Project

Reconciling precipitation with runoff: the role of understated measurement biases in the modelling of hydrological processes

Goal: Inaccurate precipitation measurements have been recently recognized as the “willfully neglected Achille’s heel” of hydro-meteorological sciences. Difficulties in achieving accurate measurements arise from various instrumental and environmental sources of systematic biases, resulting in a significant underestimation of the precipitation depth and intensity. The understated extent of the associated biases is largely unknown and varies with various environmental factors, due to the complexity of the controlling processes. Although attempts were made to standardize measurement procedures, this has never been successfully achieved.

Without any correction for or, in many cases, any awareness of such measurement errors, there is a grave risk of a breakdown in the understanding of hydro-meteorological processes in a scientific era dominated by modeling, which generally undervalues the principals of precise and accurate measurements. Implications describe an inconvenient truth in hydrological sciences, which transcends a variety of applications of precipitation data in hydrological models, from real-time flood forecasting to water resources management and urban hydrology. The calibration of satellite- and radar-based areal estimates of precipitation and the statistics derived from historic data series are also systematically affected.

The extent and implications of inherent instrumental biases and wind-induced undercatch of precipitation measurements in the modeling of hydrological processes at the basin scale is the main focus of this research project. The aim is to quantify the impact of incorrect measurements used as the forcing variable of physically based hydrological models on their typical output variables, including the flood peak and volume, time to peak, baseflow separation and the regression curves. The impact on the simulation of hydrological processes at the basin scale is investigated, such as evapotranspiration, infiltration,
interception, etc. The methods used to achieve the project objectives include theoretical analysis, numerical simulation (CFD, distributed hydrological models, interpolation and data integration, statistical analysis) and full-scale experiments performed in the laboratory (wind tunnel) and in real-world experimental basins.

The main expected result is to provide scientific evidence of precipitation measurement biases and their impact on hydrological models, by showing the improvement obtained when corrections for instrumental and environmental errors are implemented. To achieve this, other intermediate results will be obtained, e.g. the development of suitable correction curves for the wind-induced undercatch, the improvement of areal rainfall estimates based on the integration of rain gauge, radar and satellite sources, and the development of dedicated statistical tools to improve the assessment of homogeneity in
precipitation time series, climatic trends and extreme value statistics.

Date: 5 February 2017

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Project log

Luca G Lanza
added a research item
A physical full-scale experimental set-up was designed and implemented in the wind tunnel to reproduce and capture the trajectories of falling water drops when approaching the collector of catching type precipitation gauges, reproducing rainfall measurements in windy conditions. The experiment allowed to collect, for the first time, a large data set of high-resolution footages of the deviation of such trajectories, as induced by the bluff-body aerodynamics of the outer gauge shape. By processing the collected images, a consistent quantitative interpretation of each drop pattern was possible, based on a detailed Computational Fluid Dynamics simulation of the airflow updraft and acceleration features above the collector of the gauge. Numerical airflow simulations were extensively validated in the wind tunnel, using local flow measurements and Particle Image Velocimetry. Capturing the deviation of the drop trajectories in the wind tunnel allowed a clear visualization of the physical reason for the wind-induced undercatch of precipitation gauges, since drops were individually observed to fall outside instead of inside of the collector, contrary to what would be expected by extrapolating their undisturbed trajectory. The adopted Lagrangian Particle Tracking model and the formulation used for the drag coefficient were suitable to closely reproduce the observed drop trajectories when affected by the airflow deformation due to the bluff-body aerodynamics of two investigated gauge geometries. The wind tunnel experiment provided the basis for the validation of the particle tracking model in terms of the difference between simulated and observed trajectories, after initial conditions were suitably set to represent the experimental setup.
Luca G Lanza
added a research item
Numerical studies of the wind-induced bias of precipitation measurements assume that turbulence is generated by the interaction of the airflow with the gauge body, while steady and uniform free-stream conditions are imposed. However, wind is turbulent in nature due to the roughness of the site and the presence of obstacles, while precipitation gauges are immersed in a turbulent flow. Further to the turbulence generated by the flow-gauge interaction, we investigated the natural free-stream turbulence and its influence on precipitation measurement biases. Realistic turbulence intensity values at the gauge collector height were derived from 3D sonic anemometer measure-ments. Large Eddy Simulations of the turbulent flow around a chimney-shaped gauge were per-formed under uniform and turbulent free-stream conditions, using geometrical obstacles upstream of the gauge to provide the desired turbulence intensity. Catch ratios for dry snow particles were obtained using a Lagrangian particle tracking model, and the collection efficiency was calculated based on a suitable particle size distribution. The collection efficiency in turbulent conditions showed stronger undercatch at the investigated wind velocity and snowfall intensity below 10 mm h−1, demonstrating that adjustment curves based on the simplifying assumption of uniform free-stream conditions do not accurately portray the wind-induced bias of snow measurements.
Luca G Lanza
added a research item
Adjustments for the wind-induced undercatch of snowfall measurements use transfer functions to account for the expected reduction of the collection efficiency with increasing the wind speed for a particular catching-type gauge. Based on field experiments or numerical simulation, collection efficiency curves as a function of wind speed also involve further explanatory variables such as surface air temperature and/or precipitation type. However, while the wind speed or wind speed and temperature approach is generally effective at reducing the measurement bias, it does not significantly reduce the Root Mean Square Error (RMSE) of the residuals, implying that part of the variance is still unexplained. In this study, we show that using precipitation intensity as the explanatory variable significantly reduces the scatter of the residuals. This is achieved by optimized curve fitting of field measurements from the Marshall field-test site (CO, USA), using a non-gradient optimization algorithm to ensure optimal binning of experimental data. The analysis of a recent quality-controlled dataset from the Solid Precipitation InterComparison Experiment (SPICE) campaign of the World Meteorological Organization confirms the scatter reduction, showing that this approach is suitable to a variety of locations and catching-type gauges. Using Computational Fluid-Dynamics simulations, we demonstrate that the physical basis of the reduction in RMSE is the correlation of precipitation intensity with the particle size distribution. Overall, these findings could be relevant in operational conditions since the proposed adjustment of precipitation measurements only requires wind sensor and precipitation gauge data.
Luca G Lanza
added a research item
The hotplate precipitation gauge operates by means of a thermodynamic principle. It is composed by a small size disk with two thin aluminium heated plates on the upper and lower faces. Each plate has three concentric rings to prevent the hydrometeors from sliding off in strong wind. As for the more widely used tipping-bucket and weighing gauges, measurements are affected by the wind-induced bias due to the bluff-body aerodynamics of the instrument outer shape. Unsteady Reynolds-Averaged Navier-Stokes equations were numerically solved, using a k-ω shear stress transport closure model, to simulate the aerodynamic influence of the gauge body on the airflow. Wind tunnel tests were conducted to validate simulation results. Solid particle trajectories were modelled using a Lagrangian Particle Tracking model to evaluate the influence of the airflow modification on the ability of the instrument to collect the incoming hydrometeors. A suitable parameterization of the particle size distribution, as a function of the snowfall intensity, was employed to calculate the Collection Efficiency (CE) under different wind conditions. Results reveal a relevant role of the three rings in enhancing the collection performance of the gauge. Below 7.5 m s ⁻¹ , the CE curves linearly decrease with increasing the wind speed, while beyond that threshold, the blocking caused by the rings counter effects the aerodynamic induced undercatch, and the CE curves quadratically increase with the wind speed. At high wind speed, the undercatch vanishes and the instrument exhibits a rapidly increasing overcatch. For operational purposes, adjustment curves were formulated as a function of the wind speed and the measured snowfall intensity.
Luca G Lanza
added a research item
Despite the numerous contributions available in the literature about the wind-induced bias of rainfall intensity measurements, adjustments based on collection efficiency curves are rarely applied operationally to rain records obtained from catching-type rain gauges. The many influencing variables involved and the variability of the results of field experiments do not facilitate the widespread application of adjustment algorithms. In this paper, a Lagrangian particle tracking model is applied to the results of computational fluid dynamic simulations of the airflow field surrounding a rain gauge to derive a simple formulation of the collection efficiency curves as a function of wind speed. A new parameterization of the influence of rainfall intensity is proposed. The methodology was applied to a cylindrical gauge, which has the typical outer shape of tipping-bucket rain gauges, as a representative specimen of most operational measurement instruments. The wind velocity is the only ancillary variable required to calculate the adjustment, together with the measured rainfall intensity. Since wind is commonly measured by operational weather stations, its use adds no relevant burden to the cost of meteo-hydrological networks.
Mattia Stagnaro
added 4 research items
This study shows how satellite-based passive and active microwave (MW) sensors can be used in conjunction with high-resolution Numerical Weather Prediction (NWP) simulations to provide insights of the precipitation structure of the tropical-like cyclone (TLC) Numa, which occurred on 15–19 November 2017. The goal of the paper is to characterize and monitor the precipitation at the different stages of its evolution from development to TLC phase, throughout the storm transition over the Mediterranean Sea. Observations by the NASA/JAXA Global Precipitation Measurement Core Observatory (GPM-CO) and by the GPM constellation of MW radiometers are used, in conjunction with the Regional Atmospheric Modeling System (RAMS) simulations. The GPM-CO measurements are used to analyze the passive MW radiometric response to the microphysical structure of the storm, while the comparison between successive MW radiometer overpasses shows the evolution of Numa precipitation structure from its early development stage on the Ionian Sea into its TLC phase, as it persists over southern coast of Italy (Apulia region) for several hours. Measurements evidence stronger convective activity at the development phase compared to the TLC phase, when strengthening or weakening phases in the eye development, and the occurrence of warm rain processes in the areas surrounding the eye, are identified. The weak scattering and polarization signal at and above 89 GHz, the lack of scattering signal at 37 GHz, and the absence of electrical activity in correspondence of the rainbands during the TLC phase, indicate weak convection and the presence of supercooled cloud droplets at high levels. RAMS high-resolution simulations support what inferred from the observations, evidencing Numa TLC characteristics (closed circulation around a warm core, low vertical wind shear, intense surface winds, heavy precipitation), persisting for more than 24 h. Moreover, the implementation of DPR 3D reflectivity field in the RAMS data assimilation system shows a small (but non negligible) impact on the precipitation forecast over the sea up to a few hours after the DPR overpass.
Wind speed and direction are fundamental data for many application fields, such as power generation and hydrological modelling. Wind measurements are usually few and sparse; hence, spatial interpolation of wind data is required. However, in mountainous areas with complex orography, accurate interpolation of wind data should consider topographic effects. Due to computational constraints, fully physically based methods that solve thermodynamic and mass conservation equations in three dimensions cannot be applied for long-time simulations or very large areas, while fast empirical methods seem more suitable. The aim of this work is to compare fast empirical methods to interpolate wind speed against a physically based full atmospheric model in order to assess the impact of the introduced approximation in estimating the wind field and the potential evapotranspiration. Comparison is carried out over the area of the upper Po River basin, a predominantly alpine region located in northern Italy. Results show that empirical topographic correction can increase accuracy of interpolated wind speed in areas with complex topography, but it requires about 50% more computational time than simpler empirical methods that do not consider topography.
This paper describes a new rainfall rate retrieval algorithm, developed within the EUMETSAT H SAF program, based on the Passive microwave Neural network Precipitation Retrieval approach (PNPR v3), designed to work with the conically scanning Global Precipitation Measurement (GPM) Microwave Imager (GMI). A new rain/no-rain classification scheme, also based on the NN approach, which provides different rainfall masks for different minimum thresholds and degree of reliability, is also described. The algorithm is trained on an extremely large observational database, built from GPM global observations between 2014 and 2016, where the NASA 2B-CMB (V04) rainfall rate product is used as reference. In order to assess the performance of PNPR v3 over the globe, an independent part of the observational database is used in a verification study. The good results found over all surface types (CC > 0.90, ME < −0.22 mm h −1 , RMSE < 2.75 mm h −1 and FSE% < 100% for rainfall rates lower than 1 mm h −1 and around 30-50% for moderate to high rainfall rates), demonstrate the good outcome of the input selection procedure, as well as of the training and design phase of the neural network. For further verification, two case studies over Italy are also analysed and a good consistency of PNPR v3 retrievals with simultaneous ground radar observations and with the GMI GPROF V05 estimates is found. PNPR v3 is a global rainfall retrieval algorithm, able to optimally exploit the GMI multi-channel response to different surface types and precipitation structures, that provide global rainfall retrieval in a computationally very efficient way, making the product suitable for near-real time operational applications.
Luca G Lanza
added an update
Next meeting of the research units to be held next week at the kind invitation of the University of Palermo. Latest advances to be presented and discussed.
 
Mattia Stagnaro
added a research item
In operational conditions wind is the main environmental source of measurement biases for catching-type precipitation gauges. The gauge geometry induces a deformation of the surrounding airflow pattern, which is generally characterized by relevant updraft zones in front of the collector and above it. This effect deviates the trajectories of the lighter hydrometeors away from the collector, thus is responsible for a significant reduction of the collection performance. Previous approaches to this problem, using Computational Fluid Dynamic simulations and wind tunnel tests, mostly assumed steady and uniform free-stream conditions. Wind is rather turbulent in nature, though. The role of the natural free-stream turbulence on the collection performance is investigated in this work for the case study of a calix shape precipitation gauge and wind velocity between 10 and 18 ms−1. The Unsteady Reynolds Averaged Navier-Stokes model was adopted. Turbulent conditions were simulated by imposing constant free-stream velocity and introducing a fixed solid fence upstream of the gauge in order to generate the desired turbulence intensity. Wind tunnel measurements allowed validating numerical results by comparing measured and simulated velocity profiles in representative portions of the investigated domain. Results revealed that in the case of turbulent free-stream conditions both the normalized magnitude of the flow velocity and the updraft above the collector are reduced by about 20 % and 12 % respectively. The dissipative effect of the turbulent fluctuations in the free stream has a damping role on the acceleration of the flow and on the updraft. This would result in a reduced undercatch with respect to literature simulations employing the traditional uniform free-stream conditions.
Luca G Lanza
added an update
An Italian three-year research project under the MIUR PRIN 2015 call, Prot. 20154WX5NA, started February 2017, involving partners from the University of Genoa, Politechnical School of Milano, National Research Council ISAC (Rome) and ISAFOM (Cosenza) and the University of Palermo.
 
Luca G Lanza
added a project goal
Inaccurate precipitation measurements have been recently recognized as the “willfully neglected Achille’s heel” of hydro-meteorological sciences. Difficulties in achieving accurate measurements arise from various instrumental and environmental sources of systematic biases, resulting in a significant underestimation of the precipitation depth and intensity. The understated extent of the associated biases is largely unknown and varies with various environmental factors, due to the complexity of the controlling processes. Although attempts were made to standardize measurement procedures, this has never been successfully achieved.
Without any correction for or, in many cases, any awareness of such measurement errors, there is a grave risk of a breakdown in the understanding of hydro-meteorological processes in a scientific era dominated by modeling, which generally undervalues the principals of precise and accurate measurements. Implications describe an inconvenient truth in hydrological sciences, which transcends a variety of applications of precipitation data in hydrological models, from real-time flood forecasting to water resources management and urban hydrology. The calibration of satellite- and radar-based areal estimates of precipitation and the statistics derived from historic data series are also systematically affected.
The extent and implications of inherent instrumental biases and wind-induced undercatch of precipitation measurements in the modeling of hydrological processes at the basin scale is the main focus of this research project. The aim is to quantify the impact of incorrect measurements used as the forcing variable of physically based hydrological models on their typical output variables, including the flood peak and volume, time to peak, baseflow separation and the regression curves. The impact on the simulation of hydrological processes at the basin scale is investigated, such as evapotranspiration, infiltration,
interception, etc. The methods used to achieve the project objectives include theoretical analysis, numerical simulation (CFD, distributed hydrological models, interpolation and data integration, statistical analysis) and full-scale experiments performed in the laboratory (wind tunnel) and in real-world experimental basins.
The main expected result is to provide scientific evidence of precipitation measurement biases and their impact on hydrological models, by showing the improvement obtained when corrections for instrumental and environmental errors are implemented. To achieve this, other intermediate results will be obtained, e.g. the development of suitable correction curves for the wind-induced undercatch, the improvement of areal rainfall estimates based on the integration of rain gauge, radar and satellite sources, and the development of dedicated statistical tools to improve the assessment of homogeneity in
precipitation time series, climatic trends and extreme value statistics.