National Institute of Oceanography

Gravity currents are buoyancy-driven flows generated by horizontal density gradients and govern the transport of mass, momentum, and scalars in both natural and engineered systems. A detailed understanding of their near-wall behavior is essential for accurately describing the turbulent mechanisms developing in this region, which is characterized by strong spatial variability, particularly at increasing Reynolds numbers (Re=UbH/ν, with H the initial height of the dense fluid, ν the cinematic viscosity, Ub=(g’H)0.5 a velocity scale related to the reduced gravity g’=g(ρ1- ρ0)/ ρ0, where g is the gravitational acceleration, ρ1 the density of the heavier fluid and ρ0 the ambient density). Several numerical simulations were performed in straight channels under a lock-exchange configuration using a wall-resolved Large Eddy Simulation. The analyzed cases differ in terms of Reynolds number (in the range 34000-136000), both by increasing the height of the domain and by modifying the density difference. The analysis of the near-wall behavior focused on the head of the current, identified through mean density values. Subsequently, streamwise velocity profiles in the wall-normal direction were extracted, first averaged in the spanwise direction and then also along the streamwise direction. Although the latter direction is not homogeneous, this procedure provides an overall view of the behavior of the current head during its temporal evolution. The gradient of the streamwise velocity in the wall-normal direction was used to define the boundary-layer thickness δ. It was observed that the temporal evolution of the normalized thickness δ* = δ/H is similar for all the cases analyzed; moreover, after an initial increase, it tends to approach an asymptotic value during the self-similar phase. In accordance with the characteristics of this phase, it is also observed that the mean velocity profile tends to remain invariant over time during the evolution of the current. Moreover, the presence of a logarithmic region is identified, of the form u+=a(lny+)+bu+=aln⁡y++b (where u+=u/u𝜏, and y+=yu𝜏/νy+=yu𝜏/𝜈, u𝜏 denoting the friction velocity), with an increase in the slope A (in a logarithmic plot) relative to the canonical value (A=2.44), consistent with the local presence of stable stratification. The results obtained may have important implications for the parameterization of simplified large-scale circulation models, particularly with regard to the definition of appropriate boundary conditions.

Abstract Climate change is intensifying the frequency and severity of environmental hazards, with distinct impacts in urban and rural areas. Cities can experience amplified risks due to the urban heat island effect and increased exposure to extremes. We analyze climate extreme indices using Euro-CORDEX regional climate models that include urban representations, focusing on 40 cities and their surrounding rural areas. Our findings highlight that city-scale hazards intensify with warming across the domain, while precipitation responses are strongly regional. Mediterranean cities exhibit robust drying and longer dry spells, whereas central and northern European cities experience the strongest and most widespread increases in heavy-precipitation extremes. Urban–rural contrasts strengthen several hazard metrics, particularly minimum-temperature extremes, with the strongest signals found in inland cities, underscoring the need for targeted adaptation measures. Understanding how global warming impacts these hazard indices is crucial for developing climate-resilient policies and strategies tailored to both urban and rural settings.

On 2 May 2025, a Mw 7.4 earthquake occurred ~120 km off the southern tip of Tierra del Fuego, at the intersection of the Shackleton Fracture Zone and the Chilean Trench. This event is the largest recorded within this complex plate boundary, filling a

Abstract The persistence of diarrheal diseases in urban areas, even those with ostensibly safe water and sanitation systems, indicates significant underlying microbial and infrastructural vulnerabilities that pose challenges to sustainable urban development. Kavaratti Island in India serves as a pertinent example: despite achieving universal access to desalinated water and sanitation facilities, the island reports between 250 and 500 cases of acute diarrheal disease annually. This study employs a combination of household surveys and environmental monitoring to investigate the behavioural determinants of exposure and microbial contamination across various water sources, including groundwater, ponds, and adjacent coastal waters. The findings reveal that only a quarter of households depend solely on desalinated water, while the majority continue to utilize untreated groundwater, thereby reflecting a limited understanding of the associated microbial risks. Quantitative real-time PCR analysis indicates widespread fecal contamination, with concentrations of Escherichia coli surpassing 1,000 gene copies per milliliter in all surveyed ponds and in most groundwater sources during the dry season. Furthermore, Vibrio cholerae was detected in 85% of pond samples and in 20% of groundwater samples during the rainy season. Notably, more than half of the E. coli isolates, along with all Vibrio isolates, displayed resistance to ampicillin, which is a commonly prescribed antibiotic in the region. These results illustrate that infrastructure alone is insufficient to guarantee microbial safety in vulnerable coastal urban settings. The high incidence of diarrheal disease, combined with microbial pollution and antibiotic resistance, highlights the urgent need for integrated surveillance systems, enhanced public awareness, and participatory water management strategies. Community-based initiatives, such as citizen science programs and water clinics, have the potential to strengthen public health resilience in these contexts.