A Comparison of Diagnosed Vs. Predicted Precipitation Type in the Eta Forecast Model: 3-6 December 2002

Abstract

Here the diagnosed precipitation type from the NCEP (a.k.a Baldwin/Schichtel) algorithm is compared to the predicted precipitation type in the experimental/parallel version of the 12-km Eta model for an early winter storm in the southern and eastern U.S.

Learning objectives

In this case study, the main lessons are:

  1. The NCEP precipitation type algorithm overforecasts freezing rain or sleet over snow in situations where there are thick quasi-isothermal layers with temperatures between -4° and 0°C and surface temperatures below freezing. This feature of the algorithm was intentionally set up so freezing rain would not be underforecast, resulting in potentially serious consequences for public safety.
  2. It is important to understand how precipitation type post-processing schemes diagnose precipitation type, and the types of forecast situations in which the schemes tend to have problems.
  3. The forecast fraction of frozen precipitation (SNRA) from the Eta-12 takes the temperature of regions of precipitation formation and falling precipitation into account using complex microphysical parameterizations. The SNRA variable can also include sleet!
  4. The predicted SNRA fraction will generally give superior results to precipitation type diagnostic schemes because it is directly tied to the model's precipitation microphysics scheme.
  5. If in doubt about the precipitation type, check vertical structure of atmosphere at your location to see if there are any resolved layers with temperatures above 0°C. This will also help with potential sleet contained in SNRA variable.

In going through this case, when you open a separate graphic, an image or loop will pop up in a separate window. If one is already
displaying, it will be replaced by the one you just selected. Loops automatically start looping. To freeze a frame, click the Stop button .
To advance one frame, use the Plus 1 button . To resume looping, use the Forward button . Loop speed is increased by
repeatedly clicking the Fast button and decreased by doing the same to the Slow button .

Credits

by Dr. Bill Bua

Thanks to:

Eric Rogers, NCEP/EMC/MMB for graphics
Geoffrey Manikin, NCEP/EMC/MMB for review
Christopher Buonanno, SOO LZK for review
University of Wyoming and National Climatic Data Center websites for data

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