THE TROPICS: ENVIRONMENTS AND HUMAN IMPACTS UNDERSTOOD AND REINTERPRETED

In the 1950s, few people had travelled widely through different parts of the tropics and interpretations on tropical landforms, soils, vegetation and climate largely rested on impressions gained from residence or long sojourns in particular tropical countries. Most academic geography on the tropics was then written by expatriates and lacked the perspective gained from long experience of tropical regions. Particular problems of understanding arose as people attempted to extrapolate from one part of the tropics to another. Since then, understanding has advanced enormously, largely through the efforts of tropical scientists working in their own institutions and through the greater technological ability to study tropical lands both remotely and through easier field access. The establishment of a number of field stations, aimed primarily at the biological sciences, but also facilitating the work of geographers, has been a major stimulus to this effort. Concerns over practical issues, particularly local development and global climatic change have prompted new avenues of research, many of which have been assisted by the great increase in data collection in tropical countries. For the future, South-South dialogue between tropical physical geographers is needed to improve their ability to ask meaningful questions so that they can contribute to teach the rest of their discipline more about serious applied and applicable geography.     

Rain rates based on SSM/I,OLR and raingauge data sets

Summary In this paper we present a methodology for evaluating rain rates from a mix of satellite and surface based observations. The component data sets include the outgoing longwave radiation (OLR), microwave radiometric data from the special sensor microwave imager (SSM/I) and surface raingauge data from the World Weather Watch. We have noted some deficiencies in the SSM/I algorithm-based rain rate over land areas; the OLR-based rain rates exhibit a larger lateral spread and lower intensities than the observed rainfall structures. The proposed combined method assimilates these three data sets to provide improved fields of global tropical rainfall.With 3 Figures     

The fog front of 25 May 1989 — A gravity flow?

Summary During the field experiment FRONTEX 1989 a shallow cold front with a fog field behind its leading edge was observed over the North Sea. The fog field was about 70 km wide, 160 m deep at the front side and 400 m deep at the rear side and was capped by a strong frontal inversion. With increasing height of the inversion the fog broke up into cumulus clouds. The horizontal temperature gradient was largest at 500 m with 9 K/100 km. The observed segment of the front was situated near the col of a geostrophic deformation field with vanishing cross-front wind but non-vanishing cross-front confluence. The front moved at a rate of 5 m/s. It exhibits characteristics of a gravity current which propagates into a stably stratified environment.The physical processes leading to the observed frontal structure and motion are elucidated by experiments with a two-dimensional numerical model. The charateristics of the observed front can be reproduced if both, geostrophic forcing and boundary-layer processes, such as air-sea temperature difference, moisture content and cloud radiational cooling are taken into account. The development of the front under altered conditions is studied by the comparison of different simulations. The reason for the movement of the front is discussed refering to the mechanisms of geostrophic forcing and the forcing by surface friction. The analysis of the forces acting on the cold air mass shows that the Coriolis force is important at some distance behind the frontal head, but has little effect on the processes at the frontal head.With 13 Figures     

Mössbauer studies of iron silicate spinel at high pressure

We have measured in situ Mössbauer transmission spectra of iron silicate spinel (γ-Fe₂SiO₄) in a diamond anvil cell at room temperature and pressures up to 16 GPa. The observed spectra show a doublet characteristic of the paramagnetic state. The isomer shift and quadrupole splitting at atmospheric pressure are 1.10 and 2.63 mm/s, respectively, which are smaller than those of fayalite (α-Fe₂SiO₄). Both the isomer shift and quadrupole splitting decrease linearly with pressure with slope of ?0.003(1) and ?0.020(1) mm/sec · GPa, respectively. This simple linear trend suggests that no electronic or polymorphic transitions occur under 16 GPa except for those due to the small and continuous changes of volume and local symmetry under pressure. On the basis of a crystalline field calculation, the negative pressure derivative of the quadrupole splitting is associated with a trend towards an ideal cubic symmetry of the oxygen sublattice.