|Thomas, C; Mayer, JC; Meixner, FX; Foken, T: Analysis of the low-frequency turbulence above tall vegetation using a Doppler sodar, Boundary-Layer Meteorology, 119, 563-587 (2006), doi:10.1007/s10546-005-9038-0|
|Key words: Sodar,forest,coherent structures,wavelet|
This study applies acoustic sounding to observe coherent structures in the roughness sublayer (RSL) above tall vegetated surfaces. Data were collected on 22 days of measurements during two separate ¯eld experiments in summer 2003. A quality control scheme was developed to ensure high data quality of the collected time series. The data analysis was done using both discrete and continuous wavelet transform. The flow in the RSL was found to be a superposition of dynamic Kelvin-Helmholtz instabilities and convective mixing. The characteristic time scales for coherent structures resulting from the dynamic instabilities were observed to be approximately 20 s - 30 s while thermal eddies have much larger time scales of 190 s - 210 s. The degree of vertical coherency in the RSL increases with the flow evolving from neutral to near-convective conditions. This increase in the degree of organisation is attributed to the evolution of attached thermal eddies. The coherent structures resulting from instabilities were found to be present throughout the RSL but do not contribute to the increased vertical coherency. An alternate conceptual approach for the de¯nition of the RSL is proposed which yields its maximum vertical extent to 5 times the canopy height.