Application of optical techniques to the study of plumes in stratified fluids

Digital techniques for the correction of signal distortions that arise in planar laser-induced fluorescence (PLIF) measurements (by PC-based video digitizing systems) of jets and plumes and the production of flow statistics in both time and space are reviewed. The entire concentration field is repeatedly imaged in s intervals over hundreds of thousands a points in a plane. By the recognition of signal distortion sources and the employment of corrections, a clearer picture of tracer concentrations may be realized.Fluorescence studies are made with a planar sheet of laser light 430 mm tall and 1.5 mm thick. The fluorescence excitation produced from trace concentrations of Rhodamine 6G is used to visualize and measure the propagation of a jet or plume in a density stratified laboratory tank. The emitted light is collected by a CCD camera in a 512 × 480 pixel format over a 940 × 715 mm field of view. The captured images are corrected for transverse laser sheet intensity distribution; laser beam attenuation; refraction; lens vignette; time varying and spatial noise; digitization aspect ratio; camera response. The measurement and methods of correction are discussed in detail. The resulting image data can then be used to collect tracer concentration statistics for jets and plumes. Instantaneous (i.e. over of a second intervals), average, maximum, minimum, standard deviation, and coefficient of variation are given as introductory examples of image statistics realizable for a buoyant jet.     

A numerical investigation of the JASIN atmospheric boundary layer

A numerical model of the cloudy marine boundary layer is described and used to investigate the role of entrainment instability on the developing boundary layer. In general, previous studies have been limited to boundary layers capped by convectively stable inversions or have described only cumulus fields. Here we extend a stratus-capped boundary-layer model to consider the transition to a convectively unstable cloud layer capped by an inversion across which there is a rapid decrease in wet-bulb or equivalent potential temperature. In this case, the inversion is very active and the entrainment rate is determined by the active instability at the interface, in contrast to the mean turbulent motion within the boundary layer.The model is used to interpret the observed boundary layer from the JASIN experiment. Cool, dry air is modified by prolonged passage over increasingly warmer ocean which leads to the development of a convectively unstable cloud layer.     

Time-dependent electromagnetic core-mantle coupling

The magnetic field in the Earth's mantle is computed using a depth-dependent electrical conductivity, of form σ = σ_a(r/a)^?α, and an approximation scheme in which the electromagnetic time constant of the mantle is assumed small compared with the time scales of the secular variation, and in which the induced currents and fields are obtained iteratively. We first associate the toroidal fields in the mantle with motions at the core surface (r = a) which create the observed geomagnetic field by flux rearrangement, and compute the resulting couple, Γ, parallel to the geographical axis. Using only zonal core motions, and values σ_a = 3 × 10³ω^?1m^?1, α = 30 for the conductivity profile, we find that the toroidal induced fields create a couple, Γ_T, that over most of this century has been roughly ten times greater than the poloidal part, Γ_S, of Γ, and has the same sign. The total couple, Γ, has fluctuations of order 10¹⁸ Nm as required for the observed decade fluctuations in the length of the day. Its average is ～ ?1.5 × 10¹⁸ Nm, i.e., it is too large to remain unbalanced. We suppose that an equally important couple in the opposite sense is created by flux leakage from the core, and we estimate the necessary gradient of toroidal field in the core to be of order ?0.5 Gs km^?1 at the core surface. During the course of the data analysis needed for the present work, we found some evidence for a torsional wave in the Earth's core with a period of ～ 60 y.     

On the onset of entrainment instability over the ocean

The marine atmospheric boundary layer is characterized by cool temperatures and high humidity. Clouds are observed over most of the oceans. It is generally accepted that these overcast cloud decks break up into scattered fragments due to cloud-top entrainment instability. That is, if the air above the boundary layer is sufficiently cool and dry relative to cloud top, the buoyancy flux will be directed upwards and entrainment can occur freely.A boundary-layer model is used to test the sensitivity of the model atmosphere to the various processes which promote the onset of cloud-top entrainment instability. It is found that the transition from a solid cloud deck to scattered cumulus clouds depends on a rate process. The cloud cover is sensitive to mesoscale variations in sea surface temperature only if the cloud-top inversion is sufficiently weak.     

Creep parameters of rocks on an engineering scale

Vertical shafts of 137 cm diameter were excavated to measure creep deformations of carbonate shale, weathered limestone and tuff. Changes of diameter at successive time intervals were recorded under constant, radial pressure. Three typical models were chosen for a regression analysis of the experimental data and their fitness were compared to one another in a statistical approach.The introduction of a steady state flow term improves the data fit considerably, and the power law was found to yield an even better result. It was also determined that rock masses in the field contain numerous discontinuities which increase the plasticity of the three types of rock tested. The viscosities determined in-situ were found to be much lower than those derived from small intact specimens by up to several orders of magnitude.     

Statistics on the vertical structure of rainshowers

A large sample of summertime data from the McGill Radar Weather Observatory was analyzed to determine the variation with altitude of the horizontal extent of individual rainshowers. For echoes defined by a reflectivity factor of 39 dBz (equivalent to a rainfall rate of about 10 mm/h) it was found that the mean area of the total population of echoes decreases linearly with altitude from approximately 20 km² at 2 km to 11 km² at 8 km. Subsets of the total population were investigated, consisting of only the echoes penetrating the altitudes of 6, 7, and 8 km. On the average these relatively tall echoes are much greater in horizontal extent than the total population. Whereas the sizes of the total population of echoes at any altitude are distributed approximately exponentially in terms of the square root of area, the sizes of the “survivors” that extend to high altitudes may be described by the gamma distribution with a mean value decreasing approximately linearly with height above 3 km and a dispersion of 0.55. Some characteristics are also reported for echoes defined by reflectivities of 31 dBz and 47 dBz. Estimates are given of the fraction of the total area in a horizontal plane that contains echoes in each of these categories.