Evaporative Demand and Drought: Dynamics and opportunities in early warning, monitoring, and forecasting

Michael Hobbins

Tuesday, Jun 06, 2017, 2:00 pm
DSRC Room 2A305

Abstract

Despite not being an actual land-atmosphere flux and generally being poorly understood, modeled, and appreciated, atmospheric evaporative demand (E0) plays an important role in both driving and monitoring drought. For those who work in drought, this seminar aims to answer the question, “What is evaporative demand and why should I care?”



To begin, I will summarize the basic principles behind E0 and the surface assumptions that complicate its interpretation, with an emphasis on understanding the complementary and parallel interactions between E0 and actual evapotranspiration (ET) in relation to drought. While E0 is easier to estimate than ET over the time and space scales most relevant to drought monitoring and decision-making, it is also notoriously easy to estimate E0 badly. Indeed, doing so has bedeviled the drought community for decades, as I will demonstrate.



Work here at PSL is exploiting the opportunities presented by E0 as they relate to the development of a new, demand-side understanding and approach to drought at secular, operational, and climate time scales. I will summarize this work, which primarily revolves around the new Evaporative Demand Drought Index (EDDI), which has been developed in-house. EDDI has been shown to provide early warning of agricultural and hydrologic drought (as well as incipient wildfire risk); it does particularly well in spotting early the conditions that may lead to flash drought. The E0 used in EDDI further provides the opportunity for explicit attribution of the demand side of drought into its meteorological and radiative drivers. I will share some results of EDDI across CONUS and, extending this work for the Famine Early Warning System Network (FEWS NET), discuss early results relating to EDDI and the variability and attribution of E0 over Africa.



Moving beyond secular and monitoring timescales, I will discuss the opportunities presented by E0 in short-term operational forecasting and seasonal-to-subseasonal forecasting. At longer-again timescales, I will raise some conceptual and biophysical issues that must be addressed before we can successfully implement climate-scale E0 projections in drought and climate vulnerability assessments.