Spectral Structure of Turbulence and Turbulent Fluxes in the Atmospheric Boundary Layer over a Thermally Non-Homogeneous Land Surface in Eastern Siberia
Tetsuya HIYAMA (1), Michail A. STRUNIN (2)
(1) HyARC, Nagoya University, Japan
(2) Central Aerological Observatory, Russia
Data obtained during aircraft observations in atmospheric boundary layer (ABL), developed over thermally well-marked patch with mesoscale size on a land surface, were the basis for the spectral analysis of airflow. Research was conducted over a Lena River site surrounded by forest near Yakutsk City, from April to June 2000 as part of GAME-Siberia experiments. Analysis of wavelet cospectra and spectra allowed separating fluxes and some other variables into turbulent (with the scales less than depth of ABL or approximately 2 km) and mesoscale (up to 20 km) to explore them independently ("separation approach"). Analyses showed that vertical distributions of turbulent and mesoscale portions of the fluxes (sensible heat, water vapour, momentum and carbon dioxide fluxes) and other variables (wind speed and air temperature fluctuations, coherence and degree of anisotropy) obeyed different laws. "Pure" turbulent vertical scaling profiles of all fluxes followed an almost hyperbolic decay, which differed from generally accepted similarity models for the ABL. Mesoscale vertical profiles of all fluxes and other variables clearly showed sharp inflections at the same relative height of 0.55 in the ABL. Our findings suggest that conventional similarity models for sensible heat fluxes describe both turbulent and mesoscale flows. The peculiarities of vertical profiles supported the suggestion that different kinds of mesoscale motions existed in the mixed layer. The results indicate that the motion that expanded up to the relative (with respect to the depth of the ABL) level of 0.55 could have been initiated by thermal surface heterogeneity. Entrainment processes between the upper part of the ABL and the free atmosphere may have caused the other mesoscale motion.