Low Level Jets in the Pantanal Wetland Nocturnal Boundary Layer – Case Studies

Abstract

Situated in South America midwest region, Pantanal is a unique biome, alternating dry and flooded periods. An important seasonal variability characteristic from Pantanal's energy balance is the occurrence of situations in which sensible heat flux is positive (bottom-up) during night-time, when the region is flooded enough. In this study it is investigated an interesting aspect of Nocturnal Boundary Layer's (NBL) structure seasonal variability above Pantanal, that is, how Low-Level Jets (<img src=image/10.5923.j.ajee.20130301.06_001.gif></img>) occurrence and associated turbulent structure. For this, scale action of different Low-Level Jets (<img src=image/10.5923.j.ajee.20130301.06_002.gif></img>) types on Pantanal's Nocturnal Boundary Layer was investigated through case studies. Six events, distributed the following way, were used: Two events without <img src=image/10.5923.j.ajee.20130301.06_003.gif></img>, two events <img src=image/10.5923.j.ajee.20130301.06_004.gif></img> weak shear and two events <img src=image/10.5923.j.ajee.20130301.06_005.gif></img> strong shear. From the two events without <img src=image/10.5923.j.ajee.20130301.06_006.gif></img>, one of them is observed during the dry season and the other, during the flooded one. The same procedure was applied to other events (<img src=image/10.5923.j.ajee.20130301.06_007.gif></img> weak shear and <img src=image/10.5923.j.ajee.20130301.06_008.gif></img> strong shear). Vertical wind velocity and temperature variance, as well as the covariance among these variables were analyzed and investigated in scale via Wavelet Transform. Remarkable differences were observed among turbulence in NBL characteristics during the “dry” and “flooded” periods. It was observed that <img src=image/10.5923.j.ajee.20130301.06_009.gif></img> weak shear acts like a forcing that generates action of top-down mechanisms in the Pantanal's Nocturnal Boundary-layer. On the other hand, <img src=image/10.5923.j.ajee.20130301.06_010.gif></img>strong shear causes eddies blocking situations with length scales greater than the <img src=image/10.5923.j.ajee.20130301.06_011.gif></img> height. The decrease of the surface roughness at the flooded season, compared to the dry season, reduces remarkably the temperature variance in the lowest length scales below 350m. Such results are useful for a better understanding of seasonal variability in the Pantanal's Nocturnal Boundary-layer, as well as to other regions with similar environmental characteristics and must be taken into consideration in numerical simulations of the flow structure above the region.