Initial flight test results of differential absorption barometric radar for remote sensing of sea surface air pressure

The accuracy of numerical weather model predictions of the intensity and track of tropical storms may be significantly improved by large spatial coverage and frequent sampling of sea surface barometry. The availability of a radar operating at moderate-to-strong O₂ absorption bands in the frequency range 50∼56 GHz to remotely measure surface barometric pressure may provide such capability. At these frequencies, the strength of radar echoes from water surfaces has a strong gradient with frequencies owing to the absorption of atmospheric O₂. Our recent research has developed a technique based on the use of a dual-frequency, O₂-band radar to estimate surface barometric pressure from the measured attenuation due to O₂. The ratio of reflected radar signals at multiple wavelengths is used to minimize the effect of microwave absorption by liquid water and water vapor in the atmosphere, and the influences of sea surface reflection over the frequency of operation. A demonstration instrument has been developed to verify the differential O₂ absorption measurement approach. Recent test flights to evaluate the in-flight performance of the demonstration instrument have been completed. The measured radar return and differential O₂ absorption show good agreement with the modeled results. These flight test results are consistent with our instrumentation goal of ±5 mb uncertainty and indicate that our proposed differential absorption measurement approach may provide a useful measurement of sea surface pressure. Future test flights will provide higher altitude data and assess the precision of the sea surface pressure measurement for the existing demonstration radar.