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

Abstract

The 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. FIG. 10. As above, but for 850-200 hPa vertical wind shear (m s 21 ). Vertical shear from the 18 GFS-FNL analysis is averaged over an area corresponding to the 6-km domain and shown in black dots every 24 h.

North Atlantic Storm-Track Sensitivity to Warming Increases with Model Resolution

“…The Best7 models were those found to be the best at reproducing cyclone track density and central pressure distributions over the U.S. East Coast and western and central Atlantic. This pseudo global warming technique has been used previously (Schär et al 1996;Kimura and Kitoh 2007;Sato et al 2007;Mallard et al 2013) and allows for a controlled sensitivity experiment that guarantees present-day synoptic variability at the boundaries. These spatially and temporally varying changes are added to the FNL and RTG SST fields to make ''future'' initial and boundary conditions.…”

Simulated Sensitivity of Tropical Cyclone Size and Structure to the Atmospheric Temperature Profile

“…The model has been adapted for use with idealized initial conditions to permit careful control of the initial-value experiments that enables us to examine the physical mechanisms associated with TC structural changes. WRF has been used by many previous sensitivity studies that explore mechanisms concerning TC size and intensity changes (e.g., Frank and Ritchie 2001;HL09;Fang and Zhang 2012;Mallard et al 2013). …”

Sensitivity of Tropical Cyclones to Parameterized Convection in the NASA GEOS-5 Model

“…This model response is consistent with Emanuel et al (2008) and Stan (2012), who showed a sensitivity of TC activity to changes in the cumulus parameterization in the Community Climate System Model (CCSM; Gent et al 2011 Noda et al (2012) and Putman and Suarez (2011) has been shown to be beneficial for explicitly simulating convection and capturing such extremely strong hurricanes. Mallard et al (2013) explored the resolution dependence of TC simulations by comparing runs with 6-km and 18-km grid spacing, using the Weather Research and Forecasting Model (WRF). That study found that TC frequency and the detailed spatial structure of strong TC such as smallerscale maxima embedded in the eyewall, greater spiral band activity, and storm asymmetry were remarkably improved with the 6-km grid spacing.…”