Experiments with the microwave emissivity model concerning the brightness

temperature observation error&

SSM/I evaluation

Henning Wilker, MIUBMatthias Drusch, ECMWF

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

Question

Do we need a bias correction for the observed brightness

temperatures due to the uniform vertical distribution of modelled

soil moisture?

Example

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“

SGP99 site-averaged soil moisture

SGP99 soil moisture layer differences

SGP99 soil moisture layer differences

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)

Artificial soil moisture profilesone day after rain event at site

LW03

Brightness temperature distributionat site LW03 one day after rain

event

Artificial soil moisture profilessix days after rain event at site

LW03

Brightness temperature distributionat site LW03 six days after rain

event

Brightness temperature bias due to the soil moisture profile at site

LW03

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.

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

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

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

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

• 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

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)

Brightness Temperature (H-Pol.), Badajoz

Brightness Temperature (V-Pol.), Badajoz

Brightness Temperature Anomaly (H-Pol.), Badajoz

Brightness Temperature Anomaly (V-Pol.), Badajoz

Polarization Index, BadajozHV

HV

TBTB

TBTB

Anomaly of Polarization Index, Badajoz

Daily precipitation amount, Badajoz

Daily mean temperature, Badajoz

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?