Abstract
Geometric shapes of coherent structures such as ramp or cliff like signals, step changes and waves, are commonly observed in meteorological temporal series and dominate the turbulent energy and mass exchange between the atmospheric surface layer and the layers above, and also relate with low-dimensional chaotic systems. In this work a simple linear technique to extract geometrical shapes has been applied at a dataset which was obtained at a location experiencing a number of different mesoscale modes. It was found that the temperature field appears much better organized than the wind field, and that cliff-ramp structures are dominant in the temperature time series. The occurrence of structural shapes was related with the dominant flow patterns and the status of the flow field. Temperature positive cliff-ramps and ramp-cliffs appear mainly during night time and under weak flow field, while temperature step and sine structures do not show a clear preference for the period of day, flow or temperature pattern. Uniformly stable, weak flow conditions dominate across all the wind speed structures. A detailed analysis of the flow field during two case studies revealed that structural shapes might be part of larger flow structures, such as a sea-breeze front or down-slope winds. During stagnant conditions structural shapes that were associated with deceleration of the flow were observed, whilst during ventilation conditions shapes related with the acceleration of the flow.
Highlights
Geometric shapes of coherent structures such as ramp or cliff like signals, step changes and waves in meteorological temporal series, are commonly observed during atmospheric measurements
It was Taylor that as early as 1958 described “assymetrical triangular waves of temperature” in the atmospheric boundary layer and he attributed their shape to the convective plumes (Taylor, 1958)
Our results show that the step/sine ratios are close to unity (0.9 and 0.8 for wind and temperature fields respectively); ramp-cliffs to sine are lower than unity (0.7 and 0.6 for wind speed and temperature fields respectively), but the cliff-ramp to sine ratio vary significantly, being 2.2 for temperature and 0.3 for wind speed time series
Summary
Geometric shapes of coherent structures such as ramp or cliff like signals, step changes and waves in meteorological temporal series, are commonly observed during atmospheric measurements. Numerous studies have reported geometrical shapes of coherent structures over a variety of surfaces such as land (e.g., Barthlott et al, 2007), ice (e.g., Lykossov and Wamser, 1995), vegetation (e.g., Serafimovich et al, 2011), and water (e.g., Boppe et al, 1999). Ramp-like coherent structures under unstable conditions have been attributed mainly to surface-layer plumes or mixed-layer thermals (e.g., Kaimal and Businger, 1970) and superposition of shear driven and thermal structures has been reported (Thomas and Foken, 2006). Coherent structures were detected in stable flows (Kikuchi and Chiba, 1985), and they might appear without the presence of a boundary (Belušicand Mahrt, 2012)
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