Dual-polarization weather radars have improved the accuracy of precipitation estimates. However, challenges persist in evaluating hydrometeor classification (HMC) algorithms, thereby impacting the accuracy of precipitation estimates. This study proposes to use full Doppler spectra in both polarizations from a Ka- and W-band Doppler-polarimetric profiler with a 45° elevation angle to provide insights into hydrometeor characteristics. A novel methodology was developed to link the observed spectra with the output of an HMC scheme. We applied the wradlib HMC scheme using C-band weather radar data from the Netherlands for six cases (2021–2022). The HMC scheme output is used to calculate mixing ratios that are combined with the corresponding scattering properties using the Atmospheric Radiative Transfer Simulator microwave single scattering properties database (frozen hydrometeors) and T-matrix calculations (liquid hydrometeors) to simulate Doppler spectra of polarimetric variables that would be measured by the profiler. Comparing these simulations with actual profiler measurements enables a quality assessment. The method works in stratiform cases, but convective cases reveal the influence of turbulence and wind variability. Uncertainty arises from the selection of specific parameterizations for the particle size distribution and the relationship between hydrometeor size and terminal fall velocity as well as from the derived mixing ratios. Additionally, the 45° angle complicates separating horizontal wind from hydrometeor fall velocities, although the Mie notch in the dual-wavelength ratio can be effectively used to remove the radial wind component. Our results underline limitations that must be addressed but also show that integrating spectral and dual-frequency observations could yield valuable insights into hydrometeor characteristics.

River catchments are important to consider when investigating the fate of plastics once introduced into the environment. However, plastic transport at the river catchment scale is rarely quantified. In this study, we present a catchment-scale field assessment of macroplastic litter in the Odaw (270 km2). The catchment was sub-divided into non-urban riverine, urban riverine and urban tidal zones based on the urbanisation level and riverine transport across the catchment. The riverine (river and riverbank) and terrestrial environments at ten locations along the catchment were monitored on three days in December, 2021. Floating litter items in the river were monitored by visual counting. At the riverbank and terrestrial environments, litter items were sampled in a designated area (5 x 2 m2) and categorised according to the River-OSPAR list (Schone-Rivieren, 2018). Results showed a high plastic flux (1125 items/h) in the urban riverine zone, which was higher by a factor of 16 and 2 to the plastic flux at the non-urban riverine and urban tidal zones, respectively. Terrestrial and riverbank plastic density was highest closest to the river mouth (urban tidal). The factor increase between the most upstream (non-urban riverine) and downstream (urban tidal) was larger for terrestrial than for riverbank. This shows the influence of urbanisation on the generation of mismanaged plastics in the catchment. Top three plastic polymer types observed in the catchment were PO soft, EPS and Multilayer. However, at each zone, the top three plastic polymer types varied with PO Soft as the most dominant at each zone. The highest abundance of PO soft, EPS and multilayer was found at the urban riverine (56%), urban tidal (31%) and non-urban riverine (24%) zones respectively. Our findings provide information on the spatial variation of plastic transport in the Odaw catchment and therefore can help create better strategies to manage the plastic pollution problem in this catchment. Future work will further explore the potential sources and temporal sink zones in the catchment.

This paper shares an early-career perspective on potential themes for the upcoming International Association of Hydrological Sciences (IAHS) Scientific Decade (SD). This opinion paper synthesizes six discussion sessions in western Europe identifying three themes that all offer a different perspective on the hydrological threats the world faces and could serve to direct the broader hydrological community: “Tipping points and thresholds in hydrology,” “Intensification of the water cycle,” and “Water services under pressure.” Additionally, four trends were distinguished concerning the way in which hydrological research is conducted: big data, bridging science and practice, open science, and inter- and multidisciplinarity. These themes and trends will provide valuable input for future discussions on the theme for the next IAHS SD. We encourage other early-career scientists to voice their opinion by organizing their own discussion sessions and commenting on this paper to make this initiative grow from a regional initiative to a global movement.

Arctic precipitation is projected to increase more rapidly than the global mean in warming climates. However, warming-induced changes in the variability of Arctic precipitation, which are related to surface evaporation and poleward moisture transport (PMT), are currently largely unknown. This study compares the precipitation variability in different quasi-equilibrium climates simulated by a global climate model (EC-Earth) and studies the underlying mechanisms. Five quasi-equilibrium simulations of 400 years length forced with a broad range of CO2 concentrations (0.25, 0.5, 1, 2, and 4 times the current global mean) were analyzed. PMT is the dominant source of Arctic precipitation variability in colder climates when the ocean in the Arctic basin is completely covered by sea ice year-round. Arctic precipitation variability increases from colder to warmer climates, primarily in summer. In summer, the increasingly stronger relation between Arctic sea level pressure variability and precipitation variability toward warmer climates enhances variability. In winter, the severe increase in mean precipitation (due to enhanced evaporation) exerts a comparatively small increase in variability, and precipitation variability is modulated by both PMT and evaporation, which oppose each other as they both affect the vertical and meridional moisture gradients.

The correct estimation of Sensible Heat Flux (H) and Latent Heat Flux (LE) (i.e., turbulent fluxes) is vital in the understanding of exchange of energy and mass among hydrosphere, atmosphere, and biosphere in an ecosystem. One of the most popular methods to measure these fluxes is the Eddy Covariance (EC) technique; however, there are a number of setbacks to its application, especially in remote and topographically complex terrain such as the higher altitudes of the Andes. Efforts have been made by the scientific community to parameterise these fluxes based on other more commonly measured variables. One of the most widespread methods is the so-called bulk method, which relates average temperature, humidity, and wind vertical profiles to the turbulent fluxes. Another approach to estimate LE is the Penman-Monteith (PM) equation which uses meteorological measurements at a single level. The objective of this study was to validate these methods for the first time in the Tropical Andes in Southern Ecuador (in the paramo ecosystem at 3780 m a.s.l.) using EC and meteorological measurements. It was determined that the bulk method was the best to estimate H, although some adjustments had to be made to the typical assumptions used to estimate surface meteorological values. On the other hand, the PM equation yielded the best LE estimations. For both fluxes, the error in the estimations was within the uncertainty range of the EC measurements. It can be concluded that it is possible to accurately estimate H and LE using the methods described in this paper in this ecosystem when no direct measurements are available.