Snow Water Equivalent of Dry Snow Measured by Differential Interferometry

Abstract

Large scale mapping of snow water equivalent (SWE) is a long-lasting request in many scientific and economical fields. Active and passive microwave remote sensing methods are explored, as local methods cannot be generalized due to the spatial inhomogeneity of the snow pack. Microwaves interact with snow by absorption, scattering, and refraction. For dry snow of a few meters depth and frequencies below 20 GHz, absorption and scattering in the snow volume are negligible compared with the backscattered energy from the underlying ground. The signal delay caused by refraction can be measured with differential radar interferometry, but phase wrapping errors and temporal decorrelation must be considered. We demonstrate that large $\Delta\text{SWE}$ can be accurately determined from dense time series of differential interferograms at X- and Ku-band by temporal integration. Lost phase cycles are reconstructed with a two-frequency approach. Temporal decorrelation is minimized by a temporal resolution of 4h. A linear function between $\Delta\text{SWE}$ and phase difference is derived, which deviates only a few percent from the exact solution and which depends negligibly on snow density and stratigraphy. $\Delta\text{SWE}$ retrieved from observations of the SnowScat instrument (SSI) were validated against observed SWE from different reference instruments, installed at a test site near the town of Sodankyla, Finland. An accuracy below $\pm 6\;\text{mm}$ SWE was achieved at frequencies of 10 and 16 GHz for up to 200 mm of $\Delta$ SWE. An exceptionally high temporal coherence was observed for up to 30 days for dry snow, whereas for wet snow it decayed within hours.