  The location of an astronomical observatory is a key factor that affects its
scientific productivity. The best astronomical sites are generally those found
at high altitudes. Several such sites in western China and the Tibetan plateau
are presently under development for astronomy. One of these is Ali, which at
over 5000 m is one of the highest astronomical sites in the world. In order to
further investigate the astronomical potential of Ali, we have installed a
lunar scintillometer, for the primary purpose of profiling atmospheric
turbulence. This paper describes the instrument and technique, and reports
results from the first year of observations. We find that ground layer (GL)
turbulence at Ali is remarkably weak and relatively thin. The median seeing,
from turbulence in the range 11- 500 m above ground is 0.34 arcsec, with seeing
better than 0.26 arcsec occurring 25 per cent of the time. Under median
conditions, half of the GL turbulence lies below a height of 62 m. These
initial results, and the high altitude and relatively low temperatures, suggest
that Ali could prove to be an outstanding site for ground-based astronomy.
