► Recent hydrological studies based on radar data are reviewed. ► The current limitations for the hydrological use of radar are detailed. ► The main challenges for radar hydrology are proposed.
...Hydrology requires accurate and reliable rainfall input. Because of the strong spatial and temporal variability of precipitation, estimation of spatially distributed rain rates is challenging. Despite the fact that weather radars provide high-resolution (but indirect) observations of precipitation, they are not used in hydrological applications as extensively as one could expect. The goal of the present review paper is to investigate this question and to provide a clear view of the opportunities (e.g., for flash floods, urban hydrology, rainfall spatial extremes) the limitations (e.g., complicated error structure, need for adjustment) and the challenges for the use of weather radar in hydrology (i.e., validation studies, precipitation forecasting, mountainous precipitation, error propagation in hydrological models).
The raindrop size distribution (DSD) quantifies the microstructure of rainfall and is critical to studying precipitation processes. We present a method to improve the accuracy of DSD measurements ...from Parsivel (particle size and velocity) disdrometers, using a two-dimensional video disdrometer (2DVD) as a reference instrument. Parsivel disdrometers bin raindrops into velocity and equivolume diameter classes, but may misestimate the number of drops per class. In our correction method, drop velocities are corrected with reference to theoretical models of terminal drop velocity. We define a filter for raw disdrometer measurements to remove particles that are unlikely to be plausible raindrops. Drop concentrations are corrected such that on average the Parsivel concentrations match those recorded by a 2DVD. The correction can be trained on and applied to data from both generations of OTT Parsivel disdrometers, and indeed any disdrometer in general. The method was applied to data collected during field campaigns in Mediterranean France for a network of first- and second-generation Parsivel disdrometers, and on a first-generation Parsivel in Payerne, Switzerland. We compared the moments of the resulting DSDs to those of a collocated 2DVD, and the resulting DSD-derived rain rates to collocated rain gauges. The correction improved the accuracy of the moments of the Parsivel DSDs, and in the majority of cases the rain rate match with collocated rain gauges was improved. In addition, the correction was shown to be similar for two different climatologies, suggesting its general applicability.
Orographic lifting of air masses and other topographically modified flows induce cloud and precipitation formation at larger scales and preferential deposition of precipitation at smaller scales. In ...this study, we examine orographic effects on small‐scale snowfall patterns in Alpine terrain. A polarimetric X‐band radar was deployed in the area of Davos (Switzerland) to determine the spatial variability of precipitation. In order to relate measured precipitation fields to flow dynamics, we model flow fields with the atmospheric prediction model “Advanced Regional Prediction System.” Additionally, we compare radar reflectivity fields with snow accumulation at the surface as modeled by Alpine3D. We investigate the small‐scale precipitation dynamics for one heavy snowfall event in March 2011 at a high resolution of 75 m. The analysis of the vertical and horizontal distribution of radar reflectivity at horizontal polarization and differential reflectivity shows polarimetric signatures of orographic snowfall enhancement near the summit region. Increasing radar reflectivity at horizontal polarization over the windward slopes toward the crest and downwind decreasing reflectivity over the leeward slopes is observed. The temporal variation of the location of maximum concentration of snow particles is partly attributed to the effect of preferential deposition of snowfall: For situations with strong horizontal winds, the concentration maximum is shifted from the ridge crest toward the leeward slopes. Qualitatively, we discuss the relative role of cloud microphysics such as the seeder‐feeder mechanism versus atmospheric particle transport in generating the observed snow deposition at the ground.
Key Points
Orographic snowfall enhancement near the summit region
Preferred snow deposition on leeward slopes
Snow drift affects reflectivity patterns observed by radar
Microwave links can be used to estimate the path‐averaged rain rate along the link when precipitation occurs. They take advantage of the near proportionality between the specific attenuation ...affecting the link signal and the rain rate. This paper deals with the influence of the spatial variability of rainfall along the link on the accuracy of the rainfall estimates. We focus on single‐polarization single‐frequency links operating at frequencies ranging from 5 to 50 GHz and with path lengths ranging from 500 m to 30 km. A stochastic simulation framework is used to investigate the frequency at which the linearity occurs (found to be about 30 GHz) for intense Mediterranean precipitation. In addition, the error associated with path‐averaged rain rate estimates is being quantified. For instance, the bias is found to be in the order of −15% with an uncertainty of about 10% for microwave links described in the recent literature.
A data-driven approach to the classification of hydrometeors from measurements collected with polarimetric weather radars is proposed. In a first step, the optimal number of hydrometeor classes ...(nopt) that can be reliably identified from a large set of polarimetric data is determined. This is done by means of an unsupervised clustering technique guided by criteria related both to data similarity and to spatial smoothness of the classified images. In a second step, the nopt clusters are assigned to the appropriate hydrometeor class by means of human interpretation and comparisons with the output of other classification techniques. The main innovation in the proposed method is the unsupervised part: the hydrometeor classes are not defined a priori, but they are learned from data. The approach is applied to data collected by an X-band polarimetric weather radar during two field campaigns (from which about 50 precipitation events are used in the present study). Seven hydrometeor classes (nopt = 7) have been found in the data set, and they have been identified as light rain (LR), rain (RN), heavy rain (HR), melting snow (MS), ice crystals/small aggregates (CR), aggregates (AG), and rimed-ice particles (RI).
This study investigates the microphysics of winter alpine snowfall occurring in mixed-phase clouds in an inner-Alpine valley during January and February 2014. The available observations include ...high-resolution polarimetric radar and in situ measurements of the ice-phase and liquid-phase components of clouds and precipitation. Radar-based hydrometeor classification suggests that riming is an important factor to favor an efficient growth of the precipitating mass and correlates with snow accumulation rates at ground level. The time steps during which rimed precipitation is dominant are analyzed in terms of temporal evolution and vertical structure. Snowfall identified as rimed often appears after a short time period during which the atmospheric conditions favor wind gusts and updrafts and supercooled liquid water (SLW) is available. When a turbulent atmospheric layer persists for several hours and ensures continuous SLW generation, riming can be sustained longer and large accumulations of snow at ground level can be generated. The microphysical interpretation and the meteorological situation associated with one such event are detailed in the paper. The vertical structure of polarimetric radar observations during intense snowfall classified as rimed shows a peculiar maximum of specific differential phase shift Kdp, associated with large number concentrations and riming of anisotropic crystals. Below this Kdp peak there is usually an enhancement in radar reflectivity ZH, proportional to the Kdp enhancement and interpreted as aggregation of ice crystals. These signatures seem to be recurring during intense snowfall.
Rapid mass movements (RMM) pose a substantial risk to people and infrastructure. Reliable and cost-efficient measures have to be taken to reduce this risk. One of these measures includes establishing ...and advancing the state of practice in the application of early warning systems (EWSs). EWSs have been developed during the past decades and are rapidly increasing. In this paper, we focus on the technical part of EWSs, i.e., the prediction and timely recognition of imminent hazards, as well as on monitoring slopes at risk and released mass movements. Recent innovations in assessing spatial precipitation, monitoring and precursors of the triggering and deformation of RMM offer new opportunities for next-generation EWSs. However, technical advancement can only be transferred into more reliable, operational EWSs with an adequate well-instructed dedicated staff. To this end, an intense dialog between scientists, engineers and those in charge of warning, as well as further experience with new comprehensive prototype systems jointly operated by scientists and practitioners, will be essential.
Many applications in urban areas require high‐resolution rainfall measurements. Typical operational weather radars can provide rainfall intensities at 1‐km2 grid cells every 5 min. Opportunistic ...sensing with commercial microwave links yields path‐averaged rainfall intensities (typically 0.1–10 km) within urban areas. Additionally, large amounts of urban in situ rainfall measurements from amateur weather observers are obtainable in real‐time. The accuracy of these three techniques is evaluated for an urban study area of 20 × 20 km, taking into account their respective network layouts and sampling characteristics. We use two simulated rainfall events described in terms of drop size distributions on a 100‐m grid and with a temporal resolution of 30 s. Accurate radar rainfall estimation with the Z‐R relationship relies heavily on an appropriate choice of parameters, and a dual‐polarization strategy is more suitable for higher intensities. Under ideal measurement conditions, the weather station network is the most promising, with a Pearson correlation coefficient above 0.86 and a relative bias below 4% for 100‐m rainfall estimates at 5‐min resolution. Microwave link rainfall observations contain the largest error, shown by a consistently larger coefficient of variation. The accuracy of all techniques improves when considering rainfall at larger scales, especially by increasing time intervals, with the strongest improvements found for microwave links for which errors are largely caused by their temporal sampling. Sparser networks are examined, showing that the decline in measurement accuracy only becomes significant when the link and station network density are reduced to less than half their levels in Amsterdam.
Key Points
Assuming perfect measurement accuracy, the personal weather station network captures small‐scale rainfall dynamics best in Amsterdam
Measurement accuracy increases at larger temporal and spatial scales, most significantly for commercial microwave links
With current network layouts, similar accuracy is achieved by half the number of commercial microwave links and personal weather stations
Mesopelagic communities are characterized by a large biomass of diverse macrozooplankton and micronekton (MM) performing diel vertical migration (DVM) connecting the surface to the deeper ocean and ...contributing to biogeochemical fluxes. In the Southern Ocean, a prominent High Nutrient Low Chlorophyll (HNLC) and low carbon export region, the contribution of MM to the vertical carbon flux of the biological pump remains largely unknown. Furthermore, few studies have investigated MM communities and vertical flux in naturally iron fertilized areas associated with shallow bathymetry. In this study, we assessed the MM community diversity, abundance and biomass in the Kerguelen Island region, including two stations in the HNLC region upstream of the islands, and two stations in naturally iron fertilized areas, one on the Plateau, and one downstream of the Plateau. The MM community was examined using a combination of trawl sampling and acoustic measurements at 18 and 38 kHz from the surface to 800 m. A conspicuous three-layer vertical system was observed in all areas - a shallow scattering layer, SSL, between 10 and 200 m; mid-depth scattering layer, MSL, between 200 and 500 m; deep scattering layer, DSL, between 500 and 800 m - but communities differing among stations. While salps (Salpa thompsoni) dominated the biomass at the productive Kerguelen Plateau and the downstream station, they were scarce in the HNLC upstream area. In addition, crustaceans (mainly Euphausia vallentini and Themisto gaudichaudii) were particularly abundant over the Plateau, representing a large, although varying, carbon stock in the 0–500 m water layer. Mesopelagic fish were prominent below 400 m where they formed permanent or migrant layers accounting for the main source of carbon biomass. Through these spatial and temporal sources of variability, complex patterns of the MM vertical distribution and associated carbon content were identified. The total carbon flux mediated by migratory myctophids at the four stations was quantified. While this flux was likely underestimated, this study identified the main components and mechanisms of active carbon export in the region and how they are modulated by complex topography and land mass effects.
•Macrozooplankton and micronekton communities are described in the Kerguelen area using trawl and acoustic•Despite different assemblages, a consistent three-layers system occurred in the first 800 m over the Kerguelen seascape•Salps dominated global biomass in productive areas but not in HNLC region while crustaceans were abundant on the Plateau•Myctophids dominated below 400 m as permanent or migrant layers accounting for the main source of carbon biomass•We estimate total carbon flux mediated by migratory myctophids in the contrasting productive areas
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