AQUARadarERAD 2006 Barcelona Areal homogeneity of Z-R-relations Gerhard Peters, Bernd Fischer, Marco Clemens Meteorological Institute University of Hamburg

AQUARadarERAD 2006 Barcelona 1.Introduction 2.Approach 3.Experimental setup 4.Radar calibration procedure 5.Evidence of modes of Z-R-relations 6.Space correlation of adapted Z-R-relations 7.Conclusions

AQUARadarERAD 2006 Barcelona 1.Introduction There is an unquestioned need for adapted Z-R-relations for improved Radar precipitation estimation. The success of using gage networks for adapting Z-R- relation parameters (p ZR ) depends on the p ZR time-space correlation patterns, about which only little is known. p ZR, as retrieved from in-situ/radar comparisons, are uncer- tain due to distance between in-situ sensor and radar volume and due to the small sampling volume of in-situ sensors. The averaging time required, to reduce the sampling errors below the real p ZR variability, may be longer than the p ZR decorrelation time.

AQUARadarERAD 2006 Barcelona 2.Approach : During the AQUARADAR-field-campaign-2005 vertically pointing Doppler radars were used to study decorrelation properties of p ZR. Due to the sampling properties of these instruments some of the aforementioned difficulties are mitigated: The p DSD can be measured within the weather radar sampling volume and with the same time resolution as the weather radar.

AQUARadarERAD 2006 Barcelona Space correlation: In this preliminary study only the space correlation aspect of p ZR was considered. With respect to the time domain, we believed to begin with in the existence of “modes” of p ZR. The term “mode” shall express that the p ZR apparently show extended periods of persistence and a tendency of more or less sudden jumps. No attempt was made here to relate p ZR to some physical pro- perties of rain processes, which is the objective of other AQUA- RADAR sub-projects, and will be considered in a later stage. Here sub-periods of the rain time-series were assigned subjec- tively to modes, which were characterized by the application of constant adapted values of p ZR.

AQUARadarERAD 2006 Barcelona Choice of mode length: As we had no precise definition of “modes” for the purpose of this study, the division into periods of constant “modes” was arbitrary to some extent. The qualitaive guidance was as follows: Very short mode-periods, would lead to perfectly adapted but nevertheless useless Z-R-relations, as only short space correlation can be expected. In the extreme case of one sample the adapted Z-R-method becomes identical with the DSD-based results. Very long mode-periods do not yield much improvement over the use of a fixed Z-R-relation, but this small improvement is expected to show a longer space correlation. We decided intuitively between “very long” and “very short” without having yet a criterion for the optimum choice.

AQUARadarERAD 2006 Barcelona 3.Experimental Setup: Close to the DWD observatory Lindenberg in NE-Germany a chain of 13 Micro Rain Radars (MRR) was set up spanning a distance of about 6 km in SW-NE-direction. The MRRs provided DSDs with 20 s time resolution and 100 m vertical resolution on vertical profiles extending up to 3 km altitude. The MRR chain was in the range of a X-band mini weather radar (X-MWR), a modified navigation radar with pencil beam antenna and digital data acquisition. The X- MWR elevation angle was fixed at 11° leading to altitudes of common MRR and X-MWR measuring volumes between 600 and 1000 m above ground, which was safely below the melting layer.

AQUARadarERAD 2006 Barcelona Scanning x-band radar Line with 15 MRRs Berlin 30 km

AQUARadarERAD 2006 Barcelona Scanning X-Band Radar

AQUARadarERAD 2006 Barcelona Installation of one of 13 MRRs Rain Gauge

AQUARadarERAD 2006 Barcelona 4.Radar calibration procedure: MRR 1 raw dsd 1 raw rain rate 1 rain gauge cal. dsd 1 cal. Z 1 XMWRraw Z X cal. Z X MRR 3 raw dsd 3 raw Z 2 cal. Z 3 cal. dsd 3 MRR 15 raw dsd 15 raw Z 15 cal. Z 15 cal. dsd 15 MRR 2 raw dsd 2 raw Z 2 cal. Z 2 cal. dsd 2

AQUARadarERAD 2006 Barcelona MRR-XMWR comparison after calibration, 30 s averages

AQUARadarERAD 2006 Barcelona 5.Evidence of modes of Z-R-relations

AQUARadarERAD 2006 Barcelona 5.Evidence of modes of Z-R-relations cont.

AQUARadarERAD 2006 Barcelona 5.Evidence of modes of Z-R-relations cont.

AQUARadarERAD 2006 Barcelona 5.Evidence of modes of Z-R-relations cont.

AQUARadarERAD 2006 Barcelona 6.Space correlation of adapted Z-R-relations 15 Sept. – 25 Oct Events Local impact of adapted Z-R-relations

AQUARadarERAD 2006 Barcelona 6.Space correlation of adapted Z-R-relations cont. 15 Sept. – 25 Oct Events Remote impact of adapted Z-R-relations (Z-R-relation, adapted for station #2 and applied to station #15 in 6km distance)

AQUARadarERAD 2006 Barcelona Conclusions 1.Z-R-relations, after piecewise adaptation in time (  modes), were demonstrated to show reduced scatter at 6 km distance from the reference point. 2.Far excursions from a fixed Z-R-relation were removed particularly efficiently. 3.A concept for optimum and automatic choice of modes needs still to be developed.