Evolution of Orogenic Blocking

The evolution of low-level flow upstream of the Continental Divide (Rocky Mountains) from being blocked, i.e. unable to surmount the barrier, to becoming unblocked and blocked again is studied observationally and numerically. During two months in the winter of 1991/92 a transect of three wind profilers measured the wind field every few minutes. Three -dimensional numerical simulations with the Colorado State Regional Atmospheric Modeling System (RAMS) using realistic topography supplemented the observations with details of the blocked and unblocked flow. The results confirm the theory that a mountain -induced mesoscale high above the upwind slopes causes the blocking. While previous research of idealized situations focused on changes of the cross-barrier wind and stability as determining variables to build that mesoscale high, this study found synoptic and radiative forcing to drive the blocking evolution in the atmosphere. When the low -level flow was blocked by the mountain-induced mesoscale high a opposing synoptic cross-barrier pressure gradient could negate the mesoscale gradient. Therefore unblocking happened most frequently when the trough axis of a short wave was immediately upstream of the barrier, but sometimes synoptic pressure gradients caused by contrasts in vorticity and differential temperature advection were also strong enough to overpower the mesoscale pressure gradient. The flow returned to its blocked state when the ridge behind the trough approached the barrier so that the synoptic cross-barrier pressure gradient reinforced the mesoscale gradient. For a lower barrier or stronger solar insolation a well-mixed boundary layer could grow almost to the height of the barrier by afternoon and reconnect the blocked layer with the higher cross-barrier winds above the mountain. After sunset the radiative cooling stabilizes the lower atmosphere again and the transition back to the blocked state occurred. The transition between the two states happened rapidly on the order of an hour. A blocked flow event lasted on the average one and a half days. The depth of the blocked layer even during one blocking episode fluctuated considerably but never exceeded the height of the barrier.