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WEATHER AND CIRCULATION OF MARCH 1980

Record Precipitation in the South and Central High Plains

Robert E. Livezey

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Robert E. Livezey

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Robert E. Livezey

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WEATHER AND CIRCULATION OF MARCH 1981

Drought Worsens in the East and Northern Plains

Robert E. Livezey

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WEATHER AND CIRCULATION OF NOVEMBER 1981

Widespread Warmth with Storminess in the Far West

Robert E. Livezey

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WEATHER AND CIRCULATION OF NOVEMBER 1980

A Late Heat Wave and Hurricane and Early Snow

Robert E. Livezey

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Robert E. Livezey and Jae-Kyung E. Schemm

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Edward A. O'Lenic and Robert E. Livezey

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The relationship between the existence of low-frequency 700 mb height anomalies in the initial conditions of NMC's MRF global spectral model and subsequent 5-, 7-, and 10-day forecasts of 700 mb height from 1982 to 1988 is explored. Low-frequency 700 mb flow regimes are specified in each of four two-month seasons by performing a rotated principal component analysis (RPCA) on 38 or 39 year time series of daily, low-pass filtered 700 mb height analyses. In a given season, the amplitude time series (ATS) for each mode is used to decide which MRF forecast error maps should be used in forming a composite map corresponding to either the “+” phase or the “−” phase of the given mode. Several methods, including Monte Carlo simulations, are used to evaluate the statistical significance of the composite maps.

Many modes, including the Pacific North American (PNA) mode in winter and the leading summer mode, are found to be related to either unusually strong or unusually weak systematic error signature. Two different modes, one in spring and one in autumn, corresponding to quasi-stationary patterns over the United States and the North Atlantic, respectively, are related to unusually strong forecast error signatures. A statistically significant number of such modes is found in each of the four seasons, with the number of such results being smallest in autumn, and 1argest in spring. The results also indicate that the MRF model response to the presence of low-frequency regimes in the initial conditions is such that composite error signatures have a component with opposite phase and amplitude for opposite phases of a given mode (linear response). The overall results demonstrate the feasibility of using this technique to identify mode-linked forecast error signatures, and provides a potential opportunity to correct forecasts in the MRF, and possibly in other models, by removing the appropriate systematic error signatures.

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Robert E. Livezey and Sherwin W. Jamison

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Operational long-range weather prediction in the Soviet Union is reviewed. Methods for producing forecasts at the 5- and 10-day, monthly and seasonal range are described in terms of the synoptic, statistical and hydrodynamic tools available to Soviet forecasters. Skill scores for these forecasts published by the Soviets are summarized and examined.

Skill scores for Soviet operational forecasts of mean seasonal (about two months) temperature anomaly and precipitation category are computed separately for regions, seasons and years and compared to persistence skill scores. In addition, forecast-observation sets for the sign of the mean temperature anomaly are tested for “no skill.” The forecasts for the sign of the mean temperature anomaly are found to be best by region for the Arctic and by season for March through April, but generally do not outperform persistence, exhibit demonstrable skill, or show an improvement trend over the verification period. Forecasts of the mean precipitation category are shown to be consistently better than persistence, but to have quite modest skill scores.

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Robert E. Livezey and W. Y. Chen

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The effects of number and interdependence in evaluating the collective significance of finite sets of statistics are frequently non-trivial, especially for spatial networks of time-averaged meteorological data. These effects can be taken into account in two steps: By first prescreening for significance assuming data independence and then, if necessary, by taking into consideration dependence through the use of estimated effective degrees of freedom and the binomial distribution or, failing that, Monte Carlo simulation. Seasonal averages of 700 mb height data are used to illustrate the problem and to demonstrate how the data set properties are taken into account. Papers by Hancock and Yarger (1979), Nastrom and Belmont (1980) and Williams (1980) are critically examined in light of these considerations and Monte Carlo strategies for clarification of ambiguities suggested.

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