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- Author or Editor: G. G. Lala x
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Abstract
Pre-storm conditions are often characterized by an atmosphere in the presence of rather strong wind shears and a temperature inversion. The latter acts as a lid for moisture in the boundary layer. In this paper we discuss the possibility that a gravity wave generated by wind shear can reach sufficiently large amplitude to induce condensation. We show that under certain circumstances the ensuing heat release takes place in such a phase with respect to the initial gravity wave so as to reinforce it, substantially increasing its rate of growth. Thus, the lifting due to the wave will grow and so will the condensation. By showing that in the early stages after the first condensation occurs, we have a positive feedback between the gravity wave and the induced condensation, we strengthen the case for gravity waves as possible lifting agents leading to condensation and eventually to convection. The present calculations are not meant to describe the system after the onset of convection and as such they differ from existing CISK theories. The results also appear to indicate that the presence of a critical level in the region of large relative humidity may be a prerequisite for a strong feedback between the gravity wave and the induced condensation.
Abstract
Pre-storm conditions are often characterized by an atmosphere in the presence of rather strong wind shears and a temperature inversion. The latter acts as a lid for moisture in the boundary layer. In this paper we discuss the possibility that a gravity wave generated by wind shear can reach sufficiently large amplitude to induce condensation. We show that under certain circumstances the ensuing heat release takes place in such a phase with respect to the initial gravity wave so as to reinforce it, substantially increasing its rate of growth. Thus, the lifting due to the wave will grow and so will the condensation. By showing that in the early stages after the first condensation occurs, we have a positive feedback between the gravity wave and the induced condensation, we strengthen the case for gravity waves as possible lifting agents leading to condensation and eventually to convection. The present calculations are not meant to describe the system after the onset of convection and as such they differ from existing CISK theories. The results also appear to indicate that the presence of a critical level in the region of large relative humidity may be a prerequisite for a strong feedback between the gravity wave and the induced condensation.
Abstract
An extensive boundary-layer field program was conducted which included simultaneous measurements of visibility and particle size distributions during fog and haze. Several empirical expressions relating changes in visibility to characteristics of the aerosol (droplet) size spectrum and relative humidity are presented and evaluated. Detailed analysis of one evolving dense fog revealed several points of interest regarding the behavior of drop size spectra, including a scheme for approximating fog supersaturation.
Abstract
An extensive boundary-layer field program was conducted which included simultaneous measurements of visibility and particle size distributions during fog and haze. Several empirical expressions relating changes in visibility to characteristics of the aerosol (droplet) size spectrum and relative humidity are presented and evaluated. Detailed analysis of one evolving dense fog revealed several points of interest regarding the behavior of drop size spectra, including a scheme for approximating fog supersaturation.
Abstract
A numerical model of radiation fog was developed in order to test the sensitivity of variables comprising the model, and evaluate its capability for forecasting the onset of fog from standard radiosonde weather data. Four case studies were considered that included both fog and no-fog occurrences. The variables examined–initial surface temperature and moisture conditions, eddy exchange profiles, radiative flux divergence, and dew formation–were all found to influence critically the model's performance. Prediction of fog occurrence and temperature were reasonably encouraging provided a judicious (though somewhat arbitrary) choice of eddy mixing values was made.
Abstract
A numerical model of radiation fog was developed in order to test the sensitivity of variables comprising the model, and evaluate its capability for forecasting the onset of fog from standard radiosonde weather data. Four case studies were considered that included both fog and no-fog occurrences. The variables examined–initial surface temperature and moisture conditions, eddy exchange profiles, radiative flux divergence, and dew formation–were all found to influence critically the model's performance. Prediction of fog occurrence and temperature were reasonably encouraging provided a judicious (though somewhat arbitrary) choice of eddy mixing values was made.
Abstract
The characteristics of internal gravity waves generated by tropospheric jet streams are analyzed and discussed. By solving numerically the equations of motion in the linear, inviscid and Boussinesq limit, it is shown that a modal structure exists. Some of these modes have the ability to propagate vertically away from the jet and are likely to he responsible for some of the observed wave activities in the ionosphere as well as at the ground. For selected values of the minimum Richardson number of the flow, growth rates and horizontal phase velocities are given as functions of the horizontal wavenumber, for jet streams of varying width. Finally, a brief study of the stability of the so-called low-level jet, whose spectrum of generated waves undoubtedly will contribute to the dynamics of the nocturnal boundary layer, is undertaken.
Abstract
The characteristics of internal gravity waves generated by tropospheric jet streams are analyzed and discussed. By solving numerically the equations of motion in the linear, inviscid and Boussinesq limit, it is shown that a modal structure exists. Some of these modes have the ability to propagate vertically away from the jet and are likely to he responsible for some of the observed wave activities in the ionosphere as well as at the ground. For selected values of the minimum Richardson number of the flow, growth rates and horizontal phase velocities are given as functions of the horizontal wavenumber, for jet streams of varying width. Finally, a brief study of the stability of the so-called low-level jet, whose spectrum of generated waves undoubtedly will contribute to the dynamics of the nocturnal boundary layer, is undertaken.
Abstract
A laboratory model of a breaking wave or whitecap was constructed, and the aerosol produced by it was investigated intensively. Submicron- and even Aitken-sized particles were produced: the presence of salt particles of mass <10−17 g(r<0.01 μm) could be inferred. The evidence strongly suggests that the submicron fraction is composed of film drops, derived primarily from bubbles larger than 1 mm in diameter. The shape of the CCN spectrum and overall mass distribution of the model-produced aerosol were similar to what is observed in clean marine air.
Whether or not the production rate of such small particles is globally significant when the model results are applied to the oceans depends to a large extent on the set of assumptions one makes concerning aerosol residence time and fraction of sea surface covered by whitecaps. However, there are realistic choices of these parameters which suggest that appreciable fractions of both CCN and CN in the lower marine atmosphere are produced directly by the sea.
Abstract
A laboratory model of a breaking wave or whitecap was constructed, and the aerosol produced by it was investigated intensively. Submicron- and even Aitken-sized particles were produced: the presence of salt particles of mass <10−17 g(r<0.01 μm) could be inferred. The evidence strongly suggests that the submicron fraction is composed of film drops, derived primarily from bubbles larger than 1 mm in diameter. The shape of the CCN spectrum and overall mass distribution of the model-produced aerosol were similar to what is observed in clean marine air.
Whether or not the production rate of such small particles is globally significant when the model results are applied to the oceans depends to a large extent on the set of assumptions one makes concerning aerosol residence time and fraction of sea surface covered by whitecaps. However, there are realistic choices of these parameters which suggest that appreciable fractions of both CCN and CN in the lower marine atmosphere are produced directly by the sea.
