Nowcasting of Motion and Growth of Precipitation with Radar over a Complex Orography

Abstract

Motion vectors of radar echo patterns can be obtained by applying a cross-correlation method (e.g., the TREC method) to radar data collected several minutes apart Here an extension of TREC, called COTREC, is presented. Based on constraints and a variational technique, this extension is an efficient objective analysis method for smoothing the motion vectors and forcing them to fulfill the continuity equation. COTREC corrects the apparently wrong vectors that are often caused by failures of TREC. This allows us to identify regions of growth and decay of radar echoes. For different types of precipitation (convective and widespread), radar data were collected for evaluation of COTREC in complex orography. A comparison between the radial velocity components of retrieved fields of echo motion and the measured Doppler velocity has been made. A marked reduction of the differences with respect to the measured Doppler field was obtained for COTREC, as compared to TREC vectors. A retardation of COTREC-derived motion compared to Doppler-derived motion was found in orographic precipitation. This retardation may have two causes: 1) a tendency of radar patterns to become stationary (triggered) on upsloping orography; and 2) the influence of ground clutter and shielding, also highly correlated with orography. While the first reflects the fact that propagation of echoes (by growth/decay) and translation of echoes (with the wind) are two different phenomena, the second cause is an artifact produced by the method of observation (radar) but mitigated with Doppler techniques (by suppressing the stationary 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 development approximately 20 min ahead, and there is good hope to extend the forecasting period.