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To evaluate spatiotemporal backscatter sensitivity, snow property measurements collected at each site were compared against coincident estimates of integrated σ⁰. In an effort to reduce the influence of sub-scan variations in micro-topography, estimates of σ⁰ were averaged across a small range of incident angles between 30^◦ and 45^◦. In the averaged scatterometer observations, a large dynamic range of σ⁰ was found to be more than 8 dB. Copolarized σ⁰_vv and σ_hh⁰ ranged in magnitude from approximately -13 to -5 dB while cross-polarized σ_vh⁰ and σ⁰_hv responses were measured across a narrower range between -26 and -15 dB. For the entirety of the experiment period, cross-polarized σ⁰_vh and σ⁰_hv were found to be in agreement of less than 0.1 dB and therefore were considered reciprocal for the purpose of analysis. The large dynamic range of backscatter in each polarization channel suggested sensitivity to the evolving physical and dielectric properties warranting discussion of the involved interaction space.

The observed relationships between depth, SWE, and σ⁰, as shown in Figure6.5, indi-cate Ku-band sensitivity to evolving snow properties at the 26 open tundra sites. Across the small range of encountered mean depth, a clear linear relationship with σ_vv⁰ emerged

σ0(dB)

Figure 6.5: Comparison of average σ⁰ against snow depth (Left) and SWE (Right) at the open tundra observation sites. Solid circles show σ_vv⁰ response and hollow circles show σ_vh⁰ response. Measured σ_hh⁰ and σ⁰_hv responses not shown improve readability. Fit lines are linear.

where vertically polarized backscatter increased by approximately 0.22 dB for every 1-cm increase in depth (R² = 0.67). A less sensitive σ⁰_hh response, not shown in Figure 6.5, increased by 0.17 dB for every 1-cm in depth (R² = 0.44). Preferential vertical scattering was apparent throughout the experiment in which only 8 of 26 observations demonstrated a positive copolarization ratio (σ⁰_hh/ σ_vv⁰ ). The cross-polarized σ_vh⁰ response was comparable in slope to the copolarized channels, increasing by approximately 0.17 dB for every 1-cm increase in depth (R² = 0.39). Unsurprisingly, the observed cross-polarized sensitivity to depth σ⁰_vh was strongly correlated with increases in copolarization response at the same site.

As a whole, the observed increase in Ku-band backscatter response agrees with theory in which increasing path length generates additional first and upper order interactions within

the snow volume. The copolarized linear fits applied in Figure 6.5 produce root mean squared errors (RMSE) of 4.1 and 5.6 cm for σ⁰_vv and σ⁰_vh respectively. This result was encouraging as it demonstrated a strong potential to retrieve snow depth with sensitivity to small changes in the local tundra environment. Uncertainties in the potential retrieval of depth appears to be greatest with accumulation less than 5 cm where a large dynamic range of σ⁰ was observed. Under these very shallow conditions, co- and cross-polarized interactions were dominated by the response of the underlying soil rather than the snow volume. Analysis of variability of the shallow outliers was complicated by the seasonality of the measurements where all observations less than 5 cm were collected in shoulder season conditions where large differences in local soil moisture were likely influential. Without detailed inter-site measurements of soil properties it was not possible to evaluate differences found between the early season Ku-band observations and the full range of response. In spite of the large residuals observed under extremely shallow conditions, measured co- and cross- polarization backscatter demonstrated a coherent response with increased depth and potential for the retrieval under shallow tundra conditions.

Similar to snow depth, SWE accumulation in the open tundra environment was limited by persistent wind action and negligible precipitation input. Given that inter-site variations in SWE were found to be strongly related to depth, a response similar to the above result was expected between SWE and σ⁰. Figure 6.5 shows the strong Ku-band σ⁰ response identified with increasing SWE at the open tundra sites. Sensitivity to SWE was the strongest with σ_vv⁰ , increasing by 0.82 dB for every 1 cm in SWE (R² = 0.62). Similar to the observed relationship with depth, σ_hh⁰ showed weak sensitivity to SWE at 0.62 dB for every 1 cm increase (R² = 0.42). Finally, the cross-polarized σ_vh⁰ sensitivity was observed at 0.80 dB for every 1 cm increase in SWE (R² = 0.36). The observed increase in σ⁰ was large relative to the small range of observed SWE. Again, this result was encouraging for

the retrieval of tundra snow properties where strong sensitivity across a small range of SWE demonstrated potential to resolve subtle changes in snow mass.

In the observed relationship between SWE and σ⁰, backscatter variability separated sites of similar aggregate composition in some instances by several decibels. Seasonality and metamorphic state of the observed snowpack appeared to play a role in the backscatter diversity where increase in grain size was strongly correlated with increased copolarized backscatter response (R = 0.61). While this finding was of interest, the seasonality of the measurements and as a result, covariance of depth and grain growth, made it difficult to decompose causation between the evolving snow properties. Moreover, the previously demonstrated spatial variability in stratigraphy made it extremely difficult to interpolate the inter-site snow pits in the context of the larger scatterometer field of view. Overall, the distributed measurement protocol was able to show desirable Ku-band sensitivity to bulk snow properties, but did not provide the means to resolve the role of stratigraphy or grain size in observed backscatter diversity.