Investigation of nocturnal and morning transition regimes in the El Paso area

The city of El Paso is located on the Rio Grande at the Texas, New Mexico, and Mexico borders. The regional topography is uniquely defined by the Sacramento and Gila Mountains Ranges in New Mexico and the Sierra Juarez in Mexico. These large ranges create the outer rim of a broad basin. Within this basin, the smaller Franklin Mountains and the Sierra de Cristo Rey give added complexity to the topography. The Rio Grande flows through the gap between these two smaller features, and the cities of El Paso TX, Sunland Park NM, and Juarez, Mexico are located next to each other, near this gap. The combination of the very complex topographic setting and the urban sources of pollutants make this region an area where difficult air quality issues arise.

In keeping with the emphasis on mixing processes in urban basins within the Vertical Transport and Mixing Program, we have chosen to study diurnal circulations in the El Paso region and how their variability may contribute to observed poor air quality. Under the stable conditions of the night time and early morning, flows through the gap just north of the city have been observed but are not a certainty. Also, mesoscale circulations elsewhere within the basin suggest complicated interactions between drainage flow regimes. This paper addresses this complex flow structure, using numerical simulations with the Regional Atmospheric Modeling System to understand the characteristics of circulations under stable conditions that can lead to air quality degradation.