CAREER: The onset of the rainy season in Amazonia

Brown University, Providence RI

Investigators

Abstract

The Amazon basin has been called the "Green Ocean", a term inspired by the emerald expanse of its dense forest canopy. In the rainy season the analogy has physical significance as the rain falls primarily from mid-depth cumulus clouds which resemble those commonly found over tropical oceans. Also, the forested surface provides an abundant source of moisture to sustain the clouds, playing the same role as the ocean surface in maritime convection. But prior work by the PI shows that at the end of the dry season the analogy breaks down and clouds look more like the cumulus towers that form over dry grasslands. The surface also plays a different role, as the deep clouds are triggered by high values of convective available potential energy (CAPE), and the surface serves primarily as a heat source to build CAPE rather than a moisture source. Recognizing the differing roles of moist and dry land surface, the principal investigator (PI) considers the role played by plant physiology in cloud formation during the pre-onset season (also referred to as the transition season). Water stress is common at the end of the dry season and vegetation under water stress retains moisture, causing the surface to heat up and promoting higher CAPE. On the other hand clouds need a source of water vapor, which can be supplied by plant transpiration in adjacent regions where the soil is still moist. Thus the PI hypothesizes that rising motion induced by surface heating over water stressed regions draws in moisture from regions of low water stress (where plants transpire moisture into the air), so that the gradient between high and low plant water stress plays a key role in promoting convection at the onset of the rainy season. The research uses observational data from sources including satellite remote sensing and meteorological reanalysis products, together with numerical simulations, to cover the range of phenomena and spatial scales linking plant physiology with clouds and precipitation. One key data source is satellite-derived Solar Induced Fluorescence (SIF), a subtle and wavelength-specific glow emitted by chlorophyll in plants exposed to sunlight. Previous studies have shown that SIF declines when plants are under water stress, thus SIF from satellites can be used to map the spatial gradient of water stress over the Amazon. The educational component of this CAREER award focuses on outreach to K-12 and college students from underserved and underrepresented populations. At the college level the PI works with the Leadership Alliance Summer Research Early Identification Program to provide summer internships for undergraduates from minority serving institutions. K-12 education is addressed in collaboration with the Providence Public Schools (PPS), which serve a high percentage of Hispanic and African American students. The PI's team works with PPS officials to organize summer workshops for elementary and middle school teachers to assist them in creating and implementing lesson modules on weather, climate and ecosystem interactions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.