The Effect of Phase-Correlated Returns and Spatial Smoothing on the Accuracy of Radar Refractivity Retrievals

Radar refractivity retrievals have the potential to accurately capture near-surface humidity fields from the phase change of ground clutter returns. In practice, phase changes are very noisy and the required smoothing will diminish large radial phase change gradients, leading to severe underestimates of large refractivity changes (DN). Tomitigate this, themean refractivity change over the field (hDNifield) must be subtracted prior to smoothing. However, both observations and simulations indicate that highly correlated returns (e.g., when single targets straddle neighboring gates) result in underestimates of hDNifield when pulse-pair processing is used. This may contribute to reported differences of up to 30 N units between surface observations and retrievals. This effect can be avoided if hDNifield is estimated using a linear least squares fit to azimuthally averaged phase changes. Nevertheless, subsequent smoothing of the phase changes will still tend to diminish the all-important spatial perturbations in retrieved refractivity relative to hDNifield; an iterative estimation approach may be required. The uncertainty in the target location within the range gate leads to additional phase noise proportional to DN, pulse length, and radar frequency. The use of short pulse lengths is recommended, not only to reduce this noise but to increase both themaximumdetectable refractivity change and the number of suitable targets. Retrievals of refractivity fields must allow for large DN relative to an earlier reference field. This should be achievable for short pulses at S band, but phase noise due to target motionmay prevent this at C band, while at X band even the retrieval of DN over shorter periods may at times be impossible.