Known data problems

Radial velocity

The surface based velocity correction worked well the majority of the time, however there are some regions in which problems were noted. These problems manifest themselves as columns of biased radial velocity at each range bin over several rays. These so-called velocity pillars have a maximum magnitude of about 0.3 m/s.

We think these velocity pillars have two sources: i) the filtering process over-smoothing surface velocity variations due to variable pointing error (Ellis et al. 2017) and ii) the filtering process under-smoothing variations in surface velocity due to the large waves observed in the Southern Ocean. Since the pointing error and surface velocity variations can have similar scales it is difficult to smooth the pointing errors out, while not over-smoothing the velocity variations. A solution will be sought.

Backlobe echo in zenith pointing

When the HCR is pointing at zenith and the GV is near the surface, there is often an echo that results from the backlobe of the radar reflecting off of the surface. This backlobe contamination is typically characterized by a band of low reflectivity, highly variable radial velocity, and high spectrum width. The backlobe appears in the zenith data at a range equal to the altitude of the radar. So as the GV ascends or descends the backlobe contamination will recede and approach in range, respectively.

The figure below shows an example of HCR data taken as the GV descends, with the first part in nadir pointing and changing to zenith after 20:36 UTC. The location of the radar is demarcated by signals that the HCR records for engineering purposes at range bins internal to the radar, referred to as the ‘bang’, which is labeled in the figure. The backlobe contamination can be seen where the radar is pointing to zenith. It appears as a line of echo that moves closer in range as the GV descends, whereas the cloud echo in this example remains at a constant altitude.

Period in RF05 during which transmit was in H instead of V mode

During RF05, on 2018/01/26, from 01:50 to 02:15, the transmitter was transmitting in the H channel instead of the V channel. Care should be taken in using some of the data fields in this period:

  • The reflectivity calibration is incorrect for H transmit
  • The SNRVC, SNRHX, DBMVC and DBMHX fields will be missing during this period.
  • The velocity and spectrum width are unaffected, as is NCP.

Periods during which the HCR transmitter was disabled

In the HCR data, there are some short periods during which the transmitter was disabled for safety reasons. These show up as gaps in the power fields.