A Dynamical Initialization Scheme for Real-Time Forecasts of Tropical Cyclones Using the WRF Model*

Abstract

To improve the initial conditions of tropical cyclone (TC) forecast models, a dynamical initialization (DI) scheme using cycle runs is developed and implemented into a real-time forecast system for northwest Pacific TCs based on the Weather Research and Forecasting (WRF) Model. In this scheme, cycle runs with a 6-h window before the initial forecast time are repeatedly conducted to spin up the axisymmetric component of the TC vortex until the model TC intensity is comparable to the observed. This is followed by a 72-h forecast using the Global Forecast System (GFS) prediction as lateral boundary conditions. In the DI scheme, the spectral nudging technique is employed during each cycle run to reduce bias in the large-scale environmental field, and the relocation method is applied after the last cycle run to reduce the initial position error. To demonstrate the effectiveness of the proposed DI scheme, 69 forecast experiments with and without the DI are conducted for 13 TCs over the northwest Pacific in 2010 and 2011. The DI shows positive effects on both track and intensity forecasts of TCs, although its overall skill depends strongly on the performance of the GFS forecasts. Compared to the forecasts without the DI, on average, forecasts with the DI reduce the position and intensity errors by 10% and 30%, respectively. The results demonstrate that the proposed DI scheme improves the initial TC vortex structure and intensity and provides warm physics spinup, producing initial states consistent with the forecast model, thus achieving improved track and intensity forecasts.

Incorporating a Large‐Scale Constraint Into Radar Data Assimilation to Mitigate the Effects of Large‐Scale Bias on the Analysis and Forecast of a Squall Line Over the Yangtze‐Huaihe River Basin

“…Some of these methods or means have been used to deal with the problem of large‐scale errors in regional forecasts. For example, Cha and Wang () introduced the large‐scale SN into a dynamical initialization scheme of real‐time forecasts of tropical cyclones to reduce bias in the large‐scale environmental field. Wang, Wang et al () combined a dynamical initialization scheme with large‐scale spectral nudging to improve both the initial conditions and simulation of a tropical cyclone case.…”

ACCESS-TC: Vortex Specification, 4DVAR Initialization, Verification, and Structure Diagnostics

“…Since then, numerous storms [e.g., Ramsay et al (2009) and Otto (2012) for Larry 2006and, more recently, Yasi (2011] have caused serious property loss, with thankfully much less loss of life. The potential destructive power of these systems over a large area has been a major world-wide motivation for the development of dedicated numerical weather prediction (NWP) systems, specifically focused on TC forecasting (e.g., Kurihara et al 1995;Davidson and Weber 2000;Cha and Wang 2013). However, precise prediction of hurricane track, structure, and intensity requires realistic representation of the vortex in the initial conditions and therein lies part of the TC prediction problem.…”

A Highly Configurable Vortex Initialization Method for Tropical Cyclones

“…To this end, Nguyen and Chen (2011) introduced a dynamic initialization procedure in which the forward model was used in a series of 1-h cycles to intensify the analyzed vortex to one with a minimum surface pressure and maximum wind speed close to the observed values. To produce a more realistic TC boundary layer and an unbalanced secondary circulation in the initial condition, Cha and Wang (2013) utilized a 6-h cycle with a warm start model forecast initialization. Because of the warm start, spectral nudging is applied to the large scale (wavelengths greater than 1000 km) to minimize differences in the environment between the analysis and the dynamic initialization.…”