Numerical Modeling of a Heavy Rainfall Event in Present and Future Climates

Gradual flooding is a major cause of weather-related destruction. Although we have a reasonable understanding of what types of rainfall amounts and rates can cause gradual flooding in today’s climate, we know little about what to expect in future climate scenarios. While global climate models (GCMs) can be used to obtain large-scale estimates of future weather tendencies, in order to focus on how individual heavy rainfall events might behave in a warmer climate, one needs to downscale the GCM output in some way. This paper will describe our method of downscaling--start with a flooding event, simulate it using the Weather Research and Forecasting (WRF) model in the present climate, then rerun the case using WRF after altering the initial and boundary conditions based on GCM projections.

We simulate a heavy rainfall case on 8-10 October 2011 over the Kennedy Space Center (KSC) region of central Florida. This case featured a persistent pattern of sea breeze-related moist convection, followed by a “no-named” weak landfalling tropical system. Three-day regional rainfall totals ranged from 305 to 432 mm (12–17 in.), and produced flooding throughout the KSC area. Our WRF simulation reproduced the onshore rainfall amounts and patterns with a high degree of fidelity.

We next simulate the same heavy rainfall case in a future climate with the same WRF model options as our current climate simulation. However, we modify the model’s initial conditions by adding spatially varying temperature anomalies for the year 2100, based on IPCC (2013) data for two representative concentration pathways (RCPs): RCP4.5 (intermediate warming case), and RCP8.5 (strong warming case). We rerun the WRF model for the year 2100 using both RCP scenarios to see how rainfall patterns and amounts are affected. Statistical analyses are then performed to compare the rainfall patterns and totals from the current climate and future climate simulations.

Reference

IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp. doi:10.1017/CBO9781107415324.