Collaborative Research: Understanding the Role of Landcover and Landform in the Spatial Organization of Orographic Clouds and Rainfall

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Mountains supply freshwater for over 60% of the world's population making the location and timing of orographic precipitation crucial in determining the availability and utility of water resources and ecosystem services. Montane systems also harbor Earth's most important biodiversity hotspots, and species and ecosystem responses to climate variability and change in these regions are critically dependent on cloud and precipitation regime. The objective of this proposal is to investigate whether and how landform and landcover modulate the spatial and temporal variability of orographic clouds and precipitation in tropical high mountains, a high priority area for biodiversity conservation and human water supply. The central research hypothesis is that evapotranspiration is a critical source of moisture to the atmospheric boundary layer (ABL) either locally and, or remotely via moist transport by diurnal mountain-valley circulations, lowering the cloud base at high elevations during the afternoon, and enhancing thermodynamic instability at locations in the landscape where precipitable water and CAPE (Convective Available Potential Energy) attain collocated night-time maxima. Spatial patterns in the organization of clouds and precipitation should therefore be explained by the spatial variability of vegetation and soil moisture patterns on altitudinal gradients, and by how this translates into the spatial variability of the diurnal cycle of latent heating fluxes between the land surface and the lower troposphere. To evaluate this hypothesis, the research will focus on a tropical montane cloud forest in the Central Andes in Peru, leveraging on ongoing multidisciplinary, multi-institutional ecological research by the Andes Biodiversity and Ecosystem Research Group (ABERG) that includes the University of Edinburgh and Oxford University in the UK, and FIT, Wake Forest and Duke Universities among others in the US.ABERG has installed a large array of vegetation plots and associated ecosystem function measurements centered on the Kosnipata Valley, extending from the high Andes into the Amazonian lowlands. The plot network is anchored by a series of 21 mapped, measured, and vouchered 1 ha tree plots comprising ~15,000 stems. These plots are embedded in an array of existing weather stations and data loggers ranging from 3450m to 250m. The hectare plots also form the basis of an intensive ecosystem function experiment, with plots along the main transect having dendrometers on 20% of the trees chosen in a stratified random sample. Subsets of plots are also studied for leaf, fruit, and fine root productivity, forest structure and leaf area index, and are instrumented to log soil moisture, understory and canopy light levels, rainfall, winds velocity and direction. Carbon-cycle measurements on photosynthesis, leaf, stem, and soil respiration are also taken at these plots. An intensive hydrological study plot at 3000m is also instrumented for throughfall, stemflow, and cloud interception measurements. The investigation relies on diagnostic process studies integrating satellite products and surface observations from existing and augmented networks to survey the space-time relationships between hydro-eco-geomorphologic regimes and cloudiness. High-resolution simulations (1km or less) using a coupled land-atmosphere, non-hydrostatic, cloud-resolving model will be used to investigate the physical linkages among the spatial patterns of topography, vegetation, soil moisture and the diurnal cycle of monsoon rainfall in the central Andes at the valley-ridge scale. Specifically, the following science questions will be addressed: (1) What is the contribution of evapotranspiration to the diurnal cycle of the energy budget of the lower troposphere in tropical mountainous regions? How does it vary spatially with elevation and landform (ridges versus valleys, windward versus leeward slopes, foothills versus high peaks)? (2) How does transversal (lateral) mountain variability in the spatial arrangement of landform and vegetation affect the diurnal cycle of convective activity and precipitation during the monsoon? (3) What is the relationship between the observed multi-scaling behavior of cloud fields from satellite imagery and the dominant spatial scales of convective activity associated with topography and, or land-use/land-cover patterns? (4) How can the current trends of land-use/land-cover change, and in particular deforestation and extension of agricultural activity to the highlands, change the water cycle in tropical mountainous regions? What are the consequences of these changes for the long-term sustainability of tropical mountain ecosystems and water resources? Though in the tropics, the outcomes of this work are relevant for mid-latitude mountainous regions, especially the western US where orographic precipitation (rain and snowfall) is the dominant freshwater resource. Finally, the study area is undergoing strong land-use change from anthropogenic eradication of the natural tree line, and from the construction of the inter-oceanic highway. The research findings should lead to an improved science basis for sustainable environmental planning and adaptation to climate climate variability and change in mountainous regions.