Surface-atmosphere Interactions Research Articles

A spatial-temporal continuous dataset of the transpiration to evapotranspiration ratio in China from 1981\u20132015

The ratio of plant transpiration to total terrestrial evapotranspiration (T/ET) captures the role of vegetation in surface-atmosphere interactions. However, several studies have documented a large variability in T/ET. In this paper, we present a new T/ET dataset (also including transpiration, evapotranspiration data) for China from 1981 to 2015 with spatial and temporal resolutions of 0.05° and 8 days, respectively. The T/ET dataset is based on a model-data fusion method that integrates the Priestley-Taylor Jet Propulsion Laboratory (PT-JPL) model with multivariate observational datasets (transpiration and evapotranspiration). The dataset is driven by satellite-based leaf area index (LAI) data from GLASS and GLOBMAP, and climate data from the Chinese Ecosystem Research Network (CERN). Observational annual T/ET were used to validate the model, with R2 and RMSE values were 0.73 and 0.07 (12.41%), respectively. The dataset provides significant insight into T/ET and its changes over the Chinese terrestrial ecosystem and will be beneficial for understanding the hydrological cycle and energy budgets between the land and the atmosphere.

Surface weathering on Venus: Constraints from kinetic, spectroscopic, and geochemical data

On Venus, understanding of surface-atmosphere interactions resulting from chemical weathering is both critically important for constraining atmospheric chemistry and relative ages of surface features and multifaceted, requiring integration of diverse perspectives and disciplines of study. This paper evaluates the issue of surface alteration on Venus using multiple lines of evidence. Surface chemistry from Venera and Vega landers is inconsistent with significant breakdown from atmospheric interactions, with <2.0 wt% S or less observed. Consideration of kinetics and breakdown of basalt under Venus conditions indicates diffusion of Ca > Fe > Mg toward the oxidizing Venus atmosphere, favoring creation of anhydrite and carbonate-rich surfaces on basalts with minor addition of hematite. When related to Venus-analog experiments, the kinetic calculations suggest a maximum coating of ~30 μm over 500,000 years. These changes would result in a slight overall volume increase in the outermost surface materials, which in turn decreases surface rock FeO contents. Those variations can be detected from orbit because emissivity is correlated with total FeO, and the predicted magnitudes are consistent with Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) observations. Models of anhydrite and hematite coatings on basalt mixtures suggest that changes in emissivity (ε) spectra due to chemical weathering can result in ca. <0.08 shifts in total emissivity. Such gradations are small compared to the first-order effect of bulk composition on emissivity, which can cause up to ~0.80 emissivity shifts. For all these reasons, there is at present no evidence to suggest that impenetrable coatings of either hematite (ε = 0.8) or anhydrite (ε = 0.1) are present on Venus. Orbital measurements of surface emissivity on a global scale could therefore produce not only a map of rock type and surface composition based on transition metal contents (largely FeO) (Helbert et al., 2020) but also provide local scale assessments of fresh vs. mature lava flows on the surface.

Evapotranspiration flux partitioning at a multi-species shrubland with stable isotopes of soil, plant, and atmosphere water pools

Evapotranspiration is the second largest component of the hydrological cycle after rainfall precipitation in semiarid regions such as northwestern Mexico. In this study, we partitioned the evapotranspiration (ET) flux using stable isotopes of water in the soil-plant-atmosphere continuum in combination with eddy covariance flux measurements. We considered three methods for determining the isotopic composition of transpiration (δT): (1) isotopic steady state, (2) non-steady state, and (3) non-steady state weighting the relative importance of dominant species cover and accounting for the relative values of stomatal conductance. Three approaches of T/ET partitioning were estimated during several wet and dry days at the study site in Sonora, Mexico. The total ET flux was variable across years due to differences in precipitation amount between years. ET was lower during the drier-year and reached higher values during the wetter-year. Soil evaporation (E) dominated ET soon after large rain events (40 to 70 mm d–1 [DOY-196 and DOY-197] in 2008) but showed a rapid decrease in dominance as the soil surface dried. Estimated values of T/ET based on an isotopic mass balance were in the same range independent of three methods used. The mean T/ET ratio across all the periods studied was in the range of other studies in semiarid ecosystems and global trends with values of ~0.67 ± 0.02. This work contributes to a better understanding of the surface atmosphere interactions in semi-arid regions.

Impacts of land use and land cover changes on regional climate in the Lhasa River basin, Tibetan Plateau

Land use and land cover change (LUCC) can alter land surface-atmosphere interactions in the exchange of energy fluxes, with additional consequences on temperature. Understanding the impacts of LUCC on the regional climate contributes to providing fundamental information for future land use planning and regional policy orientation, especially in extremely vulnerable and sensitive plateau regions. This study was designed to explore the regional climate effects associated with LUCC in the Lhasa River basin of Tibetan Plateau using the Weather Research and Forecasting (WRF) model, with a particular focus on near-surface air temperature (Ta) and surface energy fluxes. Two numerical experiments (Case 1980 and Case 2015) were simulated, spanning from November 2014 to November 2015 with the first month as spin-up. The results indicated that the conversion from croplands or grasslands to urban and built-up land led to a noticeable increase in Ta (0.23 °C) during summer. In areas converted from grasslands to waters, Ta decreased by 0.40 °C during spring and approximately 0.50 °C during winter. The afforestation activities at this scale had an obvious impact on the Ta in spring and winter, increasing by 0.20 °C and 0.10 °C, respectively. Generally, the latent heat flux (LE) and sensible heat flux (H) were more sensitive to land conversions, while changes in other fluxes seemed to be weaker. Due to the small change in net radiation (Rn) and ground heat flux (G), the H generally showed an opposite trend with that of LE. Urbanization and reservoir construction resulted in a decrease in the LE, while afforestation construction contributed to an increase in the LE. Our study highlights the impacts of current regional development in the plateau areas on the climate, especially in spring and winter. Urbanization led to a slight warming effect; the cooling effect was more significant in spring and winter than in summer after reservoir construction, and the current afforestation project contributed to a warming effect in winter. This study contributes to the future development of environmentally compatible and sustainable land strategies.

Effects of Lateral Flow on the Convective Environment in a Coupled Hydrometeorological Modeling System in a Semiarid Environment

AbstractEvidence for surface and atmosphere coupling is corroborated in both modeling and observation-based field experiments. Recent advances in high-performance computing and development of convection-permitting regional-scale atmospheric models combined with high-resolution hydrologic models have made modeling of surface–atmosphere interactions feasible for the scientific community. These hydrological models can account for the impacts of the overland flow and subsurface flow components of the hydrologic cycle and account for the impact of lateral flow on moisture redistribution at the land surface. One such model is the Weather Research and Forecasting (WRF) regional atmospheric model that can be coupled to the WRF-Hydro hydrologic model. In the present study, both the uncoupled WRF (WRF-ARW) and otherwise identical WRF-Hydro model are executed for the 2017 and 2018 summertime North American monsoon (NAM) seasons in semiarid central Arizona. In this environment, diurnal convection is impacted by precipitation recycling from the land surface. The goal of this work is to evaluate the impacts that surface runoff and shallow subsurface flow, as depicted in WRF-Hydro, have on surface–atmosphere interactions and convection in a coupled atmospheric simulation. The current work assesses the impact of surface hydrologic processes on 1) local surface energy budgets during the NAM throughout Arizona and 2) the spectral behavior of diurnally driven NAM convection. Model results suggest that adding surface and subsurface flow from WRF-Hydro increases soil moisture and latent heat near the surface. This increases the amount of instability and moisture available for deep convection in the model simulations and enhances the organization of convection at the peak of the diurnal cycle.

Processes shaping the spatial pattern and seasonality of the surface air temperature response to anthropogenic forcing

In the period 1960–2010, the land surface air temperature (SAT) warmed more rapidly over some regions relative to the global mean. Using a set of time-slice experiments, we highlight how different physical processes shape the regional pattern of SAT warming. The results indicate an essential role of anthropogenic forcing in regional SAT changes from the 1970s to 2000s, and show that both surface–atmosphere interactions and large-scale atmospheric circulation changes can shape regional responses to forcing. Single forcing experiments show that an increase in greenhouse gases can lead to regional changes in land surface warming in winter (DJF) due to snow-albedo feedbacks, and in summer (JJA) due to soil-moisture and cloud feedbacks. Changes in anthropogenic aerosol and precursor (AA) emissions induce large spatial variations in SAT, characterized by warming over western Europe, Eurasia, and Alaska. In western Europe, SAT warming is stronger in JJA than in DJF due to substantial increases in clear sky shortwave radiation over Europe, associated with decreases in local AA emissions since the 1980s. In Alaska, the amplified SAT warming in DJF is due to increased downward longwave radiation, which is related to increased water vapor and cloud cover. In this case, although the model was able to capture the regional pattern of SAT change, and the associated local processes, it did not simulate all processes and anomalies correctly. For the Alaskan warming, the model is seen to achieve the correct regional response in the context of a wider North Pacific anomaly that is not consistent with observations. This demonstrates the importance of model evaluation that goes beyond the target variable in detection and attribution studies.

Changes in the future summer Mediterranean climate: contribution of teleconnections and local factors

Abstract. This study analyzes future climate for the Mediterranean region projected with the high-resolution coupled CM2.5 model, which incorporates a new and improved land model (LM3). The simulated climate changes suggest pronounced warming and drying over most of the region. However, the changes are distinctly smaller than those of the CMIP5 multi-model ensemble. In addition, the changes over much of southeast and central Europe indicate very modest warming compared to the CMIP5 projections and also a tendency toward wetter conditions. These differences indicate a possible role of factors such as land surface–atmospheric interactions in these regions. Our analysis also highlights the importance of correctly projecting the magnitude of changes in the summer North Atlantic Oscillation, which has the capacity to partly offset anthropogenic warming and drying over the western and central Mediterranean. Nevertheless, the projections suggest a decreasing influence of local atmospheric dynamics and teleconnections in maintaining the regional temperature and precipitation balance, in particular over arid regions like the eastern and southern Mediterranean, which show a local maximum of warming and drying. The intensification of the heat low in these regions rather suggests an increasing influence of warming land surface on the local surface atmospheric circulation and progressing desertification.

Low radar emissivity signatures on Venus volcanoes and coronae: New insights on relative composition and age

Multiple studies reveal that most of Venus highlands exhibit anomalously high radar reflectivity and low radar emissivity relative to the lowlands. This phenomenon is thought to be the result of atmosphere-surface interactions in the highlands, due to lower temperatures. These reactions are a function of rock composition, atmospheric composition, and degree of weathering. We examine the Magellan radar emissivity, altimetry and SAR data for all major volcanoes and coronae on Venus. We characterize and classify edifices according to the pattern of the variation of radar emissivity with altitude. The volcanic highlands can be classified into 7 distinct patterns of emissivity that correspond to at least 3 discrete types of mineralogy based on the altitude (temperature) of the emissivity anomalies. The majority of emissivity anomalies support the hypothesis of a weathering phenomenon at high altitude (>6053 km), but we also find strong emissivity anomalies at lower altitudes that correspond spatially to individual lava flows, indicating variations in mineralogy within an evolving volcanic system. The emissivity signature of tallest volcanoes on Venus are consistent with the presence of ferroelectric minerals in their rocks, while volcanic edifices in western Ishtar Terra and eastern Aphrodite Terra are consistent with the presence of semiconductor minerals. Sapas Mons and Pavlova Corona are also consistent with ferroelectrics, but at a different Curie temperature than the other volcanoes in Atla Regio. The spatial distribution of radar emissivity classes correlates to different geologic settings indicating that different mantle source regions (deep/shallow plumes, and possible convergence zones) may contribute to differences in mineralogy for the studied edifices. Finally, we show that the emissivity signatures of Idunn, Maat and other volcanic edifices are consistent with relatively fresh and unweathered rocks, indicating recent or possibly current volcanism on Venus.

Model representation of the coupling between evapotranspiration and soil water content at different depths

Abstract. Soil water content (θ) influences the climate system by controlling the fraction of incoming solar and longwave energy that is converted into evapotranspiration (ET). Therefore, investigating the coupling strength between θ and ET is important for the study of land surface–atmosphere interactions. Physical models are commonly tasked with representing the coupling between θ and ET; however, few studies have evaluated the accuracy of model-based estimates of θ ∕ ET coupling (especially at multiple soil depths). To address this issue, we use in situ AmeriFlux observations to evaluate θ ∕ ET coupling strength estimates acquired from multiple land surface models (LSMs) and an ET retrieval algorithm – the Global Land Evaporation Amsterdam Model (GLEAM). For maximum robustness, coupling strength is represented using the sampled normalized mutual information (NMI) between θ estimates acquired at various vertical depths and surface evaporation flux expressed as a fraction of potential evapotranspiration (fPET, the ratio of ET to potential ET). Results indicate that LSMs and GLEAM are generally in agreement with AmeriFlux measurements in that surface soil water content (θs) contains slightly more NMI with fPET than vertically integrated soil water content (θv). Overall, LSMs and GLEAM adequately capture variations in NMI between fPET and θ estimates acquired at various vertical depths. However, GLEAM significantly overestimates the NMI between θ and ET, and the relative contribution of θs to total ET. This bias appears attributable to differences in GLEAM's ET estimation scheme relative to the other two LSMs considered here (i.e., the Noah model with multi-parameterization options and the Catchment Land Surface Model, CLSM). These results provide insight into improved LSM model structure and parameter optimization for land surface–atmosphere coupling analyses.

Estimation of Surface Downward Shortwave Radiation over China from Himawari-8 AHI Data Based on Random Forest

Downward shortwave radiation (RS) drives many processes related to atmosphere–surface interactions and has great influence on the earth’s climate system. However, ground-measured RS is still insufficient to represent the land surface, so it is still critical to generate high accuracy and spatially continuous RS data. This study tries to apply the random forest (RF) method to estimate the RS from the Himawari-8 Advanced Himawari Imager (AHI) data from February to May 2016 with a two-km spatial resolution and a one-day temporal resolution. The ground-measured RS at 86 stations of the Climate Data Center of the Chinese Meteorological Administration (CDC/CMA) are collected to evaluate the estimated RS data from the RF method. The evaluation results indicate that the RF method is capable of estimating the RS well at both the daily and monthly time scales. For the daily time scale, the evaluation results based on validation data show an overall R value of 0.92, a root mean square error (RMSE) value of 35.38 (18.40%) Wm−2, and a mean bias error (MBE) value of 0.01 (0.01%) Wm−2. For the estimated monthly RS, the overall R was 0.99, the RMSE was 7.74 (4.09%) Wm−2, and the MBE was 0.03 (0.02%) Wm−2 at the selected stations. The comparison between the estimated RS data over China and the Clouds and Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) RS dataset was also conducted in this study. The comparison results indicate that the RS estimates from the RF method have comparable accuracy with the CERES-EBAF RS data over China but provide higher spatial and temporal resolution.

Seasonal Variations of Arctic Low-Level Clouds and Its Linkage to Sea Ice Seasonal Variations.

Using CALIPSO‐CloudSat‐Clouds and the Earth's Radiant Energy System‐Moderate Resolution Imaging Spectrometer data set, this study documents the seasonal variation of sea ice, cloud, and atmospheric properties in the Arctic (70°N–82°N) for 2007–2010. A surface‐type stratification—consisting Permanent Ocean, Land, Permanent Ice, and Transient Sea Ice—is used to investigate the influence of surface type on low‐level Arctic cloud liquid water path (LWP) seasonality. The results show significant variations in the Arctic low‐level cloud LWP by surface type linked to differences in thermodynamic state. Subdividing the Transient Ice region (seasonal sea ice zone) by melt/freeze season onset dates reveals a complex influence of sea ice variations on low cloud LWP seasonality. We find that lower tropospheric stability is the primary factor affecting the seasonality of cloud LWP. Our results suggest that variations in sea ice melt/freeze onset have a significant influence on the seasonality of low‐level cloud LWP by modulating the lower tropospheric thermal structure and not by modifying the surface evaporation rate in late spring and midsummer. We find no significant dependence of the May low‐level cloud LWP peak on the melt/freeze onset dates, whereas and September/October low‐level cloud LWP maximum shifts later in the season for earlier melt/later freeze onset regions. The Arctic low cloud LWP seasonality is controlled by several surface‐atmosphere interaction processes; the importance of each varies seasonally due to the thermodynamic properties of sea ice. Our results demonstrate that when analyzing Arctic cloud‐sea ice interactions, a seasonal perspective is critical.

Evapotranspiration Reconstruction Based on Land Surface Models and Observed Water Budget Components While Considering Irrigation

AbstractThe accurate estimation of evapotranspiration (ET) is essential for understanding the land surface–atmosphere interaction; however, current ET products have large uncertainties, and irrigation effects on ET are not well represented. In this study, the monthly ET was reconstructed (ETrecon) from GLDAS land surface models (LSMs) over the Yellow River basin of China, which was achieved by using observation-based precipitation, naturalized streamflow, and downscaled consumed irrigation water from the census annual data via an irrigation scheme. The results showed that the monthly ETrecon series were generally improved relative to the original LSM-based ET, with improvements in the correlation coefficient, Nash–Sutcliffe efficiency, mean absolute error, and root-mean-square error by 0.6%–1.8%, 1.2%–14.6%, 1.3%–21.0%, and 2.1%–20.4%, respectively. The ETrecon results were also superior to the collected ET synthesis products in terms of statistics, with generally higher peak values occurring in ETrecon. Regarding the annual time scale, the ETrecon values were close to the water balance ET values, which have been widely used as benchmark data. The interannual variability in ETrecon was good overall and was associated with the LSM precipitation variability and partitioning of precipitation into ET and runoff. The reconstruction method can provide an alternative ET estimate for other river basins. This study will also be valuable for studies and applications in climate change evaluation, drought assessment, and water resources management.

The annual surface temperature patterns across the Dead Sea as retrieved from thermal images

Satellite images capture a large scene instantaneously, enabling direct comparisons of surface–atmosphere interactions in a static time domain. The annual course of surface temperature of the Dead Sea is investigated using thermal images obtained from Band 10 onboard Landsat 8. Retrieved sea surface temperature varied from around 20 °C in February to ~ 35 °C in July. Retrieved sea surface temperature is commensurate with extensive field measurements, with a root mean square error of 1.25 °C. Satellite images reveal three distinct thermal patterns. The first one is the presence of temperature variations across the open sea which may reflect differences in local meteorology and/or differential mixing with the deeper layers. The second feature is the different temperature skewness patterns between winter and summer months. A slight negative temperature skewness is observed during the period November through February, and a steep positive one during the rest of the year, with large values occurring during summer. These skewness patterns reflect the variations in the annual course of heating and cooling mechanisms occurring in the open sea and along the shallow shorelines. The third distinct feature is the presence of cold finger plumes during the cold months, extending around 4 km inside the open sea and covering an area in excess of 3 km2. The influence of freshwater flux on evaporation from large hypersaline water bodies is investigated using a theoretical formulation derived in this paper. Results show that the effect of freshwater flux on evaporation from hypersaline lakes is a complex process determined by activity and temperature departures. Satellite images provide an excellent surrogate to field observations which are money and labor intensive, time consuming, and very limited in their spatial coverage.

From NEON Field Sites to Data Portal: A Community Resource for Surface–Atmosphere Research Comes Online

AbstractThe National Ecological Observatory Network (NEON) is a multidecadal and continental-scale observatory with sites across the United States. Having entered its operational phase in 2018, NEON data products, software, and services become available to facilitate research on the impacts of climate change, land-use change, and invasive species. An essential component of NEON are its 47 tower sites, where eddy-covariance (EC) sensors are operated to determine the surface–atmosphere exchange of momentum, heat, water, and CO2. EC tower networks such as AmeriFlux, the Integrated Carbon Observation System (ICOS), and NEON are vital for providing the distributed observations to address interactions at the soil–vegetation–atmosphere interface. NEON represents the largest single-provider EC network globally, with standardized observations and data processing explicitly designed for intersite comparability and analysis of feedbacks across multiple spatial and temporal scales. Furthermore, EC is tightly integrated with soil, meteorology, atmospheric chemistry, isotope, phenology, and rich contextual observations such as airborne remote sensing and in situ sampling bouts. Here, we present an overview of NEON’s observational design, field operation, and data processing that yield community resources for the study of surface–atmosphere interactions. Near-real-time data products become available from the NEON Data Portal, and EC and meteorological data are ingested into AmeriFlux and FLUXNET globally harmonized data releases. Open-source software for reproducible, extensible, and portable data analysis includes the eddy4R family of R packages underlying the EC data product generation. These resources strive to integrate with existing infrastructures and networks, to suggest novel systemic solutions, and to synergize ongoing research efforts across science communities.

IMCES Geophysical Observatory for studies of surface-atmosphere interactions

AbstarctThis paper describes a geophysical observatory in use at the Institute of Monitoring of Climatic and Ecological Systems SB RAS and the data it provides. These data can be used for the development of new parameterizations in an active layer model. This model will allow us to reproduce the physical processes in the soil and on its surface and make a detailed description of the exchanges between the surface and the atmosphere. This, in turn, could significantly increase the reliability of regional weather forecasts, especially of extreme weather events. The anticipated improvement in regional meteorological forecasting, as well as in a variety of characteristics measured at the geophysical observatory and the supporting information infrastructure open up possibilities for novel practical applications. In particular, we plan to use the ongoing measurements and data obtained for agrometeorological applications in Western Siberia.

ORACLE: A mission concept to study Mars’ climate, surface and interior

A comprehensive knowledge of the Martian climate requires a better understanding of the formation and evolution of the polar caps. Accurate measurements of mass, volume, and composition of the seasonal polar deposits would enable a precise characterization of the surface-atmosphere interactions relevant to the CO2 cycle. The acquisition of these science data is only possible through a dedicated orbital mission at Mars. ORACLE (Orbital Radio science and Altimetry for CLimate Experiment) is a mission concept that enables the monitoring of the CO2 mass exchange between the polar caps and the atmosphere with a pair of spacecraft in a 250×320 km orbit about Mars. A preliminary design of the mission configuration is presented in this study including the scientific payload that consists of a radio science system for inter-satellite tracking, a laser altimeter, and an infra-red camera. These instruments also provide data that are fundamental to investigate Mars’ internal structure and atmosphere. ORACLE is, therefore, a multi-disciplinary mission that is conceived to fulfill the constraints of a medium-class mission.

Estimating surface solar irradiance from satellites: Past, present, and future perspectives

Surface Solar Irradiance (SSI) is a key parameter dictating surface-atmosphere interactions, driving radiative, hydrological, and land surface processes, and can thus impinge greatly upon weather and climate. It is thereby a prerequisite of many studies and applications. Estimating SSI from satellites began in the 1960s, and is currently the principal way to map SSI spatiotemporal distributions from regional to global scales. Starting from an overview of historical studies carried out in the past several decades, this paper reviews the progresses made in methodology, validation, and products over these years. First, the requirements of SSI in various studies or applications are presented along with the theoretical background of SSI satellite estimation. Methods to estimate SSI from satellites are then summarized as well as their advantages and limitations. Validations of satellite-based SSI on two typical spatial scales are discussed followed by a brief description of existing products and their accuracies. Finally, the challenges faced by current SSI satellite estimation are analyzed, and possible improvements to implement in the future are suggested. This review not only updates the review paper by Pinker et al. (1995) on satellite methods to derive SSI but also offers a more comprehensive summary of the related studies and applications.

Soil Moisture Monitoring in Iran by Implementing Satellite Data into the Root-Zone SMAR Model

Monitoring Surface Soil Moisture (SSM) and Root Zone Soil Moisture (RZSM) dynamics at the regional scale is of fundamental importance to many hydrological and ecological studies. This need becomes even more critical in arid and semi-arid regions, where there are a lack of in situ observations. In this regard, satellite-based Soil Moisture (SM) data is promising due to the temporal resolution of acquisitions and the spatial coverage of observations. Satellite-based SM products are only able to estimate moisture from the soil top layer; however, linking SSM with RZSM would provide valuable information on land surface-atmosphere interactions. In the present study, satellite-based SSM data from Soil Moisture and Ocean Salinity (SMOS), Advanced Microwave Scanning Radiometer 2 (AMSR2), and Soil Moisture Active Passive (SMAP) are first compared with the few available SM in situ observations, and are then coupled with the Soil Moisture Analytical Relationship (SMAR) model to estimate RZSM in Iran. The comparison between in situ SM observations and satellite data showed that the SMAP satellite products provide more accurate description of SSM with an average correlation coefficient (R) of 0.55, root-mean-square error (RMSE) of 0.078 m3 m−3 and a Bias of 0.033 m3 m−3. Thereafter, the SMAP satellite products were coupled with SMAR model, providing a description of the RZSM with performances that are strongly influenced by the misalignment between point and pixel processes measured in the preliminary comparison of SSM data.

Titan as Revealed by the Cassini Radar

Titan was a mostly unknown world prior to the Cassini spacecraft’s arrival in July 2004. We review the major scientific advances made by Cassini’s Titan Radar Mapper (RADAR) during 13 years of Cassini’s exploration of Saturn and its moons. RADAR measurements revealed Titan’s surface geology, observed lakes and seas of mostly liquid methane in the polar regions, measured the depth of several lakes and seas, detected temporal changes on its surface, and provided key evidence that Titan contains an interior ocean. As a result of the Cassini mission, Titan has gone from an uncharted world to one that exhibits a variety of Earth-like geologic processes and surface-atmosphere interactions. Titan has also joined the ranks of “ocean worlds” along with Enceladus and Europa, which are prime targets for astrobiological research.

Caracterización de la turbulencia atmosférica en una gran zona verde de una megaciudad andina tropical

La turbulencia generada por las interacciones superficie-atmósfera, y su disipación, determinan en gran parte la dispersión de contaminantes y, por lo tanto, su impacto en la salud pública. La parametrización de la turbulencia en los modelos de transporte y dispersión de contaminantes ha sido observacionalmente obtenida y validada principalmente en latitudes medias, pero muy poco en el trópico. En este artículo se reporta la caracterización estadística de la turbulencia atmosférica en un sitio representativo del Área Metropolitana de Bogotá – Colombia (4°39’30.48”N, 74°5’2.38”O, 2577 m s.n.m.m.), una megaciudad andina tropical, a partir de mediciones de alta frecuencia con un anemómetro ultrasónico de tres ejes, validado en exposición y operado a 10 Hz, a 7.8 m de altura, entre agosto y noviembre del 2014. El análisis revela que, durante el periodo de medición, la turbulencia lateral en Bogotá, expresada como desviación estándar, fue elevada y comparable a la turbulencia longitudinal (σv ≅ σu ≈ 0.2-1.3 m s-1), lo que caracteriza su atmósfera como horizontalmente isotrópica. Las intensidades turbulentas observadas (Iu ≈ Iv ≈ 0.6, Iw ≈ 0.3) fueron ~2 a ~3 veces más altas que las reportadas para latitudes medias a las bajas velocidades del viento en Bogotá. El análisis espectral es consistente con el estadístico, con espectros longitudinales y verticales comparables a los reportados para condiciones neutrales a ligeramente inestables, y espectros laterales con frecuencias pico de un orden de magnitud menor que los reportados en la literatura, lo que indica una elevada producción de turbulencia lateral. Se estima que los remolinos al inicio de la cascada de energía tuvieron longitudes características de ~120 m y ~10 m en los planos horizontal y vertical, respectivamente. © 2019. Acad. Colomb. Cienc. Ex. Fis. Nat.