Sharp Gradients Research Articles

Shapes of the fastest fish and optimal underwater and floating hulls

A streamlined shape of the best swimmers removes the boundary-layer separation and ensures a laminar flow pattern. The fastest fish have a very sharp convex nose (rostrum), the purpose of which remains unclear. The bodies of revolution similar to their shapes are analyzed in steady underwater and floating motion. The sources and sinks were located on the axis of symmetry and above the water surface to estimate the pressure on the body and the vertical velocities on the water surface. It was shown that the flow patterns on a special shaped body with concave nose has no stagnation points and ensure small values of the water surface elevation. These fact allow diminishing the maximum pressure on the surface and wave drag. Special shapes with the sharp concave nose and negative pressure gradients on their surface could be parts of the low drag underwater and floating hulls.

The Rigidity and Stability of Gradient Estimates

In this note, we obtain the rigidity of the sharp Cheng-Yau gradient estimate for positive harmonic functions on surfaces with non-negative Gaussian curvature, the rigidity of the sharp Li-Yau gradient estimate for positive solutions to heat equations and the related estimates for Dirichlet Green’s functions on Riemannian manifolds with non-negative Ricci curvature. Moreover, we also obtain the corresponding stability results.

P11.25.B Adjuvant high precision hypofractionated radiotherapy for resected brain metastases: single-institutional experience

Abstract Background The proportion of patients with solid tumor who develop brain metastases may be as high as 35%. Whenever possible, resection should be the first treatment option. Unfortunately, resection alone has recurrence rates up to 50-60%. In order to improve local control, adjuvant radiation to the surgical cavity is highly recommended. Single dose stereotactic radiosurgery (SRS) as well as hypofractionated stereotactic radiotherapy (HFSRT) have shown one-year local control rates from 70 to 90% with low incidence of radiation-induced toxicity. High precision hypofractionated radiotherapy (HFRT) using Volumetric Modulated Arc Therapy (VMAT) combines sharp dose gradients of SRS with a fractionated scheme, allowing high radiation doses per fraction while minimizing adverse events. Therefore, it could be used as an alternative to SRS for target volumes larger than 3 cm or close to sensitive brain areas. This study aims to analyze control parameters and toxicity of High precision HFRT after brain metastasis resection. Material and Methods We retrospectively analyzed 38 patients diagnosed with brain metastases underwent complete or subtotal surgical resection followed by adjuvant High precision HFRT between January and December 2019 at Catalan Institute of Oncology, Badalona. Radiation was delivered using VMAT technique with 6 MV photon beams. Patient immobilization was achieved using commercial radiotherapy-thermoplastic-mask and daily image guide control with CBCT (cone beam computed tomography) was performed. Statistical analysis was performed with SPSS.PASW.Statistics.v18. Results We identified 38 patients (47.4% men, 52.6% women) with a median age at diagnosis of 61 years [37 - 83 years]. Most of the primary neoplasms were from lung (50%) and breast cancer (24%). The median size of cavities was 10,5cc [0.4 - 75.5cc]. Radiation scheme was selected depending on size and location of the surgical cavity, primary tumor histology and patient’s performance status. Forty-two percent of patients received 27 Gy/3 fractions, 39% received 24 Gy/3 fractions and 7% received 18Gy/3fractions. The remaining four patients received 35 Gy/5fr (3%), 15 Gy/3fr (3%), 16 Gy/2fr (3%) and 30Gy/10fr (3%) respectively. With a median follow-up of 9.9 months, brain recurrence was observed in 23 patients (60%), with only 7 patients (18.4%) showing local relapse at surgical cavity. One-year local control rate was 81% for all patients. Radiation necrosis was confirmed by Positron Emission Tomography in 3 patients (7.8%). Leptomeningeal disease was observed in 5 patients (13.2%). No significant differences were observed between radiation schemes nor primary tumor histology. Conclusion Surgery followed by High precision HFRT delivered with VMAT technique is an effective and safe treatment alternative to SRS for selected patients with brain metastases.

Hydraulic geometric optimization on stagnating flow at the inlet of main pipe of underground pumping station

The underground pumping station is widely used in many important fields. However, the stagnating flow caused by the sharp geometric gradient at the interface between the primary pump intake and the main pipeline not only causes the performance deteriorations of the whole pump station but also leads to safety and operational stability challenges. In this paper, the numerical model of the related hydraulic phenomena is presented, moreover, the effectiveness and accuracy of the numerical model are verified by comparing it with the experimental results. The simulation results indicate that the hydraulic geometry optimization scheme at the inlet of the main pipe with 0.10 D (inner diameter of the main pipe) local rounding contributes the best effect on the hydraulic efficiency and operation stability of underground pump stations. The results may give a reference for the design of underground pump stations and the optimization of hydraulic geometry in related fields.

Resonant transformation of sonic-Alfven modes in solar spicules

The photospheric 5 min oscillations of the Sun can convert to Alfven waves by some conditions such as considering of a region and of a damping mechanism. Actually, 5 min oscillations appear in the photosphere and it does not seem to be able to pierce into the solar upper atmosphere because of the acoustic cut off of stratified medium and the sharp temperature gradient in the transition region. By perturbing the equilibrium state of the magnetized plasma, we showed that the acoustic and Alfven waves can interact through the nonlinear terms of the ideal MHD equations. We found that Alfven waves with twice the period of acoustic waves can propagate into upper layers when the speeds of sound and Alfven waves are equal. This condition obtain when ϖ=1/2ϖ .

Sharp gradient stability for the Sobolev inequality

We prove a sharp quantitative version of the p-Sobolev inequality for any 1<p<n, with a control on the strongest possible distance from the class of optimal functions. Surprisingly, the sharp exponent is constant for p≤2, while it depends on p for p>2.

Crystal orientation mapping and microindentation reveal anisotropy in Porites skeletons

Structures made by scleractinian corals support diverse ocean ecosystems. Despite the importance of coral skeletons and their predicted vulnerability to climate change, few studies have examined the mechanical and crystallographic properties of coral skeletons at the micro- and nano-scales. Here, we investigated the interplay of crystallographic and microarchitectural organization with mechanical anisotropy within Porites skeletons by measuring Young’s modulus and hardness along surfaces transverse and longitudinal to the primary coral growth direction. We observed micro-scale anisotropy, where the transverse surface had greater Young’s modulus and hardness by ∼ 6 GPa and 0.2 GPa, respectively. Electron backscatter diffraction (EBSD) revealed that this surface also had a higher percentage of crystals oriented with the a-axis between ± 30–60∘, relative to the longitudinal surface, and a broader grain size distribution. Within a region containing a sharp microscale gradient in Young’s modulus, nanoscale indentation mapping, energy dispersive spectroscopy (EDS), EBSD, and Raman crystallography were performed. A correlative trend showed higher Young’s modulus and hardness in regions with individual crystal bases (c-axis) facing upward, and in crystal fibers relative to centers of calcification. These relationships highlight the difference in mechanical properties between scales (i.e. crystals, crystal bundles, grains). Observations of crystal orientation and mechanical properties suggest that anisotropy is driven by microscale organization and crystal packing rather than intrinsic crystal anisotropy. In comparison with previous observations of nanoscale isotropy in corals, our results illustrate the role of hierarchical architecture in coral skeletons and the influence of biotic and abiotic factors on mechanical properties at different scales. Statement of significanceCoral biomineralization and the ability of corals’ skeletal structure to withstand biotic and abiotic forces underpins the success of reef ecosystems. At the microscale, we show increased skeletal stiffness and hardness perpendicular to the coral growth direction. By comparing nano- and micro-scale indentation results, we also reveal an effect of hierarchical architecture on the mechanical properties of coral skeletons and hypothesize that crystal packing and orientation result in microscale anisotropy. In contrast to previous findings, we demonstrate that mechanical and crystallographic properties of coral skeletons can vary between surface planes, within surface planes, and at different analytical scales. These results improve our understanding of biomineralization and the effects of scale and direction on how biomineral structures respond to environmental stimuli.

Continuum Covariance Propagation for Understanding Variance Loss in Advective Systems

Motivated by the spurious variance loss encountered during covariance propagation in atmospheric and other large-scale data assimilation systems, we consider the problem for state dynamics governed by the continuity and related hyperbolic partial differential equations. This loss of variance is often attributed to reduced-rank representations of the covariance matrix, as in ensemble methods for example, or else to the use of dissipative numerical methods. Through a combination of analytical work and numerical experiments, we demonstrate that significant variance loss, as well as gain, typically occurs during covariance propagation, even at full rank. The cause of this unusual behavior is a discontinuous change in the continuum covariance dynamics as correlation lengths become small, for instance in the vicinity of sharp gradients in the velocity field. This discontinuity in the covariance dynamics arises from hyperbolicity: the diagonal of the kernel of the covariance operator is a characteristic surface for advective dynamics. Our numerical experiments demonstrate that standard numerical methods for evolving the state are not adequate for propagating the covariance, because they do not capture the discontinuity in the continuum covariance dynamics as correlations lengths tend to zero. Our analytical and numerical results demonstrate in the context of mass conservation that this leads to significant, spurious variance loss in regions of mass convergence and gain in regions of mass divergence. The results suggest that developing local covariance propagation methods designed specifically to capture covariance evolution near the diagonal may prove a useful alternative to current methods of covariance propagation.

Analysis of MONARC and ACTIVATE Airborne Aerosol Data for Aerosol-Cloud Interaction Investigations: Efficacy of Stairstepping Flight Legs for Airborne In Situ Sampling

A challenging aspect of conducting airborne in situ observations of the atmosphere is how to optimize flight plans for specific objectives and constraints associated with weather and flight restrictions. For aerosol-cloud interaction research, two recent campaigns utilized a “stairstepping” approach whereby an aircraft conducts level legs at various altitudes while moving forward with each subsequent leg: the 2019 MONterey Aerosol Research Campaign (MONARC) over the northeast Pacific and the 2020–2022 Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) over the northwest Atlantic. We examine the homogeneity of several atmospheric variables both vertically and horizontally in the marine boundary layer with a focus on the sub-cloud environment. In well-mixed boundary layers, there was generally good horizontal and vertical homogeneity in potential temperature, winds, water vapor mixing ratio, various trace gases, and many aerosol variables. Selected aerosol variables exhibited the most variability owing to sensitivity to humidity and near-cloud conditions (supermicrometer aerosol concentrations), coastal pollution gradients (e.g., organic aerosol mass), and small spatial scale phenomena such as new particle formation (aerosol number concentration for particles with diameter &gt;3 nm). Illustrative cases are described when stairstepping can pose issues requiring extra caution for data analysis: (i) poor vertical mixing and layers decoupled from those below; (ii) multiple cloud layers; (iii) fluctuating cloud base/top and boundary layer top heights; and (iv) horizontal variability across specific features leading to sharp gradients such as right near coastlines and over the Gulf Stream with strong sea surface temperature changes. Results from this study provide a guide both for future studies aiming to examine these mission datasets and for designing new airborne campaigns.

Numerical simulation of a relativistic magnetron using a fluid electron model

An approach to numerically modeling relativistic magnetrons, in which the electrons are represented with a relativistic fluid, is described. A principal effect in the operation of a magnetron is space-charge-limited (SCL) emission of electrons from the cathode. We have developed an approximate SCL emission boundary condition for the fluid electron model. This boundary condition prescribes the flux of electrons as a function of the normal component of the electric field on the boundary. We show the results of a benchmarking activity that applies the fluid SCL boundary condition to the one-dimensional Child–Langmuir diode problem and a canonical two-dimensional diode problem. Simulation results for a two-dimensional A6 magnetron are then presented. Computed bunching of the electron cloud occurs and coincides with significant microwave power generation. Numerical convergence of the solution is considered. Sharp gradients in the solution quantities at the diocotron resonance, spanning an interval of three to four grid cells in the most well-resolved case, are present and likely affect convergence.

Energetic electron scattering by kinetic Alfvén waves at strong magnetic field gradients of dipolarization front

Energetic electron scattering and precipitation from the Earth's plasma sheet to the ionosphere is an important contributor to magnetosphere–ionosphere coupling. In this study, we investigate the role of one of the most intense wave emissions, kinetic Alfvén waves (KAWs), in energetic electron scattering. We have evaluated the effect of KAWs on energetic electrons within a curved magnetic field configuration exhibiting sharp cross field gradients. The magnetic field in Earth's magnetotail plasma sheet with an embedded dipolarization front is used as a working example. Taking into account electron bounce motion and perpendicular guiding-center drifts, we have shown that electrons with energies of tens to hundreds of keV can be scattered by KAWs in pitch angle and momentum through Doppler-shifted Landau resonance near the magnetic equator. The bounce-averaged pitch-angle diffusion coefficients for near-loss-cone (∼2°) electrons are on the order of 10–7–10–6 rad2/s for a characteristic KAW amplitude of 1 mV/m and approach the strong diffusion limit of ∼10–4 rad2/s for amplitudes of greater than 10 mV/m. These results suggest that under such ambient conditions, KAWs can pitch-angle scatter energetic electron population into the loss cone. In Earth's plasma sheet, this scattering is, thus, very likely to cause significant precipitation during active times. The diffusion coefficients of energetic electrons at large pitch angles (∼45°–∼80°) are more than two orders of magnitude larger than those of electrons near the loss cone, suggesting that KAWs contribute to isotropization of anisotropic electrons due to adiabatic heating should they drift into the vicinity of the magnetic field gradient.

Geochemical and microbiological profiles in hydrothermal extreme acidic environments (Pisciarelli Spring, Campi Flegrei, Italy).

Although terrestrial hydrothermal systems are considered among the most fascinating environments, how their unique and extreme conditions can affect microorganisms selection and the role in biogeochemical cycles has not yet been well elucidated. A combined geochemical and microbiological exploration in waters and sediments from 10 sampling points along a sharp temperature gradient (15-90°C) within an extremely acidic hydrothermal system (Pisciarelli Spring, Campi Flegrei area, southern Italy) displayed how hydrothermal fluids influence the microbial dynamics. This area was characterized by high levels of reduced gaseous species (e.g. H2S, H2, CH4, CO) and very low pH values (<2.3). Thermodynamic calculations revealed a high microbial catabolic potential in oxidation/reduction reactions of N-, S- and Fe-bearing species. Overall, an increase of the archaeal/bacterial abundance ratio was observed by decreasing temperature and pH values. In particular, Archaea and Bacteria were present in almost equal cell abundance (up to 1.1 × 109 and 9.3 × 108 cell/g, respectively) in the <70°C sampling points (average pH = 2.09); on the contrary, the highest temperature waters (85-90°C; average pH = 2.26) were characterized by a low abundance of archaeal cells. The high-throughput sequencing of the 16S rRNA genes indicated strong differences in archaeal and bacterial communities composition along the temperature gradient. However, the microbiome in this extreme environment was mainly constituted by chemoautotrophic microorganisms that were likely involved in N-, S- and Fe-bearing species transformations (e.g. Acidianus infernus, Ferroplasma acidarmanus, Acidithiobacillus,Sulfobacillus,Thaumarchaeota), in agreement with thermodynamic calculations.

Fracture characterization from noisy displacement data using artificial neural networks

Mechanical characterization of fractures, i.e., identifying their characteristic parameters such as energy release rate, is crucial to assess the safety and stability of structural members. This is generally achieved using a combination of finite element analysis and optimization. Machine learning models are increasingly used to characterize engineering problems. While such models have shown impressive performance on smooth data, their performance diminishes significantly on data with discontinuities and sharp gradients. For fractures, this issue is more severe due to the singular solutions in the vicinity of the fracture tips. To resolve this difficulty, leveraging classical fracture mechanics, we construct custom functions, using neural networks, containing fundamental solutions of fracture mechanics. Through a series of examples, we demonstrate the proposed framework not only captures the singular solution and characteristic parameters accurately on both noise-free and noisy data, but also is highly efficient due to its simplicity and small number of parameters.

Nanoscale sharp bandgap gradient for efficiency improvement of Cu2ZnSn(S, Se)4 thin film solar cells

Due to the difficulty in controlling the post-selenization/sulfurization process accurately, single-graded structure with a lower bandgap on the surface is easy to form in the Cu 2 ZnSn(S, Se) 4 absorber. This will lower the built-in potential and lead to the formation of undesirable “cliff” type band alignment in the device, and hence a drastic V oc -deficit and lower efficiency are expected. Herein, an effective and versatile method to improve the V oc and efficiency of Cu 2 ZnSn(S, Se) 4 solar cells through bandgap engineering was presented in this work. A sharp surface bandgap gradient with larger surface bandgap was constructed simply by sputtering a nanoscale Cu 2 ZnSnS 4 layer on the surface of Cu 2 ZnSn(S, Se) 4 absorber. Under the coactions of surface sharp bandgap gradient and the single bandgap gradient existed in the Cu 2 ZnSn(S, Se) 4 absorber naturally, a quasi-U-shaped bandgap gradient was constructed inside the absorber. Benefited from the enlarged surface bandgap, favorable U-shaped bandgap gradient and appropriate band alignment at the absorber/buffer interface, a best efficiency of 9.9% with a V oc of 450 mV, a J sc of 35.26 mA/cm 2 and a fill factor (FF) of 62.73% was achieved. • Improving the V oc and efficiency of CZTSSe device by bandgap engineering. • Sharp surface bandgap gradient construction. • Formation of quasi-U-shaped bandgap gradient in the CZTSSe absorber. • Appropriate band alignment between CZTSSe/CdS heterointerface.

A moving finite element method for solving two-dimensional coupled Burgers’ equations at high Reynolds numbers

The coupled Burgers’ equations at high Reynolds numbers usually have sharp gradients or are discontinuous in the solution. Therefore, it is difficult to obtain analytical solutions. This paper aims to use the moving finite element method proposed by Li et al. (2001) to get stable and high-precision numerical solutions for the coupled Burgers’ equations at high Reynolds numbers. The method decouples the mesh equation and partial differential equation (PDE) into two unrelated parts, mesh reconstruction and PDE solver. The mesh reconstruction constructs the harmonic mapping between the physical and logical domains through an iterative method so that the mesh structure maintains harmonics after multiple numerical integrations. We compute three classic numerical examples. Numerical results show that the moving finite element method effectively solve the coupled Burgers’ equations at high Reynolds numbers, obtain stable numerical results, and achieve higher numerical accuracy. During the evolution of the solution, the mesh is always concentrated in the position where the solution has sharp gradients.

Non-intrusive balancing transformation of highly stiff systems with lightly damped impulse response

Balanced truncation (BT) is a model reduction method that uses a coordinate transformation to retain eigen-directions that are highly observable and reachable. To address realizability and scalability of BT applied to highly stiff and lightly damped systems, a non-intrusive data-driven method is developed for balancing discrete-time linear systems via the eigensystem realization algorithm (ERA). The advantage of ERA for balancing transformation makes full-state outputs tractable. Further, ERA enables balancing despite stiffness, by eliminating computation of balancing modes and adjoint simulations. As a demonstrative example, we create balanced reduced-order models (ROMs) for a one-dimensional reactive flow with pressure forcing, where the stiffness introduced by the chemical source term is extreme (condition number [Formula: see text]), preventing analytical implementation of BT. We investigate the performance of ROMs in prediction of dynamics with unseen forcing inputs and demonstrate stability and accuracy of balanced ROMs in truly predictive scenarios, whereas without ERA, proper orthogonal decomposition-Galerkin and least-squares Petrov-Galerkin projections fail to represent the true dynamics. We show that after the initial transients under unit impulse forcing, the system undergoes lightly damped oscillations, which magnifies the influence of sampling properties on predictive performance of the balanced ROMs. We propose an output domain decomposition approach and couple it with tangential interpolation to resolve sharp gradients at reduced computational costs. This article is part of the theme issue 'Data-driven prediction in dynamical systems'.

Physics-based sizing functions for ocean-to-estuary shallow-water models

Modeling ocean processes requires a discrete representation of the system geometry, which itself contains multiple degrees of freedom, especially for systems of large geographical scale. This paper assesses the unstructured meshing requirements for a large-scale, astronomic-tide model domain of the western North Atlantic Ocean, Gulf of Mexico and Caribbean Sea with telescopic focus of the South Atlantic Bight (United States southeastern seaboard) and its estuaries. The methodologies of localized truncation error analysis (LTEA), which account only for the model interior and LTEA plus complex derivatives (+CD), which account for the model interior and boundary, are exploited due to their foundations stemming directly from tidal physics. LTEA and LTEA+CD are applied for the M2 tidal harmonics resulting from linear and nonlinear solutions of the ADCIRC (advanced circulation) model based on a highly resolved uniform mesh. The resultant target element size distributions are interrogated distinctively for the inshore (estuarine) region of the South Atlantic Bight, the inlet-punctuated coastline, the shelf-slope-rise triad and deep ocean, and the open-ocean boundary. The relatively large size of the LTEA computational molecule makes it the suitable choice for the deep ocean, offshore and shelf regions where the tidal dynamics are predominantly linear and smoothly changing in space. LTEA+CD is the logical choice for the shallows and estuaries because of the spatial coverage afforded by the combined utility of interior and boundary estimators (computational molecules), as well as its accounting for the nonlinearities in the evaluation of local truncation error. The combination of the two methods enables an almost complete coverage of the domain for generating spacing requirements, with LTEA+CD extending beyond the interior-only definition of LTEA to capture the inshore regions and model boundaries. Implementing the approach with LTEA and LTEA+CD to target finer resolution in areas with sharper gradients in the hydrodynamics, while relaxing resolution in areas with smoother hydrodynamics, resulted in a mesh with a far-reduced number of elements (∼1.3M) relative to an overly resolved uniform mesh (>10M elements) that is comparable in accuracy with ±5% tidal amplitude error for 99% of the domain.

Volume imaging of aspect sensitivity in VHF radar backscatters: first results inferred from the Advanced Indian MST radar (AIR)

ABSTRACT Very-high frequency (VHF) radar echoes are known to be aspect sensitive in general, which could be either the manifestation of a thin stable layer providing sharp refractive index gradient or shear-driven steep layer structures. The present study for the first time shows the volume imaging of aspect sensitivity in the recently upgraded Advanced Indian Mesosphere Stratosphere Troposphere (MST) radar (AIR). The present configuration of radar can provide a better understanding of three-dimensional structures of turbulence and instabilities, which was not possible when the radar was operated in two orthogonal planes. Both isotropic and anisotropic layers/turbulence are observed at various height levels over the radar volume. Enhanced aspect sensitivity is observed between 6 and 9 km, which is attributed to the presence of dynamic instability arising due to strong wind shear and atmospheric stability. All the scatterers tend to become isotropic when both the square of wind shear and stability parameters are above 0.25 × 10−3 s−2. Due to the high aspect sensitivity, there may be an underestimate of 5–10% in wind velocity measurements, and it is higher for the low off-zenith beam angle.

An artificial damping method for total Lagrangian SPH method with application in biomechanics

Introducing artificial damping into the momentum equation to enhance the numerical stability of the smoothed particle hydrodynamics (SPH) method for large strain dynamics has been well established, however, implementing appropriate damping term in the constitutive model as an alternative stabilization strategy in the context of the SPH method is still not investigated. In this paper, we present a simple stabilization procedure by introducing an artificial damping term into the second Piola–Kirchhoff stress to enhance the numerical stability of the SPH method in total Lagrangian formulation. The key idea is to reformulate the constitutive equation by adding a Kelvin–Voigt (KV) type damper with a scaling factor imitating a von Neumann–Richtmyer type artificial viscosity to alleviate the spurious oscillation in the vicinity of sharp spatial gradients. The proposed method is shown to effectively eliminate the appearance of spurious non-physical instabilities and easy to be implemented into the original total Lagrangian SPH formulation. After validating the numerical stability and accuracy of the present method through a set of benchmark tests with very challenging cases, we demonstrate its applications and potentials in the field of biomechanics by simulating the deformation of complex stent structures and the electromechanical excitation–contraction of the realistic left ventricle.

Increasing footprint of climate warming on flash droughts occurrence in Europe

Flash droughts are caused by a rapid depletion of soil moisture, and they severely affect vegetation growth and agricultural production. Notwithstanding the growing importance of flash droughts under the warming climate, drivers of flash droughts across the Europe are not well understood. Here we estimate the changes in flash droughts characteristics across Europe using the ERA5 reanalysis dataset for 1950–2019 period. We find a substantial increase in the frequency and spatial extent of flash droughts across Europe (with 79% of the total area) during the growing season with at-least one fourth of domain showing two-fold increase in the recent decades. Increased occurrence of flash drought is largely attributed to frequent occurrence of warmer and drier compound extremes, with a sharp gradient of changes being noticed in Mediterranean and Central European regions. Compound meteorological extremes causing the flash drought events across Europe are pre-dominantly driven by the recent climate warming. With unabated greenhouse gas emissions and current pace of climate warming, Europe is likely to face an increased occurrence of flash droughts, requiring prompt response for effective drought adaptation and management strategies.