High frequency variability in recent climate and the north atlantic oscillation

Summary ?High-frequency temperature variability was investigated in the temperature time series measured at Prague-Sporilov (Czech Republic) between 1994–2001. The calculations were performed for time series of surface air temperature averaged for 6-hour intervals. Variability was detected by the method of absolute difference of temperature anomalies between two adjacent discrete time periods. The results indicated a frequency dependence of variability. For 24-hour intervals the variability exhibits an irregular character and decreases with time in the eight-year observation period. Variability time series calculated for the 6-hour intervals did not reveal any significant trend, however, apparent quasi-seasonal oscillations exist. A significant correlation between the North Atlantic Oscillation (NAO) activity and temperature variability can be observed. Higher NAO-index values at all frequencies tend to be associated with higher variability. Received February 28, 2002; revised March 25, 2002; accepted July 18, 2002     

Fluctuations of line integrated concentrations across plume diffusion in grid generated turbulence and in shear flows

The fluctuations of the instantaneous values of line integrated concentrations across plumes from point sources diffusing in turbulent shear flows, and in grid generated turbulence, have been studied experimentally using a fast response system which measured the attenuation of the intensity of an infrared beam crossing the plume. Analysis of the measurements show that the dimensionless statistical properties of the fluctuations at different distances from the source at each flow are approximately similar, in the sense that they depend primarily on the relative off-center location of the line of integration and almost independent of the distance from the source and the nature of the turbulence in the flows, as long as the characteristic length of the mean plume is not large compared to the size of the large eddies. The characteristic time of the fluctuations, on the other hand, was found to grow with the distance from the source and the autocorrelations of the fluctuations, particularly in the case of a plume diffusing in grid generated turbulence, were it found to be proportional to the lateral size of the mean plume. A—5/3 decay law of the power spectrum of the fluctuations was observed in the low frequency range which corresponds to the scale of the large eddies. The decay of the fluctuations caused by smaller eddies was much faster, as expected.     

Physical modelling of flow and dispersion over complex terrain

Atmospheric motion and dispersion over topography characterized by irregular (or regular) hill-valley or mountain-valley distributions are strongly dependent upon three general sets of variables. These are variables that describe topographic geometry, synoptic-scale winds and surface-air temperature distributions. In addition, pollutant concentration distributions also depend upon location and physical characteristics of the pollutant source. Overall fluid-flow complexity and variability from site to site have stimulated the development and use of physical modelling for determination of flow and dispersion in many wind-engineering applications. Models with length scales as small as 1:12,000 have been placed in boundary-layer wind tunnels to study flows in which forced convection by synoptic winds is of primary significance. Flows driven primarily by forces arising from temperature differences (gravitational or free convection) have been investigated by small-scale physical models placed in an isolated space (gravitational convection chamber). Similarity criteria and facilities for both forced and gravitational-convection flow studies are discussed. Forced-convection modelling is illustrated by application to dispersion of air pollutants by unstable flow near a paper mill in the state of Maryland and by stable flow over Point Arguello, California. Gravitational-convection modelling is demonstrated by a study of drainage flow and pollutant transport from a proposed mining operation in the Rocky Mountains of Colorado. Other studies in which field data are available for comparison with model data are reviewed.     

Precise temperature monitoring in boreholes: evidence for oscillatory convection? Part II: theory and interpretation

In the previous part of this work (Cermak, Safanda and Bodri, this volume p.MMM) we have described experimental data and quantified the heterogeneity features of the microtemperature time series. The spectral analysis and the local growth of the second moment technique revealed scaling structure of all observed time series generally similar and suggested the presence of two temperature forming processes. The longer-scale part can be attributed to the heat conduction in compositional and structural heterogeneous solid rocks, further affected by various local conditions. Short-scale temperature oscillations are produced by the intra-hole fluid convection due to inherent instability of water column filling the hole. Here we present how the observational evidence is supported by the results of the computer simulations. The exact modes of intra-hole convection may be different, ranging from quasi-periodic (“quiescent”) state to close of turbulence. As demonstrated by numerical modeling and referred on laboratory experiments, at higher Rayleigh numbers the periodic character of oscillation characteristic for “quiescent” regime is superseded by stochastic features. This so called “oscillatory” convection occurs due to instability within the horizontal boundary layers between the individual convectional cells. In spite of the fact that the basic convective cell motion is maintained and convection is characterized by slow motion, the oscillatory intra-hole flow and corresponding temperature patterns exhibit typical features of turbulence. The idea of boundary layer instability as a source of stochastic temperature fluctuations could explain many distinct features of borehole temperatures that previously cannot be interpreted.     

Thermal instability of the fluid column in a borehole: application to the Yaxcopoil hole (Mexico)

Observational evidence proved that even when a borehole is in “fully” stabilized conditions, temperature data may exhibit certain unrest resembling irregular oscillations in the order of hundredths or (in the extreme case) even tenths of degree. Temperature was monitored in complicated hydrogeological conditions in the Yaxcopoil-1 hole (Chicxulub impact structure, Mexico). Two experiments are reported: (a) 20-day monitoring when a logger was located in the center of the high temperature gradient anomaly produced by the cold wave slowly propagating downwards and (b) simultaneous three-loggers 18-day monitoring with loggers located above, in and below the anomaly. All observed temperature–time series displayed intermittent oscillations of temperature with sharp gradients and large fluctuations over all observed time scales. While the “upper” and “lower” records revealed quasi-periodic temperature variations, the “central” record shows fast temperature oscillations with strong up-and-down reversals, all with amplitudes up to a few tenths of degree. The observed temperature–time series were processed by recurrence and recurrence interval quantification as well as by spectral analyses. It is shown that fluid in a borehole, subject to thermal gradient, is stable, as far as the gradient remains below a certain critical value. At higher Rayleigh numbers, the periodic character of oscillations typical for “quiescent” regime is superseded by stochastic features. This “oscillatory” convection occurs due to instability of the horizontal boundary layers. In the specific case of the Yaxcopoil hole, the time series above and below the cold wave (characterized by relatively lower temperature gradients between 20 and 50 mK/m) contain a clear low frequency component produced by tidal forcing. This component dominates over the high frequency domain (periods from 10–15 to 1 min), which exhibit a scaling behavior. This pattern conspicuously changes in the center part of the cold wave, where the local temperature gradient exceeds 200 mK/m and where tidal forcing composes only ~3% of the signal.     

Flow over high roughness elements

The mean velocity and longitudinal turbulence-intensity distributions inside the zone of and above high roughness elements were investigated experimentally. This was accomplished by using a model forest canopy. The results indicate that the flow may be divided into transition and fully-developed flow regions, followed by a short adjustment distance near the downstream terminus of the rough boundary. The transition region has a strong effect on the flow characteristics within and above the layer of roughness elements. Generally, a similar qualitative variation for both velocity and turbulence was found inside and above the roughness zone, whose influence extends to more than three times the roughness height.Investigation of the modified universal logarithmic law for describing the velocity variation above the roughness zone revealed that both of the so-called similarity parameters, i.e., friction velocity and roughness length, are not local constants. On the contrary, for a given flow and local conditions they vary drastically with height. It is suspected that this is due to the fact that the classical assumption of constant shear stress throughout the boundary layer or significant portions of it is not satisfied in the case of roughness elements many times greater in height than the thickness of the viscous wall zone.     

Problems of atmospheric shear flows and their laboratory simulation

Shear flows generated by movement of the atmosphere near the earth's surface are accompanied by complexities not ordinarily encountered in the treatment of turbulent boundary layers. Problems arising from the following physical features are considered:

1. thermal stratification;
2. surface roughness in the form of forests and cities;
3. non-uniformity of surface roughness and/or temperature (leading to 3-dimensional turbulent boundary layers);
4. surface irregularities in the form of hilly and mountainous topography.

The complex nature of atmospheric shear flows has stimulated efforts to study their characteristics in the laboratory under controlled conditions. Accordingly, questions of similarity between the laboratory and the atmospheric flows for both mean and turbulent quantities arise. Similarity criteria, or appropriate scaling relationships, are discussed. Wind tunnels designed for investigations related to atmospheric shear flows are described. These facilities are shown to have a capability for simulating such flows for a wide range of the physical features listed above.     

Fractal aspects of integrated concentration fluctuations

Time series of vertically integrated concentrations (VIC) across neutrally buoyant plumes are used to study the fractal and multifractal characteristics of passive scalar fluctuations in turbulent flow fields. Here, the multifractal analysis is based on a novel definition of the singularity spectrum-F() of the time records. Approximations for quantities such as the fractal dimension and the spectral exponent are derived as functions ofF() and are compared with the experimental results. Among other things, we show that VIC records are characterized by two typical subdomains. One domain, which is related to integrated concentration fluctuations, is a subfractal process; whereas the second one, which is directly related to the concentration fluctuations, is a fractal process.     

Borehole temperatures, climate change and the pre-observational surface air temperature mean: allowance for hydraulic conditions

Joint analysis of surface air temperature series recorded at weather stations together with the inversion of the temperature-depth profiles logged in the near-by boreholes enables an estimate of the conditions existing prior to the beginning of the meteorological observation, the so-called pre-observational mean (POM) temperature.Such analysis is based on the presumption of pure diffusive conditions in the underground. However, in real cases a certain subsurface fluid movement cannot be excluded and the measured temperature logs may contain an advective component. The paper addresses the correction for the hydraulic conditions, which may have perturbed the climate signal penetrating from the surface into the underground. The method accounts for vertical conductive and vertical advective heat transport in a 1-D horizontally layered stratum and provides a simultaneous evaluation of the POM-temperature together with the estimate of the Darcy fluid velocity. The correction strategy is illustrated on a synthetic example and its use is demonstrated on temperature logs measured in four closely spaced boreholes drilled near Tachlovice (located about 15 km SW of Prague, Czech Republic). The results revealed that in a case of moderately advectively affected subsurface conditions (fluid velocities about 10⁻⁹ m/s), the difference between POM-values assessed for a pure conductive approach and for combined vertical conductive/advective approach may amount up to 0.3–0.5 K, the value comparable with the amount usually ascribed to the 20th century climate warming.