190 likes | 315 Views
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.
E N D
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
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
Soil Moisture Availability One Zero Soil Moisture Wilting Point Field Capacity Soil Properties Control Evaporation Eveg≈ Epotential x Soil_Moisture_Availability
Once Upon A Time • 1984: A paper is born! (Cited 838 times according to GS)
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).
Why are UM soil properties so Different from ECMWF? Much later: ~ 2006
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
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
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
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
Implementation Operational Verification of T2m T+72 RMS Error
Conclusions: The small picture • We found a bug • We fixed it • After a lot of work, everyone was happy
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