Estimation of low-level atmospheric refractivity gradient using ground echo coverage

The vertical gradient of refractivity (dN/dh) determines the path of the radar beam; e.g., the larger (smaller) negative gradient of refractivity, the more (less) bending toward the ground. This significantly affects the coverage of ground echoes in scanning radar measurements, and thus information about the propagation conditions should be useful for quality control of radar products. The vertical gradient of refractivity is usually estimated from radiosonde soundings, which are, however, hampered by their coarse temporal and spatial resolution. Since radar ground echo coverage can inform how severe the bending may be, we investigated a method that uses radar observations to characterize propagation conditions with better temporal resolution than usual soundings.

We first compared ground echo coverage in radar observations with the vertical gradient of refractivity estimated from AERI (Atmospheric Emitted Radiance Interferometer) retrieved soundings, with a resolution in time comparable to radar observations, during the field experiment IHOP_2002. The results confirmed that the echo coverage becomes larger (smaller) corresponding to a larger (smaller) negative gradient of refractivity. Encouraged by this result, we decided to compute the ground echo coverage by ray tracing depending on dn/dh over the radar domain and to compare it with observed echo coverage. The vertical gradient refractivity that gives us the best match has been validated with available sounding observations. This method showed some ability to capture information about the gradient of refractivity at low-level.