Orographic Precipitation and Regional Climates

Yale University, New Haven CT

Investigators

Abstract

Orographic precipitation, which depends profoundly on the interaction of airflow with underlying mountainous terrain, has received considerable attention though large and complex field programs. This project seeks to evaluate novel theoretical constructs in the context of comparatively simple terrain forcing. The main hypothesis to be explored is that orographic precipitation in the tropics may be described as "ascent-forced" convection, in which terrain plays a key role in modulating not only upstream triggering of new convective clouds, but their entire life cycle along trajectories across a given mountain barrier. While cordilleras at various latitudes will ultimately be examined, intensive observations will first be conducted by the NSF-supported University of Wyoming King Air research aircraft operating over/around the Caribbean Island of Dominica. During the DOMinica EXperiment (DOMEX) in March-April 2011, the King Air's in situ and cloud radar/lidar King Air measurement capabilities will be combined with data from ground-based weather radar and networked rain gauges to comprehensively describe clouds and precipitation that form when comparatively steady trade-wind flow impacts an isolated mountain ridge. These measurements will specify the nature of this convection, including its relationship to the existence and magnitude of upstream moisture and temperature fluctuations, and better describe cloud dynamics and microphysics in terms of entrainment, vertical velocity, and cloud water production. A second focus will be to determine the applicability of these results to other locations and climate zones, with candidate comparison sites including Patagonia, Costa Rica, and southeastern Alaska. Adjunct studies will evaluate stable isotope analysis of runoff water as a straightforward means of determining the average "Drying Ratio"--the ratio of precipitation to the total water vapor flux across a mountain ridge, known to vary from near 0% in certain tropical locations to 50% at higher latitudes. The Intellectual Merit of this work centers on development of improved means to explore and describe a newly hypothesized "ascent-forced" atmospheric convection, and to better relate its occurrence to differing climate zones and associated airmass modifications for flow trajectories across both isolated and more complex arrangements of mountain ridges. Broader Impacts of this research will include mentoring and field-site training of a postdoc and graduate/undergrad students pursuing work spanning several scientific disciplines, as well as extensive international cooperation. Ultimate impacts could include improved weather and climate modeling, isotope-climate interpretation, and water resources management.