NOWCASTING OF MOTION AND GROWTH OF PRECIPITATION WITH RADAR OVER A COMPLEX OROGRAPHY

Motion vectors of radar echo patterns can be obtained by applying a cr oss-correlation method (e.g., the TREC method) to radar data collected several minutes apart. Here an extension of TREC, called COTREC, is p resented. Based on constraints and a variational technique, this exten sion is an efficient objective analysis method for smoothing the motio n vectors and forcing them to fulfill the continuity equation. COTREC corrects the apparently wrong vectors that are often caused by failure s of TREC. This allows us to identify regions of growth and decay of r adar echoes. For different types of precipitation (convective and wide spread), radar data were collected for evaluation of COTREC in complex orography. A comparison between the radial velocity components of ret rieved fields of echo motion and the measured Doppler velocity has bee n made. A marked reduction of the differences with respect to the meas ured Doppler field was obtained for COTREC, as compared to TREC vector s. A retardation of COTREC-derived motion compared to Doppler-derived motion was found in orographic precipitation. This retardation may hav e two causes: 1) a tendency of radar patterns to become stationary (tr iggered) on upsloping orography; and 2) the influence of ground clutte r and shielding, also highly correlated with orography. While the firs t reflects the fact that propagation of echoes (by growth/decay) and t ranslation of echoes (with the wind) are two different phenomena, the second cause is an artifact produced by the method of observation (rad ar) but mitigated with Doppler techniques (by suppressing the stationa ry ground clutter). COTREC may be useful for nowcasting, especially in orographically complex areas: for orographic precipitation as well as for severe convective storms, the technique predicts the echo develop ment approximately 20 min ahead, and there is good hope to extend the forecasting period.