The Role of Land-Atmosphere Interactions During the CONUS 2012 Summertime Heat Wave

Abstract:

The United States summer of 2012 is remembered for record-breaking heat and devastating drought. This summertime heat and drought, however, was preceded by a heat wave in March—the March heat wave event saw a 3.7σ increase from 1948-2012 average temperature, while the July maximum saw a 2.5σ increase—that led to soil moisture depletion entering the summertime growing season. The July event was also notably difficult to predict and even to simulate in standard models. To explain this, some researchers have categorized the July event as a flash drought that was triggered by rapidly evolving dryness in the immediate lead-up to the event. Others have hypothesized that longer-term soil moisture memory played a role, where the March event was a large factor in the July event. Here we present results from controlled simulations with the NASA-Unified Weather Research and Forecasting Model (NU-WRF) in which we explore the roles of soil moisture memory and land-atmosphere coupling in the evolution of the July extreme, with a focus on near-surface air temperature. NU-WRF coupling to the Land Information System (LIS) is utilized to perform experiments with and without land coupling, with neutral versus observed initial conditions at the onset of the July event, and with lower boundary conditions prescribed to match observed 2012 anomalies or climatology. These simulations allow us to quantify the role of the land surface in driving high temperatures in 2012 and to identify mechanisms involved in land-atmosphere coupling during hot and dry extremes.