RAIN-X

Accurate estimations of precipitation over large areas is a pre-eminent need for hydrological applications. The RAIN-X research addresses this need using new radar methods. Conventional radar refelectivity measurements have long been used for estimating rainfall, but these simple estimates are prone to large errors. Various new polarization radar techniques offer the opportunity to estimate rain more accurately. The propagation differential phase shift (or Kdp) method relies on the fact that horizontally-polarized radar signals propagate more slowly through a region of slightly flattened raindrops than vertically polarized signals. This difference is measurable and more directly related to rain rate than is reflectivity. Furthermore, this new method does not depend on a radar's calibration, and it can be used in conditions where attenuation and partial beam blockage are present.

Almost all differential phase rain estimation research has used large S-band (10-cm wavelength) radars, for which the technique is best suited to estimating moderate to heavy rain rates. At ETL, the NOAA/D X-band (3 cm) radar and its new data system are being developed for differential phase estimates of light to moderate rain rates. Promising RAIN-X results from data collection in Colorado in 1997 will be extended in 1998 in the TRMM program.