The Colorado REFRACTT Demonstration

High resolution moisture (refractivity) information (a-c) and radar precipitation (reflectivity) fields (d-f) within the NE Colorado REFRACTT domain during the evening of 31 July 2006 at three time periods: 01:41 UTC (7:41 pm), 02:33 UTC (8:33 pm), and 03:45 UTC (9:45 pm). One incremental change on the color bar is approximately equivalent to 1 g/kg change in moisture (a-c) and 5 dBZ of precipitation intensity in (d-f). Magnitudes increase from bottom to top of the colorscales. A broad increase in moisture is observed from west to east across the REFRACTT domain during the two hour period (a-c) leading up to the development of two intense, N-S lines of thunderstorms at 03:45 UTC (f). In particular, a significant, localized increase in moisture (5 g/kg) was observed in the SE portion of the domain (a), one hour before the first thunderstorms developed in that area (e). Observations of high resolution moisture gradients such as this and of areas of moisture convergence are expected to improve the accuracy in time and location-specific nowcasting of thunderstorms and provide much needed information for numerical weather prediction models.

The NSF-sponsored Refractivity Experiment For H20 Research And Collaborative operational Technology Transfer (REFRACTT) was conducted in northeastern Colorado from June – August 2006. This experiment, under the umbrella of the NCAR Short Term Explicit Prediction (STEP) program and led by Rita Roberts (RAL),  included participation from scientists at McGill University, Colorado State University, University of Oklahoma, Polytechnical Institute of Catalonia (Spain), forecasters at the Denver NWS, and scientists and engineers from RAL, EOL, UCAR/COSMIC, and MMM. The REFRACTT demonstration provided a unique opportunity to collect high resolution, 2-D water vapor fields derived from refractivity data collected by three research Doppler radars and one operational NWS NEXRAD radar. There were two overarching goals of REFRACTT: 1) to improve our understanding of near-surface water vapor variability and the role it plays in the initiation of convection and thunderstorms; and 2) to build operational advocacy for the refractivity moisture retrieval technique for ultimate installation on the U.S. national network of NEXRADs. 

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 convection initiation and produce accurate quantitative precipitation forecasts (QPF) from Numerical Weather Prediction (NWP) models. Near-surface water vapor measurements can be extracted from Doppler radar based on the concept that variability in radar wave propagation between the radar and ground targets is due to changes in the properties of the air (i.e., changes in index of refraction) between the radar and targets. This “refractivity” technique, developed by Frederic Fabry of McGill University, ingests the raw I and Q (Level 1) data into a series of algorithms for computation and production of a moisture field. Moisture fields from all four radars were produced in real-time using the Fabry refractivity technique.

FY06 Accomplishments: 

The REFRACTT effort:

• Successfully demonstrated the ability to obtain and mosaic water vapor from research and operational radars.
• Was the largest domain over which high-resolution (4 km spacing), low-level water vapor has been collected.
• Collected data on a spectrum of events: stationary convergence zones, gust fronts and boundary collisions, convection initiation, rapid drying in the boundary layer, and cold front passage and post-frontal convection.
• Provided mosaics of refractivity fields in real-time to the Denver NWS forecasters for use in short term nowcasts.
• Demonstrated refractivity retrieval  on an X-band CASA radar.
• Obtained additional water vapor measurements from multiple sensors: 20 GPS receivers, mini-radiometers, MGAUS soundings, ENVISAT Synthetic Aperature Radar, and surface stations.
• Resulted in a briefing to NOAA/NWS managers in September on REFRACTT results; NWS is now considering implementation of the refractivity technique on the U. S. network of NEXRAD radars.

FY07 Plans:

The analysis of REFRACTT data and efforts to install refractivity on network radars will continue to be an active endeavor within the NCAR STEP program.  

The goals for 2007 and beyond include:

• Determine the sensitivity of thunderstorm initiation to moisture variability in the boundary layer.
• Produce 3-D water vapor fields by combining the low-level refractivity data with vertical profiles of water vapor from the GPS sensors and the radiometers.
• Continue to provide the Denver NEXRAD refractivity data to the NWS forecasters.
• Gain approval to have the refractivity technique installed on the NEXRAD radars.
• Work closely with the operational and university communities to get refractivity retrieval working on C- and X-band radars for ultimate installation on the TDWR and CASA radar networks.

Long-Term Impact:

REFRACTT-2006 is an important first step toward filling a long-standing need expressed by national committees (NRC,1998; and USWRP workshops and reports, Emmanuel et al 1995, Dabberdt and Schlatter 1996) for an improved, national, high-resolution field of moisture.  

List of Sponsors

Colorado State University, Ft. Collins, CO
McGill University, Montreal, Quebec, Canada
National Center for Atmospheric Research (RAL, EOL, MMM, STEP)
National Science Foundation
National Weather Service Office of Science and Technology (OST), Silver Spring, MD
National Weather Service OST Radar Operations Center, Norman OK
National Weather Service Weather Forecast Office, Denver CO
National Oceanic and Atmospheric Administration Global Systems Division
Politechnical University of Catalonia, Barcelona Spain
Radiometrics Corporation
University Center for Atmospheric Research (COSMIC)
University of Oklahoma, Norman, OK
University of Massachusetts, Amherst, MA