Heavy Rain Observed by InSAR and SSM; Detection and Interpretation of Tropospheric Dispersive Phase

Abstract:

Taking advantage of phase differentiation between two Synthetic Aperture Radar (SAR) observation time, Interferometric Synthetic Aperture Radar (InSAR) can reveal surface deformation induced by, for instance, earthquake and volcano eruption. Furthermore, InSAR is also beneficial to atmospheric sciences. The microwave signal transmitted by the SAR satellite and passing through the atmosphere experiences changes in its propagation speed because of the different refractivity from the vacuum. Therefore, InSAR has been used to map ionospheric anomalies, such as mid-latitude sporadic-E (Maeda et al., 2016) and an enormous water vapor associated with heavy rain in the troposphere (Kinoshita et al., 2013). Further, Mateus et al., 2018, showed a significant heavy rain forecast improvement using a high spatial resolution of water vapor map derived from InSAR.

The range split-spectrum method (SSM) (Brcic et al., 2010; Rosen et al., 2010; Gomba et al., 2016, Furuya et al., 2017) was developed to mitigate ionospheric disturbance in the InSAR data, by exploiting the dispersive nature of the ionosphere. We apply InSAR and SSM on heavy rain (rain rate exceeding 50 mm/hour) cases in Japan. Originally, we did not expect a dispersive phase detection, because we assume the Total Electron Content (TEC) is relatively low during heavy rain. However, we observed clear dispersive phases that distributed non-randomly besides the strong non-dispersive phase due to presumably to the dense water vapor during heavy rain.

Then, exploiting large bandwidth of ALOS-2/PALSAR-2, we create high and low-frequency interferograms not only in the heavy rain but also in fair weather conditions to examine the frequency dependence of the dispersive phase. We will discuss the origin of the tropospheric dispersive phase both in the fair weather and heavy rain, assuming free electron in the atmosphere is negligible.