The atmospheric turbulence at the Alcantara Space Center, Brazil

The characteristics of the atmospheric turbulence were analyzed for a data set collected at the Alcantara Space Center (ASC), Brazil. This space facility is the main Brazilian space port, it is located close to the Equator (2˚ S) and the Brazilian rockets family (e.g. VLS – Satellite Vehicle Launcher) is launched from this facility. The surface winds are very strong due to the coupling between the trade winds and the sea breeze: the maximum instantaneous winds ranging from 8-9 m/s for wet period and from 9-11 m/s for the dry season. The data were collected with a sonic anemometer 3D (Campbell Scientific CSAT3) with sample rate of 20 Hz at a height of 9 m during the dry season (august 1999) and wet period (April-may 2003). The data were split in 4 different stability conditions associated with the time: unstable period PI (from 12 until 13 Local Time - LT), stable PE (00 till 01 LT), transition from stable to unstable PEI (5:30 till 6:30 LT) and transition from unstable to stable PIE (17:30 till 18:30 LT) and for each stability conditions a sub-set of 100 samples were used (with 10 min each summing 12000 instantaneous measurements) in order to determine the atmospheric turbulence characteristics. The windspeed was more intense during the dry period (ranging from 4.5 to 5.5 m/s) than during the wet period (ranging from 3.9 to 4.9 m/s). For both cases there was a small diurnal cycle with more intensive winds for the subgroups PI e PEI. This does not modify the turbulent intensity which was 0.22 (dimensionless) independent of the stability conditions and/or season. The gust factor ranged from 1.82 (dimensionless) and 1.83 for the dry period, reducing to 1.69 up to 1.77 during the wet period. The highest variation of the atmospheric turbulence was held during the dry season. There is no diurnal variation among the 4 different groups and this is due to the high mechanical mixing of the surface layer. The amplitude of the gusts was also computed for the dataset available and it varies from -0.7 to + 0.7 m/s during the dry season and from -0.48 to +0.45 m/s for the wet period. The normalized standard deviation for zonal (σu/u*) and for vertical (σw/u*) winds presented values of 2.3 (dimensionless) and 1.4 (dimensionless), respectively.