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Impact of Convective Triggering Mechanisms on CAM2 Model Simulations

Impact of Convective Triggering Mechanisms on CAM2 Model Simulations Shaocheng Xie, Gerald L. Potter, Richard T. Cederwall, and James S. Boyle Atmospheric Science Division Lawrence Livermore National Laboratory Minghua Zhang and Wuyin Lin Marine Sciences Research Center

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Impact of Convective Triggering Mechanisms on CAM2 Model Simulations

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  1. Impact of Convective Triggering Mechanisms on CAM2 Model Simulations • Shaocheng Xie, Gerald L. Potter, Richard T. Cederwall, and James S. Boyle • Atmospheric Science Division • Lawrence Livermore National Laboratory • Minghua Zhang and Wuyin Lin • Marine Sciences Research Center • State University of New York at Stony Brook • CCSM AMWG Meeting, Boulder, Colorado, 9-10 March, 2004

  2. Outline • Problems with the original trigger in CAM2 and a revised trigger developed by Xie and Zhang (2000) • Impacts of the new trigger on model simulations • SCM tests • CAPT tests (CCPP-ARM Parameterization Testbed) • Climate simulations (AMIP run) • Summary

  3. Problems with the Original Trigger (CAPE>0) • Convection over land is triggered too often during the day in warm season SGP SCM Test ARM 97 Summer IOP

  4. Problems with the Original Trigger (CAPE>0) • Strong diurnal variations in CAPE over land in warm season Time (days since 2330 GMT Jun. 18, 97) ARM 97 Summer IOP

  5. Convective Triggering Mechanisms • An air parcel is energetic enough to penetrate the layer of convection inhibition == > Convection Occurs • Large-scale upward motion • Lifting associated with fronts and orography • Pre-existing convection • Subgrid-scale dynamic instability • Surface heterogeneity • Growth of the boundary layer

  6. A Revised Trigger (DCAPE >0) for CAM2 • Xie and Zhang (2000) introduced a positive dynamic CAPE generation rate (DCAPE) that describes a combined measurement of lifting and inhibition effects to control the onset of deep convection. • DCAPE = {CAPE*(T*, q*) – CAPE(T,q)}/t • Where • T* = T + (∂ T/∂ t)adv * t • q* = q + (∂ q/∂ t)adv * t • New Trigger: CAPE>0 & DCAPE>0 • The new triggering function links cumulus convection directly to the large-scale dynamic forcing, such as lower level convergence.

  7. A Revised Trigger (DCAPE >0) • Observations show a strong correlation between positive DCAPE and convection (and associated precipitation). ARM 97 Summer IOP

  8. CCM3 SCM Tests • The new trigger largely reduces the effect of the strong diurnal variations in the surface isolation on the initiation of convection. ARM 97 Summer IOP

  9. CAM2 CAPT Tests (SGP Site) ARM 97 Summer IOP • The new trigger produces less frequent precipitation.

  10. CAM2 CAPT Tests (United States) Daily Precipitation Rates (mm/day) • The original model overestimates the observed precipitation in most parts of the United States while the excessive precipitation is clearly reduced using the new trigger 30-dayensemble mean of 0-24 hour forecasts(June 18 – July 17, 1997)

  11. CAM2 CAPT Tests (the Globe) • The new trigger gives a more realistic spatial pattern of precipitation in the tropics 30-dayensemble mean of 0-24 hour forecasts(June 18 – July 17, 1997)

  12. CAM2 Climate Simulations (10 yr AMIP run) Precipitation Rates (mm/day) CAM2O CAM2M JJA Xie-Arkin • Double ITCZ has been noticeably reduced

  13. CAM2 Climate Simulations (10 yr AMIP run) Precipitation Rates (mm/day) DJF CAM2O CAM2M Xie-Arkin • Improved simulations are seen over SPCZ and India Ocean

  14. newsstamip06 • High Clouds (ANN) • CAM2 • CAM2M • ISCCP • CAM2O • CAM2 –ISCCP • CAM2M –CAM2O • High Clouds are considerably reduced, especially in the tropical and sub-tropical regions.

  15. Temperature JJA • CAM2M • CAM2O • CAM2M –CAM2O • Slightly Warmer tropopause and colder troposphere. • CAM2M –CAM2O

  16. Specific Humidity • CAM2O • Yap (9.4N, 138.1 E) • Panama (9N, 79W) • CAM2M • JAN • APR • JAN • APR • JUL • JUL • OCT • OCT • Easter Island (27S, 110W) • Resolute_Canada (74.7N, 95W) • JAN • APR • JAN • APR • JUL • OCT • JUL • OCT

  17. Taylor Diagram(10 yr AMIP run) JJA Climatology: Zonal Mean 20N to 90N JJA Climatology: Zonal Mean 20S to 20N

  18. Summary • The old trigger (CAPE > 0) ==> overactive warm season convection in CAM2 and its previous version CCM3 (SCM, CAPT, Climate Simulations). • The revised trigger (DCAPE>0) ==>emulates the collective effects of large-scale lower level moistening and upward motion==> effectively reduces the problem • Improved results are seen at ARM SGP site in SCM and CAPT tests, • and in US and around the globe in CAPT and climate (AMIP) simulations, in both winter and summer • Collaborate with Dr. Jim Hack from NCAR and Dr. Minghua Zhang from SUNY Stony Brook to test the revised triggering scheme in CAM3

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