Assessment of absorbing aerosols on austral spring snow albedo reduction by several basins in the Central Andes of Chile from daily satellite observations (2000–2016) and a case study with the WRF-Chem model
Changes in snow albedo (SA) on the Limari, Choapá, Aconcagua and Maipo basins of the Central Andes of Chile (CAC) are associated with the possible deposition of light-absorbing particles in the austral spring. We correlate SA with daily data of snow cover, aerosol optical depth (AOD) and land surface temperature (LST) available from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the NASA Terra satellite between 2000 and 2016, and other derived parameters such as days after albedo (DAS) and snow precipitation (SP). We used satellite pixels with 100% snow cover to obtain monthly average value of SA, LST, AOD, DAS and SP from September to November performing multiple regression analysis. We show that in Maipo, after considering LST, AOD represents an important role in changes induced to SA. The multiple regression model illustrates that AOD increases can reduce the SA during spring months by 13.59, 0.01, 0.77 and 3.8% in Limari, Choapá, Aconcagua and Maipo, respectively. In addition, we used a numerical prediction Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), showing that the black carbon distribution and average daily AOD are associated with the SA decrease of 0.15 in the Maipo basin between September 29 and 30, 2016. The WRF-Chem output showed aerosols are transported mainly with dominating westerly winds to the Limari and Maipo basins. Our results further suggest that SA decrease due to AOD may be originated in the largest industrial and urban areas in Chile, producing a negative impact on the hydrological resource, generated in the CAC.
KeywordsSnow albedo MODIS AOD Snow darkening effect WRF-Chem Central Andes in Chile
Snow albedo is a parameter of great importance to determine the amount of solar radiation adsorbed in the cryosphere and is defined as a relationship between incoming and reflected solar radiation by a surface. Fresh snow can reflect almost completely incoming solar radiation (0.95) but can decrease to 0.45 due to metamorphism [1, 2, 3, 4]. Snow albedo variations are influenced, among others by the surface temperature, snowfall, snow age and snow impurities [2, 5, 6, 7]. Snow impurities (LAP) reduce the snow albedo and absorb more solar radiation (also called, snow darkening effect—SDE), which further accelerates the snow aging process and the melting rate of the layer of snow [8, 9, 10, 11, 12, 13]. LAP and its SDE were identified as the main forcing agents that affect climate change [14, 15].
Besides the above-mentioned studies, we are not aware of any other regional study in CAC on the effect of deposition of absorbent aerosols on snow surfaces and their impact on the increase in snow ablation melting in each of the basins where they feed rivers. To improve this understanding, the main objective of this study was to investigate the aerosols effect on the snow albedo reduction in several basins of the CAC during the spring season, based on satellite remote sensing data for the years 2000–2016. The role aerosols that play in the negative snow albedo trend have been treated in , and its relationship with the distribution and local deposition of aerosols in snow has been identified in previous studies for this region [23, 25, 26, 27].
2 Data and methodology
To analyze the albedo reduction in the CAC (30.2°S and 34.3°S), we selected the area of four basins with an elevation greater than 2000 m to observe the behavior on the Chilean side of the mountain range (western), as shown in Fig. 1. The Global Digital Elevation Model (GTOPO30) developed by the US Geological Survey (USGS) National Center for Earth Resources Observation and Science (EROS) Data Center with a 1 km resolution was used as surface topography (https://www.usgs.gov/centers/eros/science/usgs-eros-archive-digital-elevation-global-30-arc-second-elevation-gtopo30).
Data retrieved from used satellite products
Used satellite product
Spatial resolution (km)
All pixels with 100% snow cover
Clouds presence produced very frequently missing satellite data (daily). Therefore, we used a series of 17 years of daily data of the austral spring (September to November from 2000 to 2016) to minimize gaps and produce a more reliable statistic. Since TRMM detects only liquid precipitation, a parameter was generated to estimate the days where precipitation was snow, by detecting temperatures below the freezing level at − 4 °C, taking the uncertainties of MOD11 measurements into account . The number of days between snowfalls was also estimated by a parameter called days after snow (DAS).
The daily data sets were analyzed by means of box and whisker plot allowing to observe the variability and skewness of the data and if so, in what direction. To estimate the effect of aerosol deposition on the SA reduction in each basin, the correlation of SA to LST, AOD, snow precipitation (SP) and DAS was investigated by means of a multiple regression analysis for all available data sets per basin. Finally, in a case study we ran the numerical prediction Weather Research and Forecasting model coupled with Chemistry (WRF-Chem)  for a few selected days and compared them with the average of snow albedo (SA) and AOD data retrieved over two basins. To estimate the regional source of black carbon (BC), we studied the west winds that drag air masses toward the high areas of the basins studied using WRF-Chem and backward trajectories using NOAA HYSPLIT [40, 41].
3 Analysis and discussion
3.1 Observational data
Correlation coefficients (r) of SA with LST, ADO, DAS and SP for each basin of the study area
Contribution percentage to SA of each parameter for the data sets and basins analyzed using the multiple regression model
3.2 Case study using WRF-Chem model
3.2.1 WRF-Chem modeling
To analyze the contribution of aerosol to the studied basins, we run a numerical meteorological prediction model, the Weather Research and Forecast with Chemistry (WRF-Chem) . This model was used with a parametrization shown in Suppl. Mat. Table S1, which was already tested in this region [47, 48, 49, 50]. WRF-Chem model was run using three nested domains (shown in Suppl. Mat. Fig. S1) and was verified using vertical profiles of several meteorological variables for September 28 and 29, 2016 (shown in Suppl. Mat. Figs. S2 and S3, respectively). Aerosol deposition as light-absorbing particles (BC) is very efficient to decrease the snow albedo [34, 51, 52, 53, 54], and we used BC to simulate and estimate the distributions over the basins of the study area (Fig. 1). We expected the aerosol to come mainly from dominating westerly wind in the Central Andes [55, 56, 57] from the urban and industrial areas of Chile and moving toward the higher altitudes of the basins covered with snow . For the two selected basins (with higher influence of the aerosols in the decrease in snow albedo, according to the regression model in Table 3), we recovered as an output from the WRF-Chem model, the daily averages of SA and AOD for pixels with 100% snow cover (shown in Fig. 5).
The Limari and Maipo basins have an average slope of 35.87 and 5.23%, respectively, in addition to a height of almost 5000 m a.s.l. Other studies in mountainous areas have shown that aerosol removal by deposition is efficient in high mountains [5, 58, 59, 60]. Therefore, our BC distribution and transport by westerly winds confirm the prediction of our multiple regression model on the influence of aerosols on the SA decrease for the period of time analyzed.
The snowpack in the studied area is a very important source of water supply in central Chile because snow stores fresh water during the cold and wet season and then gradually releases water during warm season, representing an important contribution to the river flows of this region [67, 68]. In addition, several studies suggest that a snowpack reduction in this mountain area has impacts on the hydrological cycle and the water supply for the region [57, 68, 69]. The variations and high absorbent aerosols deposited in the snow analyzed in this study suggest that the sources of anthropogenic aerosols may be playing a role in the availability of water through a positive effect with solar radiation.
Our research on the effect of aerosols on the snow albedo (SA) shows that the SA average does not exceed 0.55 for the whole study area. The Limari and Maipo basins located more to the north and south, respectively, were the only ones with extreme values higher than 0.9. The average SA was the lowest in Choapá due to the increase in LST and DAS. Snow precipitation (SP) showed to be the most important parameter contributing to the increase in SA in Limari, despite a high DAS, the high SP (in comparison with the other basins) produced the lowest LST and the highest SA average. In Limari, the AOD average was the highest. The multiple linear regression model including the LST, AOD, SP and DAS explains 84, 87, 74 and 76% of the variation in snow albedo in the Limari, Choapá, Aconcagua and Maipo basins, respectively. In addition, we evaluated the regression model with the maximum differences observed in the study period. The prediction of the regression equation shows that approximately 13.59, 0.01, 0.77 and 3.8% of the snow albedo reduction in the spring months in Limari, Choapá, Aconcagua and Maipo, respectively, are due to an increase in AOD. Our results suggest that decreases in SA due to AOD are correlated with the air masses originating from the largest industrial and urban areas in Chile. WRF-Chem modeling of a case study in the Limari and Maipo basins showed that, especially in the Maipo basin, the BC-modeled distribution and the average daily AOD are associated with a decrease in the SA. Also, there is a predominance of westerly winds bringing in air masses, containing high BC content, generated by anthropogenic activities from the region of Santiago de Chile and the V region as shown on backward trajectories simulated by NOAA HYSPLIT. Future research should consider the potential effects of LAP over the local radiative forcing, as well as model improvements for the analysis of WRF-Chem simulations to improve knowledge on the impact of such impurities in snow on surface energy and water budgets in this region.
This work was supported by Universidad Tecnológica Nacional (UTN IFI Projects PID 1799 and 1487, CONICET (CONICET PIP 112 201101 00673 and PICT 2016 1115), and the authors are thankful to FCE UNCUYO for allowing to run the WRF-Chem model on its cluster. We also acknowledge the MODIS, SRTM DEM and NOAA HYSPLIT mission scientists and associated personnel for the production of the data used in this research effort. All data requests should be addressed to the first author.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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