The Snowline and 0°C Isotherm Altitudes During Precipitation Events in the Dry Subtropical Chilean Andes as Seen by Citizen Science, Surface Stations, and ERA5 Reanalysis Data

Abstract

Understanding the variability of the snowline and the closely related 0°C isotherm during infrequent precipitation events in the dry Andes in Chile is fundamental for precipitation, snow cover, and discharge predictions. For instance, it is known that on the windward side of mountains, the 0°C isotherm can be several hundreds of meters lower than on the free air upwind counterpart, but little is understood about such effects in the Andes due to missing in situ evidence on the precipitation phase. To bridge this gap, 111 photographs of the snowline after precipitation events between 2011 and 2021 were gathered in the frame of a citizen science programme to estimate the snowline altitude. Since photographs of the mountain snowline are in good agreement with Sentinel-2 imagery, they have great potential to validate empirical snowline estimations. Using the snowline altitude from the photos, we evaluated different methods to estimate the snowline and 0°C isotherm altitude during precipitation events based on surface meteorological observations and ERA5 reanalysis data. We found a high correlation between the observed snowline altitude and the extrapolated 0°C isotherm based on constant lapse rates (−5.5 to −6.5°C km−1) applied to air temperature from single, near stations. However, uncertainty increases for distances >10 km. The results also indicate that the linear regression method is a good option to estimate ZSL, but the results strongly depend on the availability of high-elevation station datasets. During half of the precipitation events, the 0°C isotherm lies between ∼1,800 and ∼2,400–2,500 m asl. in winter, and the snowline is on average ∼280 m below this altitude. Our results indicate the presence of a mesoscale lowering of the 0°C isotherm over the windward slopes compared to the free-air upwind value during precipitation events and a possible isothermal layer of near-freezing air temperatures comparable to other mountain ranges. Due to this mesoscale and local behavior, ERA5 data generally overestimate the snow–rain transition in high-elevation areas, especially for relatively intense events. On the other hand, the 0°C isotherm altitude is underestimated if only low-elevation valley stations are considered, highlighting the importance of high-altitude meteorological stations in the network.

Thermal simulations on periglacial soils of the Central Andes, Argentina

“…We applied a simple standard air temperature lapse rate (−6.5°C km −1 ) for bias correction of the MAT parameter at the study site. Even though it is an oversimplification for the MAAT calculation and may not reflect the local lapse rates on complex terrain surfaces, it could be accepted as the mean value in long‐time reconstructions 53 …”

“…Even though it is an oversimplification for the MAAT calculation and may not reflect the local lapse rates on complex terrain surfaces, it could be accepted as the mean value in long-time reconstructions. 53 Precipitation was also estimated using gridded daily data from the Climate Hazards Group InfraRed Precipitation with Station (CHIRPS) database (https://www.chc.ucsb.edu/data/chirps). The CHIRPS data set shows a good agreement for the representation of the seasonal and interannual precipitation variability in the Central Western Andes, 54 thus being suitable for this study.…”

A conceptual hydrological model of semiarid Andean headwater systems in Chile

“…The 0°C isotherm is generally below 2000 m a.s.l. during significant precipitation events (Schauwecker et al, 2022), and even though the snow-rain transition is highly variable in the SA due to atmospheric processes (Scaff et al, 2017; Schauwecker et al, 2022), the relatively low elevation suggests that very little rainfall occurs in headwater catchments. In fact, Ayala et al (2020) found that rain only accounts for 3% of the total runoff contribution at the annual scale of the Maipo headwaters between 1955 and 2016.…”

Spatial distribution and controls of snowmelt runoff in a sublimation-dominated environment in the semiarid Andes of Chile

“…as important for snow accumulation in this type of environment (Schauwecker et al, 2022). Second, the high sublimation rates (of up to 300 mm a −1 , Figure 9) use large amounts of energy from the snowpack that would be otherwise available for melt.…”

“…A particularly critical case of snowmelt dependency is that of arid and semiarid high-elevation mountain ranges, such as the semiarid Andes, Central Asia, and Southwestern USA (Huning and AghaKouchak, 2020). The climate of these mountain ranges is characterized by low temperatures and little precipitation (hereafter we refer to these ranges as cold and dry mountain regions), and pose characteristic challenges to estimate snowmelt runoff, such as episodic precipitation events (Schauwecker et al, 2022), shallow snowpacks (Zhang and Ishikawa, 2008) and high sublimation rates (Stigter et al, 2018).…”