Factors Affecting the Precision of Radar Measurement of Rain. An Assessment From an Hydrological Perspective

A great variety of errors affect radar measurements of rain and an important effort has been devoted to investigate them. From the point of view of radar rainfall estimates the relative importance of most of these errors has been evaluated and several correction procedures have been suggested. Besides, some effort has been devoted to analyze their importance from a hydrological perspective, as well as to assess the degree of performance of the different procedures of correction.

The aim of this work is to present a simulation based study of some of these errors and their hydrological consequences. The followed procedure uses radar measurements to generate a high resolution three-dimensional rainfall field that is used as reference. Then, through simulation, this field is ‘degraded’ in order to reproduce a series on radar errors. Next, different usual correction procedures are applied to the degraded fields to obtain the ‘corrected’ ones.

The hydrological analysis is made by using the reference field and the degraded and corrected ones as input of a basin with a previously adjusted hydrological model. Thus, the runoffs at the outlet of the basin for the degraded and corrected fields are compared to those associated with the reference field. This allowed us to analyze the importance of each error or combination of errors and the effectiveness of the corrections from the hydrological perspective.

The errors studied include those related to the distance to the radar, in particular the decrease of resolution, the variation of the reflectivity with height and the bright band contamination. The importance of the Z-R variation as a function of the type of precipitation has been also analyzed. Rainfall events registered at the C-Band radar of the CMTC (Instituto Nacional de Meteorologia, Barcelona) were used to generate the different rainfall fields. The results from simulation for these events will be presented.