Analysis of the impact of supplemental dropwindsonde and rawinsonde observations on model track forecasts of Hurricane Irene (2011)

As Hurricane Irene approached the east coast of the United States in August 2011 a large number of supplemental observations were taken in an effort to improve operational analyses and model forecasts of the cyclone. The NOAA Gulfstream-IV jet flew 10 synoptic surveillance missions from 23-27 August and deployed between 22 and 36 dropwindsondes during each mission to collect data near and upstream of Irene. In addition, supplemental 0600 and 1800 UTC rawinsondes were launched beginning at 1800 UTC 22 August from upper-air stations in the southeastern United States. In an unprecedented move, the coverage of the supplemental rawinsondes was then expanded to include all of the continental United States from the Rocky Mountains eastward beginning at 0600 UTC 25 August in an effort to better sample synoptic-scale flow features upstream of Irene.

In an attempt to quantify the impact of these supplemental data, data denial studies have been performed for both the dropwindsonde and rawinsonde observations. Analyses were created excluding both supplemental data types using the National Centers for Environmental Prediction (NCEP) Gridpoint Statistical Interpolation (GSI) data assimilation scheme. Then both the NCEP Global Forecast System (GFS) and Hurricane Research and Weather Forecasting (HWRF) models were run using the GSI analyses that excluded the supplemental observations. Differences between the operational forecasts of the GFS and HWRF and the forecasts from the “data-denial” runs will be used to quantify the impact of the supplemental observations on the forecast of Hurricane Irene in both models. Results from this study may be useful in determining when and where to task supplemental observations in future cases.