Statistics of turbulence profile at Cerro Tololo

Abstract

The results of a three-month continuous monitoring of the turbulence profile and seeing at Cerro Tololo (Chile) in 2002 May-July are presented. Some 28 000 low-resolution profiles were measured by a new MASS single-star turbulence monitor, accompanied by seeing data from DIMM. The median seeing was 0.95 arcsec. The first 500 m contribute 60 per cent to the total seeing, and the free-atmosphere median seeing was 0.55 arcsec. The free-atmosphere seeing is almost never better than 0.15 arcsec because there is always some turbulence above 12 km. A 4-d period of calm upper atmosphere with a stable free-atmosphere seeing of 0.20.3 arcsec was noted. The gain in resolution from adaptive compensation of the ground layer will be typically 1.7 times, and 2-3 times during such calm periods. Correlations of the free-atmosphere turbulence with the wind speed at the tropopause and of the ground-layer turbulence with the ground wind are studied. The temporal evolution of turbulence is characterized by recurrent bursts, their typical duration increasing from 15 min in low layers to 1-2 h in high layers. The large data base of turbulence profiles can be used to test mesoscale modelling of astronomical seeing.