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Isolating the Impact of Height Dependence on Cumulus Entrainment

This study isolates the impact of height dependence on cumulus entrainment, addressing unrealistic tropical wave variability and offering an improved model. Emphasizing large-scale vertical velocity, environmental humidity, and cloud base height, the research introduces a novel approach that proves effective in enhancing performance and addressing trigger conditions. The paper discusses the importance of adjusting cloud base entrainment levels and explores the impacts on diurnal timing of precipitation. With a focus on entrainment and detrainment as sources and sinks of cloud air, the study provides insights on how to optimize cumulus schemes for more accurate simulations. For further questions or suggestions, please feel free to reach out.

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Isolating the Impact of Height Dependence on Cumulus Entrainment

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  1. Isolating the Impact of Height Dependence onCumulus Entrainment Walter Hannah May 25th, 2011

  2. EAPSI • 8 week visit (10 weeks or Japan) • Myong-In Lee at Ulsan National Institute of Science and Technology in South Korea

  3. Relaxed Arakawa-Schubert

  4. Problems • Most cumulus schemes based on the “entraining plume” are not sensitive enough to the environmental conditions • Precipitation occurs too often • Precipitation occurs too early • Tropical wave and MJO variability is unrealistic • These problems are typically addressed with arbitrary trigger conditions • Environmental Humidity (Wang and Schlesinger, 1999) • Entrainment (Tokioka et al, 1988) • Large scale vertical velocity (Kain and Fritsch, 1992) • Cloud base height (Lee et al, 2008) • Precipitation evaporation (Sud and Walker, 1993) • Convective inhibition (Kain and Fritsch, 1992)

  5. Chikira and Sugiyama (2010) • Closure based on kinetic energy instead of mass flux • Entrainment rate is assumed to be inversely related to vertical velocity • Improved performance is credited to having large entrainment at low levels Gregory (2001) Neggers et al. (2002) Pan and Randall (1998)

  6. Height Dependent Entrainment • RAS constant entrainment • RAS linear entrainment

  7. The Normalized Mass Flux

  8. Chikira’s Test Case • Input profile calculated from TOGA-COARE • Precipitation rate > 20 mm/day

  9. SCM Results: GATE

  10. SCM Results: TOGA-COARE

  11. Diurnal Cycle Application • Lee et al. (2007) showed that only certain trigger mechanisms can significantlyeffect the diurnal timing of precipitation • Height of starting level • Defined as level of maximum MSE within 300 hPa from the surface • Convective inhibition • The LFC is constrained to be within 150 hPa from the starting level for convection to activate • Can modifying the cloud base entrainment have the same effect? Lee et al. (2007)

  12. Diurnal Cycle Application Lee et al. (2007)

  13. Is Height Dependence Realistic? • Romps (2010) presented a method for a direct measure of entrainment rate in LES models • Entrainment and detrainment are sources and sinks of cloud air

  14. Romps (2010) • Entrainment rate was found not to vary significantly with height • Entrainment did not scale with vertical velocity or buoyancy

  15. Romps (2010) Cloud air defined by moisture only Cloud air defined by moisture and vertical velocity

  16. Questions? Suggestions?

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