Mode-concept for individualising the Z-R-relation for precipitation radar

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The remote sensing technique for precipitation by means of radar generally allows for the online determination of areal precipitation, as needed for example as input data for runoff and flood forecast simulations. Traditionally the calculation of precipitation rates from radar reflectivities applies a power law conversion function which is either derived empirically as a climatological mean or theoreticall based on the ideal equilibrium case. A comparison of derived rainrates with measured ones shows errors up to a factor of 5 at radar pixel level when a static power law Z-R relation is applied. Many previous attempts to characterise drop size distributions in terms of precipitation or weather types have not been successful mainly because it was assumed that only large scale meteorological structures are of importance for discrimination. In the mode concept to be developed in this proposal the Z-R relation shall be determined by superimposing several drop modes, thus allowing to take also small scale meteorological phenomena into account. The relative contributions of the different drop modes will be determined systematically from available long-term observations of drop size distributions and ancillary meteorological criteria like precipitation intensity, vertical extension of the cloud, stratiform or convective type of precipitation, height of the zero degree layer, type of vertical reflectivity profile, and others. Primarily the average relative contributions of up to four discrete dropmodes shall be determined for typical precipitation types and processes. With the possibility to identify these precipitation processes and types in the radar image or data sets the precipitation rate can be later derived from look-up tables allowing for the derivation of optimal combinations of the drop modes. The validity of the empirically determined relations will be investigated and validated by the use of the comprehensive droplet spectra measured during the Special Observation Period (SOP) from ground based disdrometers and Micro Rain Radars of the partners witing AQUARadar.