L.G. Lanza

Rainfall intermittency and the sampling error of tipping-bucket rain gauges

Abstract This work amplifies upon the influence of rainfall intermittency on pluviometric time series. Intermittency is here defined as the percentage of no-rain periods within a rainfall event, and can be interpreted as a sort of ‘stochastic intermittency’. When measuring an intermittent or, in general, ‘erratic’ signal, a relevant source of error is associated with the sampling procedure. By using numerical simulation of intermittent rainfall events, we focus on the deriving pattern of sampling errors within rainfall measurements obtained from any classic tipping-bucket rain gauge. The analysis, performed in both the time and frequency domain, reveals a strong inverse linear dependence between rainfall intermittency and the sampling errors, while a weak dependence on the autocorrelation of synthetic rainfall events is shown. The average absolute error is around 30% for the events analysed while the error at each sampling period peaks values higher than 100%. A direct influence of intermittency on the statistical characteristics of measured rainfall events is observed.

The Role of GIS as A Tool for the Assessment of Flood Hazard at the Regional Scale

A review of some recent applications of “hydrologically oriented” GIS in flood forecasting and related issues is presented. A regional approach to the assessment of flood hazard in orographically accentuated areas is also described on the basis of the supporting data handling capabilities of Geographical Information Systems. In particular, the development of operational methodologies aimed at the issuing of effective and timely warnings at the regional scale is presented both in a deterministic perspective — by simple enumeration of a series of pre-defined targets — and in a probabilistic one, based on the most recent advances in drainage geomorphology. The use of multisensor monitoring devices for the detection of extreme events, from the overall meteorological scenario down to the scale of the local variability of rainfall in space, is proposed on the basis of the experience of some international research projects recently activated in the field. The multi-faceted role of GIS in the collection of rainfall data from the whole set of available sensors, the identification of areas of potential occurrence of extreme meteorological events, the operational support to distributed rainfall-runoff models and the mapping of flood prone areas and landscape vulnerability is highlighted throughout the paper.

An Improved Trajectory Model to Evaluate the Collection Performance of Snow Gauges

Recent studies have used numerical models to estimate the collection efficiency of solid precipitation gauges when exposed to the wind in both shielded and unshielded configurations. The models used computational fluid dynamics (CFD) simulations of the airflow pattern generated by the aerodynamic response to the gauge‐shield geometry. These are used as initial conditions to perform Lagrangian tracking of solid precipitation particles. Validation of the results against field observations yielded similarities in the overall behavior, but the model output only approximately reproduced the dependence of the experimental collection efficiency on wind speed. This paper presents an improved snowflake trajectory modeling scheme due to the inclusion of a dynamically determined drag coefficient. The drag coefficient was estimated using the local Reynolds number as derived from CFD simulations within a time-independent Reynoldsaveraged Navier‐Stokes approach. The proposed dynamic model greatly improves the consistency of results with the field observations recently obtained at the Marshall Field winter precipitation test bed in Boulder, Colorado.

The Collection Efficiency of Shielded and Unshielded Precipitation Gauges. Part II: Modeling Particle Trajectories

AbstractThe use of windshields to reduce the impact of wind on snow measurements is common. This paper investigates the catching performance of shielded and unshielded gauges using numerical simulations. In Part II, the role of the windshield and gauge aerodynamics, as well as the varying flow field due to the turbulence generated by the shield–gauge configuration, in reducing the catch efficiency is investigated. This builds on the computational fluid dynamics results obtained in Part I, where the airflow patterns in the proximity of an unshielded and single Alter shielded Geonor T-200B gauge are obtained using both time-independent [Reynolds-averaged Navier–Stokes (RANS)] and time-dependent [large-eddy simulation (LES)] approaches. A Lagrangian trajectory model is used to track different types of snowflakes (wet and dry snow) and to assess the variation of the resulting gauge catching performance with the wind speed. The collection efficiency obtained with the LES approach is generally lower than the on...

The Collection Efficiency of Shielded and Unshielded Precipitation Gauges. Part I: CFD Airflow Modeling

AbstractThe aerodynamic response of snow gauges when exposed to the wind is responsible for a significant reduction of their collection performance. The modifications induced by the gauge and the windshield onto the space–time patterns of the undisturbed airflow deviate the snowflake trajectories. In Part I, the disturbed air velocity field in the vicinity of shielded and unshielded gauge configurations is investigated. In Part II, the airflow is the basis for a particle tracking model of snowflake trajectories to estimate the collection efficiency. A Geonor T-200B gauge inside a single Alter shield is simulated for wind speeds varying from 1 to 8 m s−1. Both time-averaged and time-dependent computational fluid dynamics simulations are performed, based on Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) models, respectively. A shear stress tensor k–Ω model (where k is the turbulent kinetic energy and Ω is the turbulent specific dissipation rate) is used for the RANS formulation and s...

Influence of stratigraphy and slope on the drainage capacity of permeable pavements: laboratory results

A small size laboratory test-bed was realized at the University of Genoa in order to evaluate the drainage capacity of permeable pavements by monitoring inflow, runoff and sub-surface outflow. The laboratory test programme was designed to investigate the influence of rainfall intensity and pavement slope on the hydrologic response of permeable pavements. Four permeable pavement systems combining two paving types (concrete cell and pervious brick) with two filter layers made of recycled glass aggregate and a mix of gravel and coarse sand are tested. The hydrologic response of permeable pavements is analysed by using a dimensionless volume index (discharge coefficient) and a timing index. Laboratory results reveal that the hydrologic performance is fairly consistent for all the investigated permeable pavements. The recycled glass aggregate turns out to be a valid solution. No surface runoff occurs even at 98 mm/h rainfall intensity.

Impact of Wind Direction, Wind Speed, and Particle Characteristics on the Collection Efficiency of the Double Fence Intercomparison Reference

The accurate measurement of snowfall is important in various fields of study such as climate variability, transportation, and water resources. A major concern is that snowfall measurements are difficult and can result in significant errors. For example, collection efficiency of most gauge–shield configurations generally decreases with increasing wind speed. In addition, much scatter is observed for a given wind speed, which is thought to be caused by the type of snowflake. Furthermore, the collection efficiency depends strongly on the reference used to correct the data, which is often the Double Fence Intercomparison Reference (DFIR) recommended by the World Meteorological Organization. The goal of this study is to assess the impact of weather conditions on the collection efficiency of the DFIR. Note that the DFIR is defined as a manual gauge placed in a double fence. In this study, however, only the double fence is being investigated while still being called DFIR. To address this issue, a detailed analysis of the flow field in the vicinity of the DFIR is conducted using computational fluid dynamics. Particle trajectories are obtained to compute the collection efficiency associated with different precipitation types for varying wind speed. The results show that the precipitation reaching the center of the DFIR can exceed 100% of the actual precipitation, and it depends on the snowflake type, wind speed, and direction. Overall, this study contributes to a better understanding of the sources of uncertainty associated with the use of the DFIR as a reference gauge to measure snowfall.

Multisensor observations during the flood event of 4–6 November, 1994 over Northern Italy

In the perspective of flood hazard multisensor monitoring, the event of 4–6 November, 1994 over Northern Italy is described and analyzed in this paper using traditional ground‐based rainfall observations and remote sensing techniques. Satellite imagery is used with different objectives: the interpretation of SAR measurements allows the identification of flooded areas, Meteosat infrared images show high intensity rain areas and SSM/I passive microwave data provide estimates of the rain‐rates. The IFA‐SAP algorithm, a profile based retrieval technique for estimating rainfall rates and precipitating cloud parameters from spacebome multifrequency microwave radiometers, has been used in the latter case. The use of multisensor observations considerably supplements conventional monitoring systems which prove inadequate for many hydrological purposes. The present paper is written in the spirit of encouraging a posteriori analyses of observed heavy rainfall events so as to explore the potential of a real‐time oper...

Cloud tracking using satellite data for predicting the probability of heavy rainfall events in the Mediterranean area

The use of cloud tracking techniques and storm identification procedures is proposed in this paper with the aim of predicting the evolution of cloud entities associated with the highest rainfall probability within a given meteorological scenario. Suitable algorithms for this kind of analysis are based on the processing of digital images in the thermal infrared (IR) band from geostationary satellites: a selection of such algorithms is described in some detail together with a few real case applications. Three heavy rainfall events have been selected for this purpose with reference to the extreme meteorological situation observed during Fall 1992 and 1993 over the Mediterranean area. A window from 30 to 60 °N and from 20 °W to 30 °E has been identified for the analysis of data from the radiometer on board the ESA Meteosat platform. In conclusion, the suitability of cloud tracking techniques for predicting the probability of heavy rainfall events is discussed provided that the former are associated with proper modeling of small scale rainfall distribution.

The tracking and prediction of high intensity rainstorms

The paper addresses some concepts and issues relevant to the use of satellite imagery, as provided by the infrared radiometers flying on board geostationary orbiting platforms, in the tracking and prediction of typical mid‐latitude Mesoscale Convective Complexes (MCCs) associated with high intensity rainstorms over the Mediterranean area. The predictive content of sequences of Meteosat half‐hourly images is exploited in this work, aiming at the development of storm identification and cloud tracking procedures suitable for operational use in flash flood forecasting applications. Though relying essentially on image processing techniques, the cloud tracking approach seems quite useful in the short term prediction of the dynamics of MCCs as the resolution scale of the temporal sampling provided by the satellite sensor is short enough to ensure that abrupt changes in the cloud characteristics are not likely to occur between two subsequent images. Some studies are presented to show the potential of the procedur...