Ensemble-based targeting experiments during FASTEX: The effect of dropsonde data from the Lear jet

In this study we evaluate the performance of the Ensemble Transform (ET) te chnique, which is one of several targeting methods used in real time during the Fronts and Atlantic Storm-Track EXperiment (FASTEX). 'Targeted' observ ations were taken adaptively in those upstream areas identified in real tim e as most relevant for improving the initial conditions for forecasts of sy noptic-scale storms developing downstream. The upstream areas were identifi ed as regions where the effect of extra observations at a future analysis t ime could produce the largest decrease in the largest likely forecast error at a preselected later verification time at a given downstream location. T he ET technique selects these observational areas out of a large number of possible deployment locations of observational resources via a linear trans formation of an ensemble of forecasts. The analysis and forecast effects of special targeted observations associat ed with seven Intensive Observing Periods (IOPs) during FASTEX were investi gated. The most important result of the present study is that the ET techni que, based on the National Centers for Environmental Prediction (NCEP) oper ational global ensemble, was able to identify upstream areas that had signi ficant contribution to the quality of selected future downstream forecast f eatures. Moreover, the technique could reliably distinguish between the are as of greatest contribution. Through the overall impact of the targeted data on forecast quality is posi tive, there were cases when the extra data;degraded the forecasts. Our anal ysis indicates that large amplification of ensemble perturbations from the targeted area into the verification area is a good indicator of potential f orecast improvement.