## 3 Results
### Regional Case Studies
#### 3.5.1 Spring Onset Characteristics over Alaska  
The spring onset is closely linked with seasonal thaw timing and the initiation of the vegetation growing season, where the frozen season is a significant constraint on annual vegetation growth [61]. The primary spring thaw date was determined from the 9 km SMAP daily global FT record over Alaska as the first date (DOY) for which 12 out of 15 successive days from March to July are classified as non-frozen for each year of record [1]. The growing season onset was also determined for each grid cell and study year as the mean DOY when GPP was between 10% and 20% of the annual maximum daily rate (GPP\({}_{\text{max}}\), [4]). Here, daily GPP over Alaska was obtained for the study period from the SMAP L4_C global product [62], where GPP and other carbon flux variables are derived using a satellite-driven terrestrial carbon model informed by MODIS vegetation and SMAP assimilation enhanced surface-to-root zone soil moisture and temperature inputs [37]. The SMAP L4_C product is produced in the same 9 km global grid format as the L3_FT_E global product.  
The resulting pattern of spring thaw timing and growing season onset from the SMAP record is shown in Figure 7. The 2016 record was associated with a major El Nino and showed generally earlier thaw timing over Alaska, which is consistent with above-normal spring temperatures relative to 2017 [63; 64]. However, the spatial pattern of thaw timing was similar for both years, beginning in mid-March over southwest Alaska and progressing into mid-May at higher latitudes and elevations.  
The growing season onset indicated by the SMAP L4_C GPP record lagged the primary thaw timing by approximately 46 \(\pm\) 24 (spatial SD) days over Alaska (Figure 7). The SMAP FT record indicates generally earlier spring thaw timing than the FT retrievals from higher frequency (\(\sim\)37 GHz) Tb measurements [4; 65]. These results, which are consistent with earlier studies, indicate that the SMAP L-band FT record may be more sensitive to initial surface thawing and wet snow conditions within a deeper snow layer, relative to higher frequency retrievals that are sensitive to more ephemeral surface FT variations [9; 24; 66]. The seasonal progression in the proportional area of non-frozen conditions preceded the spring GPP growing season onset in Alaska for both study years. The timing and rate of increase in thawed area for each year was also inversely proportional to spring productivity, whereby cumulative daily GPP showed higher productivity in 2016 due to earlier spring thaw than in 2017 (Figure 7). These results confirm the importance of the spring FT signal as an effective proxy for the seasonal release of frozen temperature constraints on ecosystem productivity and terrestrial carbon (CO\({}_{2}\)) sink activity in the boreal-Arctic [4; 61], while the SMAP L-band signal is expected to have greater sensitivity to soil FT conditions relative to higher frequency Tb based FT records [1].  
Figure 7: Spring thaw onset pattern over Alaska derived from SMAP global daily 9 km FT retrievals for 2016 and 2017, and the corresponding GPP onset indicated from the SMAP L4_C record. The grey shading in the images denotes where the SMAP data are unavailable. The lower plot shows the seasonal progression in the proportional area (% of Alaska domain) of the FT classified non-frozen (NF, in black) conditions and the cumulative daily GPP (in red) over Alaska from January to July, in 2016 and 2017.