Differential absorption microwave radar measurements for remote sensing of atmospheric pressure

The accuracy of numerical weather model predictions of the intensity and track of tropical storms may be improved by large spatial coverage and frequent sampling of sea surface barometry. The feasibility of a microwave radar operating at moderate to strong O₂ absorption bands in the frequency range of 50~56 GHz to remotely measure surface barometric pressure may provide such capability. At these radar wavelengths, the reflection of radar echoes from water surfaces is strongly attenuated by atmospheric column O₂. Because of the uniform mixture of O₂ gases within the atmosphere, the total atmospheric column O₂ is proportional to atmospheric path lengths and the total atmospheric column air, and thus, to surface barometric pressures. Recent research has developed a technique based on the use of a dual-frequency, O₂-band radar to overcome many of the difficulties associated with techniques requiring larger frequency separation. The ratio of reflected radar signals at multiple wavelengths are used to minimize the effect of microwave absorption by liquid water, water vapor in the atmosphere and the influences of sea surface reflection over the frequency of operation. Langley Research Center (LaRC) has developed a radar based on this measurement technique to estimate barometric pressure. This paper will present an overview of the differential absorption measurement concept and will discuss a radar instrument to verify the differential O₂ absorption measurement approach. Results of instrument functional testing and initial measurements will be presented.