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Christopher Davis, Barbara Brown, and Randy Bullock

Abstract

The authors develop and apply an algorithm to define coherent areas of precipitation, emphasizing mesoscale convection, and compare properties of these areas with observations obtained from NCEP stage-IV precipitation analyses (gauge and radar combined). In Part II, fully explicit 12–36-h forecasts of rainfall from the Weather Research and Forecasting model (WRF) are evaluated. These forecasts are integrated on a 4-km mesh without a cumulus parameterization. Rain areas are defined similarly to Part I, but emphasize more intense, smaller areas. Furthermore, a time-matching algorithm is devised to group spatially and temporally coherent areas into rain systems that approximate mesoscale convective systems. In general, the WRF model produces too many rain areas with length scales of 80 km or greater. Rain systems typically last too long, and are forecast to occur 1–2 h later than observed. The intensity distribution among rain systems in the 4-km forecasts is generally too broad, especially in the late afternoon, in sharp contrast to the intensity distribution obtained on a coarser grid with parameterized convection in Part I. The model exhibits the largest positive size and intensity bias associated with systems over the Midwest and Mississippi Valley regions, but little size bias over the High Plains, Ohio Valley, and the southeast United States. For rain systems lasting 6 h or more, the critical success index for matching forecast and observed rain systems agrees closely with that obtained in a related study using manually determined rain systems.

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Allan H. Murphy and Barbara G. Brown

This paper reports some results of a study in which two groups of individuals—undergraduate students and professional meteorologists at Oregon State University—completed a short questionnaire concerning their interpretations of terminology commonly used in public weather forecasts. The questions related to terms and phrases associated with three elements: 1) cloudiness—fraction of sky cover; 2) precipitation—spatial and/or temporal variations; and 3) temperature—specification of intervals.

The students' responses indicate that cloudiness terms are subject to wide and overlapping ranges of interpretation, although the interpretations of these terms correspond quite well to National Weather Service definitions. Their responses to the precipitation and temperature questions reveal that some confusion exists concerning the meaning of spatial and temporal modifiers in precipitation forecasts and that some individuals interpret temperature ranges in terms of asymmetric intervals. When compared to the students' responses, the meteorologists' responses exhibit narrower ranges of interpretation of the cloudiness terms and less confusion about the meaning of spatial/temporal precipitation modifiers.

The study was not intended to be a definitive analysis of public understanding of forecast terminology. Instead, it should be viewed as a primitive form of the type of forecast-terminology study that must be undertaken in the future. Some implications of this investigation for future work in the area are discussed briefly.

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Barbara G. Brown and Richard W. Katz

Abstract

The statistical theory of extreme values is applied to daily minimum and maximum temperature time series in the U.S. Midwest and Southeast. If the spatial pattern in the frequency of extreme temperature events can be explained simply by shifts in location and scale parameters (e.g., the mean and standard deviation) of the underlying temperature distribution, then the area under consideration could be termed a “region.” A regional analysis of temperature extremes suggests that the Type I extreme value distribution is a satisfactory model for extreme high temperatures. On the other hand, the Type III extreme value distribution (possibly with common shape parameter) is often a better model for extreme low temperatures. Hence, our concept of a region is appropriate when considering maximum temperature extremes, and perhaps also for minimum temperature extremes.

Based on this regional analysis, if a temporal climate change were analogous to a spatial relocation, then it would be possible to anticipate how the frequency of extreme temperature events might change. Moreover, if the Type III extreme value distribution were assumed instead of the more common Type I, then the sensitivity of the frequency of extremes to changes in the location and scale parameters would be greater.

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Christopher Davis, Barbara Brown, and Randy Bullock

Abstract

A recently developed method of defining rain areas for the purpose of verifying precipitation produced by numerical weather prediction models is described. Precipitation objects are defined in both forecasts and observations based on a convolution (smoothing) and thresholding procedure. In an application of the new verification approach, the forecasts produced by the Weather Research and Forecasting (WRF) model are evaluated on a 22-km grid covering the continental United States during July–August 2001. Observed rainfall is derived from the stage-IV product from NCEP on a 4-km grid (averaged to a 22-km grid). It is found that the WRF produces too many large rain areas, and the spatial and temporal distribution of the rain areas reveals regional underestimates of the diurnal cycle in rain-area occurrence frequency. Objects in the two datasets are then matched according to the separation distance of their centroids. Overall, WRF rain errors exhibit no large biases in location, but do suffer from a positive size bias that maximizes during the later afternoon. This coincides with an excessive narrowing of the rainfall intensity range, consistent with the dominance of parameterized convection. Finally, matching ability has a strong dependence on object size and is interpreted as the influence of relatively predictable synoptic-scale systems on the larger areas.

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Barbara G. Brown and Allan H. Murphy

Abstract

Fire-weather forecasts (FWFs) prepared by National Weather Service (NWS) forecasters on an operational basis are traditionally expressed in categorical terms. However, to make rational and optimal use of such forecasts, fire managers need quantitative information concerning the uncertainty inherent in the forecasts. This paper reports the results of two studies related to the quantification of uncertainty in operational and experimental FWFs.

Evaluation of samples of operational categorical FWFs reveals that these forecasts contain considerable uncertainty. The forecasts also exhibit modest but consistent biases which suggest that the forecasters are influenced by the impacts of the relevant events on fire behavior. These results underscore the need for probabilistic FWFs.

The results of a probabilistic fire-weather forecasting experiment indicate that NWS forecasters are able to make quite reliable and reasonably precise credible interval temperature forecasts. However, the experimental relative humidity and wind speed forecasts exhibit considerable overforecasting and minimal skill. Although somewhat disappointing, these results are not too surprising in view of the fact that (a) the forecasters had little, if any, experience in probability forecasting; (b) no feedback was provided to the forecasters during the experimental period; and (c) the experiment was of quite limited duration. More extensive experimental and operational probability forecasting trials as well as user-oriented studies are required to enhance the quality of FWFs and to ensure that the forecasts are used in an optimal manner.

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Allan H. Murphy and Barbara G. Brown

Worded forecasts, which generally consist of both verbal and numerical expressions, play an important role in the communication of weather information to the general public. However, relatively few studies of the composition and interpretation of such forecasts have been conducted. Moreover, the studies that have been undertaken to date indicate that many expressions currently used in public forecasts are subject to wide ranges of interpretation (and to misinterpretation) and that the ability of individuals to recall the content of worded forecasts is quite limited. This paper focuses on forecast terminology and the understanding of such terminology in the context of short-range public weather forecasts.

The results of previous studies of forecast terminology (and related issues) are summarized with respect to six basic aspects or facets of worded forecasts. These facets include: 1) events (the values of the meteorological variables): 2) terminology (the words used to describe the events); 3) words versus numbers (the use of verbal and/or numerical expressions); 4) uncertainty (the mode of expression of uncertainty); 5) amount of information (the number of items of information); and 6) content and format (the selection of items of information and their placement). In addition, some related topics are treated briefly, including the impact of verification systems, the role of computer-worded forecasts, the implications of new modes of communication, and the use of weather forecasts.

Some conclusions and inferences that can be drawn from this review of previous work are discussed briefly, and a set of recommendations are presented regarding steps that should be taken to raise the level of understanding and enhance the usefulness of worded forecasts. These recommendations are organized under four headings: 1) studies of public understanding, interpretation, and use; 2) management practices; 3) forecaster training and education; and 4) public education.

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Scott Sandgathe, Barbara Brown, Brian Etherton, and Edward Tollerud
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Barbara G. Brown, Richard W. Katz, and Allan H. Murphy

Abstract

The use of a concept called a precipitation “event” to obtain information regarding certain statistical properties of precipitation time series at a particular location and for a specific application (e.g., for modeling erosion) is described. Exploratory data analysis is used to examine several characteristics of more than 31 years of primitive precipitation events based on hourly precipitation data at Salem, Oregon. A primitive precipitation event is defined as one or more consecutive hours with at least 0.01 inches (0.25 mm) of precipitation. The characteristics of the events that are considered include the duration, magnitude, average intensity and maximum intensity of the event and the number of hours separating consecutive events.

By means of exploratory analysis of the characteristics of the precipitation events, it is demonstrated that the marginal (i.e., unconditional) distributions of the characteristics are positively skewed. Examination of the conditional distributions of some pairs of characteristics indicates the existence of some relationships among the characteristics. For example, it is found that average intensity and maximum intensity are quite dependent on the event duration. The existence and forms of these relationships indicate that the assumption commonly made in stochastic models of hourly precipitation time series that the intensities (i.e., hourly amounts within an event) are independent and identically distributed must be violated. Again using exploratory data analysis, it is shown that the hourly intensities at Salem are, in fact, stochastically increasing and positively associated within a precipitation event.

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Gregory Thompson, Roelof T. Bruintjes, Barbara G. Brown, and Frank Hage

Abstract

The purpose of the Federal Aviation Administration’s Icing Forecasting Improvement Program is to conduct research on icing conditions both in flight and on the ground. This paper describes a portion of the in-flight aircraft icing prediction effort through a comprehensive icing prediction and evaluation project conducted by the Research Applications Program at the National Center for Atmospheric Research. During this project, in- flight icing potential was forecast using algorithms developed by RAP, the National Weather Service’s National Aviation Weather Advisory Unit, and the Air Force Global Weather Center in conjunction with numerical model data from the Eta, MAPS, and MM5 models. Furthermore, explicit predictions of cloud liquid water were available from the Eta and MM5 models and were also used to forecast icing potential.

To compare subjectively the different algorithms, predicted icing regions and observed pilot reports were viewed simultaneously on an interactive, real-time display. To measure objectively the skill of icing predictions, a rigorous statistical evaluation was performed in order to compare the different algorithms (details and results are provided in Part II). Both the subjective and objective comparisons are presented here for a particular case study, whereas results from the entire project are found in Part II. By statistically analyzing 2 months worth of data, it appears that further advances in temperature and relative-humidity-based algorithms are unlikely. Explicit cloud liquid water predictions, however, show promising results although still relatively new in operational numerical models.

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Barbara G. Brown, Richard W. Katz, and Allan H. Murphy

Abstract

A general approach for modeling wind speed and wind power is described. Because wind power is a function of wind speed, the methodology is based on the development of a model of wind speed. Values of wind power are estimated by applying the appropriate transformations to values of wind speed. The wind speed modeling approach takes into account several basic features of wind speed data, including autocorrelation, non-Gaussian distribution, and diurnal nonstationarity. The positive correlation between consecutive wind speed observations is taken into account by fitting an autoregressive process to wind speed data transformed to make their distribution approximately Gaussian and standardized to remove diurnal nonstationarity.

As an example, the modeling approach is applied to a small set of hourly wind speed data from the Pacific Northwest. Use of the methodology for simulating and forecasting wind speed and wind power is discussed and an illustration of each of these types of applications is presented. To take into account the uncertainty of wind speed and wind power forecasts, techniques are presented for expressing the forecasts either in terms of confidence intervals or in terms of probabilities.

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