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Experiments with the microwave emissivity model concerning the brightness
temperature observation error&
SSM/I evaluation
Henning Wilker, MIUBMatthias Drusch, ECMWF
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Extended Kalman Filter Analysis
aMxx
][ bba xHyKxx 1111 ][ RHHRHBK TT
x soil moisture vector to forecastxa soil moisture analysis vectorxb soil moisture background vectory observationsM forecast operatorH observation operatorB background error covariance matrixR observation error covariance matrix
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Question
Do we need a bias correction for the observed brightness
temperatures due to the uniform vertical distribution of modelled
soil moisture?
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Example
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Approach
SGP99 gravimetric soil moisture measurements(0 - 2.5 cm and 2.5 - 5 cm)
Layer Difference Statistics
Artificial soil moisture profiles
Set of brightness temperatures
Brightness temperature bias and rms error
Microwave emissivity model
Classified by „days after rain event“
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SGP99 site-averaged soil moisture
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SGP99 soil moisture layer differences
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SGP99 soil moisture layer differences
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Creation of soil moisture profiles
Assumptions: cbzaz )ln()( cba ,0,0
2) mz
m zdzz )(0])([
3)
101.0 b bzbz
a
1
2
12
ln
srzz )()( 21
cm75.3 cm,25.1 21 zz
= mean soil moisture layer difference of all SGP99 sites
r = Gaussian random number
s= standard deviation of soil moisture layer difference
m = measured mean soil moisture (0-5cm)
)ln( bzac m
1)
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Artificial soil moisture profilesone day after rain event at site
LW03
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Brightness temperature distributionat site LW03 one day after rain
event
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Artificial soil moisture profilessix days after rain event at site
LW03
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Brightness temperature distributionat site LW03 six days after rain
event
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Brightness temperature bias due to the soil moisture profile at site
LW03
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Conclusions Radiative transfer modelling indicates that
the lacking knowledge of the soil moisture profile could cause systematic errors of about 5 K in modelled brightness temperatures.
The TB-bias is a function of precipitation and dry-down time and could be taken into account in the assimilation scheme (further studies are needed).
Vertical structures in soil moisture which are not represented by the hydrological model can cause rms-errors of ~6 K. In the assimilation experiments an error of 2 K was assumed.
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Preparations for ELDAS soil moisture validation with SSM/I brightness
temperatures
Workpackage 4200
Objective:
Validation of ELDAS soil moisture fields with SSM/I brightness temperatures
over sparsely vegetated regions
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SSM/I: Few details
SSM/I = Special Sensor Microwave Imager
passive microwave radiometer onboard the DMSP satellites
Frequencies Footprint (Ghz) Pol. Size (km²)---------------------------------------- 19.35 h,v 43 x 69 22.235 v 40 x 60 37.0 h,v 30 x 38 85.5 h,v 13 x 15
Start: 1987
Incidence angle: 53.1°
Revisit time: ~ twice per day
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SSM/I: Available data
Brightness temperatures aggregated to anEqual-Area Scalable Earth (EASE) grid
Horizontal resolution: 25 km (global)Temporal resolution: daily (1988 - 2001)
Example of EASE-griddedbrightness temperatures(horizontally polarized),
15. September 2000
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Validation approach
Land surface microwave emissivity model
ELDAS soilmoisture fields
Additional soil, vegetationand atmospheric parameters
TOA brightness temperatures
Comparison of SSM/I and ELDAS-derived TBs
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• Examine time series at selected validation sites (Badajoz, Tomelloso?).
• Purpose: Investigation of temporal development of ELDAS-derived brightness temperatures (including major rain event and following dry-down).
• Maybe additional soil, vegetation and/or atmospheric parameters available?
• Selected case study (central Spain?).
• Purpose: Examination of spatial distribution of ELDAS-derived brightness temperatures (for example rain events touching only limited areas).
• Merging of ELDAS grid and EASE grid required.
Point validation Area validation
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Required input data for the microwave model
Volumetric water content MARS archiveSoil temperature MARS archiveSand/Clay percentage MARS? / soil type?Bulk density ? / average value?Surf. roughness (rms height) ????
Atm. Profiles of p, T, RH MARS
Parameter Source
Vegetation cover MARSVegetation temperature Soil surface temperature (MARS)Vegetation water content ? / vegetation type (MARS)Other vegetation parameters Vegetation type (MARS)
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Brightness Temperature (H-Pol.), Badajoz
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Brightness Temperature (V-Pol.), Badajoz
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Brightness Temperature Anomaly (H-Pol.), Badajoz
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Brightness Temperature Anomaly (V-Pol.), Badajoz
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Polarization Index, BadajozHV
HV
TBTB
TBTB
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Anomaly of Polarization Index, Badajoz
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Daily precipitation amount, Badajoz
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Daily mean temperature, Badajoz
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Summary
Preparation of SSM/I-data completed. First look at validation site Badajoz is
promising. Availability of additional soil and vegetation
parameters …? Open issue yet: merging of ELDAS and
SSM/I EASE grid. Untill ELDAS fields are available, testing of
technical procedures with reanalysis data?