| We analyze the behavior of the maximum Thorpe displacement (dT)max and the Thorpe
scale LTat the atmospheric boundary layer (ABL), extending previous research with new
data and improving our studies related to the novel use of the Thorpe method applied to ABL.
The maximum Thorpe displacements vary between -900 m and 950 m for the different field
campaigns. The maximum Thorpe displacement is always greater under convective
conditions than under stable ones, independently of its sign. The Thorpe scale LT ranges
between 0.2 m and 680 m for the different data sets which cover different stratified mixing
conditions (turbulence shear-driven and convective regions). The Thorpe scale does not
exceed several tens of meters under stable and neutral stratification conditions related to
instantaneous density gradients. In contrast, under convective conditions, Thorpe scales are
relatively large, they exceed hundreds of meters which may be related to convective
bursts.
We analyze the relation between (dT)max and the Thorpe scale LT and we deduce that
they verify a power law. We also deduce that there is a difference in exponents of the power
laws for convective conditions and shear-driven conditions. These different power laws could
identify overturns created under different mechanisms.
References
Cuxart, J., Yagüe, C., Morales, G., Terradellas, E., Orbe, J., Calvo, J., Fernández, A.,
Soler, M., Infante, C., Buenestado, P., Espinalt, Joergensen, H., Rees, J., Vilà, J.,
Redondo, J., Cantalapiedra, I. and Conangla, L.: Stable atmospheric boundary-layer
experiment in Spain (Sables 98). A report, Boundary-Layer Meteorology, 96, 337-370,
2000.
Dillon, T. M.: Vertical Overturns: A Comparison of Thorpe and Ozmidov Length Scales,
J. Geophys. Res., 87(C12), 9601-9613, 1982.
Itsweire, E. C.: Measurements of vertical overturns in stably stratified turbulent flow,
Phys. Fluids, 27(4), 764-766, 1984.
Kitade, Y., Matsuyama, M. and Yoshida, J.: Distribution of overturn induced by internal
tides and Thorpe scale in Uchiura Bay, Journal of Oceanography, 59, 845-850,
2003.
López P., Cano J. L., Cano D. and Tijera M.: Thorpe method applied to
planetary boundary layer data, Il Nuovo Cimento, 31C(5-6), 881-892, 2008. DOI:
10.1393/ncc/i2009-10338-3.
Lorke A. and Wüest A.: Probability density of displacement and overturning
length scales under diverse stratification, J. Geophys. Res., 107 (C12), 3214-3225,
2002.
Piera, J., Roget, E. and Catalan, J.: Turbulent patch identification in microstructure
profiles: a method based on wavelet denoising and Thorpe displacement analysis, J.
Atmospheric and Oceanic Technology, 19, 1390-1402, 2002.
Piera, J.: Signal processing of microstructure profiles: integrating turbulent spatial
scales in aquatic ecological modelling, Ph. D. Thesis, Gerona University, Spain,
2004.
Smyth, W. D. and Moum, J. N.: Length scales of turbulence in stably stratified mixing
layers, Phys. Fluids., 12, 1327-1342, 2000.
Thorpe, S.A.: Turbulence and Mixing in a Scottish Loch, Philos. Trans. R. Soc. London
(Ser. A), 286(1334), 125-18, 1977. |