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Abstract
A steady-state two-layer model has been developed for the baroclinic boundary layer. The lower layer is the constant flux surface layer (SL) in which the eddy viscosity K varies with height and stability according to the Monin-Obukhov similarity theory; the upper one is the Ekman layer in which K is fixed at the value attained at the top of the SL. The equations of motion in the Ekman layer are solved using the Green's function approach. The lower boundary condition gives two equations from which the nondi-mensionalized friction velocity u */V g0 and the cross-isobaric angle α0 can be obtained in terms of the other known parameters. These equations are compared with the resistance laws. The boundary condition also is given a geometrical interpretation. It has been shown that if Vg (z) is linear, the variation of α0 and u */V g0 with θ, the angle between Vg (0) (surface isobars) and thermal wind (isotherms), is sinusoidal. Also, there is a phase difference of 90° between the variation of α0 and u */V g0, and the amplitude of variation of α0 is found to be proportional to the non-dimensionalized magnitude of thermal wind. ATEX 1969 observations are used to test the wind profiles obtained by the model.
Abstract
A steady-state two-layer model has been developed for the baroclinic boundary layer. The lower layer is the constant flux surface layer (SL) in which the eddy viscosity K varies with height and stability according to the Monin-Obukhov similarity theory; the upper one is the Ekman layer in which K is fixed at the value attained at the top of the SL. The equations of motion in the Ekman layer are solved using the Green's function approach. The lower boundary condition gives two equations from which the nondi-mensionalized friction velocity u */V g0 and the cross-isobaric angle α0 can be obtained in terms of the other known parameters. These equations are compared with the resistance laws. The boundary condition also is given a geometrical interpretation. It has been shown that if Vg (z) is linear, the variation of α0 and u */V g0 with θ, the angle between Vg (0) (surface isobars) and thermal wind (isotherms), is sinusoidal. Also, there is a phase difference of 90° between the variation of α0 and u */V g0, and the amplitude of variation of α0 is found to be proportional to the non-dimensionalized magnitude of thermal wind. ATEX 1969 observations are used to test the wind profiles obtained by the model.
Abstract
Regular radiosonde measurements conducted by the India Meteorological Department at eleven stations spread over a latitude range of 8.5°–28.6°N for a period of nine years are used for the, study of the tropical tropopause characteristics, altitude (H), temperature (T) and pressure (P). The results are examined in the light of the hypothesis of Reid and Gage. It has been shown that large-scale cloud cover affects the tropopause parameters for annual and semiannual periods, while for longer periods (biennial) the cloud cover effect is not significant.
Abstract
Regular radiosonde measurements conducted by the India Meteorological Department at eleven stations spread over a latitude range of 8.5°–28.6°N for a period of nine years are used for the, study of the tropical tropopause characteristics, altitude (H), temperature (T) and pressure (P). The results are examined in the light of the hypothesis of Reid and Gage. It has been shown that large-scale cloud cover affects the tropopause parameters for annual and semiannual periods, while for longer periods (biennial) the cloud cover effect is not significant.
Abstract
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
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
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
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.
