Forecasting Tropical High-Impact Rainfall Events Using a Hybrid Statistical Dynamical Technique Based on Equatorial Waves

Abstract: Equatorially trapped waves, such as Kelvin Waves, Equatorial Rossby Waves and Westward-moving Mixed Rossby-Gravity (WMRG) Waves, play a major role in organising tropical convection on synoptic to sub-seasonal timescales. These waves have the potential to provide an important source of predictability for high impact weather in South East (SE) Asia and the tropics more widely. Global models can adequately predict the evolution of dynamical structure of equatorial waves on time-scales of several days, but they do not predict the relationship between waves and rainfall well. Therefore, hybrid statistical-dynamical forecasting techniques combining model ensemble forecasts of equatorial waves, and large-scale atmospheric conditions, with climatological rainfall statistics are compared with forecasts of rainfall probability taken directly from models over SE Asia. It is hypothesised that forecasts of wave activity may be used to more accurately predict upcoming heavy rainfall events. In tests using the Met Office Global and Regional Forecasting System (MOGREPS) and the Met Office seasonal prediction system (GLOSEA6) the hybrid forecasts outperform model rainfall forecasts in a number of regions. It is also demonstrated that combining forecasts of multiple equatorial wave types into one hybrid forecast can provide an improvement in hybrid forecast skill, relative to a forecast built on a single equatorial wave. However, errors in forecasting equatorial waves diminish the hybrid forecast's skill, with the most significant reduction observed for Kelvin waves, suggesting that a significant improvement in the prediction of the propagation of equatorial waves would have a significant impact on rainfall prediction in the tropics.

Biography: Sam Ferrett is currently a Research Scientist at the University of Reading. She is currently working on the FORecasting high-impact Weather And extreme Rainfall Drivers and dynamics for Southeast Asia (FORWARDS) project as part of WCSSP: SE Asia in partnership with the Met Office. Research interests include tropical climate and weather, uncertainty in future changes in climate and the dynamics and teleconnections of modes of variability (e.g. equatorial waves, El Nino-Southern Oscillation etc.) in the Tropical Pacific.