Poldirad

POLDIRAD Quicklooks

Martin Hagen

Quicklooks of DLR's polarimetric Doppler weather radar POLDIRAD are intended to give an overview on the precipitation fields as seen by the radar located at Oberpfaffenhofen. More radar data can be found in the data section of this CD.

Radar-loop

The quicklooks are based on the surveillance scans (data section). Shown are the reflectivity data of PPI scans at an elevation of 0.8°. The image covers a region in southern Germany, Austria and Switzerland extending from about Zurich to Salzburg (west to east) and from the Inn valley in the south to the Main river in the north. The area is 330 by 330 km². Geographic landmarks are international borders (black), the border between the staates of Baden-Württemberg and Bayern in Germany (black), rivers (blue), cities (brown), lakes (blue) and autobahns (yellow). Cities and lakes are filled with colour if no radar echos are present at this location.

Reflectivity is (among others) a measure of the amount of precipitation in the air. Values below 15 dBZ correspond to very light rain (0.3 mm/h), clouds and clear-air echos. Clear-air echos are observed frequently in the region close to the radar (up to 60 km range during summer time). Reflectivity values above 55 dBZ refer to graupel or hail in most situations.

Some data processing was done to make the radar images easier to interpret: First, ground clutter (see below) was suppressed by a cancelation of all data points where the depolarisation ratio is above -14 dBZ. Therefore, in regions with strong ground clutter no clear-air echos are visible any more. Second, range bins which are not surrounded by usable data points have been omitted.

When interpreting radar images, a few things have to be considered to avoid misunderstanding:

• Earth is a sphere: Even when looking horizontally, the radar beam will be at a considerable height above ground at far distances. Here are a few examples for the case of an elevation of 0.8°:
  range 50 km: 0.8 km height
  range 100 km: 2.0 km height
  range 150 km: 3.4 km height
  range 200 km: 5.1 km height
  range 250 km: 7.1 km height
  for that reason, only deep convection can be seen with the radar at the western edge of the images.
• Ground clutter: Besides precipitation, mountains and other obstacles (buildings) reflect the transmitted power back to the radar. For quicklooks these signals have been considerably suppressed, however, still some ground clutter is visible in the region around the radar, within Munich, and the northern edge of the Alps along the Austrian - Germany border.
• Attenuation: During the passage of the radar pulse through heavy precipitation a certain amount of the transmitted energy is dissipated, and no longer available for scattering at ranges further out. This results in sometimes easy-to-see gaps behind storm cells, as well as invisible precipitation areas at the rear of the squall line. However, not always attenuation is clearly visible. At the time, there is no stable algorithm available for the correction of attenuation. In composite images this problem can be overcome by other radars looking from the rear side.
• Beam blockage: When there are obstacles close to the radar, the radar beam is blocked, and no data are available beyond that point. A problem with POLDIRAD is, that the antenna is not sheltered by a radome. In situations with strong wind from behind, the radar beam is pointed into the ground (e.g. images at 2026 and 2046 UTC). Also, precipitation south of the Alpine main ridge is normally invisible.

Quicklook-Files

11:02 UTC	11:08 UTC	11:23 UTC	11:38 UTC
11:54 UTC	12:09 UTC	12:17 UTC	12:32 UTC
12:49 UTC	13:06 UTC	13:32 UTC	13:40 UTC
14:12 UTC	14:24 UTC	14:39 UTC	14:46 UTC
15:01 UTC	15:16 UTC	15:52 UTC	16:07 UTC
16:17 UTC	16:27 UTC	16:37 UTC	16:47 UTC
16:58 UTC	17:08 UTC	17:19 UTC	17:33 UTC
17:42 UTC	17:52 UTC	18:02 UTC	18:12 UTC
18:22 UTC	18:34 UTC	18:44 UTC	18:54 UTC
19:09 UTC	19:27 UTC	19:37 UTC	19:53 UTC
20:05 UTC	20:26 UTC	20:48 UTC	20:58 UTC
21:08 UTC	21:21 UTC	21:29 UTC	21:39 UTC
21:57 UTC	22:07 UTC	22:17 UTC	22:28 UTC
22:38 UTC	22:48 UTC	23:03 UTC	23:13 UTC

Poldirad