Estabilidade e estrutura da turbulência sob a influência de Jatos de Baixos Níveis Noturnos no sudoeste Amazônia

Abstract

Low level jets (LLJs) are wind-speed maxima in the vertical profile within the Atmosphere Boundary Layer (ABL). Such jets are common above the ABL during night time; however, how LLJs influence turbulence and stability near the surface is still widely debated. This study aims at evaluating how the occurrence of LLJs could influence turbulence structure and stability above a pasture area in the southwest Amazon. Using data collected by radiosondes and by Eddy Covariance Systems during the WetAMC-LBA campaign, (January and February 1999), LLJs were identified in 60% of all analyzed wind profiles, of which 53% were classified as weak and 15% classified as strong. The majority of cases of weak jets occurred at low heights, probably due to the reduction of friction between the surface and the atmospheric flow. On the other hand, strong jets occurred at greater heights, most likely resulting from large and meso scale effects. Based on the turbulence data, three atmosphere stability regimes to surface were defined: weakly stable; transition and very stable. In relation to the structure of turbulence, higher values of the friction velocity and of the Turbulent Kinetic Energy were showed during strong LJJs than weak LJJs. There was also a good relationship between strong jets and transition regime. However there was no a clear tendency between the occurrence of weak jets and any stability regime. In general, the results from this study suggest that the occurrence of LLJs, depending on their intensity, could generate turbulence at higher levels in the atmosphere and propagate it downward towards the surface. This could influence both stability and the structure of turbulence indicating that the theory of Shear Sheltering does not apply to the experimental site studied.