92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012: 11:15 AM
Accuracy Issues of Temperature Measurements for CT2óDetermination Using Sonic Anemometer-Thermometers
Room 239 (New Orleans Convention Center )
Ulrich Weisensee, Deutscher Wetterdienst, Tauche OT Lindenberg, Germany; and J. P. Leps

As an alternative to the fast direct fluctuation measurements for deriving fluxes of energy and matter using eddy-covariance techniques, structure parameters are of special interest in order to derive fluxes from remote sensing measurements, e.g., from scintillometers. Structure parameter derivation is usually based on a special analysis of the high-resolution time series of the variables of interest, which means either the calculation of the structure function or spectral analysis. In many cases both methods are applied to get an additional impression on the statistical characteristics of the data and to decide whether or not all basic assumptions are fulfilled. In contrary to the direct flux derivation from the statistical behaviour of a time series, which can be considered as relatively robust, the spectral signal analysis for deriving structure parameters tends to be more sensitive with respect to different conditions. Not least the characteristics of the measurement device itself can influence the process of signal analysis remarkably. This shall be discussed on the example of temperature structure parameter ( CT2 ) derivation.

Fast temperature fluctuation measurements for the determination of CT2 are usually performed by fine wire sensors (resistance wire or thermocouple) or they are based on the virtual temperature measurements of sonic anemometers. The former are mostly used in short time scientific experimental setup, the latter in more long term operational measurement programs. Both assemblies have been working reliably and became more and more user- friendly over the past decades such that accurate and adequate results are often taken as granted. Unfortunately, the instrumental characteristics and configuration of just these devices can have a significant influence on the determined spectra and the derived structure parameters under certain conditions. Especially more or less undocumented device-internal algorithms, unnoticed changes in technical specifications and numerical aspects may lead to unexpected results.

Based on the LITFASS-2009 data and on laboratory investigations, the influence of resolution and accuracy of temperature measurements on the magnitude, the quality and the availability of derived CT2 values shall be illustrated by some typical examples. Based on this, consequences for experimental design and setup, device selection and configuration, analysis algorithms and data interpretation shall be discussed briefly.

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