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Log e or not Log e ? Some Questions with Answers about Soil Properties

Log e or not Log e ? Some Questions with Answers about Soil Properties. Talk by Imtiaz Dharssi September 2013 Based on work by Dharssi, Verhoef, Vidale, Macpherson, Jones and Best Met R&D Technical Report 528, 2009.

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Log e or not Log e ? Some Questions with Answers about Soil Properties

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  1. Loge or not Loge?Some Questions with Answers about Soil Properties Talk by Imtiaz Dharssi September 2013 Based on work by Dharssi, Verhoef, Vidale, Macpherson, Jones and Best Met R&D Technical Report 528, 2009. http://research.metoffice.gov.uk/research/nwp/publications/papers/technical_reports/reports/528.pdf

  2. What Are Soil Properties? • Soil Properties Affect: • The ability of vegetation to extract soil water • The thermal properties of the soil and consequently the ground flux • The vertical movement of soil water and runoff • In UM jargon the soil properties are called the soil ancillaries • They are assumed to be determined by the soil texture • Time independent

  3. Soil Moisture Availability One Zero Soil Moisture Wilting Point Field Capacity Soil Properties Control Evaporation Eveg≈ Epotential x Soil_Moisture_Availability

  4. Once Upon A Time • 1984: A paper is born! (Cited 838 times according to GS)

  5. Cosby paper Uses Log Functions

  6. A New Land Surface Model • Late 1990s: The UK Met Office develop a new land surface model • Met Office Surface Exchange Scheme (MOSES) • The Cosby equations are used to calculate the soil hydraulic properties • The new model assumes that the log functions used by the Cosby paper are natural log functions (i.e. Loge).

  7. Why are UM soil properties so Different from ECMWF? Much later: ~ 2006

  8. Do Cosby mean Loge or Log10?

  9. Cosby used Log10 not Loge

  10. How we Tested the Bug Fix • Testing performed by scientists at: • Reading University, Climate Model • Met Office, Weather forecasting model • Hadley Centre, Climate Model • Centre for Ecology and Hydrology, Land Surface Model

  11. Impact of Bug fix • All the tests showed a consistent and surprising result: • Model Soil moisture increased significantly • Model Evaporation decreased significantly • The bug fix significantly improved estimates of evaporation in the off-line land surface model • The bug fix made the NH summer warm bias worse

  12. Soil Moisture Availability One Zero Soil Moisture Wilting Point Field Capacity Soil Moisture Increases But Evaporation Decreases! How? • Evaporation doesn’t depend on the absolute value of soil moisture! • Soil Moisture increased but the Soil Moisture Availability decreased! • The new Wilting point and Field capacity values are much higher • Crucially, after the bug fix the Field capacity minus Wilting point values increased significantly

  13. Don’t Panic! • The Bug Fix made the NH summer warm bias worse! • So we created a package of changes, including the bug fix, that significantly improved model performance as well as improving the science • most notably an improved parameterisation of soil thermal conductivity

  14. Implementation Operational Verification of T2m T+72 RMS Error

  15. Conclusions: The small picture • We found a bug • We fixed it • After a lot of work, everyone was happy

  16. Conclusions: The BIG picture • The soil properties are as important as soil moisture • Soil properties have a significant impact on the soil moisture dynamic range • Soil properties have a significant impact on evaporation • We should work with soil scientists at CSIRO (e.g. ASRIS) to improve the UM soil properties for Australia • The vertical variation of soil texture is ignored by most models but might be very important

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