Estimating Topographic Blocking Using a Froude Number When the Static Stability Is Nonuniform

Abstract

A parameter widely used to predict topographic flow blocking is the nondimensional mountain height or, synonymously, the inverse Froude number. Predictions using this parameter are based on the morphology of flows with uniform upstream static stability and wind speed, which rarely occur in the real world. The appropriateness of applying this theory in the presence of nontrivial background stability is therefore investigated using a numerical model. Two methods were considered to estimate the low-level stability, averaging the Brunt-Väisälä frequency below the crest and using the bulk change in between the ground and crest level.No single best method emerged for estimating the upstream static stability and thereby mapping the simulations with inversions onto the set of solutions with constant stratification. Instead, the best method depended on the application at hand. To predict the onset of flow stagnation, averaging the low-level stability worked best, while to predict low-level flow diversion the bulk estimate of low-level stability was most appropriate. These results are consistent across a range of inversion thicknesses and strengths. In addition, it is shown that variations in static stability above the mountain crest have little impact on flow blocking.

Dynamics of Orographic Gravity Waves Observed in the Mesosphere over the Auckland Islands during the Deep Propagating Gravity Wave Experiment (DEEPWAVE)

“…1) yielded Fr ; 4 6 1 throughout the RF23 period. Modified formulas that account for low-level vertical gradients (Reinecke and Durran 2008) produced similar findings. Since the surface forcing environment is linear, we computed Fourier wave field solutions from z 5 0 to 100 km for a range of propagation cutoff times t c using upstream wind, height, stability, and density profiles from the NAVGEM reanalysis from 0400 to 1200 UTC (see Fig.…”

Influence of the wind profile on the initiation of convection in mountainous terrain

“…Table I), thus the flow is nearly neutrally stratified and the estimation of N as well as Fr is critical and different techniques can be applied (e.g. Reinecke and Durran, 2008).…”

Disentangling the Forcing Mechanisms of a Heavy Precipitation Event along the Alpine South Side Using Potential Vorticity Inversion

“…Reinecke and Durran (2008) investigated different methods to characterize nonuniform flows using the inverse Froude number. We follow their approach and calculate the inverse Froude number based on the bulk value N 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (g/u 00 )(u H 0 À u 00 )/H 0 q where u 00 is the reference potential temperature at the ground and u H 0 the potential temperature at H 0 .…”

“…We follow their approach and calculate the inverse Froude number based on the bulk value N 5 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (g/u 00 )(u H 0 À u 00 )/H 0 q where u 00 is the reference potential temperature at the ground and u H 0 the potential temperature at H 0 . Following Reinecke and Durran (2008), the bulk method is the better predictor of the low-level flow diversion. The U and N values are evaluated along a cross section across the Alps, parallel to the wind field at 850 hPa.…”