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Robert F. Dale and Robert H. Shaw

An upward bias exists in probabilities of 0 or trace weekly total precipitation since small amounts often occur undetected and are recorded as 0 or trace at climatological stations where observations are made only once a day. Exact evaluation and correction of this bias is difficult, but individual estimates of 1-week P(0,T) for substations in the north-central region of the United States can be multiplied by a factor of 0.8 to reduce them to more reasonable values. Although the opportunity for low precipitation bias decreases with increasing length of period, significant bias still persists in the 2- and 3-week estimates in the western part of the north-central region during the winter season. Since the probability of measurable precipitation is 1–P(0,T), the P(0,T) bias is carried into the precipitation probabilities, but compensating biases in the gamma-distribution parameter estimates apparently contain most of the bias in the 0.01 to 0.09-inch interval.

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Robert F. Dale and Robert H. Shaw

Abstract

The average seasonal march and frequency of soil moisture in the corn root zone at Ames, Iowa, during a 30-yr period was estimated for a well-drained 5-ft soil profile holding 9 inches of available water at field capacity. Average seasonal marches of soil moisture in the top 5 ft were also prepared from simulated water balance computations for soils with three different available field capacities (6, 9 and 12 inches) and, for each capacity, three different 1 April soil moisture profiles (20, 60 and 100 per cent of available field capacity) from which to begin the moisture budget calculations. The average seasonal march and frequencies of evaporation from a Weather Bureau Class A evaporation pan and from corn with soil moisture not limiting were estimated. Using an experimentally derived atmospheric-soil moisture stress relation for corn, the climatology of potential evapotranspiration from corn was expressed as the soil moisture necessary in the corn root zone to prevent moisture stress in corn on any day of the season. The estimation of moisture stress in corn showed at least some stress days to occur in every one of the 30 years and an average of 40 non-stress days in the critical 63-day period for corn six weeks before silking to three weeks after silking.

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ROBERT F. SHAW and ELLIS J. JOSEPH

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No Abstract Available.

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Vincent J. Oliver and Robert F. Shaw

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W.R. Moninger, J. Bullas, B. de Lorenzis, E. Ellison, J. Flueck, J.C. McLeod, C. Lusk, P.D. Lampru, R.S. Phillips, W.F. Roberts, R. Shaw, T.R. Stewart, J. Weaver, K.C. Young, and S.M. Zubrick

During the summer of 1989, the Forecast Systems Laboratory of the National Oceanic and Atmospheric Administration sponsored an evaluation of artificial-intelligence-based systems that forecast severe convective storms. The evaluation experiment, called Shootout-89, took place in Boulder, Colorado, and focused on storms over the northeastern Colorado foothills and plains.

Six systems participated in Shootout-89: three traditional expert systems, a hybrid system including a linear model augmented by a small expert system, an analogue-based system, and a system developed using methods from the cognitive science/judgment analysis tradition.

Each day of the exercise, the systems generated 2–9-h forecasts of the probabilities of occurrence of nonsignificant weather, significant weather, and severe weather in each of four regions in northeastern Colorado. A verification coordinator working at the Denver Weather Service Forecast Office gathered ground-truth data from a network of observers.

The systems were evaluated on several measures of forecast skill, on timeliness, on ease of learning, and on ease of use. They were generally easy to operate; however, they required substantially different levels of meteorological expertise on the part of their users, reflecting the various operational environments for which they had been designed. The systems varied in their statistical behavior, but on this difficult forecast problem, they generally showed a skill approximately equal to that of persistence forecasts and climatological forecasts.

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