A numerical study to investigate the relationship between moisture convergence patterns and orography in central Mexico

This study examines small-scale orographic effects on atmospheric moisture convergence at the ridge valley scale in the Grande de Santiago River basin in central Mexico during a major monsoon storm on 13-14 August 1999. The simulation was performed using a coupled land-cloud resolving model on three nested grids (at 12-, 3-, and 1-km resolutions). The specific objective is to investigate the physical mechanisms that explain the regional space-time organization of orographic precipitation and cloudiness identified in the region from satellite data. The overarching goals of the research were 1) to characterize the effects of landform and topography-flow geometry relationships on the spatial distribution of precipitation and clouds, especially with regard to the role of mountain winds and lateral drainage flows, and 2) to assess the influence of land-atmosphere interactions (specifically latent and sensible heat fluxes) on moisture convergence patterns during monsoon storms. The model results indicate that large-scale moisture convergence dominates the distribution of total water in the troposphere during monsoon storms, which is modulated by topographically induced gravity waves and thermodynamic gradients associated with the land-sea contrast in the coastal zone. In the Grande de Santiago River basin, mountain-plain differences in thermodynamic response control mesoscale moisture convergence patterns leading to nocturnal buildup in the valleys. At the ridge-valley scale, strong convergence and strong winds (up to 15 m s(-1)) occur on the lee side of ridges oriented perpendicularly to the impinging synoptic flow with the development of transient flow conditions (from supercritical to subcritical) in the valley, independently of the time of day. In turn, this localized hydraulic-jump-like circulation drives strong return winds (i.e., cold outflows in the downvalley direction) that converge in the central lowlands preceding nighttime rainfall, lifting warm moist air at the mouth of the valley, thus initiating nocturnal convection. Simulated moisture convergence patterns are clustered along the ridges and against the foot slopes at the outlet of the north-south oriented catchments consistent with the space-time distribution of satellite observations of precipitation and clouds in the region overall, and with precipitation features detected during the simulated event in particular.