Time as an independent variable for current ripples developing towards linguoid equilibrium morphology

Flume experiments show that current ripples on very fine sand surfaces always develop towards a linguoid shape with constant height and wavelength provided that sufficient time is allowed for their formation. Straight and sinuous current ripples only reflect intermediate stages in ripple development and may be regarded as non-equilibrium bedforms. The time period which current ripples require to reach linguoid equilibrium morphology is related to an inverse power of flow velocity. In the transitional stage from current ripples to upper stage plane bed (i.e. washed-out ripple stage) only the equilibrium wavelength remains constant, whereas equilibrium height rapidly decreases to zero. Our observations imply that bed-roughness parameters in sediment transport calculations can be simplified when equilibrium conditions are attained, and that inferences about flow energy from the dimensions of current ripples in very fine sand need to be regarded with caution.     

Book reviews

Ohne Zusammenfassung

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Book reviews

     

The ACPI Project, Element 1: Initializing a Coupled Climate Model from Observed Conditions

A problem for climate change studies with coupled ocean-atmosphere models has been how to incorporate observed initial conditions into the ocean, which holds most of the `memory' of anthropogenic forcing effects. The first difficulty is the lack of comprehensive three-dimensional observations of the current ocean temperature (T) and salinity (S) fields to initialize to. The second problem is that directly imposing observed T and S fields into the model results in rapid drift back to the model climatology, with the corresponding loss of the observed information. Anthropogenic forcing scenarios therefore typically initialize future runs by starting with pre-industrial conditions. However, if the future climate depends on the details of the present climate, then initializing the model to observations may provide more accurate forecasts. Also, this ～ 130 yr spin up imposes substantial overhead if only a few decades of predictions are desired. A new technique to address these problems is presented. In lieu of observed T and S, assimilated ocean data were used. To reduce model drift, an anomaly coupling scheme was devised. This consists of letting the model's climatological (pre-industrial) oceanic and atmospheric heat contents and transports balance each other, while adding on the (much smaller) changes in heat content since the pre-industrial era as anomalies. The result is model drift of no more than 0.2 K over 50 years, significantly smaller than the forced response of 1.0 K. An ensemble of runs with these assimilated initial conditions is then compared to a set spun up from pre-industrial conditions. No systematic differences were found, i.e., the model simulation of the ocean temperature structure in the late 1990s is statistically indistinguishable from the assimilated observations. However, a model with a worse representation of the late 20th century climate might show significant differences if initialized in this way.     

Rotational distortion of polytropes by analytic approximation,III

Paper II of this series was less rigorous than desirable, particularly in the fitting of internal and external potentials. A method is developed in this paper properly to fit the external and internal potentials on the distorted configuration boundary. Displacement of the fiting radius from the radius of a nonrotating configuration in general changes the distorted boundary location by an insignificant amount.A comparison of this theory with the numerical integrations of James (1964) shows excellent agreement to at least an angular velocity equalling 70% of critical rotation. The comparison is for polytropic indices of 1.0, 1.5, 2.0 and 3.0.     

Participation in Field Learning and Teaching Opportunities: Avenues to Research and Publication

Field experiences continue to be a hallmark of a geographer's education and, for that matter, reeducation, as we all strive to remain current in the real world. Academic geographers beginning their ascent towards tenure and promotion might consider augmenting their portfolios with materials emerging from field teaching and learning activities.