The role of small‐scale orographic features in the spatial distribution of precipitation

Abstract

SUMMARYRadar images from the Cévennes experiment, which took place during the autumns of 1986-88 on the southeastern ank of the Massif Central in France, reveal a characteristic and repetitive structure of the precipitation distribution. In southerly Mediterranean ows, which are frequent there in autumn, precipitation is often organized into narrow bands or plumes, oriented south-north.The period 14 and 15 November 1986 has been selected as an example on which to perform numerical simulations and analyse the mechanism yielding this speci c precipitation organization. The numerical model correctly reproduces the bands, especially at high horizontal resolution (1 km). These rainfall bands are focused on the small-scale topography, oriented east-west, composed of a succession of ridges and penetrating valleys. The bands are generated upstream of these ridges and enhanced on the lee side by convergence created by de ection around the obstacle and penetration of the ow into the valleys.Idealized simulations and sensitivity experiments have been carried out to verify these hypotheses. These simulations suggest that small-scale orography plays a decisive role in forcing the precipitation into bands, while upstream moisture conditions mainly affect the precipitation intensity and its south-north extent.

Least square support vector and multi‐linear regression for statistically downscaling general circulation model outputs to catchment streamflows

“…Geopotential heights depict the heights of the pressure surfaces of equal pressure above the mean sea level. These geopotential heights are indicators of atmospheric troughs and ridges (Cosma et al, 2002), which are related with relatively low and relatively high atmospheric pressure fields causing rainfall and relatively dry weather conditions, respectively. Therefore, various geopotential heights that showed a good relationship with streamflows were included in the sets of potential predictors.…”

TECHNICAL NOTE: First evidence of a structured and dynamic spatial pattern of rainfall within a small humid tropical catchment

“…In areas of larger relief, rainfall maxima typically occur on upwind slopes (Cosma et al, 2002) but, in areas of shallow relief, maxima can occur just downwind of obstacles (Sharon and Arazi, 1997). Monsoonal changes in the regional wind direction may, therefore, result in different sides of topographic obstacles receiving the greater rainfall at different times of the year.…”

“…Basist et al, 1994;Thielen and Gadian, 1996;Lyons and Bonell, 1994;Weston and Roy, 1994;Cosma et al, 2002). Outside the humid tropics, Goodrich et al (1995), Shah et al (1996) and Sharon and Arazi (1996) have shown that topography may also affect rainfall patterns at the very fine scale of a few 10s or 100s of metres.…”

“…The potential effects of aspect and relief on rainfall pattern because of differential heating and uplift (Thielen and Gadian, 1996;Cosma et al, 2002), were examined with further correlation analyses. Given the changes in dominant wind direction from the south-west to north-east monsoon in this part of the SE Asian region of the humid tropics (Estoque et al, 2000), seasonal changes in rainfall pattern were also examined.…”

Small‐scale variability of orographic precipitation in the Alps: Case studies and semi‐idealized numerical simulations

“…As demonstrated by Cosma et al (2002) and Anquetin et al (2003), quasistationary rain bands leading to locally very large rainfall accumulations can be triggered by fairly small mountains, because of either upstream lifting or lee-side flow convergence. In a more systematic study on orographic triggering of convection, Durran (2005a, 2005b) found that even smaller orographic features than those considered by Cosma et al (2002) can be sufficient to trigger quasi-stationary rain bands. In the presence of marginal orographic forcing (a simple ridge with some noise added), stationary rain bands are favoured by weak convective instability and strong forward shear 1702 G. ZÄNGL without significant directional turning.…”