A Numerical Study to Investigate the Relationship between Moisture Convergence Patterns and Orography in Central Mexico*

Abstract

This study examines small-scale orographic effects on atmospheric moisture convergence at the ridgevalley 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 Ϫ1 ) 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.

An Object-Based Approach for Quantification of GCM Biases of the Simulation of Orographic Precipitation. Part I: Idealized Simulations

“…6c (on the mountain peak) is where the small-scale precipitation is observed. Previous studies also showed a strong relationship between surface MFC and the precipitation (Becker et al 2011;Bhushan and Barros 2007;Holman and Vavrus 2012). This convergence can have multiple reasons, such as moisture transport from the moist air impinging on the mountain and local evaporation over the mountain peak where precipitation is observed.…”

Orographic effects on precipitating clouds

“… Frame and Markowski [2006] found that the cold pool of a squall line system passing over a relatively low mountain range can undergo this effect, producing a leeward side regeneration of the squall line at the location of the hydraulic jump (Figure 11). Studying conditions in the Sierra Madre in Mexico, Bhushan and Barros [2007] showed how a hydraulic jump associated with synoptic‐scale flow over a region of larger mountains can produce a line of moisture convergence and trigger convection in a valley.…”

The nonconvective/convective structural transition in stochastic scaling of atmospheric fields

“…The geometry and orientation of topographic features dynamically modulates synoptic scale systems by a broad variety of physical mechanisms, including orographic uplift, blocking, differential heating, moisture convergence, convective triggering, and gravity waves (see, e.g., Barros and Lettenmaier [], Roe [], Smith [], and Houze [], for review). Stationary orographic forcing has been shown to dominate the hydroclimatology over massive mountainous regions at climate scales and can give rise to some of the most extreme rainfall intensities and gradients at weather time scales [e.g., Barros and Lettenmaier , ; Lang and Barros , ; Barros and Lang , ; Garreaud et al ., ; Barros et al ., , ; Roe , ; Zipser et al ., 2006; Bhushan and Barros , ; Giovannettone and Barros , ; Romatschke and Houze , ; Rasmussen and Houze , ; Houze , ]. The resulting moisture fields are modified by numerous nonlinear interactions giving rise to complex orographic cloud and rainfall spatial patterns with variability over a wide scale spectrum [ Harris et al ., ; Barros et al ., , ; Nykanen , ; Nogueira et al ., ].…”