Wednesday, 15 January 2020
As Earth warms, tools are needed to provide knowledge about our future, for building resilience and for protecting our safety and economic prosperity. Flash floods, like hurricanes, tornadoes, and heatwaves, threaten life, property, and commerce. Projecting the region-specific behavior of flood-producing storms, as they unfold in the future, could lead to dependable and cost-effective adaptation strategies. For example, knowing whether the 100-year flood of today will become a 10-year flood a few decades from now could inform the design and implementation of these strategies for many uses, such as storm water management, erosion control, and protection of infrastructure.
Such projections, however, are difficult to obtain. Flood-producing storms are typically associated with intense convection and other meteorological processes not resolved by climate models, and climate model simulations that resolve convection would be impracticably computationally-intensive and time-consuming. This study addresses these difficulties by employing convection-permitting weather model runs within a pseudo global warming (PGW) context, such that the processes inherent to flood-producing storms and their responses to climate change can be simulated with readily available computational resources.
We use a small Weather Research and Forecasting (WRF) physics ensemble, a suite of ten simulations for both current and future climate conditions that are run with varying physical parameterizations, to understand the behavior of a specific flood-producing storm and the changes that would result if it had occurred in a future, and thus warmer, environment. Our study focuses on the Raleigh, North Carolina rain event of July 16-17, 2016, which produced over 100 mm of rain in the span of a few hours. This resulted in significant damage, most notably flooding busy streets and a parking garage in a shopping mall, inundating vehicles and triggering dangerous water rescues. WRF simulations are run at a convection-permitting grid-spacing of 3km and use boundary and initial conditions from High-Resolution Rapid Refresh model data.
To assess the effects of a future, warmer environment, the PGW method is used, where initial and boundary conditions are modified to include temperature changes for the late 21st century. Temperature changes are obtained from an ensemble of 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) projections under the Representative Concentration Pathway 8.5 (RCP8.5) trajectory. We find noticeable increases in storm intensity and moisture content, and we assess whether these increases follow or exceed Clausius-Clapeyron scaling. We also investigate the sources of a small but significant increase in the speed of storm motion in the future simulations.
Such projections, however, are difficult to obtain. Flood-producing storms are typically associated with intense convection and other meteorological processes not resolved by climate models, and climate model simulations that resolve convection would be impracticably computationally-intensive and time-consuming. This study addresses these difficulties by employing convection-permitting weather model runs within a pseudo global warming (PGW) context, such that the processes inherent to flood-producing storms and their responses to climate change can be simulated with readily available computational resources.
We use a small Weather Research and Forecasting (WRF) physics ensemble, a suite of ten simulations for both current and future climate conditions that are run with varying physical parameterizations, to understand the behavior of a specific flood-producing storm and the changes that would result if it had occurred in a future, and thus warmer, environment. Our study focuses on the Raleigh, North Carolina rain event of July 16-17, 2016, which produced over 100 mm of rain in the span of a few hours. This resulted in significant damage, most notably flooding busy streets and a parking garage in a shopping mall, inundating vehicles and triggering dangerous water rescues. WRF simulations are run at a convection-permitting grid-spacing of 3km and use boundary and initial conditions from High-Resolution Rapid Refresh model data.
To assess the effects of a future, warmer environment, the PGW method is used, where initial and boundary conditions are modified to include temperature changes for the late 21st century. Temperature changes are obtained from an ensemble of 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) projections under the Representative Concentration Pathway 8.5 (RCP8.5) trajectory. We find noticeable increases in storm intensity and moisture content, and we assess whether these increases follow or exceed Clausius-Clapeyron scaling. We also investigate the sources of a small but significant increase in the speed of storm motion in the future simulations.
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