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Electro-optical sensors detect the radiation emitted and/or reflected by objects. Outside the laboratory, where the distance between sensor and object may become large, the intervening atmosphere interacts with the radiation while it propagates to the sensor. The major propagation effects are (1) macroscopic refraction effects, (2) turbulence (or microscopic refraction effects), and (3) transmission losses. Propagation effects cause image distortions and a loss of contrast and thereby affect military relevant products such as detection and classification distance. This paper provides an overview of the major propagation effects and their relation to the environmental conditions, and discusses a few recent advantages.

The Advanced Navy Aerosol Model provides a quick and simple method to estimate aerosol properties in the maritime atmosphere using standard meteorological parameters

We have re-exanined the established data for the mean velocity profile for turbulent flow over a smooth surface and found its Reynolds number dependence in the inner region to be remarkably large. By combining two eddy-viscosity models (with overlapping validity in the inertial sublayer), this feature is shown (both analytically and numerically) to arise from inner-outer sublayer interaction, involving a nonzero viscous correlation length.

The knowledge of the optical contrast of an oil layer on the sea under various surface roughness conditions is of great interest for oil slick monitoring techniques. This paper presents a 3D simulation of a dynamic sea surface contaminated by a floating oil film. The simulation considers the damping influence of oil on the ocean waves and its physical properties. It calculates the radiance contrast of the sea surface polluted by the oil film in relation to a clean sea surface for the SWIR spectral band. Our computer simulation combines the 3D simulation of a maritime scene (open clear sea/clear sky) with an oil film at the sea surface. The basic geometry of a clean sea surface is modeled by a composition of smooth wind driven gravity waves. Oil on the sea surface attenuates the capillary and short gravity waves modulating the wave power density spectrum of these waves. The radiance of the maritime scene is calculated in the SWIR spectral band with the emitted sea surface radiance and the specularly reflected sky radiance as components. Wave hiding and shadowing, especially occurring at low viewing angles, are considered. The specular reflection of the sky radiance at the clean sea surface is modeled by an analytical statistical bidirectional reflectance distribution function (BRDF) of the sea surface. For oil at the sea surface, a specific BRDF is used influenced by the reduced surface roughness, i.e., the modulated wave density spectrum. The radiance contrast of an oil film in relation to the clean sea surface is calculated for different viewing angles, wind speeds, and oil types characterized by their specific physical properties. © 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

This paper reviews three phenomena that affect the propagation of electro-optical radiation through the atmosphere: absorption and scattering, refraction and turbulence. The net effect on imaging or laser systems is a net reduction of the effective range, or a degradation of the information contained in the electro-optical radiation. Recent advances in techniques to assess the concentration and composition of atmospheric aerosols, and to assess the strength of optical turbulence are discussed in more detail. © 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Weather Research and Forecasting (WRF) model predictions using different boundary layer schemes and horizontal grid spacings were compared with observational and numerical large-eddy simulation data for conditions corresponding to a dry atmospheric convective boundary layer (CBL) over the southern Great Plains (SGP). The first studied case exhibited a dryline passage during the simulation window, and the second studied case was used to examine the CBL in a post-cold-frontal environment. The model runs were conducted with three boundary layer parameterization schemes (Yonsei University, Mellor-Yamada-Janjic, and asymmetrical convective) commonly employed within the WRF model environment to represent effects of small-scale turbulent transport. A study domain was centered over the Atmospheric Radiation Measurement Program SGP site in Lamont, Oklahoma. Results show that near-surface flow and turbulence parameters are predicted reasonably well with all tested horizontal grid spacings (1, 2, and 4 km) and that value added through refining grid spacing was minimal at best for conditions considered in this study. In accord with this result, it was suggested that the 16-fold increase in computing overhead associated with changing from4- to 1-km grid spacing was not justified. Therefore, only differences among schemes at 4-km spacing were presented in detail. WRF model predictions generally overestimated the contribution to turbulence generation by mechanical forcing over buoyancy forcing in both studied CBL cases. Nonlocal parameterization schemes were found to match observational datamore closely than did the local scheme, although differences among the predictions with all three schemes were relatively small.

A study is carried out to classify possible combinations of refractivity conditions for RF and IR over a wide range of meteorological conditions using different micrometeorological bulk models. The calculated refractivity profiles are analyzed for evaporation duct height (EDH), mainly relevant for RF propagation, and for gradients of the modified refractivity at different heights, relevant for both RF and IR propagation. These refractivity gradients are a direct indicator for the occurrence of sub- or super refraction at the height of interest. The present study reveals that under humid and unstable conditions evaporation ducts are found at approximately 3±2 m above cold (5°C) waters and at approximately 8±5 m over warm waters (25°C). Under dry conditions, these duct heights are approximately 9±5 m and 20±10 m, respectively. Duct heights decrease with increasing wind speed. Under humid and near-neutral conditions, duct heights range from 1 to 25 m, and decrease with increasing air temperature and/or wind speed. On the other hand, for dry and near-neutral conditions, and also for neutral conditions, the duct height is not well defined. Values between 1 m and 100 m are found, with an irregular dependence on air temperature and wind speed. Reliable modeling of duct height under these conditions remains questionable due to a lack of vertical mixing in the surface layer. The paper also shows that all four combinations of RF and IR sub- and super-refraction can occur in the atmosphere. The occurrence of a specific combination depends predominantly on temperature and humidity, and to a relatively minor part on wind speed. The magnitude of refraction effects in the two spectral bands is not necessarily coupled but varies with environmental conditions and height. Sub-sub refraction is generally weak and occurs under neutral conditions or at large heights. Super-super refraction occurs under warm and dry conditions and can reach medium strengths. RF-super refraction in combination with IR-sub refraction occurs under strong unstable conditions (e.g., surface temperature higher than air temperature) and can reach medium strengths. RF-sub refraction in combination with IR-super refraction occurs under stable and warm conditions. The magnitude of refraction can be very large, especially at low altitude.

Aerosols are important to a large number of processes in the marine boundary layer. On a micro-meteorological scale, they influence the heat and moisture budgets near the sea surface. Since the ocean acts both as a source and a sink for aerosols, the sea spray droplets may transfer water vapour and heat (as well as pollutants and bacteria) through the air-sea interface. While aloft, sea-salt particles shrink by evaporation or grow by condensation through interaction with the humidity field. Hence, they may affect the fluxes of water vapour and heat, which may have an impact on larger scale meteorological processes and climatology.

A commercial long-range scintillometer was deployed over a 2-km path in False Bay, South Africa, for a timeframe of one year. The turbulence data retrieved from the instrument are compared to turbulence parameters inferred from micrometeorological data and models, and the relation between experimental and model-data is explored.

Atmospheric turbulence can cause severe blurring and scintillation in images. These distortions result in less detailed images. Using only image enhancement techniques can improve the quality of the images to some extend, but usually not enough to see all details. Turbulence correction techniques, such as post-processing techniques using a deconvolution, are developed and show good results. A more profound understanding of atmospheric turbulence, and especially the impact of turbulence on the image quality, might help to improve the quality of these images even further. To this end, we present a simple method of estimating the shape and size of the point spread function. The method is tested using images from an ongoing trial in South-Africa.

The Advanced Navy Aerosol model (ANAM), being a modified version of the Navy Aerosol Model (NAM), is a wellknown engineering tool providing a quick and reasonable estimate of the aerosol extinction in the marine near-surface environment on the basis of simple meteorological input data. The original NAM consists of 3 lognormal distributions, which describe freshly produced marine aerosols, aged marine aerosols (produced elsewhere and advected to the measurement site) and a background concentration of marine aerosols. The ANAM adds a 4th lognormal mode to NAM to account for the largest marine particles. To account for non-marine particles, a special lognormal mode, called “dust mode” was included in NAM. The relative importance of the dust mode versus the marine background concentration is governed by a special input parameter known as the air mass parameter (AMP). Unfortunately, the AMP is ill-defined and the NAM user community has found it difficult to attribute a proper value to the AMP. This inconvenience became even more stressing when NAM was used for assessing aerosol extinction in the coastal zone. To overcome this inconvenience, a new approach is suggested which involves replacement of the AMP by the Ångström coefficient. The advantage is that the latter parameter can be directly measured and has a physical relation to the aerosol size distribution. When the particle size distribution is dominated by small particles, usually associated with pollution, the Ångström coefficients are high; in clear conditions they are usually low. Therefore this parameter is a good tracer of the aerosols originated over land and hence a good replacement for the AMP.

The effective field-of-view of an electro-optical sensor in a given meteorological scenario can be evaluated using a ray-tracer. The resulting ray trace diagram also provides information pertinent to the quality (distortion, mirages) of the image being viewed by the sensor. The EOSTAR (Electro Optical Signal Transmission And Ranging) model suite contains a ray tracer that has been upgraded to take into account horizontal inhomogeneities in the atmosphere, such as temperature gradients as observed in coastal areas where (e.g.) cold air flows out over warm waters. Initial results for horizontally inhomogeneous atmospheres are presented and compared to calculations for horizontally homogeneous atmospheres. It is shown that the horizontal inhomogeneity of temperature should be taken into account when assessing sensor performance.

A methodology is presented to infer the refractive-index structure function parameter and the structure parameters for temperature and humidity from numerical simulations of the turbulent atmospheric convective boundary layer (CBL). The method employs spatial and temporal averaging of multiple realizations of the CBL flow field reproduced by a large-eddy simulation (LES) of the atmosphere. The values yielded by LES-based approach agree fairly well with values predicted by the Monin-Obukhov similarity theory In this respect, the cn2 retrieval from the LES data is promising for evaluating the vertical profile of cn2 throughout the entire CBL Under the considered CBL conditions and for the selected optical wavelength of 055 μm the value of cn2 was found to be dominated by the cn2 contribution in the first few hundred meters above the surface, whereas the ctqcontribution became significant aloft. © 2013 SPIE.

The work performed in 1992 in the framework of the EUROTRAC subproject ASE was mainly focused on three topics. The first was the extension of the modified CLUSE numerical model [Rouault et al., 1991; De Leeuw et al., 1992a] to over-ocean conditions. The modifications in the new code (SEACLUSE) include the influence of waves on the air flow and the evaporation of salt-water droplets. The second aim was to finalize the analysis of the TWO-PIE experimental data on tracer aerosol deposition on water surfaces in the presence of a simulated whitecap and associated sea spray aerosol. The CLUSE and TWO-PIE efforts are part of EUROTRAC ASE subtopic 5: Factors determining particle dynamics over the air-sea interface, in cooperation with Riso Nationäl Laboratory, Roskilde, Denmark and Ecole Centrale de Nantes, France. The third effort was the participation in the field experiment from FPN in September 1992. The cooperation to integrate our modeling efforts with atmospheric chemistry models to study wet-phase chemistry and scavenging processes has been intensified. The work that is part of EUROTRAG ASE subcopic 5 is described in De Leeuw et al. [1993a] and is therefore only briefly discussed in this contribution

The variation of the extinction coefficient with wavelength can be presented as a power law function with a constant (related to the power factor) known as the Ångström coefficient. When the particle size distribution is dominated by small particles, usually associated with pollution, the Ångström coefficients are high; in clear conditions they are usually low. Long residence time of air masses over land and in particular the passage over large urban areas cause high concentrations of fine particles and thus high values of the Ångström coefficients. The opposite effect can be observed over water. The longer the time that the air masses spent over water the more evident is a change in the aerosol size distribution caused by the deposition of continental aerosols. As a result of this process the measured Ångström coefficient values become much smaller. Therefore this parameter is a good tracer for the concentration of aerosols originated over land. The relation between the Ångström coefficient and TOS (time over sea) is demonstrated on three data sets. The first data set includes measurements collected at the Irish Atlantic coast in 1994 and 1995, the second one, data collected within the Rough Evaporation Duct (RED) experiment that took place off Oahu, Hawaii in 2001. The third one represents data collected at the Baltic Sea during cruises in 1997 and 1998.

The Navy Aerosol Model (NAM, available in MODTRAN) is widely used as a tool to assess the aerosol extinction in the marine atmospheric surface layer. NAM was built as a regression model in the 1980s to represent the aerosol extinction at deck height as a function of the meteorological conditions. The recently developed Advanced Navy Aerosol Model (ANAM) utilizes additional experimental evidence to supersede NAM by correcting the underestimation of the concentration of aerosols larger than a few microns. More importantly, ANAM provides the aerosol extinction as a function of height between the surface and several tens of meters. Present-day naval surveillance and threat scenarios require detection of targets at the horizon, such as sea-skimming missiles, or small targets such as rubber boats. In either case, the propagation path from sensor to target is likely to come very close to the wave surface and in order to estimate detection ranges, an assessment of the transmission losses along the path is necessary. To answer the question posed in the title, we assess the two models using two meteorological data sets (784 cases) representative of diverse maritime conditions in regions of interest around the world.

The work performed in 1990 in the framework of EUROTRAC and related research is stummarized. This includes the analysls of the CLUSE data to derive the aerosol source functions specific for the CLUSE experiments in fresh and salt water, a study on the source function for the marine aerosol, and comparison of aerosol profiles in laboratory and field situations. Particle deposition on water surfaces was studied during the TWO-PIE expertments in June 1990 in Marseille.

The results of applying the empirical orthogonal functions (EOF) method to decomposition and approximation of aerosol size distributions are presented. A comparison was made for two aerosol data sets, representing coastal and oceanic environments. The first data set includes measurements collected at the Irish Atlantic coast in 1994 and 1995, the second one data collected during the Rough Evaporation Duct (RED) experiment that took place off Oahu, Hawaii in 2001. The main finding is that aerosol size distributions can be represented by a superposition of the mean size distribution and the first eigenvector multiplied by an amplitude function. For the two aerosol data sets the mean size distribution is very similar in the range of small particles sizes (radius < 1μm) but the main difference appears for larger aerosols (radius > 1μm). It is also reflected by the spectral shape of the eigenvector. The differences can be related to the type of aerosols present at both locations, and the amplitude function can be associated to meteorological conditions. The amplitude function also indicates the episodes with the maximum/minimum continental influence. The results of this analysis will be used in upgrades of the ANAM model.

The Navy Aerosol Model (NAM) is widely used as an engineering tool to provide a quick estimate of the aerosol extinction in the marine environment. Since its introduction, several shortcomings of NAM have been identified that are being addressed by the development of the Advanced Navy Aerosol Model (ANAM). At present, the Advanced Navy Aerosol Model has been reviewed as concerns its production mode. The two separate production modes (3rd and 4th modes in ANAM4) have been replaced by a single production mode in ANAM5. The shape of the new production mode is given by two sea spray source functions taken from literature, Vignati et al. and Smith and Harrison. The intensity of the new production mode in ANAM5 at a particular height above the surface is governed by a transfer function that depends on radius and wind speed. The production mode in ANAM5 has several tuning parameters that have been optimized by comparing ANAM5 concentration predictions to experimental aerosol data. ANAM5 performs better than ANAM4 in predicting the concentrations of large aerosols in open ocean conditions, but the performance is reduced in the coastal zone. This may be due to the presence of a strong advection mode that is currently not well taken into account by the ANAM.