Precipitation is a fundamental driver of erosional processes and strongly influences topographic evolution. Topography affects the spatial distribution of precipitation in mountain ranges such that precipitation patterns and topography form a coupled system and evolve together. The Western Ghats of India provide a natural laboratory for interdisciplinary study of the interactions between precipitation and topography. A steep and high escarpment stretching for more than 1500 kilometers has persisted over tens of millions of years. The South Asian Summer Monsoon produces heavy precipitation across the region that but does not penetrate into narrow canyons in the Ghats Breaches. The relationship between topography and precipitation patterns will be better constrained with existing TRMM (Total Rainfall Measuring Mission) satellite precipitation radar data and a new in-situ precipitation gauge network. A mesoscale atmospheric model, the Weather Research and Forecasting (WRF) model, will be used to investigate airflow, cloud, and precipitation processes through case studies and idealized simulations. This model will establish the dominant precipitation processes and clarify the dynamics that govern the observed relationships between precipitation patterns and topography. The relationship between precipitation patterns and erosion will be examined by comparing suspended sediment yield and reservoir sedimentation rates with precipitation patterns. The long-term relationship between climate and topography will be investigated with a numerical landscape evolution model. It is expected that the decrease in precipitation toward the headwaters of narrow canyons will limit incision and favor a transition toward broad embayments. This mechanism has the potential to heal irregularities in the escarpment front and help to maintain steep topography over very long time scales.
This project asks two fundamental questions: how does precipitation influence the evolution of topography and what feedback mechanisms exist? These questions are inherently interdisciplinary, and the idea that atmospheric and geologic processes are intimately linked is an important research area in the earth sciences. The relationship between rainfall patterns and mountain ranges is particularly important, since these may affect how climate changes in the future. The use of satellite measurements, numerical models, and field measurements will provide a quantitative assessment of the interaction among precipitation patterns, erosion rates, and topography. This approach allows for examination of both the effects of topography on local climate and the impact of climate on physical landscapes down to the scale of individual river basins. Such knowledge will be useful in land-use planning and in protection against natural hazards. Collaboration with Indian scientists ensures that the knowledge gained will be available for application to development and conservation planning in the region. Additionally, graduate and undergraduate students in India and the United States will participate in the research.
Intellectual Merit: We established a high-resolution rain gauge network across a region of complex topography over three monsoon seasons. Rain-gauge data were compared with satellite precipitation radar to evaluate radar performance and compare short and long timescales. We find large precipitation gradients associated with topography and demonstrate the reliability of satellite precipitation radar in this region. Mesoscale atmospheric modeling has isolated key parameters influencing precipitation distribution including the atmospheric stability and topographic slope. A coupled model of precipitation and landscape evolution was developed and highlights the importance of a relatively low-elevation precipitation maximum in driving landscape evolution toward steep and long-lived escarpment forms. Variability in precipitation at small spatial scales is partially driven by topography and exerts control on long-term topographic evolution. Broader Impact: One Ph.D. student and three M.S. students completed their degrees while working on this project. Two undergraduates also participated, completing senior theses and presenting at confer-ences. Outreach toward people in the region studied included a lecture series at Goa University and demonstrations for hundreds of children at 10 primary schools in rural India. This project was the first NSF grant to PI-Anders, who is female. Publications/data: 1) Nesbitt, S.W., and Anders, A.M., 2009, Very high resolution precipitation climatologies from the Tropical Rainfall Measuring Mission precipitation radar, Geophysical Research Letters, 36, 15, 2)Colberg, J.S., and Anders, A.M., in review, Numerical Modeling of Spatially-variable Precipitation and Passive Margin Escarpment Evolution, Geomorphology 3)Precipitation radar data and gauge data available at: www.atmos.uiuc.edu/precip
