Traditionally, 850 mb and 700 mb trajectories have been used because they
were manually calculated using data on 850 mb and 700 mb charts. In other
words, wind data were available at those levels. 1000 mb trajectories
would generally not be better, even though there are more surface
observations, because 1000 mb can be below ground, and because boundary
layer transport is usually better represented by mid-layer winds rather
than near-surface winds.
Barbara Stunder