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Caren Marzban and Scott Sandgathe

Abstract

Modern numerical weather prediction (NWP) models produce forecasts that are gridded spatial fields. Digital images can also be viewed as gridded spatial fields, and as such, techniques from image analysis can be employed to address the problem of verification of NWP forecasts. One technique for estimating how images change temporally is called optical flow, where it is assumed that temporal changes in images (e.g., in a video) can be represented as a fluid flowing in some manner. Multiple realizations of the general idea have already been employed in verification problems as well as in data assimilation. Here, a specific formulation of optical flow, called Lucas–Kanade, is reviewed and generalized as a tool for estimating three components of forecast error: intensity and two components of displacement, direction and distance. The method is illustrated first on simulated data, and then on a 418-day series of 24-h forecasts of sea level pressure from one member [the Global Forecast System (GFS)–fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5)] of the University of Washington’s Mesoscale Ensemble system. The simulation study confirms (and quantifies) the expectation that the method correctly assesses forecast errors. The method is also applied to a real dataset consisting of 418 twenty-four-hour forecasts spanning 2 April 2008–2 November 2009, demonstrating its value for analyzing NWP model performance. Results reveal a significant intensity bias in the subtropics, especially in the southern California region. They also expose a systematic east-northeast or downstream bias of approximately 50 km over land, possibly due to the treatment of terrain in the coarse-resolution model.

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Caren Marzban and Scott Sandgathe

Abstract

In a recent paper, a statistical method referred to as cluster analysis was employed to identify clusters in forecast and observed fields. Further criteria were also proposed for matching the identified clusters in one field with those in the other. As such, the proposed methodology was designed to perform an automated form of what has been called object-oriented verification. Herein, a variation of that methodology is proposed that effectively avoids (or simplifies) the criteria for matching the objects. The basic idea is to perform cluster analysis on the combined set of observations and forecasts, rather than on the individual fields separately. This method will be referred to as combinative cluster analysis (CCA). CCA naturally lends itself to the computation of false alarms, hits, and misses, and therefore, to the critical success index (CSI). A desirable feature of the previous method—the ability to assess performance on different spatial scales—is maintained. The method is demonstrated on reflectivity data and corresponding forecasts for three dates using three mesoscale numerical weather prediction model formulations—the NCEP/NWS Nonhydrostatic Mesoscale Model (NMM) at 4-km resolution (nmm4), the University of Oklahoma’s Center for Analysis and Prediction of Storms (CAPS) Weather Research and Forecasting Model (WRF) at 2-km resolution (arw2), and the NCAR WRF at 4-km resolution (arw4). In the small demonstration sample herein, model forecast quality is efficiently differentiated when performance is assessed in terms of the CSI. In this sample, arw2 appears to outperform the other two model formulations across all scales when the cluster analysis is performed in the space of spatial coordinates and reflectivity. However, when the analysis is performed only on spatial data (i.e., when only the spatial placement of the reflectivity is assessed), the difference is not significant. This result has been verified both visually and using a standard gridpoint verification, and seems to provide a reasonable assessment of model performance. This demonstration of CCA indicates promise in quickly evaluating mesoscale model performance while avoiding the subjectivity and labor intensiveness of human evaluation or the pitfalls of non-object-oriented automated verification.

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Caren Marzban and Scott Sandgathe

Abstract

The verification of a gridded forecast field, for example, one produced by numerical weather prediction (NWP) models, cannot be performed on a gridpoint-by-gridpoint basis; that type of approach would ignore the spatial structures present in both forecast and observation fields, leading to misinformative or noninformative verification results. A variety of methods have been proposed to acknowledge the spatial structure of the fields. Here, a method is examined that compares the two fields in terms of their variograms. Two types of variograms are examined: one examines correlation on different spatial scales and is a measure of texture; the other type of variogram is additionally sensitive to the size and location of objects in a field and can assess size and location errors. Using these variograms, the forecasts of three NWP model formulations are compared with observations/analysis, on a dataset consisting of 30 days in spring 2005. It is found that within statistical uncertainty the three formulations are comparable with one another in terms of forecasting the spatial structure of observed reflectivity fields. None, however, produce the observed structure across all scales, and all tend to overforecast the spatial extent and also forecast a smoother precipitation (reflectivity) field. A finer comparison suggests that the University of Oklahoma 2-km resolution Advanced Research Weather Research and Forecasting (WRF-ARW) model and the National Center for Atmospheric Research (NCAR) 4-km resolution WRF-ARW slightly outperform the 4.5-km WRF-Nonhydrostatic Mesoscale Model (NMM), developed by the National Oceanic and Atmospheric Administration/National Centers for Environmental Prediction (NOAA/NCEP), in terms of producing forecasts whose spatial structures are closer to that of the observed field.

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Caren Marzban and Scott Sandgathe

Abstract

A statistical method referred to as cluster analysis is employed to identify features in forecast and observation fields. These features qualify as natural candidates for events or objects in terms of which verification can be performed. The methodology is introduced and illustrated on synthetic and real quantitative precipitation data. First, it is shown that the method correctly identifies clusters that are in agreement with what most experts might interpret as features or objects in the field. Then, it is shown that the verification of the forecasts can be performed within an event-based framework, with the events identified as the clusters. The number of clusters in a field is interpreted as a measure of scale, and the final “product” of the methodology is an “error surface” representing the error in the forecasts as a function of the number of clusters in the forecast and observation fields. This allows for the examination of forecast error as a function of scale.

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Caren Marzban, Scott Sandgathe, and Hilary Lyons

Abstract

Recently, an object-oriented verification scheme was developed for assessing errors in forecasts of spatial fields. The main goal of the scheme was to allow the automatic and objective evaluation of a large number of forecasts. However, processing speed was an obstacle. Here, it is shown that the methodology can be revised to increase efficiency, allowing for the evaluation of 32 days of reflectivity forecasts from three different mesoscale numerical weather prediction model formulations. It is demonstrated that the methodology can address not only spatial errors, but also intensity and timing errors. The results of the verification are compared with those performed by a human expert.

For the case when the analysis involves only spatial information (and not intensity), although there exist variations from day to day, it is found that the three model formulations perform comparably, over the 32 days examined and across a wide range of spatial scales. However, the higher-resolution model formulation appears to have a slight edge over the other two; the statistical significance of that conclusion is weak but nontrivial. When intensity is included in the analysis, it is found that these conclusions are generally unaffected. As for timing errors, although for specific dates a model may have different timing errors on different spatial scales, over the 32-day period the three models are mostly “on time.” Moreover, although the method is nonsubjective, its results are shown to be consistent with an expert’s analysis of the 32 forecasts. This conclusion is tentative because of the focused nature of the data, spanning only one season in one year. But the proposed methodology now allows for the verification of many more forecasts.

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Caren Marzban, Scott Sandgathe, and James D. Doyle

Abstract

Knowledge of the relationship between model parameters and forecast quantities is useful because it can aid in setting the values of the former for the purpose of having a desired effect on the latter. Here it is proposed that a well-established multivariate statistical method known as canonical correlation analysis can be formulated to gauge the strength of that relationship. The method is applied to several model parameters in the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) for the purpose of “controlling” three forecast quantities: 1) convective precipitation, 2) stable precipitation, and 3) snow. It is shown that the model parameters employed here can be set to affect the sum, and the difference between convective and stable precipitation, while keeping snow mostly constant; a different combination of model parameters is shown to mostly affect the difference between stable precipitation and snow, with minimal effect on convective precipitation. In short, the proposed method cannot only capture the complex relationship between model parameters and forecast quantities, it can also be utilized to optimally control certain combinations of the latter.

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Caren Marzban, Robert Tardif, and Scott Sandgathe

Abstract

In a recent work, a sensitivity analysis methodology was described that allows for a visual display of forecast sensitivity, with respect to model parameters, across a gridded forecast field. In that approach, sensitivity was assessed with respect to model parameters that are continuous in nature. Here, the analogous methodology is developed for situations involving noncontinuous (discrete or categorical) model parameters. The method is variance based, and the variances are estimated via a random-effects model based on 2kp fractional factorial designs and Graeco-Latin square designs. The development is guided by its application to model parameters in the stochastic kinetic energy backscatter scheme (SKEBS), which control perturbations at unresolved, subgrid scales. In addition to the SKEBS parameters, the effect of daily variability and replication (both, discrete factors) are also examined. The forecasts examined are for precipitation, temperature, and wind speed. In this particular application, it is found that the model parameters have a much weaker effect on the forecasts as compared to the effect of daily variability and replication, and that sensitivities, weak or strong, often have a distinctive spatial structure that reflects underlying topography and/or weather patterns. These findings caution against fine-tuning methods that disregard 1) sources of variability other than those due to model parameters, and 2) spatial structure in the forecasts.

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Caren Marzban, Robert Tardif, and Scott Sandgathe

Abstract

A sensitivity analysis methodology recently developed by the authors is applied to COAMPS and WRF. The method involves varying model parameters according to Latin Hypercube Sampling, and developing multivariate multiple regression models that map the model parameters to forecasts over a spatial domain. The regression coefficients and p values testing whether the coefficients are zero serve as measures of sensitivity of forecasts with respect to model parameters. Nine model parameters are selected from COAMPS and WRF, and their impact is examined on nine forecast quantities (water vapor, convective and gridscale precipitation, and air temperature and wind speed at three altitudes). Although the conclusions depend on the model parameters and specific forecast quantities, it is shown that sensitivity to model parameters is often accompanied by nontrivial spatial structure, which itself depends on the underlying forecast model (i.e., COAMPS vs WRF). One specific difference between these models is in their sensitivity with respect to a parameter that controls temperature increments in the Kain–Fritsch trigger function; whereas this parameter has a distinct spatial structure in COAMPS, that structure is completely absent in WRF. The differences between COAMPS and WRF also extend to the quality of the statistical models used to assess sensitivity; specifically, the differences are largest over the waters off the southeastern coast of the United States. The implication of these findings is twofold: not only is the spatial structure of sensitivities different between COAMPS and WRF, the underlying relationship between the model parameters and the forecasts is also different between the two models.

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Caren Marzban, Scott Sandgathe, and Eugenia Kalnay

Abstract

Statistical postprocessing methods have been successful in correcting many defects inherent in numerical weather prediction model forecasts. Among them, model output statistics (MOS) and perfect prog have been most common, each with its own strengths and weaknesses. Here, an alternative method (called RAN) is examined that combines the two, while at the same time utilizes the information in reanalysis data. The three methods are examined from a purely formal/mathematical point of view. The results suggest that whereas MOS is expected to outperform perfect prog and RAN in terms of mean squared error, bias, and error variance, the RAN approach is expected to yield more certain and bias-free forecasts. It is suggested therefore that a real-time RAN-based postprocessor be developed for further testing.

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Scott Sandgathe, Barbara Brown, Brian Etherton, and Edward Tollerud
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