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Weather Radar Products

Figure 1: Radar product PE of the DWD: a local echo top, the reflectivity is only displayed above a given size (here 18 dBZ), the colors correspond to the height of the cell with the measured value,
(© 2020 German Weather Service)

Figure 1: Radar product PE of the DWD: a local echo top, the reflectivity is only displayed above a given size (here 18 dBZ), the colors correspond to the height of the cell with the measured value,
(© 2020 German Weather Service)

Weather Radar Products

From the data measured by the weather radar, different local radar products and radar composite products are generated. If the radar data are mathematically combined, they are called composite products or a composite image. It can be either a local composite (when data from several antenna rotations are superimposed on each other) or a composite of data from different radars and radar sites. The images of, for example, the so-called “rain radar” which are distributed on the Internet, are always composites.

Local Radar Products

Primary data by the radar method (or its base data) are only the reflectivity and possible radial velocities. Depending on the scanning method used, the data is displayed either as PPI (with antenna rotating in azimuth) or RHI (range-height indication with antenna rotating in elevation). The base reflectivity and the base speed are each a basic product measured by the radar and can be displayed in both PPI and RHI.

Figure 2: RHI display (Range-Height-Indication)

Figure 2: RHI display (Range-Height-Indication)

Different radar products are created from these data by using different numbers of classes (number of intensity levels represented by color variants), a different resolution in distance or angle, different maximum ranges, different primary data (volume scan or precipitation scan) as well as (due to software) different compression algorithms.

With a PPI, the meteorological objects are represented in a plane (similar to a map), and in the center is the weather radar. The position of the echoes is determined by the direction and the distance, (in a polar coordinate system, thus related to the radar station). A color scale is used to display the intensity of the echoes. Each rotation is done with a different elevation angle. In the original, this PPI therefore only consists of a single elevation scan, which is recorded during one antenna rotation. The colors of the representation correspond to the reflected power received by the radar antenna from a specific location. However, a single PPI cannot give a complete picture of the vertical structure of a precipitation area. It only shows a conical section through the weather.

In an RHI (see Figure 2), only a single side angle is displayed. The radar antenna swings up and down in the elevation angle. An RHI enables the investigation of the vertical extension and distribution of the echoes (for example, to determine the height of the cloud base as well as the cloud top of precipitation areas or thunderclouds). With modern weather radars, however, the antenna does not have to stop and start to swivel: The RHI representation is composed as a local composite from the data of several antenna rotations.

The classic RHI always starts at the radar's location and ends at any side angle at the edge of the radar's detection range. With an “Arbitrary Vertical Cross-section” (AVCS), the user can determine the start and endpoint of this vertical display. Thus, a vertical section across a weather area is possible.

Figure 3: A combination of different elevation scans to one CAPPI (Constant Altitude PPI)

Figure 3: A combination of different elevation scans to one CAPPI (Constant Altitude PPI)

Local composites

A combination of data from several antenna rotations of weather radar is called local composite or site composite. They consist of the different image sequences of all elevation scans (the loop).

Figure 4: Radar product PZ of the DWD: a local CAPPI showing the weather in an altitude of 3 000 m (10 000 feet),
(© 2020 German Weather Service)

Figure 4: Radar product PZ of the DWD: a local CAPPI showing the weather in an altitude of 3 000 m (10 000 feet),
(© 2020 German Weather Service)

Composite reflectivity

In a composite reflectivity, all data of the volume scan are superimposed. The strongest echo signal from the elevation scans is displayed. Each point of the radar image shows the maximum value (in dBZ) of reflectivity in the air column above that point.

Echo top

In the case of an echo top, the data of the elevation scans are superimposed also. However, the different colors indicate the height level of the strongest reflectivity above the ground. This map has a special meaning for the meteorological flight advice of pilots.

CAPPI

A CAPPI (Constant Altitude PPI) is a horizontal, two-dimensional representation of radar measurement variables at a certain height. A CAPPI is calculated and interpolated from several PPI measurements at different elevations. On a PPI- Scope, the different heights will be mostly represented by color variations. A so-called pseudo-CAPPI (pCAPPI) differs from a CAPPI in that additional values in the far zone are extrapolated.

cross-
sections
vertical cross section
ground plan
horizontal cross section
medional cut
zonal cut

Figure 5: Structure of a Vol-CAPPI-Display

cross-
sections
vertical cross section
ground plan
horizontal cross section
medional cut
zonal cut

Figure 5: Structure of a Vol-CAPPI-Display

Vol-CAPPI

Another display option is called Volume Constant Altitude Plan Position Indicator (Volume-CAPPI) and allows a clear echo assignment in all three cross-sections. In addition to a floor plan, an elevation (corresponds to a side elevation from east to west) and a single cross elevation (corresponds to a side elevation from north to south) are shown. With the help of a crosshair-cursor, the positions of these cross-sections can be selected.

MAX-CAPPI

In a MAX-CAPPI (Maximum Constant Altitude Plan Position Indicator) maxima of CAPPI data from different heights are displayed. This is done on the one hand by a vertical maximum projection in an (image) elevation, on the other hand by horizontal maximum projections from south to north or from west to east in side elevations. In the side elevations, the lateral scale indicates the height in km. In a MAX-CAPPI mostly reflectivities are shown.

Composites across multiple sites

Combining the radar images of different locations into a composite image opens up further possibilities for weather analysis and forecasting. In most cases, only local reflectivity composites are further processed across locations. The advantage is a large-scale overview. The disadvantage is the poorer accuracy compared to local products.

WX product and RX product

The superposition of the reflectivities of all radar sites in Germany forms a two-dimensional standard evaluation of the radar reflectivity distribution with a resolution of 1 km × 1 km. Only the precipitation scan of the weather radar units is used for the WX product. However, this precipitation scan is updated only every 5 minutes. These images or their sequence as animation are published as “Rain Radar”. For the RX product the local reflectivity composites, i.e. the data from all elevation scans, are used.

Vertically Integrated Liquid (VIL)

Vertically Integrated Liquid (VIL) is the total amount of rain that would fall if all the liquid water in a column within a rain cloud (usually a thunderstorm) reached the surface of the earth. Basically, all water quantities estimated on the basis of reflectivity are added over a grid-like distributed unit of area.

VIL is not observed or measured, but calculated based on the reflectivity measured by the radar. Thus, everything that influences the reflectivity has an effect on the VIL.