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Intensities from KNMI radar data

Iwan Holleman / Hans Beekhuis

Radar in De Bilt


Contents:

Reflectivity:

The echo power of calibrated weather radars is converted to units of the radar reflectivity Z, a property of distributed scatterers [dimension mm^6/m^3]. These echo values have been normalised for range. The reflectivity is usually given in logaritmic units (base 10):

dBZ = 10 LOG Z

In the radar HDF5 files the reflectivity data are converted to 8-bit integers (0-254) and 255 is used to indicate "nodata". The scale and offset of the conversion are given in the HDF5 file:

dBZ = 0.5*value-31.5

For a table with the calculated values click on: table with reflectivities.

Rainfall Intensity:

There is a fixed range-independent relation between the reflectivity value Z and the rainfall intensity R [mm/h]. The following semi-empirical relation, so-called Z-R relationship, is used:

Z = 200*R^1.6

or using the logarithmic reflectivity:

dBZ = 16.0*LOG R +23.0

Examples: 7 dBZ for 0.1 mm/h, 23 dBZ for 1 mm/h, and so on.

In the radar HDF5 files the reflectivity data are converted to 8-bit integers (0-254) and 255 is used to indicate "nodata". The scale and offset of the conversion are given in the HDF5 file:

dBZ = 0.5*value-31.5

Using the Z-R relationship, the logarithm (base 10) of the rainfall intensity can be obtained from:

log R = (value-109)/32

A rainfall intensity of 0.1 mm/hr is represented by the value 77, 1 mm/hr by 109, and so on. A data value 255 is used to indicate that no measurement is available, i.e., beyond 320 km from the radar.

For a table with the calculated values click on: table with rainfall intensities.

Radar Echotop Heights:

In the radar HDF5 files the radar echotop heights in km are converted to 8-bit integers (0-254) and 255 is used to indicate "nodata". The scale and offset of the conversion are given in the HDF5 file:

Height = 0.06299*value

For a table with the calculated values click on: table with echotop heights.

Probability of Hail:

In the radar HDF5 files the probability of hail (POH) in % is converted to 8-bit integers (0-254) and 255 is used to indicate "nodata". The scale and offset of the conversion are given in the HDF5 file:

POH = 0.665*value-10.66

For a table with the calculated values click on: table with probability of hail.


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Hans Beekhuis