Multiresolution Ensemble Forecasts of an Observed Tornadic Thunderstorm System. Part II: Storm-Scale Experiments

Abstract

In Part I, the authors used a full physics, nonhydrostatic numerical model with horizontal grid spacing of 24 km and nested grids of 6- and 3-km spacing to generate the ensemble forecasts of an observed tornadic thunderstorm complex. The principal goal was to quantify the value added by fine grid spacing, as well as the assimilation of Doppler radar data, in both probabilistic and deterministic frameworks. The present paper focuses exclusively on 3-km horizontal grid spacing ensembles and the associated impacts on the forecast quality of temporal forecast sequencing, the construction of initial perturbations, and data assimilation. As in Part I, the authors employ a modified form of the scaled lagged average forecasting technique and use Stage IV accumulated precipitation estimates for verification. The ensemble mean and spread of accumulated precipitation are found to be similar in structure, mimicking their behavior in global models. Both the assimilation of Doppler radar data and the use of shorter (1–2 versus 3–5 h) forecast lead times improve ensemble precipitation forecasts. However, even at longer lead times and in certain situations without assimilated radar data, the ensembles are able to capture storm-scale features when the associated control forecast in a deterministic framework fails to do so. This indicates the potential value added by ensembles although this single case is not sufficient for drawing general conclusions. The creation of initial perturbations using forecasts of the same grid spacing shows no significant improvement over simply extracting perturbations from forecasts made at coarser spacing and interpolating them to finer grids. However, forecast quality is somewhat dependent upon perturbation amplitude, with smaller scaling values leading to significant underdispersion. Traditional forecast skill scores show somewhat contradictory results for accumulated precipitation, with the equitable threat score most consistent with qualitative performance.

Convective-Scale Warn-on-Forecast System

“…system performance for a wide variety of convective weather events, and obtain forecaster input on system design. Recent real-time experiments and post-realtime case studies that use radar data assimilation have produced useful ensemble thunderstorm forecasts with horizontal grid spacing of 1-4 km (Kong et al 2007a(Kong et al , 2008Gao et al 2008;Xue et al 2009), suggesting that an evaluation of a 1-4 km grid spacing warn-on-forecast system can provide useful results and be used to help inform choices relating to system development. Second, research studies need to be undertaken to address the many scientific, technical, and sociological challenges that remain before a warn-on-forecast system can be implemented in operations.…”

“…Collaborative research activities between researchers and operational forecasters within the NOAA Hazardous Weather Testbed (HWT) have already begun to address some of these warn-on-forecast challenges (Kain et al 2003(Kain et al , 2006Stumpf et al 2008). 2008, the HWT experimental forecast program examined output from an experimental 10-member storm-scale ensemble forecast system (Kong et al 2007b) and evaluated the probabilistic watch guidance derived for high-impact convective weather events. In 2008, the HWT experimental warning program explored the development of probabilistic hazard information for severe weather warnings (Kuhlman et al 2008).…”

“…The concept of numerically predicting thunderstorms was proposed nearly two decades ago (Lilly 1990;Droegemeier 1997). More recent demonstrations of the utility of convectivescale numerical weather prediction (Xue et al 1996(Xue et al , 2007bDone et al 2004;Kain et al 2006;Kong et al 2007b;Smith et al 2008;Weisman et al 2008) and the continued rapid increase in affordable computational resources suggest that numerical forecasts can become an important component of convectivescale warning operations in the future. The general lifetime and gross evolution of thunderstorms are already predicted by real-time experimental convectivescale model forecasts (Fig.…”

Ensemble predictability of an isolated mountain thunderstorm in a high‐resolution model

“…Marsigli et al, 2004Marsigli et al, , 2005 and has therefore resulted in a movement towards convective-scale ensembles at O(1 km) grid spacing (e.g. Kong et al, 2006Kong et al, , 2007Hohenegger et al, 2008;Clark et al, 2010;Leoncini et al, 2010).…”

Transforming the sensing and numerical prediction of high-impact local weather through dynamic adaptation

“…Adapted from Xue et al (2007). precipitation intensity, with the composite value computed as the maximum over all altitudes within a given vertical column) at 1800 on 24 May 2007, and figure 7 shows the ensemble mean and spread of forecast reflectivity, the ensemble-derived probability of reflectivity exceeding 35 dBZ and a 'spaghetti' plot of 40 dBZ reflectivity contours, valid at 1800 on 24 May 2007. Owing to the spatially discrete nature of the convection, the magnitude of the ensemble mean is not very meaningful (Kong et al 2007b)-the ensemble mean will almost surely underestimate intensity.…”