Bimodal Regime Research Articles

Barchan swarm dynamics from a Two-Flank Agent-Based Model

Abstract. In this work, we simulate barchan swarms using the Two-Flank Agent-Based Model and investigate how changes to model parameters and environmental drivers lead to different swarm dynamics. In particular, we explore how the parameter qshift, which controls the rate of equilibration between the flanks of an asymmetric dune and thereby the stability of an asymmetric morphology, influences the frequencies of different collision types in the swarm and can be adjusted to produce swarms with dune sizes that are longitudinally homogeneous. Such size stability has been observed in real-world swarms but has not been obtained by previous agent-based models. We also find that, for certain densities of newly added barchans, the dune number density remains constant with downwind distance, something which has also been reported in nature but not in previous agent-based models. We also investigated how boundary condition changes propagate through swarms, something which will become increasingly important in the face of human infrastructure expansion and anthropogenic climate change. Finally, we are able to investigate how asymmetry and spatially patterning in the swarms are influenced by bimodal wind regimes. Since these simulations produce many more realistic phenomena than previous models, this work represents a significant step forward in the our understanding of the interplay between environmental conditions and dune interactions in shaping the dynamics of barchan swarms.

The potential of mixing model of wind speed distribution in Algerian High Plateaus

The evaluation of wind energy relies primarily on the probability density function PDF, which corresponds well with the wind speed data. Single PDFs are widely used in the assessment of wind. In contrast, homogeneous or heterogeneous mixed models are rarely used, especially in Algeria, where the bimodal wind speed distribution is expected. This research aims to investigate the potential of heterogeneous PDFs Generalized Extreme Value-Weibul and Normal-Extreme Value PDFs in assessing wind energy at three meteorological stations in the high plateau against the single widespread PDFs Weibull and GEV by analyzing five years of archived wind speed data. The estimation of mixed model parameters is obtained by applying the Expectation-Maximization algorithm, and the identification of the appropriate PDF is obtained by four goodness-of-fit (GOF) criteria and compared with the widespread single distributions. The results show that the mixed model surpasses widely the single model for all the GOF criteria used at the three selected sites. The proposed mixed model fits all the wind speed distributions related to unimodal and bimodal regimes.

The bimodal regime of ocean waves and winds over the continental shelf of Maranhão

This study analyzed surface waves and winds from ERA5 reanalysis dataset in the Western Equatorial Atlantic (WEA) Ocean in order to properly understand the long-term regime of oceanic waves over the Continental Shelf of Maranhão (CSMA), pointing to the changes that occur at the western and eastern sectors as the waves propagate to the coastal zone. A data validation with observational data collected in situ by the PNBOIA and PIRATA projects has been conducted. The parameters analyzed were significant wave height (Hs), peak period (Tp), mean wave direction (Dm), and the zonal (u10) and meridional (v10) components of wind at 10 m. The analysis covered a period of 43 years (1979–2023). The monthly climatology of the Hs, Tp and Dm found has revealed a semi-annual cycle in the waves pattern in the region. The wave direction variability executes a clockwise motion from northeast-east-southeast throughout a year. The annual climatology analysis reveals that 59 % of sea states in the region are characterized by swell conditions in the region, while wind-sea dominates in 41 % of cases, with seasonal variability. During the austral summer (autumn), swells prevail over wind-seas along the WEA, accounting for 77 % (84 %) of the cases. This coincides with the boreal winter, characterized by high storm activity, whose storms generate swells that traverse the ocean, reaching Maranhão's waters and resulting in the highest waves in the region (3.54 m in summer). In contrast, during the spring season, sea states are dominated by wind-sea conditions (70 %) due to the increase in southeast trade winds. The highest Hs observed from wind-sea conditions is 2.94 m. The largest contribution for the maxima Hs in the region is given by swells in the summer/autumn and by wind-seas in the winter/spring seasons, with swells propagating more energy and power to cause erosional processes on the eastern sector of Maranhão coast. Oppositely, in the western sector of the littoral, whose geomorphology is dominated by tidal flats, cheniers plains and mangroves, the Hs is small, indicating a depositional environment. The algorithm indicated that swells generated by tropical (extratropical) storms typically take approximately 3.5–4.5 (6−7) days to travel the distance to the CSMA, while in the equatorial zone, the arrival of swells at the CSMA averages between 3.5 and 7 days. In this study we demonstrated the CSMA region is dominated by a bimodal wave regime with a strong semi-annual cycle, which is driven by the tropical/extratropical storms propagating from the North Atlantic Ocean toward the study area and by trade winds variability.

Quantifying the Strength of the El Niño Southern Oscillation and the Indian Ocean Dipole in Influencing the OND Rainfall Season in Tanzania

The current paper examines the strength of variability between the El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) in influencing October to December (OND) rainfall over the bimodal rainfall regime of Tanzania. The empirical orthogonal function (EOF), correlation analysis, and composite analysis were used during the data analysis. The results show more rainfall distribution over the western part of the Lake Victoria basin and to the peripheral of the northern coast of the country, thus suggesting that, during the OND rainfall season, the onset starts in the western part of the Lake Victoria basin, then spreads to the rest of the areas under investigation as the season progresses. Furthermore, on the spatial scale, the findings revealed that there is a strong correlation between IOD and ENSO indices and OND rains in the northeastern highlands. Furthermore, a robust temporal correlation is revealed between the mean OND rains over the bimodal rainfall areas and IOD (r = 0.70) compared to ENSO (r = 0.62). The anomalous warming over the western Indian Ocean (positive IOD) has a faster response to OND rains over the bimodal rainfall regime of Tanzania compared to the remote influence induced by anomalous warming from the central equatorial Pacific Ocean (warm phase of ENSO). Meanwhile, dry years are associated with negative IOD and the cold phase of ENSO conditions. The findings offer valuable insights on strategies for mitigating the effects associated with extreme weather events and improving resilience in Tanzania.

Forecasting wildfire hazard across northwestern south America

Fire hazard is often mapped as a static conditional probability of fire characteristics’ occurrence. We developed a dynamic product for operational risk management to forecast the probability of occurrence of fire radiative power in the locally possible near-maximum fire intensity range. We applied standard machine learning techniques to remotely sensed data. We used a block maxima approach to sample the most extreme fire radiative power (FRP) MODIS retrievals in free-burning fuels for each fire season between 2001 and 2020 and associated weather, fuel, and topography features in northwestern south America. We used the random forest algorithm for both classification and regression, implementing the backward stepwise repression procedure. We solved the classification problem predicting the probability of occurrence of near-maximum wildfire intensity with 75% recall out-of-sample in ten annual test sets running time series cross validation, and 77% recall and 85% ROC-AUC out-of-sample in a twenty-fold cross-validation to gauge a realistic expectation of model performance in production. We solved the regression problem predicting FRP with 86% r2 in-sample, but out-of-sample performance was unsatisfactory. Our model predicts well fatal and near-fatal incidents reported in Peru and Colombia out-of-sample in mountainous areas and unimodal fire regimes, the signal decays in bimodal fire regimes.

Contrarian Majority Rule Model with External Oscillating Propaganda and Individual Inertias.

We study the Galam majority rule dynamics with contrarian behavior and an oscillating external propaganda in a population of agents that can adopt one of two possible opinions. In an iteration step, a random agent interacts with three other random agents and takes the majority opinion among the agents with probability p(t) (majority behavior) or the opposite opinion with probability 1-p(t) (contrarian behavior). The probability of following the majority rule p(t) varies with the temperature T and is coupled to a time-dependent oscillating field that mimics a mass media propaganda, in a way that agents are more likely to adopt the majority opinion when it is aligned with the sign of the field. We investigate the dynamics of this model on a complete graph and find various regimes as T is varied. A transition temperature Tc separates a bimodal oscillatory regime for T<Tc, where the population's mean opinion m oscillates around a positive or a negative value from a unimodal oscillatory regime for T>Tc in which m oscillates around zero. These regimes are characterized by the distribution of residence times that exhibit a unique peak for a resonance temperature T*, where the response of the system is maximum. An insight into these results is given by a mean-field approach, which also shows that T* and Tc are closely related.

Evaluation of CMIP6 historical simulations over IGAD region of Eastern Africa

The Accuracy of model simulations is critical for climate change and its socio-economic impact. This study evaluated23 Global climate models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6). The main objective was to identify the top 10 best performance models in capturing patterns of rainfall for the 1981–2014 period over the Intergovernmental Authority on Development (IGAD) region of Eastern Africa. The total rainfall, annual cycle, continuous, categorical and Volumatic statistical metrics, scatter plots, Cumulative Distribution Function (CDF), and colored code portrait were used to assess the patterns of total rainfall. Results indicate that most CMIP6 models generally capture the characteristics of the observed climatology pattern of total rainfall, bimodal and unimodal rainfall regimes. The majority of models over Arid and Semi-Arid Lands (ASALs) in Kenya, Somalia, Ethiopia, and Sudan scored the lowest skills, highest bias, and over-estimated rainfall, and lower skills over June–September (JJAS) compared to March–May (MAM) and October-December (OND). Quantitatively, a high percent of bias exceeding 80% scored over ASALs, a high correlation coefficient ranging between 0.6 and 0.7 across Ethiopia’s highlands, and a 5–40 as the lowest Root Mean Squared Error scored over the majority of the region. In addition, 21 out of 23 CMIP6 over-estimated rainfall over most parts of the region. The ACCESS-ESM1-5 and MIROC6 are the most over-estimated models as opposed to CNRM-CM6-1HR as the most model under-estimated rainfall, highest bias, and RMSE values. The regional and sub-national analysis showed it is inconclusive to select best-performed models based on individual metrics and sub-national analysis. Out of 23 models, the INM-CM5-0, HadGEM3-GC31-MM, CMCC-CM2-HR4, IPSL-CM6A-LR, KACE-1-0-G, EC-Earth3, NorESM2-MM, GFDL-ESM4, TaiESM1, and KIOST-ESM are the best 10 performance models over IGAD region. These findings highlight the importance of selecting the best performance models for mapping present and future hotspots and extreme rainfall events over the IGAD region of Eastern Africa.

Pattern of Litterfall Production Throughout a Tropical Humid Forest Chronosequence

Neotropical landscapes are often characterized as mosaics of actively grazed pastures and forest fragments at various stages of succession. Restoring abandoned pastures requires intervening with actions informed by local ecological succession. Here we evaluate how litter production, a key component of nutrient cycling in forests, varies between physiographic landscapes (hill and lowland mountain), among stand age groups (early-secondary: 0–20 year, intermediate-secondary: 20–40 year, and old-growth forests: > 40 year), and in response to climatic (precipitation and irradiation) and forest structure and biodiversity variables (stand density, basal area, species density, diversity), in a tropical humid forest chronosequence. Total and leaf litter production was higher in the lower elevation hill landscape. Leaf litter production increased with stand age in both landscapes but only correlated positively with forest structure and biodiversity in the hill landscape. Lack of correlation in the mountain landscape is attributed to greater variance in structure and species composition in the subset of plots selected from this landscape for this study. Both landscapes exhibited an asynchronous, bimodal regime of leaf fall in response to precipitation. Leaf litter peaks prior to the wet season likely occur in response to water stress, while peaks after the wet season that coincided with increasing solar irradiation are likely the result of a leaf turnover strategy to optimize photosynthesis. Ecological succession in these landscapes does not appear limited in leaf litter production, but further research on the drivers of leaf litter production recovery is necessary to adequately guide intervention efforts.

Evaluation of Reanalysis Estimates of Precipitation, Radiation, and Temperature over Benin (West Africa)

Abstract In West Africa, climatic data issues, especially availability and quality, remain a significant constraint to the development and application of distributed hydrological modeling. As alternatives to ground-based observations, reanalysis products have received increasing attention in recent years. This study aims to evaluate three reanalysis products, namely, ERA5, Water and Global Change (WATCH) Forcing Data (WFD) ERA5 (WFDE5), and MERRA-2, from 1981 to 2019 to determine their ability to represent four hydrological climates variables over a range of space and time scales in Benin. The variables from the reanalysis products are compared with point station databased metrics Kling–Gupta efficiency (KGE), mean absolute error (MAE), correlation, and relative error in precipitation annual (REPA). The results show that ERA5 presents a better correlation for annual mean temperature (between 0.74 and 0.90) than do WFDE5 (0.63–0.78) and MERRA-2 (0.25–0.65). Both ERA5 and WFDE5 are able to reproduce the observed upward trend of temperature (0.2°C decade−1) in the region. We noted a systematic cold bias of ∼1.3°C in all reanalyses except WFDE5 (∼0.1°C). On the monthly time scale, the temperature of the region is better reproduced by ERA5 and WFDE5 (KGE ≥ 0.80) than by MERRA-2 (KGE &lt; 0.5). At all time scales, WFDE5 produces the best MAE scores for longwave (LW) and shortwave (SW) radiation, followed by ERA5. WFDE5 also provides the best estimates for the annual precipitation (REPA ∈ ]−25, 25[ and KGE ≥ 50% at most stations). ERA5 produces similar results, but MERRA-2 performs poorly in all the metrics. In addition, ERA5 and WFDE5 reproduce the bimodal rainfall regime in southern Benin, unlike MERRA-2, but all products have too many small rainfall events.

Recurring summer and winter droughts from 4.2-3.97 thousand years ago in north India

The 4.2-kiloyear event has been described as a global megadrought that transformed multiple Bronze Age complex societies, including the Indus Civilization, located in a sensitive transition zone with a bimodal (summer and winter) rainfall regime. Here we reconstruct changes in summer and winter rainfall from trace elements and oxygen, carbon, and calcium isotopes of a speleothem from Dharamjali Cave in the Himalaya spanning 4.2–3.1 thousand years ago. We find a 230-year period of increased summer and winter drought frequency between 4.2 and 3.97 thousand years ago, with multi-decadal aridity events centered on 4.19, 4.11, and 4.02 thousand years ago. The sub-annually resolved record puts seasonal variability on a human decision-making timescale, and shows that repeated intensely dry periods spanned multiple generations. The record highlights the deficits in winter and summer rainfall during the urban phase of the Indus Civilization, which prompted adaptation through flexible, self-reliant, and drought-resistant agricultural strategies.

Spatiotemporal Variability of Extreme Rainfall in Southern Benin in the Context of Global Warming

Changes in the frequency and timing of extreme precipitation in southern Benin are assessed in the context of global warming. The peak-over-threshold (POT) is used for this purpose, with the six (06) year return period daily rainfall as the threshold over seventeen (17) weather stations between 1960 and 2018. The results show that the South Benin experienced extreme rainfall on many occasions between 1960 and 2018 with a nonuniform spatiotemporal distribution of this category of rainfall. No statistically significant trend in the frequency and variation of extreme rainfall intensities is revealed over the study period. Despite the low rate of extreme rainfall, the monthly trend is consistent with the bimodal rainfall regime in southern Benin. The global warming highlighted in its last decades in southern Benin is accompanied by a slightly upward trend in extreme rainfall compared to the period before 1990.

VULNERABILITE DE LA PRODUCTION DU MANIOC AUX CHANGEMENTS CLIMATIQUES ET PRIORITE D’ADAPTATION DANS LES AGGLOMERATIONS FRONTALIERES BENIN-TOGO (VALLEE DU MONO, AFRIQUE DE L’OUEST)

In the south of the Mono Valley and in its Benin-Togo border towns, the production of cassava roots is hampered by fluctuating climatic conditions. It is in this context that this study is conducted, in order to analyze climatic variations and their impact on cassava production. Thus, climatological data (rainfall, temperature, relative humidity, potential evapotranspiration) and agricultural data (sown areas, yield and production of cassava) made it possible to characterize the major agroclimatic situations. Documentary research and field surveys formed the basis of the research methodology. In addition, data analysis reveals that the study area has a bimodal rainfall regime. The annual evolution of rainfall over the period 1988-2018 is marked by 45.16% of surplus years, 12.90% of normal years and 41.93% of deficit years. Therefore, the index of the deviation from the minimum value was calculated in order to see the years where the minimum need was not satisfied for certain years. In addition, an upward trend in average temperatures is noted with an average annual rate of 0.04°C/year. The linear correlations established between cassava yields and climatic parameters showed that there is no explicit linear relationship between cassava production and temperature and between cassava production and insolation. The rainfall and humidity dependence of cassava production are also low: 27% and 20% respectively. Thus, climatic parameters taken in isolation are not sufficient to explain agricultural yields. In addition, other factors could also play an inhibiting or enhancing role in yields. However, the producers have developed adaptation strategies in the face of climatic variability in the study area that deserve to be accompanied. KEYWORDS : Climate change, Cassava, Lower Mono Valley, border agglomeration, Benin and Togo

COUPLED EFFECT OF SOLID FRACTION AND INTERNAL ENERGY SOURCE ON CONVECTIVE HEAT TRANSPORT IN THREE-LAYERED AIR-POROUS-AIR DOMAIN: COMPARISON WITH DIFFERENT TWO-LAYERED DOMAINS

This paper studies convective heat transport driven by a uniform internal energy source coupled with the solid fraction in the partial air-saturated porous domains bounded by the top and bottom impermeable thermally conductive surfaces. The consideration of the coupled effect distinguishes our work from the papers of previous authors. We provide a comprehensive numerical study of internal convection and compare results of the linear stability analysis for three distinct domains. The first domain is air-porous-air (APA), where the heat-generating porous matrix is between the upper and lower air layers. In the second air-porous (AP) domain, the air layer overlays the porous medium. In the third porous-air (PA) domain, the air layer underlays the porous medium. The bimodal marginal stability curves and regime map with a demarcation line between the local and large-scale convective flows are obtained only for the APA and AP domains due to the division of each domain into the upper unstably stratified and lower stably stratified parts. The local convection cannot originate in the PA domain because the air layer belongs to the lower stably stratified part. A remarkable destabilizing effect of additional air layers has been revealed. For example, at the fixed solid fraction of 0.1, we achieve a 40-fold reduction of the critical Darcy-Rayleigh number in the APA domain by increasing the depth ratio from 0 to 0.5, and by 14 times in the AP domain and only by 3.5 times in the PA domain. The destabilizing effect enhances with increases in the solid fraction.

Performance of the ‘Anna’ apple (Malus domestica Borkh.) in Tropical Highlands: A review

The 'Anna' apple is a variety of low requirements of winter chill (250 to 300 chilling hours ≤7.2°C). This apple has essential health benefits and remarkable adaptive potential in tropical and subtropical areas affected by climate change. Thus, this review presents the significance of the 'Anna' apple cultivation, the phenological and eco-physiological modifications, and the current state of agronomic management when continuous crops are managed in tropical highlands. The production of this apple in tropical highlands has outstanding potential to obtain cyclical or continuous harvests (two harvests per year) in certain areas with specific environmental conditions, implementing a particular management system. In plantations, it is crucial to carry out some agronomic practices during the reproductive phenology so that the apple tree does not enter into an endodormancy. These are water stress - defoliation – tie-down branches, and the application of dormancy-breaking agents (flower-inducing compounds). In Colombia, ‘Anna’ variety was introduced in 1985 and is grown in areas with temperatures between 14 and 20°C, altitudes between 1700 and 2800 meters above sea level (m a.s.l.), with bimodal and monomodal rain regimes, and solar brightness between 800 and 2000 hours a year. The harvest is between 100 to 120 days after anthesis, with firmness values of 38.38N, a soluble solids content of 8.58°Brix, and total titratable acidity of 0.7% of the fruit. This documentation indicates a good production with great potential in terms of growth and development, earliness, and quality of the 'Anna' apple tree in Colombian highlands.

Morphology and distribution of Martian wedge dunes and their terrestrial analogs

Wedge dunes are widespread on Mars and contain important information about Mars surface processes and environmental characteristics. These dunes are wedge-shaped in plan-view, similar in scale to a barchan dune, with two slip faces intersecting at an obtuse angle and extending outward toward the main ridge downwind. And, the length of its main ridge does not exceed its width. At present, our understanding of wedge dunes and their development in nature is limited. The type, morphology, distribution and spatial composition of wedge dunes on Mars and Earth were investigated using high-resolution satellite image data. The results reveal that wedge dunes are simple in type and similar in size to barchan dunes. Martian wedge dunes are similar in shape to terrestrial wedge dunes but larger in size. The average angles between the sub-ridges of Martian and terrestrial wedge dunes are similar, at about 116°. Martian wedge dunes are mainly located in Abalos Undae, Siton Undae, and Aspledon Undae in the North Polar region, accounting for about 5% of the total area covered by dune fields. Wedge dunes are rare on Earth and can only be found on the edge of a few contiguous dune fields. These dunes indicate acute bimodal or obtuse bimodal wind regimes. Wedge dunes have distinct transitional characteristics and may be the initial stage of the development of various dune patterns. The implications of these findings are discussed, in particular the periodic changes in the regional wind direction reflected by the wedge dunes, as well as the significance of the distinctive transitional features of the wedge dunes for improving our understanding of the genesis of complex dune patterns.

Evidence for Fluctuating Wind in Shaping an Ancient Martian Dune Field: The Stimson Formation at the Greenheugh Pediment, Gale Crater

AbstractTemporal fluctuations of wind strength and direction can influence aeolian bedform morphology and orientation, which can be encoded into the architecture of aeolian deposits. These strata represent a direct record of atmospheric processes and can be used to understand ancient Martian atmospheric processes as well as those on Earth. The strata can: give insight to ancient atmospheric circulation, how the atmosphere evolved in response to global changes in habitability, and how ancient processes differ from modern processes. The Stimson formation at the Greenheugh pediment (Gale crater) records evidence of fluctuating wind across multiple temporal scales. The strata can be subdivided into three intervals–Gleann Beag, Ladder, and Edinburgh intervals. Internally, the intervals record changes of dune morphology and orientation, correlatable to wind fluctuations at multiple temporal scales. The basal Gleann Beag interval comprises compound cross‐strata, deposited by oblique compound dunes. These dunes record a bimodal wind regime, resulting in net sediment transport toward the north. The Ladder interval records a reversal of sediment transport to the south, where straight‐crested simple‐dunes shaped by a seasonally variable winds formed. Finally, the Edinburgh interval records sediment transport to the west, where a unimodal wind formed sinuous‐crested simple dunes. These observations demonstrate active and variable atmospheric circulation in Gale crater during the accumulation of the Stimson dune field, at multiple temporal scales from seasonally driven winds to much longer time‐frames, during the Hesperian. These observations can be used to further understand ancient atmospheric conditions and processes, at a high temporal resolution on Mars.

Analysis of Spatial and Temporal Variations of the Near-Surface Wind Regime and Their Influencing Factors in the Badain Jaran Desert, China

Wind regime is one of the main natural factors controlling the evolution and distribution of aeolian sand landforms, and sand drift potential (DP) is usually used to study the capacity of aeolian sand transport. The Badain Jaran Desert (BJD) is located where polar cold air frequently enters China. Based on wind data of eight nearby meteorological stations, this research is intended to explore the temporal variation and spatial distribution features of wind speed and DP using linear regression and cumulative anomaly method, and reveal the relationship between atmospheric circulation and wind speed with correlation analysis. We found that the wind speed and frequency of sand-blowing wind in the BJD decreased significantly during 1971–2016, and the wind speed obviously mutated in 1987. The regional wind speed change was affected by the Asian polar vortex, the northern hemisphere polar vortex and the Tibet Plateau circulation. The wind rose of the annual sand-blowing wind in this region was the “acute bimodal” type. Most of the annual wind directions clustered into the W-NW, and the prevailing wind direction was WNW. During 1971–2016, the annual DP, the resultant drift potential (RDP) and the directional variability (PDP/DP) in the desert showed an obvious downtrend, with a “cliff-like” decline in the 1980s and relative stable fluctuation thereafter. The BJD was under a low-energy wind environment with the acute bimodal wind regime. Wind speed, sand-blowing wind frequency and DP were high in the northeast and low in the southwest.

Nucleation effects on cavitation about a sphere

The influence of nucleation on cavitation about a sphere from inception through to supercavitation at a transcritical Reynolds number of$1.5\times 10^{6}$is investigated experimentally. Two extreme free-stream nuclei populations, deplete and abundant, were investigated. Unsteady surface pressures from two sensors on opposing sides of the sphere were acquired simultaneous with high-resolution high-speed photography at cavitation numbers between 1.0 and 0.3. High-resolution spectrograms derived from these measurements reveal principally bi-modal shedding in attached and detached regimes. Correlations between unsteady pressure measurements show the high modes to be axisymmetric and low modes asymmetric. Modal topology is also discerned from the high-speed imaging. The bi-modal shedding for lower cavitation numbers is driven by coupled re-entrant jet formation and upstream shockwave propagation. The attached regime is shown to have two sub-regimes. For the abundant case, the continuous supply of activated nuclei around the sphere periphery in the first bi-modal regime has the effect of driving the high symmetric mode preferentially over the asymmetric low mode compared with the deplete case. For the first bi-modal regime, frequencies were unaffected by nucleation changes although peak responses were centred at a cavitation number of about 0.8 for the deplete and 0.825 for the abundant. For the second attached regime, where cavity lengths are of the order of the sphere size, changes in nucleation altered frequencies and amplitudes of peak unsteady pressures. For the abundant case, the continuous nuclei supply significantly reduced coherence with modal peak amplitudes reduced by an order compared with the deplete case. Continuous nuclei activation increased the probability of the high mode over the low compared with the deplete case but to a lesser extent than the first regime. Nuclei activations also significantly reduced inter-cavity and cavity durations, but not growth and collapse phases, which increased modal frequencies compared with the deplete case. The second regime asymmetric low mode topology, for both nucleation cases, is shown to be alternate shedding of oblique vortices from diametrically opposing sides of the sphere similar to low Reynolds number shedding about spheres and other axisymmetric bluff bodies in single-phase flows.

Seasonal carryover of water and effects on carbon dynamics in a dryland ecosystem

AbstractNet primary productivity in arid and semiarid regions is controlled by water availability for which rainfall has been a commonly used proxy at annual scales. However, the hydrological partitioning occurring through the water balance can also shape both seasonal and annual net ecosystem productivity. In this study, we used 10 years of water and carbon flux measurements in a mixed shrubland watershed of the Chihuahuan Desert to investigate the seasonal variability and controls on net ecosystem production. Over this period, the site exhibited a bimodal rainfall regime with an average of 211 and 67 mm for the wet (July–December) and dry (January–June) periods of the year, respectively. During the wet season, soil infiltration and channel transmission losses led to an average of 35.8 mm of water stored in the subsurface for subsequent dry periods. By contrast, dry seasons consumed 30.3 mm of stored subsurface water to fulfill the ecosystem water demand, particularly during the springtime. In response, gross primary productivity occurred in equal amounts for both seasons, while ecosystem respiration was substantially higher during the wet season. This resulted in the mixed shrubland acting as an annual net sink of carbon (average of 153.2 g C m−2 year−1) of which 65% occurred during the dry season. We determined that the wet season provided water in excess of vegetation demands in that season, a portion of which was stored in the watershed subsurface for subsequent use, leading to a legacy effect. Gross primary productivity during the dry season was dependent on carryover soil moisture accessed by deep‐rooted shrubs. Our coordinated observations of water–carbon dynamics revealed that water availability during the wet season can sustain the annual ecosystem productivity by impacting the subsequent dry season carbon uptake.

Seasonal inundation dynamics and water balance of the Mara Wetland, Tanzania based on multi-temporal Sentinel-2 image classification

Anthropogenic hydrologic alternations are a threat to flood dependent wetland ecosystems like The Mara Wetland in Tanzania, which plays an important role in regulating the quality, timing and magnitude of the flow of water into Lake Victoria and provides natural resources and habitat for a variety of species. Baseline information on the spatiotemporal dynamics of the seasonal inundation is critical to manage potential threats such as dam construction and local climate change, but is challenging to acquire due to the heterogeneity of the landscape caused by the interspersion of open water and emergent vegetation. In this study, intra- and inter-annual inundation pattern of the Mara Wetland are reproduced for 2017–2019 using remotely sensed data. A Random Forest (RF) algorithm is trained bi-seasonally, using eight Sentinel-2 bands, five water- and vegetation-indices and a Digital Surface Model (DSM) to classify the land cover of the wetland region in a semi-automated way, by processing a total of 73 Sentinel-2 scenes based on wet- and dry-state training images for 2017. The scenes are classified into seven classes; three wetland- and four dryland-classes. The overall classification accuracy achieved (based on an independent validation set) is 98.6%. The inundation extents are used in combination with available hydrological field-data to estimate the water balance. The seasonal expansion and contraction of the wetland follows a consistent bi-modal regime, and the results from the water balance affirm the importance of river-flow during the dry-season and local precipitation for the seasonal fluctuations in water-availability.