Toward a national high resolution field of water vapor

The lack of detailed, high resolution water vapor measurements in the atmospheric boundary layer is one of the primary limiting factors in being able to predict the initiation of convection and produce accurate quantitative precipitation forecasts (QPF) from Numerical Weather Prediction (NWP) models. One of the most promising outcomes of the International H₂O Project (IHOP) was the near surface water vapor measurements extracted from radar using the index of refraction (refractivity) technique developed by Fabry et al (1997). A recent study by Weckwerth et al (2005) comparing Fabry's radar refractivity technique with a variety of surface-based and airborne moisture measurements has verified that this retrieval technique can be used to accurately estimate the near surface field of water vapor. Now, for the first time, an opportunity exists to demonstrate the value and impact of this high resolution, 2-D moisture field to the operational community through the collection of refractivity data over a multi-radar domain that includes the Denver KFTG NEXRAD radar.

This paper will document the preliminary results from the Refractivity Experiment For H₂O Research And Collaborative operational Technology Transfer (REFRACTT) conducted in NE Colorado from May-August 2005. In addition to the Denver KFTG radar, two other radars are being used to collect the average phase (I and Q) information used as input to the Fabry refractivity technique: the Colorado State University CHILL and the NCAR S-Pol 10 cm wavelength radars. The locations of the three radars are fortuitous as the overlapping coverage from the radars will enable us to mosaic the refractivity fields together to provide a continuous picture of moisture transport over an approximately 120 x 175 km domain. Highlights of data collected on the evolution of the near-surface boundary layer moisture field leading up to convection initiation and storm evolution will be presented. Simultaneous collection of moisture information over a much larger domain than has been previously possible will allow this data to be assimilated into mesoscale numerical models and automated nowcasting systems with the goal of improving quantitative precipitation forecasts. The potential use of the refractivity data by NWS forecasters for improving short term forecasts of thunderstorms and convective outlooks will be discussed. The challenges faced and still to be met in producing refractivity fields from an operational radar will also be discussed, as the ultimate goal of REFRACTT is to pave the way for installation of the refractivity technique on the national network of NEXRAD radars.