Friday, May 11, 2012

Radar Echoes Over Southwest

Regional radar images from the Southwest are interesting this morning. A visible satellite image (above, for 1345 UTC) indicates that clear skies dominate, except for the ocean stratus along the west coast. However, a regional radar chart (below, for 1400 UTC - from NCAR website) shows widespread echoes over the northern GoC and southern California (reflectivities of 20-25 dBZ over a fairly large area). Out to the northwest of Phoenix there is what appears, on radar, to be a convective storm with reflectivities of 50+ dBZ. This echo had a radar-estimated rainfall of around half an inch when I checked (I refer to this kind of radar-estimated, false rainfall as "rock rain"). The echoes are mostly ground clutter that was not removed by the filters on the NWS WSR-88D radars at Phoenix and Yuma in Arizona. Ground clutter that passes through the filters has long been a problem on these two radars. However, the problem seems to have worsened some (after several years of improvement) with the addition of dual polarization - at least at Tucson and Phoenix (I don't know whether the Yuma radar has been upgraded yet). Radar loops indicate that there are two regions of translating echo - one is in southern California and the other along the east coast of the GoC in Mexico. These echos are likely insects or birds, however, the bulk of the echoes at 1400 UTC are just morning ground-clutter.

The primary issue related to ground-clutter rain occurs in long-term research efforts that use radar-estimated rainfall from the WSR-88D archives as "ground truth". Over long-periods of time, there can be considerable contamination of the archived data. Researchers would have to detect bogus rain estimates via some form of quality control efforts. The ground clutter to the northwest of Phoenix, for example, used to produce substantial, rock rainfall accumulations (I have observed amounts over an inch on many occasions). Thus, yet another caution for users of NWS-archived data.

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