The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss the interaction between a tropical cyclone and the midlatitude environment, the role of latent heat release, convection and the underlying surface in ET, the structural changes due to frontogenesis, the mechanisms responsible for precipitation, and the energy budget during ET. Finally, a summary of the future directions for research into ET is given

Comparative Assessment of Various Machine Learning-Based Bias Correction Methods for Numerical Weather Prediction Model Forecasts of Extreme Air Temperatures in Urban Areas

Forecasts of maximum and minimum air temperatures are essential to mitigate the damage of extreme weather events such as heat waves and tropical nights. The Numerical Weather Prediction (NWP) model has been widely used for forecasting air temperature, but generally it has a systematic bias due to its coarse grid resolution and lack of parametrizations. This study used random forest (RF), support vector regression (SVR), artificial neural network (ANN) and a multi-model ensemble (MME) to correct the Local Data Assimilation and Prediction System (LDAPS; a local NWP model over Korea) model outputs of next-day maximum and minimum air temperatures ( Tmaxt+1 and Tmint+1) in Seoul, South Korea. A total of 14 LDAPS model forecast data, the daily maximum and minimum air temperatures of in-situ observations, and five auxiliary data were used as input variables. The results showed that the LDAPS model had an R-2 of 0.69, a bias of -0.85 degrees C and an RMSE of 2.08 degrees C for Tmaxt+1 forecast, whereas the proposed models resulted in the improvement with R-2 from 0.75 to 0.78, bias from -0.16 to -0.07 degrees C and RMSE from 1.55 to 1.66 degrees C by hindcast validation. For forecasting Tmint+1, the LDAPS model had an R-2 of 0.77, a bias of 0.51 degrees C and an RMSE of 1.43 degrees C by hindcast, while the bias correction models showed R-2 values ranging from 0.86 to 0.87, biases from -0.03 to 0.03 degrees C, and RMSEs from 0.98 to 1.02 degrees C. The MME model had better generalization performance than the three single machine learning models by hindcast validation and leave-one-station-out cross-validation

The Extratropical Transition of Tropical Cyclones. Part II: Interaction with the Midlatitude Flow, Downstream Impacts, and Implications for Predictability

The article of record as published may be found at http://dx.doi.org/10.1175/MWR-D-17-0329.1This review was partly initiated at the World Meteorological Organization’s Eighth International Workshop on Tropical Cyclones in 2014.The extratropical transition (ET) of tropical cyclones often has an important impact on the nature and predictability of the midlatitude flow. This review synthesizes the current understanding of the dynamical and physical processes that govern this impact and highlights the relationship of downstream development during ET to highimpact weather, with a focus on downstreamregions. It updates a previous review from2003 and identifies new and emerging challenges and future research needs. First, the mechanisms through which the transitioning cyclone impacts the midlatitude flow in its immediate vicinity are discussed. This ‘‘direct impact’’manifests in the formation of a jet streak and the amplification of a ridge directly downstream of the cyclone. This initial flow modification triggers or amplifies amidlatitude Rossby wave packet,which disperses the impact ofETinto downstream regions (downstream impact) and may contribute to the formation of high-impact weather. Details are provided concerning the impact of ET on forecast uncertainty in downstream regions and on the impact of observations on forecast skill. The sources and characteristics of the following key features and processes thatmay determine the manifestation of the impact of ET on the midlatitude flow are discussed: the upper-tropospheric divergent outflow, mainly associated with latent heat release in the troposphere below, and the phasing between the transitioning cyclone and the midlatitude wave pattern. Improving the representation of diabatic processes during ET in models and a climatological assessment of the ET’s impact on downstream high-impact weather are examples for future research directions.German Science Foundation (DFG)Swiss National Science Foundation (SNSF) PZ00P2_148177/1Helmholtz Association VH-NG-1243Transregional Collaborative Research Center SFB/TRR 165NSF AGS-1240502NSF AGS- 1355960NRL Base Program PE 0601153NONR PE 0602435NAustralian Research Council Centre of Excellence CE110001028NSF ATM-1461753ONR N00014091052