## 3 Results
### Regional Case Studies
#### 3.5.2 Australian Frost Events  
Recent studies have reported that the frequency of annual frost events has increased across southern Australia [67; 68]. Southern Australia has a characteristic temperate climate with little contrast between mean summer and winter temperatures, which may lead to fewer cold weather adaptations and greater vegetation susceptibility to potentially damaging frost. Frost occurrence in the region has resulted in significant agricultural crop damage and productivity losses [60; 69], while the wheat belt of southwest Australia experienced unusual frost events before harvest in September 2016 [70].  
The SMAP 9 km global daily FT records were analyzed over Australia to evaluate the spatial and seasonal pattern of frost occurrence over the region in 2016 and 2017. Here, frost occurrence represented days with classified frozen or transitional (AM or PM) FT conditions [59]. The seasonal pattern of frost-affected areas over the Australian portion of the global L3_FT domain is shown for each year of record in Figure 8. The SMAP-derived frost distribution in both years is consistent with the annual frost potential pattern established from long-term surface air temperature measurements using a standard 2 \({}^{\circ}\)C FT threshold [68; 71; 72]. Frost occurrence is primarily the result of nighttime freezing followed by subsequent daytime thawing, indicated by the transitional (AM frozen, PM thaw) FT condition from the SMAP FT daily composite. However, the SMAP results indicate a larger number of frost events in 2016 than in 2017. These results contrast with regional weather station observations that indicate frequent nighttime frosts across southern Australia from June to September 2017 [73], but warmer-than-normal air temperatures and lower documented frost occurrence in 2016 [74]. The apparent discrepancy may be attributed to annual climate variability and differences in physical sensing depth between the SMAP L-band FT retrievals representing surface soil and snow cover (when present) conditions, and a weather station frost condition defined from overlying daily minimum air temperatures. In 2016, Australia had above-average rainfall associated with a strong El Nino, whereas the rainfall in 2017 was below normal. Generally, drier soil conditions may account for fewer detected SMAP FT frost events in 2017, owing to the degraded FT dielectric contrast at low soil moisture levels [75]. Despite generally dry conditions, a succession of cold fronts and cold polar air intrusions from June to August resulted in significant snowfall across higher elevation areas in southeastern Australia in 2017 [76; 77]. The insulating effect of snow cover provides a strong thermal buffer between SAT extremes, including ephemeral frost events, and more stable underlying snow and soil layers [7; 78]; this may also account for the lower number of SMAP FT-derived frost days in 2017 over the mountain areas of southeast Australia relative to SAT-derived frost events reported from the regional weather station network. Therefore, relatively warmer and drier climate areas may promote a lower frost classification accuracy in relation to in situ SAT observations.