The Atmospheric River Reconnaissance Dropsonde Data Impact on GFS Forecasts

Atmospheric Rivers (ARs) are a global weather phenomenon that are responsible for most of the horizontal water vapor flux outside of the tropics, a key feature in the global water cycle, and closely tied to both water supply and flood risks, particularly in the western United States. AR Reconnaissance (AR Recon) observations are officially part of the U.S. National Winter Season Operations Plan (NWSOP). For the Water Year 2023 AR Recon campaigns, there were 39 intensive observing periods (IOPs) with 46 flights, which provided additional dropsonde observations of the full atmospheric profile of water vapor, temperature, and winds within and around ARs. Data impact experiments have been carried out with the NCEP operational global forecast system (GFS) version 16 (GFSv16) to study the dropsonde impact on GFS forecast, including landfalling ARs and associated precipitation. GFSv16 is with the finite volume cubed-sphere dynamical core and improved GFDL (Geophysical Fluid Dynamics Laboratory) microphysics. The model has 127 vertical layers, with the model top at 80 km, and a horizontal grid with 13 km resolution. The data assimilation system uses a 4-Dimensional Hybrid EnVar with Incremental Analysis Update (4D-IAU) technique. The GFSv16 data assimilation and forecast results indicated improvements in the GFS initial conditions when dropsonde data are assimilated, and improvements in GFS precipitation forecast skill over the U.S. West Coast, in particular over California, for 48- to 72-hour forecast. The improvement in the precipitation forecast is associated with improvement of the forecast for other meteorological fields (such as temperature, moisture, and wind) and the AR landfalling along the U.S. West Coast. We will present the results in detail of dropsondes impacts on GFS forecasts for the Water Year 2023.