Noise correction to reflectivity calibration

The dBZ0 number in the above equation is the number which sets the sensitivity of the radar. Lower numbers mean greater sensitivity. In $Z = C S r^{2} = [\frac{C r_{0}^{2} N}{g^{r} g^{t}}] [\frac{r^{2}}{r_{0}^{2}}] [\frac{T_{0} - N}{N}]$ it was assumed that the noise level at calibration time is the same as the noise level at run time. And that any changes in measured noise level were due to changes in receiver gain not sensitivity.

Modern digital receivers and low-noise amplifiers are very gain stable, and this is generally not true. One example of a relatively large noise level variation is the thermal noise from the relatively warm earth and atmosphere. So, the bottom degree or so of elevation has a different noise level, and thus a different sensitivity, and thus a different dBZ0. Normally we calibrate while aiming the antenna up in the air in a direction away from the surface, or the sun. For this discussion, let us define two new noise values:

N sub c: Noise level at calibration time.
N sub r: Noise level at ray processing time.

If we answer yes to "Enable noise power based correction of Z0" in the Mp non-volatile setup section, then the new radar equation becomes:

Z = C S r^{2} = [\frac{C r_{0}^{2} N c}{g^{r} g^{t}}] [\frac{N r}{N c}] [\frac{r^{2}}{R_{0}^{2}}] [\frac{T_{0} - N r}{N r}]

In this equation, the dBZ0 is the term $[\frac{C r_{0}^{2} N c}{g^{r} g^{t}}] [\frac{N r}{N c}]$

The dBZ0 fed into RVP10 is the basic $[\frac{C r_{0}^{2} N c}{g^{r} g^{t}}]$ , while the processor modifies the value by the ratio Nr/Nc. Values read out by the GPARM command, and so on are the modified value.

This is the only place where the calibration-time noise level Nc is used in the processing.

It is possible for this value to be unknown, in which case it is set to nan internally. In this case, if you request the corrected values, the correction is not applied, and you get the message "Cannot enable Z0 noise correction because calibration is missing" in the rvp log file.