| The microclimate of the surface layer can be strongly affected by turbulent flux divergence. This effect can be enhanced at temporal or spatial transitions.; In chapter 2, the early evening transition (EET) is investigated. This is the period of turbulence decay, when mixing activity gets constrained to a shallow stable layer. Data from 26 stations located in an area of 30 x 30 km are used. Mean variables are subject to appreciable changes: a jump in specific humidity and a temporal inflection point in temperature. The horizontal variability of these variables increases at the period. A large eddy simulation is made, confirming that turbulent flux convergence is the dominant process at the period.; In chapter 3, we study intermittent mixing in the core of the night in cold nights. Turbulent activity happens only in sporadic events. Higher, open locations remain turbulent, and their temperature can be used as an estimation of the temperature at a higher level (Hc) in all network. It allows the estimation of surface fluxes from a bulk parameterization. Sensible and latent heat flux are restricted mostly to high, open places, with an important effect on the spatially averaged fluxes.; In chapter 4, a large eddy model is used to simulate the effects of obstacles and gaps on mean variables and surface fluxes. Simulations are used to determine the wind reduction at a location from the information on distance and height of the obstacle. The presence of gaps in forest canopies induces circulations similar to that of flow over a backward facing step. Scalar fluxes around the gap can be as much as 3 times larger than the domain averages. The gap alters the wind and flux profiles at its location and downwind of it.; In chapter 5, the vernal transition in eastern United States is studied. A method is presented to determine the arrival of spring from simple climatological records. The position of the vernal front is determined in good agreement to phenological data. The method is used to estimate the evolution of spring arrival along the 20th century. |
