The foehn wind east of the Andes in a 20-year climate simulation

Abstract

This study investigates the spatial structure and the seasonal occurrence of foehn wind to the east of the Andes using a flow blocking analysis in a 20-year climate simulation. The latter was performed by the Eta-CPTEC regional model at 50-km horizontal resolution. This version of the model includes a cut-cell scheme to represent topography and a finite-volume vertical advection scheme for dynamic variables. The results indicate that foehn wind more frequently blows during winter and spring on the eastern slopes of the Andes, except to the south of 37° S where it blows at all seasons. Higher mountains of the Central Andes (27° S–35° S) and the High Plateau (15° S–27° S) result in blocked foehn events, with a weak adjustment to the geostrophic balance. On the Central Andes, rain and snow on mountain tops may also contribute to generate foehn wind on the eastern slopes. The results show that a low pressure develops to the east of the Central Andes, and also to the east of the High Plateau when foehn blows. Lower mountains in Patagonia (to the south of 37° S) result in more frequent non-blocked foehn event, with better adjustment to the geostrophic balance.

Appendix A

Non-dimensional mountain height and blocking diagnosis.

The parameter h as defined by Smith (1988), also known as the inverse Froude number, represents the non-dimensional mountain height. It depends on the mountain height, the wind velocity, and the static stability of the incident flow, as follows:

$$h=\frac{N{h}_{0}}{U},$$

where N is the Brunt–Väisälä frequency, h₀ is the relative height of the mountain crest defined as Z_crest − Z₀, where Z_crest is the altitude of the mountain crest and Z₀ the altitude of the 1000 hPa pressure level, U is the velocity of the wind perpendicular to the mountain at the crest level.

The bulk method of Reinecke and Durran (2008) is applied to calculate N, as follows:

$$N=\sqrt{\frac{g}{{\theta }_{0}}\frac{{\theta }_{{h}_{0}}-{\theta }_{0}}{{h}_{0}}},$$

where g = 9.8 m s^–2 is the gravity acceleration, ${\theta }_{0}$ is the air temperature at the 1000 hPa level upwind of the mountain, and ${\theta }_{{h}_{0}}=T{\left(\frac{1000}{P}\right)}^{\frac{R}{{c}_{p}}}$ is the potential temperature at the crest level upwind of the mountain, where T is the air temperature at the crest level upwind of the mountain, p is the air pressure at the crest level, R = 287 J kg^–1 K^–1 is the gas constant for dry air and c_p = 1004 J kg^–1 K^–1 is the specific heat at constant pressure for dry air.

The theory presented by Smith (1988) estimates conditions under which orographic blocking occurs based on the linear theory for air flow over an isolated mountain. The following conditions are obtained:

0 < h < ½	Flux overpasses the mountain
h = ½	Streamlines collapse on the lee slope
½ < h < 1	Region of collapsed streamlines spreads to cover more of the lee slope
h = 1	Collapsed region reaches the mountain crest
1 < h	Windward blocking occurs. Flow may overpass aloft