Atlantic Hurricanes and Climate Change. Part I: Experimental Design and Isolation of Thermodynamic Effects

Abstract

he Weather Research and Forecasting (WRF) model is used in a downscaling experiment to simulate a portion of the Atlantic hurricane season both in present-day conditions and with modifications to include future thermodynamic changes.Temperature and moisture changes are derived from an ensemble of climate simulations from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) A1B scenario and added to analyzed initial and lateral boundary conditions, leaving horizontal temperature gradients and vertical wind shear unaltered. This method of downscaling excludes future changes in shear and incipient disturbances, thereby isolating the thermodynamic component of climate change and its effect on tropical cyclone (TC) activity.The North Atlantic basin is simulated with 18- and 6-km grid spacing, and a four-member physics ensemble is composed by varying microphysical and boundary layer parameterization schemes. This ensemble is used in monthly simulations during an active (2005) and inactive (2009) season, and the simulations are able to capture the change in activity between the different years. TC frequency is better reproduced with use of 6-km grid spacing and explicitly simulated convection, relative to simulations with 18-km grid spacing. A detailed comparison of present-day and future ensemble results is provided in a companion study.