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li 380 PJ. BREMAUDAND V.B. POINTlN FORECASTING HEAVY RAINFALL FROM RAIN CELL MOTION 3''- 20- Fig. 4. EC echo round a eEL echo or one picture matched with a eF.l echo of the previous picture. 10- Forecast by extrapolation RADAR Through the matching procedure we are able to define with accuracy the motion of the CEL echoes, Le, the real motion of rain cells associated with convective cells, Moreover, the high spatial and temporal resolution of the radar data permits us to observe that these structures are very evolutive to such an ext~nt that this dynamical tendency introduces a random noise in the estimation of the motion of the CEL echoes, So, to avoid too dissimilar an extrapolation speed relative to the mean motion really observed, we have chosen to extrapolate the motion of each of the echoes in which the CEL echoes are embedded with a speed which is equal to the arithmetic average of the speeds of the CEL echoes, As illustrated in Fig, 4, these 'EC echoes' (for 'Enclosing ,Cell' echoes) are plotted round each matched CEL echo by using a threshold value which is equal to the reflectivity minimum of the less intense CEL echo, Moreover, an EC echo is plotted if a CEL echo in one picture is not matched with a"eEL echo in the next picture; in this case, the plot is centred on the point which corresponds to the centroid of the CEL echo translated with the mean motion of all the CEL echoes, This definition implies that one EC echo can enclose zero, one or more eEL echoes, The main advantage of this definition for hydrological purposes is that multiple mergers of rain cells in strong convective bands can be taken into account by enclosing them in one structure, Indeed, the rain cells in a convective band may have somewhat homogeneous motions which can be different from the apparent motion of the entire band, This definition of EC echo allows us to find this coherent motion and to forecast with accuracy the location of dangerous hydrological phenomena (for example, the frontal band shown in Fig, I has generated more than 70 mm of rain during less than I h), This definition is illuStrated, for the rain b,md data of Fig, I, in Fig. 5, which shows a single EC echo enclosing the four CEL echoes of Fig. 3. -30 o 10 20 30 E.-t. West dista.nce in km 40 381 .... 6<l '0 40 30 20 10 50 6<l Fig. S. EC echo dctcctc:d on the image observed on 11 October 1988 at lOb 42min above the CCvcnnes (gray scale in dB). RESULTS To evaluate the real capabilities of the PARAPLUIE method, we have compared it with three other methods, for four events (Bremaud, 1991). The three other methods are the persistence 'PERS',(which assumes no motion), the global extrapolation of the entire image 'EXTRA' (cross..correhition technique) and the SCOUT 11,0 method (an earlier 'complex method'), which is used operationally in Seine-Saint-Oenis county (near Paris) for sewer management (Einfalt et aI., 1990), The four events studied are characterized by different weather situations - convective cells embedded within a large ~