Carl R. Zeisse

Estimating the Refractive Index Structure Parameter (Cn2) over the Ocean Using Bulk Methods

Abstract Infrared scintillation measurements were obtained along a 7-km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter were computed from the buoy data and compared with scintillation-derived values. The bulk estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk estimates became increasingly higher than the scintillation values as the air–sea temperature difference increased. This disagreement may be due to enhanced wave-induced mixing of the lower atmosphere that decreases the vertical temperature and humidity gradients in stable conditions from the assumed Monin–Obukhov similarity (MOS) theory forms, resulting in bulk values that are too high. The bulk estimates decrease rapidly when the absolute air–sea temperature difference approaches small positive values. These predicted decreases in were not observed in either ...

Low-altitude infrared propagation in a coastal zone: refraction and scattering

Midwave and long-wave infrared propagation were measured in the marine atmosphere close to the surface of the ocean. Data were collected near San Diego Bay for two weeks in November 1996 over a 15-km horizontal path. The data are interpreted in terms of effects expected from molecules, aerosol particles, and refraction. Aerosol particles are a dominant influence in this coastal zone. They induce a diurnal variation in transmission as their character changes with regular changes in wind direction. A refractive propagation factor calculation is introduced, and it is systematically applied to the model and to the data analysis. It is shown that this refractive propagation factor is a necessary component of a complete near-sea-surface infrared transmission model.

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. ©

Near-surface scintillation in a coastal ocean region

Infrared scintillation measurements were obtained along a 7 km path over San Diego Bay concurrently with meteorological measurements obtained from a buoy at the midpoint of the path. Bulk estimates of the refractive index structure parameter Cn2, were computed from the buoy data and compared with scintillation-derived Cn2 values. The bulk Cn2 estimates agreed well with the scintillation measurements in unstable conditions. In stable conditions the bulk Cn2 estimates were higher than the scintillation data, by up to an order of magnitude on average. This disagreement may be due to the effects of ocean waves in decreasing the vertical temperature and humidity gradients in stable conditions from the assumed Monin-Obukhov similarity theory forms, resulting in bulk Cn2 values that are too high. The bulk Cn2 estimates decrease rapidly when the absolute air-sea temperature difference approaches small positive values. These predicted decreases in Cn2 were not observed in the path-averaged scintillation measurements or in single-point turbulence measurements, demonstrating that bulk models which estimate structure parameters based on mean air-sea differences are not valid when the mean air-sea difference approaches zero. It is believed that obtaining a better understanding of surface wave modification of near- surface atmospheric gradients represents the most promising means toward improving the bulk model.

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.

Comparison of near-surface bulk and scintillation C2n measurements during EOPACE

During the Electro-Optical Propagation Assessment in a Coastal Environment (EOPACE) experiment of May-June 1998, IR scintillation measurements were obtained along a 7 km path over San Diego Bay. Simultaneous meteorological measurement were obtained from a buoy located at the midpoint of the transmission path. Bulk estimates of the refractive index structure parameter, Cn2 were computed from the buoy data and compared with scintillation-derived Cn2 values. The bulk Cn2 estimates agreed well with the scintillation measurements in unstable conditions. The agreement between the two methods was poor for near-neutral and stable conditions. In particular, when the air-sea temperature difference has small positive values the bulk model predicts the vertical refractive index gradient approaches zero, resulting in rapid decrease in bulk Cn2 estimates. These predicted decreases in Cn2 were not observed in the path-averaged scintillation measurements.

Low-elevation transmission measurements at EOPACE I. Molecular and aerosol effects

An analysis is presented showing the effects of molecules and aerosols on atmospheric transmission data obtained during the Electro-Optical Propagation Assessment in Coastal Environments (EOPACE) campaign carried out in San Diego during March and April, 1996. Mid wave infrared transmission was measured over San Diego Bay along a 14.9 km path and a 7.0 km path at heights less than 4 meters above the water. Simultaneous meteorological measurements were obtained from two buoys placed at the mid-points of each path. An aerosol spectrometer was used to measure the aerosol size distribution over each transmission path. Data were analyzed with MODTRAN and Mie theory. The conclusion of this and the next two papers is that low altitude infrared transmission is a complex phenomenon whose mean value may be controlled either by molecular absorption, aerosol scattering, or refractive focusing, and whose fluctuating value is controlled by scintillation.

Refraction and scintillation in the atmospheric surface layer

An infrared or optical signal propagating along a line-of-sight horizontal path near the earths surface can encounter substantial perturbations. There are two prominent factors which can generate fluctuations: first, refractive distortions are low-frequency modulations which can amplify or reduce a signal, and second, scintillation is a higher frequency fluctuation in signal intensity. We will discuss models developed to predict these effects, and associated field test efforts to corroborate and correct the model predictions. The field efforts include tests along horizontal near-surface paths over land and over the coastal ocean surface. Previous transmission field tests have revealed that slow-scale refractive effects can create very pronounced changes in the recorded one-minute average intensity of a source. We will show the results of an analysis of this signal based upon wavelet transforms and filtering. We focus on the detection of a detectable signature frequency of the variations in signal intensity that is based upon the Fresnel zone size. This Fresnel frequency is correlated with the location of the Kolmogorov power law scaling.

Marine aerosol particles and infrared transmission

Abstract : The propagation of infrared radiation close to the ocean surface is controlled by three effects: (1) extinction (absorption and scattering) by aerosol particles, (2) extinction by molecules, and (3) refraction. Molecular extinction can be predicted with fair accuracy by transmission codes such as MODTRAN, and refraction can often be ignored along paths shorter than 10 km. Hence, by making continuous measurements of infrared transmission, a continuous record of aerosol transmission after removing the molecular transmission should be obtainable. This paper shows that this is indeed the case for mid- and longwave infrared transmission measured several meters above San Diego Bay.