The effect of targeted dropsonde observations during the 1999 winter stormreconnaissance program

In this paper, the effects of targeted dropsonde observations on operationa l global numerical weather analyses and forecasts made at the National Cent ers for Environmental Prediction (NCEP) are evaluated. The data were collec ted during the 1999 Winter Storm Reconnaissance field program at locations that were found optimal by the ensemble transform technique for reducing sp ecific forecast errors over the continental United States and Alaska. Two p arallel analysis-forecast cycles are compared; one assimilates all operatio nally available data including those from the targeted dropsondes, whereas the other is identical except that it excludes all dropsonde data collected during the program. It was found that large analysis errors appear in areas of intense baroclin ic energy conversion over the northeast Pacific and are strongly associated with errors in the first-guess field. The "signal," defined by the differe nce between analysis-forecast cycles with and without the dropsonde data, p ropagates at an average speed of 30 degrees per day along the storm track t o the east. Hovmoller diagrams and eddy statistics suggest that downstream development plays a significant role in spreading out the effect of the dro psondes in space and time. On average, the largest rms surface pressure err ors are reduced by 10%-20% associated with the eastward-propagating leading edge of the signal. The dropsonde data seem to be more effective in reduci ng forecast errors when zonal how prevails over the eastern Pacific. Result s from combined verification statistics (based on surface pressure, troposp heric winds, and precipitation amount) indicate that the dropsonde data imp roved the forecasts in 18 of the 25 targeted cases, while the impact was ne gative (neutral) in only 5 (2) cases.