An evaluation of the impact of RAWS observations on surface objective analyses over the western United States

Federal, state, and other wildland resource management agencies contribute to the collection of weather observations from approximately 1000 Remote Automated Weather Stations (RAWS) across the western United States. To support the operations and decision making of these resource agencies, RAWS stations in the West tend to be located in remote locations away from other weather observing sites. Hence, they fill in data gaps in the surface observing network and improve monitoring of current conditions and may be of benefit to specify the initial state of the atmosphere in data assimilation systems. The goal of this study is to assess objectively the impact of the RAWS observations on objective analyses of surface weather parameters. In other words, if the RAWS observations are removed entirely from surface objective analyses, does the analysis quality degrade relative to randomly removing a similar number of observations from all available sources?

RAWS observations from the 2003-2004 winter season have been used in a data denial study to assess their impact on surface temperature and wind speed analyses over the western United States. The surface objective analyses were created using the ARPS Data Assimilation System (ADAS). A set of control analyses were created each day at 0000 and 1200 UTC by assimilating in ADAS all available observations (approximately 4000 observations from MesoWest). Analyses were also generated withholding all of the RAWS observations. In addition, 10 additional sets of analyses were generated by randomly withholding 1000 out of the 4000 observations from MesoWest.

The impact of RAWS observations on ADAS analysis error statistics is diagnosed as a function of location and synoptic regime. Estimates of the sensitivity of the ADAS analyses to the RAWS observations are examined. Results suggest that withholding RAWS observations is more detrimental to the analyses than randomly withholding a comparable number of observations from other sources. The results also indicate that the RAWS observations are particularly valuable to analyses during winter season cold pool events in the West.