Simulating the Effects of Hydroclimate Stress on BVOC-Chemistry-Climate Interactions in NASA GISS ModelE

Drought is a hydroclimatic extreme that causes perturbations to the terrestrial biosphere which causes biosphere-atmosphere feedbacks. Drought acts as a stressor on vegetation, which causes a unique biogenic volatile organic carbon (BVOC) emission response during severe or prolonged drought. In the first chapter, we introduce the concept of drought and the four main types: meteorological, agricultural, hydrological, and socioeconomic. Then we expand upon how drought affects vegetation on the physiological level and how drought stress impacts vegetative isoprene emissions and their impact on atmospheric composition and climate, as isoprene is the dominantly emitted BVOC and is a precursor to ozone (O3) and secondary organic aerosols (SOA). In the second chapter, the implementation of isoprene drought stress into the Model of Emissions and Gases and Aerosols from Nature (MEGAN) module in the NASA GISS ModelE, an earth system model, is described along with validation of isoprene emissions during the 2012 drought at the Missouri Ozarks Ameriflux site. In the third chapter, the development of a model tuned isoprene drought stress parameterization is described along with validation of O3, PM2.5 and HCHO tropospheric columns compared to observations during drought periods. We found the isoprene drought stress parameterization reduced the normalized mean bias in tropospheric HCHO column, a proxy for isoprene emissions, by ~ 19.3% in the southeast U.S. during the 2007 drought and ~ 15.3% during the 2011 drought. In the fourth chapter, the ModelE tuned isoprene drought stress ( ) parameterization is applied in free-running experiments to simulate the historic 1950s decadal drought that occurred from 1948-1957 and in present-day experiments from 2003-2013. It was found the inclusion of isoprene drought stress in both the historic and present-day experiments reduced the averaged precipitation in the warm season (APR-SEP) and reduced the frequency of “rainy-days”. With decreasing isoprene emissions and decreasing SOA during drought, its theorized a biosphere- atmosphere feedback is causing reduced precipitation during the warm season through BVOC- drought-aerosol-climate interactions. We speculate that this feedback during our simulations is exacerbating the already existing precipitation deficit during drought periods.