Aerosol optical depth measurements over Bahrain acquired through the ground-based Aerosol Robotic Network are analyzed. Optical depths obtained from ground-based sun/sky radiometers showed a pronounced temporal trend, with a maximum dust aerosol loading observed during the March to July period. The aerosol optical depth probability distribution is rather narrow with a modal value of about 0.25. The Angstrom parameter frequency distribution has two peaks. One peak around 0.7 characterizes a situation when dust aerosol is more dominant, the second peak around 1.2 corresponds to relatively dust-free cases. The correlation between aerosol optical depth and water vapor content in the total atmospheric column is strong (correlation coefficient of 0.82) when dust aerosol is almost absent (Angstrom parameter is greater than 0.7), suggesting possible hygroscopic growth of fine mode particles or source region correlation, and much weaker (correlation coefficient of 0.45) in the presence of dust (Angstrom parameter is less than 0.7). Diurnal variations of the aerosol optical depth and precipitable water were insignificant. Angstrom parameter diurnal variability (~20-25%) is evident during the April-May period, when dust dominated the atmospheric optical conditions. Variations in the aerosol volume size distributions retrieved from spectral sun and sky radiance data are mainly associated with the changes in the concentration of the coarse aerosol fraction (variation coefficient of 61%). Geometric mean radii for the fine and coarse aerosol fractions are 0.14 µm (s.d.=0.02) and 2.57

Ducting of microwave radiation is a common phenomenon over the oceans. The height and strength of the duct are controlling factors for radar propagation and must be determined accurately to assess propagation ranges. A surface evaporation duct commonly forms due to the large gradient in specific humidity just above the sea surface; a deeper surface-based or elevated duct frequently is associated with the sudden change in temperature and humidity across the boundary layer inversion. In April 1996 the U.K. Meteorological Office C-130 Hercules research aircraft took part in the U.S. Navy Ship Antisubmarine Warfare Readiness/Effectiveness Measuring exercise (SHAREM-115) in the Persian Gulf by providing meteorological support and making measurements for the study of electromagnetic and electrooptical propagation. The boundary layer structure over the Gulf is influenced strongly by the surrounding desert landmass. Warm dry air flows from the desert over the cooler waters of the Gulf. Heat loss to the surface results in the formation of a stable internal boundary layer. The layer evolves continuously along wind, eventually forming a new marine atmospheric boundary layer. The stable stratification suppresses vertical mixing, trapping moisture within the layer and leading to an increase in refractive index and the formation of a strong boundary layer duct. A surface evaporation duct coexists with the boundary layer duct. In this paper the authors present aircraft-and ship-based observations of both the surface evaporation and boundary layer ducts. A series of sawtooth aircraft profiles map the boundary layer structure and provide spatially distributed estimates of the duct depth. The boundary layer duct is found to have considerable spatial variability in both depth and strength, and to evolve along wind over distances significant to naval operations (ϳ100 km). The depth of the evaporation duct is derived from a bulk parameterization based on Monin-Obukhov similarity theory using nearsurface data taken by the C-130 during low-level (30 m) flight legs and by ship-based instrumentation. Good agreement is found between the two datasets. The estimated evaporation ducts are found to be generally uniform in depth; however, localized regions of greatly increased depth are observed on one day, and a marked change in boundary layer structure resulting in merging of the surface evaporation duct with the deeper boundary layer duct was observed on another. Both of these cases occurred within exceptionally shallow boundary layers (Յ100 m), where the mean evaporation duct depths were estimated to be between 12 and 17 m. On the remaining three days the boundary layer depth was between 200 and 300 m, and evaporation duct depths were estimated to be between 20 and 35 m, varying by just a few meters over ranges of up to 200 km. The one-way radar propagation factor is modeled for a case with a pronounced change in duct depth. The case is modeled first with a series of measured profiles to define as accurately as possible the refractivity st...

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A two-week overwater experiment has been performed to verify the bulk aerodynamic method for calculating the index of refraction structure function parameter, CN. Meteorological data were obtained on shipboard adjacent to a 13-km optical path over Monterey Bay. Model CQ and measured CN values agree to within 33% on the average when there is spatial homogeneity. During periods of strong sea-surface temperature gradients, disagreements by a factor of 10 are common.

The usual description of the vertical distribution of aerosol concentrations assumes neutral atmospheric stratification. This work examines the influence of atmospheric stability on vertical aerosol concentration profile. A model is developed to describe the aerosol transport in the atmospheric surface layer. The model describes a balance between upward aerosol flux due to atmospheric turbulence and downward flux due to gravitational deposition. Monin-Obukhov similarity theory describes the turbulent profiles of wind, temperature and specific humidity. An approximate equilibrium expression relates particle size to ambient relative humidity.To isolate the role of transport and growth, we first use the model to describe the surface layer concentration of inert particles unaffected by relative humidity. The calculations show that growth phenomena must be included to correctly describe aerosol concentrations in the presence of relative humidity gradients. Complete calculations of aerosols concentrations, including growth effects, show that the concentration of large particles (radius greater than 5 pm) can vary by an order of magnitude depending on stability. With light winds, the stability effect on large aerosols substantially affects the slope of the expected aerosol size distribution. With strong winds the aerosol distribution is not affected by changes in temperature and humidity lapse rates, but instead is governed by wind-shear-produced turbulent mixing.