Evaporation in the Atacama Desert

Understanding evaporation in arid regions is essential for climate change assessment and optimizing water resources management under a changing climate. This thesis analyses the physical processes that govern local evaporation in a representative salt flat setting at the Altiplano region of the Atacama Desert. Such physical processes are analyzed from climatic to sub-diurnal scales. Moreover, regional (>100 km) to local (<1 km) scales are integrated, through the analysis of the ocean-desert circulation and its influences on the atmospheric boundary layer and surface fluxes. Firstly, findings show that wind-driven turbulence is the primary evaporation controller at the sub-diurnal scale, whereas radiation plays a major role at the seasonal scale. Secondly, at the local scale, surface fluxes are mainly controlled by mechanical turbulence, which is only present in the afternoon due to a regional-scale flow resulting from the thermal contrast between the cool Pacific Ocean and the warm Atacama Desert. Thirdly, the regional flow that triggers evaporation in the Altiplano origins from the marine boundary layer, whose stability regime dominates the formation of fog and the inland moisture transport. Finally, the role of surface heterogeneity on turbulent fluxes measurements is quantified at the sub-kilometer scale, where footprint and MOST functions play an important role. This thesis contributes to untangling and linking processes driving evaporation from local to regional-scale and from sub-diurnal to inter-annual scale, across confined saline lakes in arid regions

Characterizing the influence of the marine stratocumulus cloud on the land fog at the Atacama Desert

Fog in the Atacama Desert is a virtually untapped source of fresh water in the driest place on Earth. Focusing on understanding the role played by marine stratocumulus (Sc) in the development of land-fog, we analyse surface observations made along a steep transect at different heights. These observations are combined with numerical experiments performed using the Weather Research and Forecasting Model (WRF). We find two main diurnal regimes based on atmospheric thermal stability, both of which determine the formation and dissipation of fog. These are (a) a well-mixed regime characterized by low gradients of potential temperature and specific humidity, low diurnal variability, and presence of Sc cloud-fog. (b) A stratified regime characterized by high gradients of potential temperature and specific humidity, high diurnal variability, but no Sc clouds nor presence of fog. By using the parcel method, initialised with surface observations, we characterize the Sc cloud of nine typical fog events, estimating a mean cloud depth of 566 m between 740 m (±150 m) and 1307 m (±30 m). Fog observations at ground level agree with these cloud-base and cloud-top estimates, showing a liquid water mixing ratio of Sc cloud-fog in the range 0.3–0.7 g kg−1. The study reveals that the advection of marine Sc cloud and the stability of the boundary layer are key processes in the formation and dissipation of fog. Sc cloud advection over land is modulated by upwind and driven by topography and local circulation. We conclude that a realistic characterization of Sc cloud-fog is possible by combining limited surface observations and numerical experiments.</p

Optical-Microwave Scintillometer Evaporation measurements over a Saline Lake in a Heterogeneous Setting in the Atacama Desert

Estimating lake evaporation is a challenge due to both practical considerations and theoretical assumptions embedded in indirect methods. For the first time, we evaluated measurements from an optical microwave scintillometer (OMS) system over an open water body under arid conditions. The OMS is a line-of-sight remote sensing technique that can be used to measure the sensible- and latent heat fluxes over horizontal areas with path lengths ranging from 0.5-10 km. We installed an OMS at a saline lake surrounded by a wet-salt crust in the Salar del Huasco, a heterogeneous desert landscape in the Atacama Desert. As a reference, we used Eddy Covariance systems installed over the two main surfaces in the OMS footprint. We performed a footprint analysis to reconstruct the surface contribution to the OMS measured fluxes (80% water and 20% wet-salt). Furthermore, we investigated the applicability of the Monin-Obukhov Similarity Theory (MOST) which was needed to infer fluxes from the OMS-derived structure parameters to the fluxes. The OMS structure parameters and MOST were compromised, which We mitigated by fitting MOST coefficients to the site conditions. We argue that the MOST deviation from values found in the literature due to the effects of the surface heterogeneity and the non-local processes induced by regional circulation. With the available data set we were not able to rule out instrument issues, such as additional fluctuations to the scintillation signal due to absorption or the effect of vibration in high wind conditions. The adjusted MOST coefficients lowered by a factor of 1.64 compared to using standard MOST coefficients. For H and K=LvE, we obtained zero-intercept linear regressions with correlations, R2, of 0.92 and 0.96, respectively. We conclude that advances in MOST are needed to successfully apply the OMS method in landscapes characterized by complex heterogeneity such as the Salar del Huasco

Midday Boundary-Layer Collapse in the Altiplano Desert : The Combined Effect of Advection and Subsidence

Observations in the Altiplano region of the Atacama Desert show that the atmospheric boundary layer (ABL) suddenly collapses at noon. This rapid decrease occurs simultaneously to the entrance of a thermally driven, regional flow that causes a rise in wind speed and a marked temperature decrease. We identify the main drivers that cause the observed ABL collapse by using a land–atmosphere model. The free atmosphere lapse rate and regional forcings, such as advection of mass and cold air as well as subsidence, are first estimated by combining observations from a comprehensive field campaign and a regional model. Then, to disentangle the ABL collapse, we perform a suite of numerical experiments with increasing level of complexity: from only considering local land–atmosphere interactions, to systematically including the regional contributions of mass advection, cold air advection, and subsidence. Our results show that non-local processes related to the arrival of the regional flow are the main factors explaining the boundary-layer collapse. The advection of a shallower boundary layer (≈ - 250 m h- 1 at noon) causes an immediate decrease in the ABL height (h) at midday. This occurs simultaneously with the arrival of a cold air mass, which reaches a strength of ≈ - 4 K h- 1 at 1400 LT. These two external forcings become dominant over entrainment and surface processes that warm the atmosphere and increase h. As a consequence, the ABL growth is capped during the afternoon. Finally, a wind divergence of ≈ 8 × 10 - 5 s- 1 contributes to the collapse by causing subsidence motions over the ABL from 1200 LT onward. Our findings show the relevance of treating large and small-scale processes as a continuum to be able to understand the ABL dynamics

Multi-scale temporal analysis of evaporation on a saline lake in the Atacama Desert

We investigate how evaporation changes depending on the scales in the Altiplano region of the Atacama Desert. More specifically, we focus on the temporal evolution from the climatological to the sub-diurnal scales on a high-altitude saline lake ecosystem. We analyze the evaporation trends over 70 years (1950–2020) at a high-spatial resolution. The method is based on the downscaling of 30 km ERA5 reanalysis data at hourly resolution to 0.1 km spatial resolution data, using artificial neural networks to analyze the main drivers of evaporation. To this end, we use the Penman open-water evaporation equation, modified to compensate for the energy balance non-closure and the ice cover formation on the lake during the night. Our estimation of the hourly climatology of evaporation shows a consistent agreement with eddy-covariance (EC) measurements and reveals that evaporation is controlled by different drivers depending on the time scale. At the sub-diurnal scale, mechanical turbulence is the primary driver of evaporation, and at this scale, it is not radiation-limited. At the seasonal scale, more than 70 % of the evaporation variability is explained by the radiative contribution term. At the same scale, and using a large-scale moisture tracking model, we identify the main sources of moisture to the Chilean Altiplano. In all cases, our regime of precipitation is controlled by large-scale weather patterns closely linked to climatological fluctuations. Moreover, seasonal evaporation significantly influences the saline lake surface spatial changes. From an interannual scale perspective, evaporation increased by 2.1 mm yr−1 during the entire study period, according to global temperature increases. Finally, we find that yearly evaporation depends on the El Niño–Southern Oscillation (ENSO), where warm and cool ENSO phases are associated with higher evaporation and precipitation rates, respectively. Our results show that warm ENSO phases increase evaporation rates by 15 %, whereas cold phases decrease it by 2 %

Spatial distribution and interannual variability of coastal fog and low clouds cover in the hyperarid Atacama Desert and implications for past and present Tillandsia landbeckii ecosystems

The hyperarid Atacama Desert coast receives scarce moisture inputs mainly from the Pacific Ocean in the form of marine advective fog. The collected moisture supports highly specialized ecosystems, where the bromeliad Tillandsia landbeckii is the dominant species. The fog and low clouds (FLCs) on which these ecosystems depend are affected in their interannual variability and spatial distribution by global phenomena, such as ENSO. Yet, there is a lack of understanding of how ENSO influences recent FLCs spatial changes and their interconnections and how these variations can affect existing Tillandsia stands. In this study, we analyze FLCs occurrence, its trends and the influence of ENSO on the interannual variations of FLCs presence by processing GOES satellite images (1995–2017). Our results show that ENSO exerts a significant influence over FLCs interannual variability in the Atacama at ~ 20°S. Linear regression analyses reveal a relation between ENSO3.4 anomalies and FLCs with opposite seasonal effects depending on the ENSO phase. During summer (winter), the ENSO warm phase is associated with an increase (decrease) of the FLCs occurrence, whereas the opposite occurs during ENSO cool phases. In addition, the ONI Index explains up to ~ 50 and ~ 60% variance of the interannual FLCs presence in the T. landbeckii site during summer and winter, respectively. Finally, weak negative (positive) trends of FLCs presence are observed above (below) 1000 m a. s. l. These results have direct implications for understanding the present and past distribution of Tillandsia ecosystems under the extreme conditions characterizing our study area

Climate and coastal low-cloud dynamic in the hyperarid Atacama fog Desert and the geographic distribution of Tillandsia landbeckii (Bromeliaceae) dune ecosystems

Despite the extensive area covered by the coastal Atacama fog Desert (18-32 degrees S), there is a lack of understanding of its most notorious characteristics, including fog water potential, frequency of fog presence, spatial fog gradients or fog effect in ecosystems, such as Tillandsia fields. Here we discuss new meteorological data for the foggiest season (July-August-September, JAS) in 2018 and 2019. Our meteorological stations lie between 750 and 1211 m a. s. l. at two sites within the Cordillera de la Costa in the hyperarid Atacama (20 degrees S): Cerro Oyarbide and Alto Patache. The data show steep spatial gradients together with rapid changes in the low atmosphere linked to the advection of contrasting coastal (humid and cold) and continental (dry and warm) air masses. One main implication is that fog presence and fog water yields tend to be negatively related to both distance to the coast and elevation. Strong afternoon SW winds advect moisture inland, which take the form of fog in only about 6% of the JAS at 1211 m a. s. l., but 65% at 750 m a. s. l. on the coastal cliff. Although sporadic, long lasting fog events embrace well-mixed marine boundary layer conditions and thick fog cloud between 750 and 1211 m a. s. l. These fog events are thought to be the main source of water for the Tillandsia ecosystems and relate their geographic distribution to the lowest fog water yields recorded. Future climate trends may leave fog-dependent Tillandsia even less exposed to the already infrequent fog resulting in rapid vegetation decline

Advancing understanding of land–atmosphere interactions by breaking discipline and scale barriers

Vegetation and atmosphere processes are coupled through a myriad of interactions linking plant transpiration, carbon dioxide assimilation, turbulent transport of moisture, heat and atmospheric constituents, aerosol formation, moist convection, and precipitation. Advances in our understanding are hampered by discipline barriers and challenges in understanding the role of small spatiotemporal scales. In this perspective, we propose to study the atmosphere–ecosystem interaction as a continuum by integrating leaf to regional scales (multiscale) and integrating biochemical and physical processes (multiprocesses). The challenges ahead are (1) How do clouds and canopies affect the transferring and in-canopy penetration of radiation, thereby impacting photosynthesis and biogenic chemical transformations? (2) How is the radiative energy spatially distributed and converted into turbulent fluxes of heat, moisture, carbon, and reactive compounds? (3) How do local (leaf-canopy-clouds, 1 m to kilometers) biochemical and physical processes interact with regional meteorology and atmospheric composition (kilometers to 100 km)? (4) How can we integrate the feedbacks between cloud radiative effects and plant physiology to reduce uncertainties in our climate projections driven by regional warming and enhanced carbon dioxide levels? Our methodology integrates fine-scale explicit simulations with new observational techniques to determine the role of unresolved small-scale spatiotemporal processes in weather and climate models