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Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g-1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal. [ABSTRACT FROM AUTHOR]

Radiative forcing of aerosols is much more difficult to estimate than that of well-mixed gases due to the large spatial variability of aerosols and the lack of an adequate database on their radiative properties. Estimation of aerosol radiative forcing generally requires knowledge of its chemical composition, which is sparse. Ground-based sky radiance measurements [e.g., aerosol robotic network (AERONET)] can provide key parameters such as the single-scattering albedo, but in shipborne experiments over the ocean it is difficult to make sky radiance measurements and hence these experiments cannot provide parameters such as the single-scattering albedo. However, aerosol spectral optical depth data (cruise based as well as satellite retrieved) are available quite extensively over the ocean. Spectral optical depth measurements have been available since the 1970s, and spectral turbidity measurements (carried out at meteorological departments all over the world) have been available for several decades, while long-term continuous chemical composition information is not available. A new method to differentiate between scattering and absorbing aerosols is proposed here. This can be used to derive simple aerosol models that are optically equivalent and can simulate the observed aerosol optical properties and radiative fluxes, from spectral optical depth measurements. Thus, aerosol single-scattering albedo and, hence, aerosol radiative forcing can be estimated. Note that the proposed method is to estimate clear-sky aerosol radiative forcing (over regions where chemical composition data or sky radiance data are not available) and not to infer its exact chemical composition. Using several independent datasets from field experiments, it is demonstrated that the proposed method can be used to estimate aerosol radiative forcing (from spectral optical depths) with an accuracy of ±2 W m-2. [ABSTRACT FROM AUTHOR]

During the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II), extensive measurements of spectral aerosol optical depth, mass concentration, and mass size distribution of ambient aerosols as well as mass concentration of aerosol black carbon (BC) were made onboard a research vessel during the intermonsoon period (i.e., when the monsoon winds are in transition from northeasterlies to westerlies/southwesterlies) over the Arabian Sea (AS) adjoining the Indian Peninsula. Simultaneous measurements of spectral aerosol optical depths (AODs) were made at different regions over the adjoining Indian landmass. Mean AODs (at 500-nm wavelength) over the ocean (∼0.44) were comparable to those over the coastal land (∼0.47), but were lower than the values observed over the plateau regions of central Indian Peninsula (∼0.61). The aerosol properties were found to respond distinctly with respect to change in the trajectories, with higher optical depths and flatter AOD spectra associated with trajectories indicating advection from west Asia, and northwest and west-coastal India. On average, BC constituted only ∼2.2% to total aerosol mass compared to the climatological values of ∼6% over the coastal land during the same season. These data are used to characterize the physical properties of aerosols and to assess the resulting short-wave direct aerosol forcing. The mean values were –27 W m-2 at the surface and -12 W m-2 at the top of the atmosphere (TOA), resulting in a net atmospheric forcing of +15 W m-2. The forcing also depended on the region from where the advection predominates. The surface and atmospheric forcing were in the range -40 to -57 W m-2 and +27 to +39 W m-2, respectively, corresponding to advection from the west Asian and western coastal India where they were as low as -19 and +10 W m-2, respectively, when the advection was mainly from the Bay of Bengal and from central/peninsular India. In all these cases, the net atmospheric forcing (heating) efficiency was lower than the values reported for northern Indian Ocean during northern winter, which is attributed to the reduced BC mass fraction. [ABSTRACT FROM AUTHOR]

A response to a letter to the editor on the article on the mechanisms of mammatus formation in the previous issue is presented.