Radar refractivity retrieval: validation and application to short-term forecasting

The International H₂O Project (IHOP_2002) was designed to sample the three-dimensional time-varying moisture field to better understand convective processes. Numerous research and operational water vapor measuring systems and retrievals were operated in the U.S. Southern Great Plains from 13 May to 25 June 2002. This was done in combination with more traditional observations of wind and temperature. The radar refractivity retrieval was run on NCAR’s S-Pol Doppler radar. This study will assess the representativeness of radar refractivity and discuss possible applications to short-term convective forecasting.

The refractivity signal is derived from the difference in radar signal phase delay between two ground targets measured at a reference time exhibiting no refractivity gradients and a time of meteorological interest. This signal delay difference is due to atmospheric refractivity variations which are caused by changes in pressure, temperature and most strongly, water vapor. During IHOP a field of refractivity measurements was routinely obtained from the S-Pol radar in the Oklahoma panhandle. The nominal retrieval range was approximately 40 km but it extended 60 km toward the northwest due to the more numerous ground targets and more optimal slope of the land.

This study will examine the representative horizontal scale and vertical depth of the refractivity retrievals. Comparisons between the refractivity field and fixed and mobile mesonets within refractivity range show a strong correlation. Comparisons between the refractivity field and low-flying aircraft support these results. Numerous instruments that profile the temperature and moisture structure are compared with the refractivity field to assess the layer over which refractivity is representative.

The application of the real-time refractivity field to short-term weather forecasting is assessed by evaluating several IHOP cases. One day illustrated the potential for refractivity to identify boundaries prior to the more traditional radar reflectivity and Doppler velocity fields. Two days showed some value in monitoring the refractivity field prior to convective development. This technique shows great promise for both research and operational applications.