Search Results

You are looking at 1 - 8 of 8 items for :

  • Author or Editor: Kenneth F. Heideman x
  • Weather and Forecasting x
  • All content x
Clear All Modify Search
Kenneth F. Heideman
Full access
Kenneth F. Heideman
Full access
Kenneth F. Heideman
Full access
Kenneth F. Heideman and J. Michael Fritsch

Abstract

Satellite, radar, surface, and upper-air data from the June–August periods of 1982–83 are examined to determine the mechanisms, and their relative contributions, for producing warm-season precipitation in the United States. Only areas where rainfall equaled or exceeded 12.7 mm during the 24-h period ending at 1200 UTC were considered.

Rainfall associated with extratropical cyclones accounted for about half of the warm-season precipitation. Most of the remaining precipitation was produced by mesoscale forcing mechanisms acting independently from the traveling extratropical cyclones. Nearly all the warm-season precipitation was convective; over 80% of the total was directly or indirectly associated with thunderstorms. Nearly three-fourths of the precipitation that occurred between the Rockies and the Mississippi Valley was nocturnal. Conversely, about three-fourths of the precipitation east of the Mississippi Valley fell during the daylight hours.

During the 1983 July-August drought, the area of precipitation from extratropical cyclones was significantly smaller than the precipitation area during the corresponding period in 1982. The decrease was the result of a sharp drop in the frequency of cyclone events. Conversely, the frequency of mesoscale events and associated precipitation increased during the drought year. This difference in precipitation mode from one year to the next may help explain the large differences in numerical model forecasting skill from summer to summer. The large proportion of warm-season precipitation produced by mesescale forcing mechanisms suggests that further improvement in forecasting cyclonic storms and fronts will likely result in only limited increases in summertime quantitative precipitation forecast (QPF) skill. Moreover, the historically smaller values of warm-season QPF skill, relative to the cool-season levels, are likely to continue until operational numerical models can predict mesoscale convective systems as well as they predict extratropical cyclones and attendant fronts.

Full access
J. Michael Fritsch and Kenneth F. Heideman

Abstract

The skill of the limited-area fine-mesh (LFM) model in making 24-h areal quantitative precipitation forecasts (QPF) of ⩾ 0.5 in. (12.7 mm) is evaluated and analyzed for two warm seasons; 1982 and 1983. Differences in skill between the eastern and western United States are investigated and the impact of updated initial conditions is explored. The skill in predicting precipitation from cyclonic versus mesoscale systems is also examined.

It is found that the model skill changed drastically from the summers of 1982 to 1983. Skill levels for 1982 were about 25% to 35%. In 1983, they dropped to an average value of less than 10% and often to zero on a daily basis. The precipitous drop in skill appears to be the result of changes made in key model threshold parameters. These parameters include grid-resolvable saturation criterion and convective cloud base boost. The introduction of constraints on convective precipitation and the vertical advection of moisture also appear to have reduced model skill.

A comparison of skill between the eastern and western United States indicates that the model scored substantially better in the east. It is also evident that, in general, the model skill for quantitative precipitation forecasts for the 12-h period 0000 to 1200 UTC was much greater with initial conditions from 0000 UTC, than with initial conditions from 12 h earlier at 1200 UTC; i.e., updated initial conditions had a large positive impact on the model skill. This was particularly true for mesoscale convective systems. Longer term (> 12 h) predictions of the 0000–1200 UTC period scored better for cyclone-related precipitation than for precipitation from mesoscale systems. When the model was reinitialized at 0000 UTC, there was virtually no difference between skill levels for cyclone-related and mesoscale events.

The results indicate that the lack of a realistic diurnal heating cycle in the LFM may be particularly detrimental to the model's ability to forecast convectively dominated warm-season rainfall. The results also indicate that threshold or critical values for initiating resolvable-scale precipitation or making convective adjustments may introduce artificial seasonal, diurnal, and regional variations in quantitative precipitation forecasts. Due to the inherent nonlinearity of numerical models, attempts to improve model skill for a particular region, season, time of day, or precipitation type by adjusting threshold or critical values may introduce bias in other model parameters and negatively impact overall model skill.

Full access
Wayne H. Schubert, Joseph B. Klemp, and Kenneth F. Heideman
Full access
David M. Schultz, Robert M. Rauber, and Kenneth F. Heideman
Full access
Kenneth F. Heideman, Thomas R. Stewart, William R. Moninger, and Patricia Reagan-Cirincione

Abstract

The relationship between the quality and quantity of information available to meteorologists and the skill of their forecasts was investigated. Twelve meteorologists were asked to make probabilistic forecasts of significant and severe weather events under three information conditions. Forecast accuracy was generally low. As the amount and quality of the information increased substantially, there was a modest increase in the accuracy of the forecasts. However, the results suggest that the forecasters were least consistent when they had the most information to work with, partially reducing the benefits of the increased information.

Full access