DWD's new operational scan strategy

DWD's new operational scan strategy

The German Meteorological service serves a wide spectrum of weather radar users, whose sometimes diverging requirements translate into competing operations requirements. Over the past 25 years, DWD's solution to this dilemma has been a succession of dedicated scan modes. In "Doppler mode", a dense 18 elevation volume has been scanned downwardly every 15 minutes in staggered PRF (1200 Hz / 800 Hz) in order to obtain Doppler wind data unfolded up to +-32 m/s. To extend the unambiguous range of the volume for reflectivity surveillance, this had been complemented by five low elevation sweeps moving upward ("intensity mode") at 600 Hz (250 km), likewise every 15 min. Combined products could be created from both volumes. To obtain QPE measurements ("precipitation mode"), one sweep has been taken at a roughly terrain-following lowest elevation at slow azimuth rate and low PRF every five minutes. This left 4 minutes of idle time before the next volume scan. As data homogeneity is one of DWD's major concerns, this sequence of 18+5+3 elevations has been strictly followed within the German radar network for over 20 years, testifying to its usefulness.

Meanwhile, national foci worldwide have changed from 'QPE only' to nowcasting, warning, and data assimilation. At the same time, increasing international data exchange (in Europe: OPERA) is pushing towards harmonization, and new technical features are available to fulfill these requirements. Within DWD, several realtime applications in radar hydrology, ATC, NWP, and warning systems would benefit from a rapid volume update. Specifically, the present 5 min cell tracking sorely lacks height information, while 15 min update rate turned out to be too low for cell tracking, as the time scale of convective processes may be significantly lower. So far, only 2D mosaics could be used in QPE, cell recognition and severe weather warning. Thus, a higher volume update rate became crucial within DWD's strategy of automated forecast and warning products.

To latch with satellite intervals, 5 min volumes have been decided upon. Boundary conditions were to preserve full radar coverage of Germany, minimize elevation gaps, maintain the present unambiguous velocity, possibly one low elevation sweep every 2.5 min, and to minimize elevation gaps as well as detrimental effects on existing or future radar algorithms. Given the coverage and technical possibilities of the German radar network, several scan patterns have been set up and tested at the Hohenpeissenberg research radar, including theoretical and statistically based considerations e.g. on VIL integration of a model cell under the different scan strategies. 
 As a result, a reduced volume consisting of 10 sweeps has been designed preserving the lowest 6 elevations between 0.5° and 5.5°, as they contain ~80% of the data. This yields continuous height coverage up to 12 km (8 km) starting from a range of 100 km (70 km). The upper elevations have been thinned out, as interpolation in space is considered more robust than in time. The mode concept has been abandoned in that no elevation is scanned twice in a volume (eliminating the "intensity" scan) to save time. On the other hand, a vertical calibration scan has been introduced (48°/sec, PRF 3000Hz, 25m range resolution, DAS 5°) which also yields boundary layer vertical profiles over radar (cf. Frech, this conference).

The +-32 m/s Nyquist interval is maintained lowering the high PRF to 800 Hz and increasing the unfolding ratio to 3:4 at low elevations. At an elevation of 8° (where VAD is taken), the unfolding ratio is lowered to 2:3. At still higher elevations where 60 km range is sufficient, staggered PRF is switched off in favor of a uniform PRF of 2400 Hz (pulse with 0.4 us, azimuth rate 30°/sec)

The temporal sequence of elevations has been optimized to minimize antenna movement and to provide a lowest elevation scan every 2.5 min. Starting at 5.5° moving downward, el=0.5° is reached after 2.5 minutes at an azimuth rate of 3 rpm, the outcome for improving precipitation estimation needs to be tested. After that, the upper 4 elevations are scanned. Hence the sequence is 5.5°, 4,5°, 3,5°, 2,5°, 1,5°, 0.5°, 8.0°, 12.0°, 17.0°, 25,0°. Neither an interleaved ("the famous "Swiss Scan" of 1993) nor an adaptive scan (NEXRAD VCPs) have been favored by users in DWD. Considering the automated operation and data evaluation in masaicked data, a homogenous scan strategy has been preferred.

The new scan strategy has been introduced into network operation throughout Germany in the course of a current system replacement project. Qualitative products are only slightly affected by gaps (CAPPI rings) and higher de-aliasing. The former is partly a visual effect only as most automated procedures are sweep based. One drawback of the higher unfolding ratio is a decreased tolerance (2.56 m/s instead of 5.3 m/s formerly) of the unfolding algorithm against velocity inhomogeneities in neighboring rays. A correction algorithm has been developed to take care of unfolding errors. VAD is taken at a lower elevation than before, tightening the requirement on wind field homogeneity. The resulting decrease in range to 180 km and a slightly larger "cone of silence" (25 km at a height of 1200 m instead of 15 km due to highest elevation 25° instead of 37° formerly) are deemed tolerable due to the high network coverage. Over all, the benefit of the new scan pattern is considered to outweigh these effects by far, given a threefold update rate between, and a minimized time lag within volumes. For instance, 5 min 3D cell recognition, evaluation and tracking becomes possible. New algorithms are being developed based on the new volumes (cf. Steinert, Tracksdorf this conference). The precipitation scan has not been altered except switching from H-only (former single-pol systems) to hybrid or SHV-Mode in order to make copolar dualpol moments available instead of LDR. A design review and fine tuning of scan parameters will be conducted when all network radars have been replaced.