Targeted observations in FASTEX: Adjoint-based targeting procedures and data impact experiments in IOP17 and IOP18

The Fronts and Atlantic Storm-Track EXperiment (FASTEX) provided an opportu nity for testing targeted-observing procedures in a real-time framework dur ing January and February 1997. This study describes the use of singular vec tors (SVs) for objective targeting during FASTEX, and the evaluation of the impact obtained from targeted dropsonde data, satellite wind data, and oth er observations on 1-2 day forecast skill in intensive observation periods (IOPs) 17 and 18. In IOP17, targeted dropsondes improve a 42 h forecast of L41 (Low 41; cyclo nes were numbered in sequence throughout FASTEX) in terms of sea-level pres sure, but the forecast skill is degraded in the upper troposphere. It is su ggested that the degraded forecast may be caused by an incomplete survey of the SV target area, that improved the analysis in one region, bur made the analysis less accurate in an adjacent part of the target area where no dro psonde data were provided. In a series of experiments, the best 42 h foreca st of L41 is obtained by the addition of a few radiosonde profiles provided specially for FASTEX at off-times, that provide observational data in the most sensitive part of the SV target area. The analysis differences introdu ced by the radiosonde profiles are much smaller in magnitude than those fro m the dropsonde data, but have a larger forecast impact, because they occur in an area that has larger error growth rates in this forecast. In a series of experiments for IOP18, the best 24 h forecast of L44 is obta ined using a combination of targeted-dropsonde data and satellite wind data . Both data types can also be used separately to improve this forecast. The assimilation of satellite wind data and ship-based soundings in areas of w eak initial-condition sensitivity ('null' areas) is shown to have minimal i mpact on the forecast error. The target areas identified by SVs in these tw o IOPs occur in strongly baroclinic regions, tending to favour the right-en trance and left-exit regions of the upper-level jet, but with greatest sens itivity near 600 hPa.