Modeling Isoprene Emission Response to Drought and Heatwaves Within MEGAN Using Evapotranspiration Data and by Coupling With the Community Land Model.

We introduce two new drought stress algorithms designed to simulate isoprene emission with the Model of Emissions of Gases and Aerosols from Nature (MEGAN) model. The two approaches include the representation of the impact of drought on isoprene emission with a simple empirical approach for offline MEGAN applications and a more process‐based approach for online MEGAN in Community Land Model (CLM) simulations. The two versions differ in their implementation of leaf‐temperature impacts of mild drought. For the online version of MEGAN that is coupled to CLM, the impact of drought on leaf temperature is simulated directly and the calculated leaf temperature is considered for the estimation of isoprene emission. For the offline version, we apply an empirical algorithm derived from whole‐canopy flux measurements for simulating the impact of drought ranging from mild to severe stage. In addition, the offline approach adopts the ratio (fPET) of actual evapotranspiration to potential evapotranspiration to quantify the severity of drought instead of using soil moisture. We applied the two algorithms in the CLM‐CAM‐chem (the Community Atmosphere Model with Chemistry) model to simulate the impact of drought on isoprene emission and found that drought can decrease isoprene emission globally by 11% in 2012. We further compared the formaldehyde (HCHO) vertical column density simulated by CAM‐chem to satellite HCHO observations. We found that the proposed drought algorithm can improve the match with the HCHO observations during droughts, but the performance of the drought algorithm is limited by the capacity of the model to capture the severity of drought. Plain Language Summary: Isoprene from terrestrial vegetation plays an important role in the climate system and for air quality, so it is critical to quantitatively understand the controlling processes in order to accurately represent isoprene emission in numerical models. Drought is an extreme event that is known to be important for regulating isoprene emission but is omitted in most isoprene emission models and is poorly represented in the most widely used model, the Model of Emissions of Gases and Aerosols from Nature (MEGAN). In this study, we introduced two methods to describe the relationship between drought and MEGAN isoprene emission estimates. The new drought algorithms improved the simulation of formaldehyde, a major isoprene oxidation product, predicted by a chemistry and transport model during the drought period, but the performance of the drought algorithm is limited by the land model capacity to simulate drought. Key Points: New drought stress algorithms improve capability of Model of Emissions of Gases and Aerosols from Nature isoprene emission estimatesDrought algorithms can improve the match with the HCHO observations during droughtsThe performance of the drought algorithm is limited by the capacity of the model to capture the severity of drought [ABSTRACT FROM AUTHOR]