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NAM Rime Factor and Ice Accumulation Rate

NAM Rime Factor and Ice Accumulation Rate. Faye Barthold — HMT Meteorologist Mike Bodner — DTB November 4, 2009. What is rime factor?. Defined as the total growth of ice by vapor deposition and accretion divided by the growth of ice by vapor deposition alone

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NAM Rime Factor and Ice Accumulation Rate

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  1. NAM Rime Factor and Ice Accumulation Rate Faye Barthold — HMT Meteorologist Mike Bodner — DTB November 4, 2009

  2. What is rime factor? • Defined as the total growth of ice by vapor deposition and accretion divided by the growth of ice by vapor deposition alone • Indicates the amount of riming present on an individual ice particle • Derived directly from the model’s microphysics scheme • Currently available in the NAM (Ferrier microphysics) only

  3. Rime Factor Snowflake images from: http://www.its.caltech.edu/~atomic/snowcrystals/

  4. Ice Accumulation Rate • Can derive an “ice accumulation rate” in inches per hour using rime factor value in the lowest layer of the model • Median point between the surface and the interface with the next layer • Combines snow and precipitation ice • Does not give information about freezing rain rates

  5. Ice Accumulation Rate • Algorithm depends on an assumption of a maximum possible snow to liquid ratio Where: IAR = ice accumulation rate PRATE = precipitation rate POFP = percent of frozen precipitation DR = density ratio FRIME = rime factor

  6. Ice Accumulation Rate • Assuming a 30:1 maximum snow to liquid ratio • Assuming a 20:1 maximum snow to liquid ratio • Assuming a 10:1 maximum snow to liquid ratio

  7. Limitations • Doesn’t take ground temperature or surface heat flux into account • Accumulation rates will always increase as the assigned snow to liquid ratio increases • Model doesn’t make any decision about whether a given snow to liquid ratio is realistic • Instantaneous value only—doesn’t capture what happens at intermediate time steps • Based on NAM QPF, temperature profile, etc. • If NAM sounding and temperature forecasts are questionable, rime factor and ice accumulation rate forecasts will also be questionable

  8. Cases • Used CoCoRaHS new snow reports to determine usefulness of ice accumulation rate output • October 2008—northern Rockies snowstorm • December 2008—eastern U.S. mixed precipitation event • January 2009—southern plains to northeast US winter storm

  9. October 2008 • Early season mountain snow event • Snowfall totals (maximum observed)

  10. 30:1 20:1 10:1

  11. December 2008 • East coast mixed precipitation event • Snowfall totals (maximum observed)

  12. 30:1 20:1 10:1

  13. 30:1 20:1 10:1

  14. How can this tool improve forecasts? • More objective way to estimate snowfall accumulations • Uses cloud physics parameters, surface and vertical temperature data • No QPF conversion required • Accumulation rates can be used to indicate intensity • Better indication of event duration, thus providing better guidance for advisory/warning issuance

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