Three dimensional water vapor distribution based on InSAR data during the heavy rain over Shizuoka on 3 July 2008

Interferometric synthetic aperture radar (InSAR) is a technique for mapping the phase difference between two observations and used for detecting ground deformation associated with, for instace, earthquakes and volcanic activities. Like GPS, the phase signal in InSAR is affected by the earth's atmosphere which has a different refractive index relative to vacuum. In the neutral atmosphere, water vapor has the largest impact for InSAR phase signals due to its spatiotemporal heterogeneity. However, we can take advantage of this to detect an extremely detailed spatial distribution of precipitable water vapor in the absence of any ground displacements. On 3-4 July 2008, a heavy rain battered Shizuoka prefecture, the east side of Tokai region. The Advanced Land Observing Satellite (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) observed this area on 3 July 2008, and the interferogram obtained from that PALSAR data contained the localized signal near Mishima city, the horizontal width of which is on the order of 10 km and the amplitude reached 15 cm in radar line-of-sight. In the weather radar data, there was the strong echo equivalent to 80 mm per an hour in the west-southwest of the localized signal in the interferogram. Comparing this with other interferograms over the same region, we verified that the localized signal is not due to any ground displacements, but due to water vapor in the troposphere. Here we estimated three dimensional water vapor distribution during the heavy rain based on the interferogram including the water vapor-derived localized delay signal with the ray tracing method (Hobiger et al., 2008). The three dimensional refractivity field is a function of dry air density, temperature and water vapor density (Aparicio and Laroche, 2011), and is needed to estimate the delay amount by ray tracing. In this study, we first put values of dry air density and temperature which are spatially less variable to that of Mesoscale Model (MSM) data provided by Japan Meteorological Agency (JMA), then estimated the water vapor distribution by trial and error to correspond with the localized delay signal of the interferogram. Here we assumed that saturated water vapor extends upward from the lower troposphere. As a result, it turns out that there is the dry region lower than 50 % in relative humidity above 5000 m altitude, and the saturated water vapor within 10 km square in the horizontal and from the surface to 10000 m in the vertical, the axis of which is slightly tilting toward the west, at the localized signal in the interferogram. Calculating the zenith precipitable water vapor (PWV) from the estimated water vapor field, we found that the amount of PWV at the signal in the interferogram is 20 mm higher than that around the signal, and the location of high PWV exists at the location of the strong echo of the weather radar. This indicates that huge amount of water vapor locally exists in the convective cloud. We are trying to reproduce the situation of the heavy rain on 3 July 2008 using Weather and Research Forecasting model (WRF) for the comparison of that interferogram with this. In this presentation, we will show results discussed above and what we could understand from the result of the numerical weather simulation.