Extratropical Transition of Hurricane Irene (2011) in a Changing Climate

As tropical cyclones (TCs) move out of the tropics, they begin to interact with extratropical
weather systems, leading to a loss of their tropical characteristics. While many TCs weaken in
this stage, a subset of these systems can re-intensify and induce torrential precipitation, very
large waves, and hurricane-force winds in areas rarely affected by TCs.
Although the numerous studies have been devoted to the extratropical transition (ET) process
and climate warming effects on tropical cyclones, relatively less attention has been paid to
the question of how climate change will affect the characteristics of TCs undergoing ET. Here, to
examine how the characteristics of extratropically transitioning TCs will be affected by climate
change, a series of simulations are conducted for Hurricane Irene, which affected East Coast of
the U.S. in late August 2011. Irene was absorbed by a frontal system on August 30 after
transitioning to an extratropical cyclone. A set of numerical simulations consisting of 10
members with varying microphysics, cumulus parameterization, boundary layer physics, and
initial time is conducted using the Weather Research and Forecasting (WRF) model. To assess
the effects of climate change, a pseudo-global warming (PGW) method is used in which
thermodynamic changes between end of 21st century and 20th century, derived from an
ensemble of 20 CMIP5 GCMs under RCP 8.5 scenario, are applied to initial and lateral boundary
conditions.

Preliminary results demonstrate that area-and- time averaged 3-hourly storm-relative precipitation
increase during ET are found to ~16%. Given an approximate 4.3K increase in area-and- time
averaged storm-relative 850 hPa temperature, the atmospheric water vapor is expected to
increase to ~30% in accordance with the Clausius-Clapeyron relation. The rate of increase in
storm-relative precipitation is far below the Clausius-Clapeyron driven water vapor increase
during ET in the future. However, projected changes in area-and-time averaged storm-relative
precipitation based on Clausius-Clapeyron relation does not account for changes in temperature
within Irene due to shift in its track. To isolate the changes due to thermodynamic changes by
eliminating changes in land interaction between present and future simulated Irene, pseudo-
idealized experiments with no orography and entirely covered by ocean are utilized. The results
of the land effect on precipitation changes are presented. In addition, enhanced latent heat release
in warmer climate is found responsible for an increase in mean sea level pressure and wind speed
of Irene. Taking the mitigating effect on future TCs intensity in the tropics into consideration,
robust recurving TCs, such as Hurricane Irene will show a greater intensity increase in a warmer
climate compared to TCs remaining in the tropics. This hypothesis enables us to quantify the
strengthening of future recurving TCs by comparing the rate of increase in mean sea level
pressure between Irene and previously investigated future strong storms in the deep tropics (e.g.,
Hill and Lackmann 2011; Knutson et al. 2010). These analyses allow determination of changes
in the characteristics of Hurricane Irene undergoing ET in response to climate change.