Soil Moisture
Assimilation in the NSIPP Land Surface Model
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1,2Jeffrey P. Walker and 2Paul
R. Houser |
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1Universities Space Research
Association |
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2NASA/Goddard Space Flight
Center |
LSM v Remote Sensing: 1
LSM v Remote Sensing: 2
Why Assimilate Soil
Moisture?
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Accurate soil moisture forecasts are
necessary for accurate predictions of precipitation. |
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LSM spin-up does not guarantee correct
soil moisture initialization. |
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Errors in soil moisture forecasts
result from: |
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The memory of errors in LSM soil
moisture initialization. |
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Errors in atmospheric forcing data. |
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Errors in LSM physics. |
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Errors in soil and topographic data. |
What is Data Assimilation
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The process of finding the model
representation which is most consistent with the observations. |
The Kalman-Filter
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Forecasting Equations |
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States: |
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Covariances: |
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Observation Equation |
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Surface Moisture: |
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Updating Equations |
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States: |
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Covariances: |
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The Catchment-Based LSM
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srfexc = srfexc – es – srflow +
i + a |
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rzexc = rzexc – ev + srflow –
rzflow + b |
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catdef = catdef + et – rzflow +
baseflow + c |
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where a, b and c are correction
terms to ensure
mass balance |
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Using a first-order Taylor
series expansion: |
Numerical Experiments
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“Truth” Data Set |
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Spin-up catchment-based LSM for 1987
using ISLSCP forcing data. |
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Run the simulation for 1987 using
ISLSCP forcing data and output surface moisture data as “observations”. |
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Open Loop Data Set |
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Degrade the soil moisture prognostic
variable spin-up states to make the catchments artificially wet. |
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Run the simulation for 1987 using
ISLSCP forcing data. |
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Assimilated Data Set |
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Degrade the soil moisture prognostic
variable spin-up states to make the catchments artificially wet (as for the
open loop). |
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Run the simulation for 1987 using
ISLSCP forcing data. |
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Assimilate the surface “observations”
once every 3 days using the Kalman-filter. |
Soil Moisture: January 30
Soil Moisture: July 31
Soil Moisture: December
29
“Errors” in Assimilated
Moisture: 1
“Errors” in Assimilated
Moisture: 2
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Full covariance forecasting |
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Partial covariance forecasting |
“Errors” in Assimilated
Moisture: 3
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Full covariance forecasting |
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Partial covariance forecasting |
Monthly
Evapotranspiration
Monthly Runoff
Short Term Goals
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Identify why the correct soil moisture
is retrieved more quickly for some catchments compared to others. |
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Identify the maximum level of error
which surface observations can have before the assimilation is no longer
useful. |
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Identify the minimum frequency of
surface observations before there is a significant degradation of the
assimilation results. |
Longer Term Goals
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Assimilate Manfred’s surface soil
moisture data over North America and evaluate. |
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Move towards a global implementation of
the assimilation. |
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Develop a set of global soil moisture
initialization states for the start of each month from 1978 to 1994. |
Evaluation of
Assimilation?
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How does NSIPP plan to evaluate the
soil moisture assimilation? |
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Soil moisture - ideal but data not
available. |
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Evapotranspiration - data not available
and assumes that soil moisture is the only reason for poor estimates. |
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Precipitation forecasts - assumes soil
moisture is the only reason for poor forecasts. |
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Other ? |