The Impact of NORPEX Targeted Dropsondes on the Analysis and 2–3-Day Forecasts of a Landfalling Pacific Winter Storm Using NCEP 3DVAR and 4DVAR Systems

Abstract

During the North Pacific Experiment (NORPEX), both the Navy Operational Global Atmospheric Prediction System and the National Centers for Environmental Prediction (NCEP) operational forecast systems found a 48-h forecast degradation over the NORPEX forecast verification region due to the inclusion of a set of NORPEX targeted dropsondes deployed north of Hawaii during 29?30 January 1998. The NCEP three- and four-dimensional varitional data assimilation (3DVAR and 4DVAR) systems are used here to reassess the impact of these dropsonde observations on model predictions. The assimilation of these targeted dropsondes excluding the conventional observations improved the 48-h forecast over the NORPEX forecast verification region. However, the addition of the dropsonde data to an analysis that already contained various conventional observations degraded the 48-h forecast over the NORPEX forecast verification region. In the later case, the dropsonde data still improved and had its largest impact on the forecast over the northeast Pacific (outside of the forecast verification region). In this region, errors in the forecast using only conventional observations were largest. Furthermore, assimilation of the targeted dropsonde data using the 4DVAR approach produced greater improvements in the 1?3-day forecasts over the Pacific Ocean than the 3DVAR approach did in both cases, with and without conventional observations.