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<br />~ <br /> <br />376 <br /> <br />PJ. BR~MAUD AND Y.B. POINT1N <br /> <br />FORECASTING HEAVY RAINFALL FROM RAIN CELL MOTION <br /> <br />371 <br /> <br /> <br />80 <br /> <br />70 <br /> <br />30 <br /> <br />-' <br />0 <br />!;( 60 <br />z <br />< <br />~ <br />'0 50 <br />l! <br />"' <br />N .0 <br />~ <br />~ <br />1i <br />> JO <br />~ <br />~ <br />~ 20 <br />l! <br />~ <br />~ <br /> 10 <br /> 0 <br /> 0 <br /> <br /> <br />6 dB <br /> <br />60 <br /> <br />50 <br /> <br />40 <br /> <br />10 <br /> <br />20 <br /> <br />JO <br /> <br />'0 <br /> <br />50 <br /> <br />60 <br /> <br />East.West distance in km <br />Fig. 2. eEL echoes detected on the horizontal reflectivity profile drawn in Fig. 1. <br /> <br />10 <br /> <br />order of the life-time of the convective cells, i.e, usually more than half an <br />hour. Moreover. the convective cells often lead to heavy precipitation at the <br />ground and the accurate forecast of their future position is very important for <br />urban hydrologists. Second, this definition avoids the use of a fixed and <br />arbitrary reflectivity threshold which cannot be appropriate for detection of <br />the characteristic motions for every weather situation. For example, the <br />detection of convective cells embedded within a large stratiform rain area <br />requires a higher reflectivity threshold than the detection of isolated storms, <br />Furthermore, the motion of heavy rain cells is often different from the motion, <br />of the large rain area within which they are embedded, In that case, the <br />tracking of the large rain area rather than the CEL echoes can lead to <br />considerable underestimates of the observed rain over a drainage basin. <br /> <br />20 <br /> <br />o 10 ~ W ~ M ~ <br />But.Weat distance in kIn <br />Fig. I. Image observed on II October 1988 at lOb 42min above the Cevcnnes (gray scale in dB). <br /> <br />- - <br /> <br />the -6dB contour (one pixel represents 240m x 240m for this event) and <br />by their persistent position detected during the tracked entity matching step <br />described below, The four resulting CEL echoes, shown in Fig, 3 for the radar <br />dala corresponding to the PPI of Fig. I, mainly have a size of about 8km and <br />20 km in the cross-front and along-front directions respectively, <br />There are Iwo main advantages of this CEL echo definition, First, in the <br />case of critical hydrological weather situations, (convective or frontal <br />situations), the CEL echoes are associated with hl;.vy rain cells which are <br />closely related to conveCtive cells characterized by the particular dynamics of <br />the cloudy air, Therefore, each CEL echo can be tracked generally as long as <br />this individual convective circulation is maintained which, typically. is of the <br /> <br />Tracked entity characterization <br /> <br />To recognize which one of the eEL echoes of one picture corresponds to <br />the' CEL echo of the same rain/cell in: the next picture (the. matching <br />procedure), we have charactenzed each CEL echo by the following charac- <br />teristic attributes: area; reflectivity distribution characteristics (median and <br />maximum); coordinates of the centroid; elongation of the inertia matrix <br /> <br />