19th Conf. on weather Analysis and Forecasting/15th Conf. on Numerical Weather Prediction


Determining the resolved spatial scales of Eta model precipitation forecasts

Michael E. Baldwin, CIMMS/Univ. of Oklahoma, Norman, OK; and M. S. Wandishin

We plan to examine different measures of spatial structure as a function of spatial scale in forecast fields from Eta model forecasts, comparing these measure to those found in observed fields, following the work of Harris et al (2001, Journal of Hydromet.) In particular, we plan to compare precipitation fields from NCEP’s operational Eta model, which runs at 12km grid-spacing with the Betts-Miller-Janjic convective scheme, to an experimental version of the Eta running at NSSL, with 22km grid-spacing and the Kain-Fritsch convective parameterization (known as “Eta-KF”). Observed precipitation fields will be obtained by the operational 4km national hourly “stage IV” analysis available at NCEP. Visual comparison of rainfall fields from these different versions of the Eta has indicated that, outside of the region of steeply varying terrain in the Western U.S., the experimental NSSL Eta-KF produces forecasts containing more spatial detail and variability than those produced by the operational Eta model. This is despite the fact that the operational Eta runs with nearly half the grid spacing of the NSSL Eta-KF. Fourier power spectrum, structure function, and scale-moment analyses will be used to show which horizontal scales are resolved by each version of the Eta model. Another factor separating the experimental NSSL Eta-KF run from the operational Eta is the use of a higher order horizontal diffusion operator (4th-order versus 2nd-order in the operational Eta), which we believe is partially responsible for producing greater detail in the Eta-KF. We will examine the impact of each of these factors (grid spacing, convective scheme, horizontal diffusion) on the true model “resolution” by comparing the results from off-line runs where each of these factors is varied. Results from this analysis will be presented at the conference.

extended abstract  Extended Abstract (136K)

Session 3, Statistical Evaluation I
Monday, 12 August 2002, 4:30 PM-5:45 PM

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