE Intensive Operational Periods (TOPs) have been conducted during 1996 and 1 997. Two more TOPs are planned along with one Extended Operational Period (EOP). In situ and remote sensing techniques have been used to infer the impact of surf-generated aerosols, air mass parameterization required for propagation codes, near ocean surface infrared transmission properties, and IRST/FLIR systems performance in coastal environments. Initial results concern coastal aerosols. This paper gives an overview of the EOPACE effort and discusses the initial observations relative to: (1) the impact of surf generated aerosols on visual and JR extinction in a coastal environment, (2) establishing the variability of aerosol concentrations and composition for coastal air masses for the development of a Coastal Aerosol Model (CAM), and (3) quantifying JR propagation characteristics for two wavebands (3- 5 and 8-12 microns) for near ocean transmission.