In order to investigate flows over topography in an atmospheric contex
t, we have studied experimentally the wake structure of axi-symmetric
Gaussian obstacles towed through a linearly stratified fluid. Three di
mensionless parameters govern the flow dynamics: F, the Froude number
based on the topography height h; Re, the Reynolds number and the aspe
ct ratio r = h/L, where L is the topography horizontal scale. Two-dime
nsional (2-D), saturated lee wave (SLW) and three-dimensional (3-D) re
gimes, as defined in Chomaz et al. (1993), are found to be functions o
f F and r only (Fig. 1) as soon as Re is larger than Re-c approximate
to 2000. For F < 0.7 the flow goes around the obstacle and the motion
in the wake is quasi-two-dimensional. This 2-D layer is topped by a re
gion affected by lee wave motions with amplitude increasing with r and
F. For 0.7 < F < 1/r, the flow is entirely dominated by a lee wave of
saturated amplitude which suppresses the separation of the boundary l
ayer from the obstacle. Above the critical value 1/r, the lee wave amp
litude decreases with F and a recirculating zone appears behind the ob
stacle. Simultaneously, coherent large-scale vortices start to be shed
periodically from the wake at a Strouhal number which decreases as 1/
F until it reaches its neutral asymptotic value.