Pressure drag and momentum fluxes due to the Alps. I: Comparison between numerical simulations and observations

Abstract

AbstractA three‐dimensional, non‐hydrostatic, anelastic model using interactive grid nesting is employed to simulate the airflow over and around the Alps during a strong foehn event on 8 November 1982. In the model, a single upstream sounding is used to initialize the mean flow. This single sounding was obtained from the output of a three‐dimensional objective analysis of rawinsonde data. The model results show that in low levels the airflow is forced around the Alps, in particular at its western edges, whereas above 700 hPa the flow is over the Alps. Above northern Italy a convergence line between the easterly flow and the foehn forced by the Apennine Mountains is simulated in accordance with observations. To the west of Milan vertical vortices are simulated in agreement with the complex circulation observed in this region.The simulations show that there is significant horizontal variability in the local vectors of the pressure drag and the momentum flux. This variability complicates the comparison between model and observational data as well as restricting one's ability to extrapolate accurately local cross‐sectional observations of pressure drag and momentum flux to values representative for the entire Alpine complex. Unfortunately, there were no measurements of surface pressure drag for the entire Alps on 8 November 1982. However, measurements during ALPEX during four foehn events obtained meridional and zonal values of −4.3 and 1.8 × 1011 N. The current values of −4.7 and 1.7 × 1011 N obtained for the simulations of the 8 November 1982 case using 5 km horizontal resolution are very close to the previous observations. There were observations on a cross‐section between Vicenza and Munich on this day. The values obtained were −0.67 × 106 N m−1 for the surface pressure drag and −0.75 × 105 N m−1 for the momentum flux averaged between 5 and 10 km above mean sea level. The simulated values for the same cross‐section were −0.62 and −0.50 × 106Nm−1 for the pressure drag and momentum flux, respectively. Overall, the averaged simulated meridional momentum flux between 5 and 10 km was −2.2 × 1011 N which is about 47% of the surface pressure drag. These results and comparisons with observations suggest that while the model appears to predict reasonable values for the surface pressure drag the amplitude of the momentum flux values is too large. The simulated values are, for example, six times those observed for the Vicenza to Munich cross‐section. Simulations using even finer resolution over a reduced region suggest that poorly simulated dissipative forces by the model may be responsible for some of this discrepancy. Some other factors such as surface friction are also discussed in the text.