A comparison of aerosol data acquired at five different sites around the globe is presented. All data has been acquired with the same instrumentation and representative size distributions for marine air masses at 10 m/s wind speed have been selected for comparison. Differences in the concentrations of larger and smaller aerosols at the various sites are explained in terms of fetch, trade winds, shielding, pollution, seawater temperature and phytoplankton bloom. The differences in size distribution induce significant differences in the extinction coefficients from the VIS to the LWIR at the various sites. Consequently, the transmission over a specific range also varies significantly. This suggests that a detailed analysis of the conditions at each site is necessary in order to understand the exact aerosol behavior and to correctly predict electro-optical propagation effects due to aerosols.
The performance of electro-optical systems can be substantially affected by aerosol particles that scatter and absorb electromagnetic radiation. While molecular extinction can be calculated using propagation codes such as MODTRAN (Berk et al., 1989), the influence of aerosols is much less easy to account for. However, concentrations and optical properties of aerosol particles in the atmosphere are quite variable both in time and space. Very few relevant models for the aerosol size distributions have been published during the last decades. One of the most used is the Navy aerosol model (NAM, Gathman, 1983), which is currently being upgraded to the advanced Navy aerosol model (ANAM, van Eijk and Merritt, 2006). NAM and ANAM are dedicated to open ocean, while coastal areas induce specific processes (Piazzola et al., 2000, and Bendersky et al., 2004). To include coastal effects for the prediction of aerosol concentrations and their effects on the extinction, Piazzola et al. (2003) proposed the aerosol extinction code MEDEX. We deal with the extension of the predictions of MEDEX on a regional scale. To achieve it, MEDEX is forced by the regional mesoscale meteorological model RAMS (Cotton et al., 2003) to account for the fine details of the coastal orography. Simulations are then compared to aerosol size distributions recorded in the Mediterranean. The results show a nonhomogeneous spatial coverage of the aerosol concentrations over the northern Mediterranean. In addition, we show the least performance of the coupling for unsteady conditions of the wave field.
The performance of electro-optical systems can be substantially affected by aerosol particles that scatter and absorb electromagnetic radiation. The performance assessment of EO systems by propagation prediction codes then requires accurate aerosol models. However, concentrations and optical properties of aerosol particles in the atmosphere are very variable both in time and space. In particular, coastal areas induce specific physical processes. To include coastal effects in the model for the prediction of aerosol concentrations, Piazzola et al.1 proposed an extension of the Navy Aerosol Model (NAM2) to coastal areas. This work has been coupled with Mie theory to develop the aerosol extinction code MEDEX,3 which is based on an extensive series of measurements in the Mediterranean.
The present paper deals with the extension of the predictions of MEDEX to a regional scale. To achieve it, MEDEX has been coupled with a regional meso-scale meteorological model (RAMS). This allows taking into account the details of the orography of the coast and a better modelling of the unsteadiness for both meteorological and oceanic conditions. The results show the feasibility of extending the predictions of MEDEX to any coastal site.
The authors describe the behavior of particles (connected with Laser Doppler Anemometry measurements) in high velocity flows. The different flow regimes are exposed, as well as the associated usual drag laws for a sphere. A unique relation is proposed for high velocities and for all flow regimes. Calculated values obtained with this law and experimental results are compared in the case of particle motion through an oblique shock wave. They are found in good agreement. Different effects due to rarefaction, compressibility and high relative Reynolds number are discussed.