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How Might Orographic Precipitation Respond to Global Warming?

How Might Orographic Precipitation Respond to Global Warming?. Study with a Simple Model of Mid-latitude Orographic Precipitation. Xiaoming Shi. Outline. Background and Motivation Simple Model of Orographic Precipitation Numerical Experiments Conclusion. Moisture.

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How Might Orographic Precipitation Respond to Global Warming?

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  1. How Might Orographic Precipitation Respond to Global Warming? Study with a Simple Model of Mid-latitude Orographic Precipitation Xiaoming Shi

  2. Outline • Background and Motivation • Simple Model of Orographic Precipitation • Numerical Experiments • Conclusion

  3. Moisture • Models tend to maintain a fixed relative humidity (RH) as they warm. • Then Clausius-Clapeyron equation implies moisture will increase at 7%/K (CC Scaling). (Held & Soden 2006; Allan & Soden 2008)

  4. Precipitation • ~ 2%/K, result from energy constraints. (Allen & Ingram, 2002; Takahashi 2009; O’Gorman et al, 2011) From O’Gorman et al, 2011

  5. Precipitation • The energetic constraints can’t determine regional precipitation. • Mid-latitude precipitation may increase at different rate. • Orographic precipitation ?

  6. Orographic Precipitation • Central part of the interaction between the land surface and the atmosphere. • Important for ecosystems and management of human water resources. (Roe 2005 and references therein)

  7. Parcel Model (Kirshbaum & Smith 2008; Alpert 1986) • Assumptions: • Airflow parallel to terrain at all levels; • Exponential decrease of saturation vapor density; • Flow impinging on mountains is saturated. • Condensation rate of one level:

  8. Parcel Model • Assuming α is constant: (or ) • Precipitation: • Moisture flux determines orographic precipitation in this simple model.

  9. Experiments • GFDL atmosphere and land model (AM2-LM2)

  10. Experiments • Bottom Boundary: LM2 + Slab Ocean (Frierson et al, 2006). • Resolution: 2olatitude × 2.5o longitude, 24 levels • Constant concentration of CO2, O3, CH4 … • Orographic Precipitation feeds back on dynamics by latent heating and radiation.

  11. Experiments • AM: Atmosphere Modeling, NO orographic precipitation, CO2 ~ 330ppm. • AM2CO2: … , CO2~ 660ppm. • AMO: having orographic precipitation, CO2 ~ 330ppm • AMO2CO2: … , CO2 ~ 660ppm

  12. Experiments • Run for 16 years. Data of last 8 years are usedfor averaging.

  13. Temperature • Orographic precipitation can suppress warming. Mean Temperature Unit: K

  14. Moisture • Clausius-Clapeyron Scaling.

  15. Precipitation AM AMO

  16. Precipitation • Island precipitation increases at rate ofCC scaling. • The increase of pure orographic precipitation is 9.3%/K.

  17. Orographic Precipitation • Increase of Orographic Precipitation: 9.3%/K (Drop eddy flux term, )

  18. Orographic Precipitation 9.3%/K 2.4%/K 7.5%/K • Change of wind speed will enhance the increase of orographic precipitation. • (In AM & AM2CO2, wind increased 0.5%/K)

  19. Conclusion • Wind above mountains may increase in global warming. • Together with increase of moisture, it would make orographic precipitation increase faster than 7%/K. • Hypothesis: • Stationary Thermal forcing caused by orographic precipitation can increase wind speed. • Latent heating + IR cooling

  20. References Allen, M. R. & W. J. Ingram, 2002: Constraints on future changes in climate and the hydrologic cycle. Nature419, 224-232, doi:10.1038/nature01092s Allan, R. P., B. J. Soden, 2008: Atmospheric Warming and the Amplification of Precipitation Extremes. Science321, 1481 Alpert P. 1986. Mesoscale indexing of the distribution of orographic precipitation over high mountains. J. Appl. Meteorol. 25: 532–545. Frierson, Dargan M. W., Isaac M. Held, Pablo Zurita-Gotor, 2006: A Gray-Radiation Aquaplanet Moist GCM. Part I: Static Stability and Eddy Scale. J. Atmos. Sci., 63, 2548–2566. doi: 10.1175/JAS3753.1 Held, Isaac M., Brian J. Soden, 2006: Robust Responses of the Hydrological Cycle to Global Warming. J. Climate, 19, 5686–5699. doi: 10.1175/JCLI3990.1 Kirshbaum, D. J., and R. B. Smith, 2008: Temperature and moist- stability effects on midlatitude orographic precipitation. Quart. J. Roy. Meteor. Soc., 134, 1183–1199. O'Gorman, P. A., Allan, R. P., Byrne, M. P. & Previdi, M. 2011: 
Energetic constraints on precipitation under climate change, Surveys in Geophysics (Submitted) Roe, G. H. 2005: Orographic Precipitation. Annu. Rev. Earth Planet. Sci. 2005. 33:645–71 doi: 10.1146/annurev.earth.33.092203.122541 Takahashi K, 2009: Radiative constraints on the hydrological cycle in an idealized radiative-convective equilibrium model. J AtmosSci 66:77–91

  21. Moisture • Clausius-Clapeyron Scaling. Column Integrated Water Vapor Unit: kg/m2

  22. ΔP in simulation without orographic precipitation.

  23. Precipitation • Orographic Precipitation can obey CC scaling. Mean Precipitation Unit: mm/day

  24. Orographic Precipitation • Increase of Orographic Precipitation: 9.3%/K

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