Mountain Drag along the Gotthard Section during ALPEX

Abstract

An estimate of the time variation of the pressure drag component across the inner portion of the St. Gotthard section of the Alps is derived for the special observational period (March?April 1982) of the ALPEX project. Surface pressure data obtained from a purpose-designed, mesoscale array of microbarograph stations are used, and the results indicate that this drag component amounted to a value per unit area that varied on the order of ±5 Pa with a mean ?0.78 Pa. It is shown that the phase variations of the time-trace of the drag are directly related to synoptic scale flow developments. However, the magnitude of the drag is substantially influenced by mesoscale effects. Further, an indication is given of the contribution to the drag of the effects of diurnal variation, vertically propagating buoyancy waves, and low-level flow blocking/deflection. The order of magnitude of these drag values (notwithstanding the potential errors in the estimates) indicates that the Alps exert a substantial influence upon the atmospheric momentum budget of the region.