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Single Column Experiments with a
Microwave Radiative Transfer Model
Henning Wilker, Meteorological Institute of the University of Bonn (MIUB)
Gisela Seuffert, ECMWFMatthias Drusch, ECMWF / MIUB
Pedro Viterbo, ECMWF
Overview
1. Motivation.2. The microwave radiative transfer model.3. The SGP97 dataset.4. First results of single column experiments.5. Outlook.
Motivation: Theory• In the microwave region and at terrestrial temperatures the radiation intensity is proportional to brightness temperature:
• The dielectric constants of water and dry soil show large differences in the microwave region.
→ dry soil: e > 0.9, very wet soil: e ≈ 0.6
• The atmosphere is largely transparent for low frequency microwaves.
→ Satellite remote sensing.SSSBR TreTTI )1(~ ),( frS
Motivation:
Passive Microwaves within ELDAS
• Satellite observations of microwave brightness temperatures have the potential of getting valuable information about soil moisture.
• If brightness temperature observations are assimilated into NWP models what kind of improvements do we get in modeled soil water content?
→ single column experiments
• Verification of soil moisture fields produced within ELDAS with SSM/I and eventually AMSR data in low vegetated areas.
SB TT
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BSBVB TccTT )1(
*))]exp()1(1(
*))exp(1*)(1(*)exp()[exp(
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atskyadatauBV
TT
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Passive Microwaves:Solutions of the Radiative Transfer Equation
a) No atmosphere, no vegetation (2)
b) No vegetation (1), (2), (3)
c) ‚full‘ solution (1) – (5)
(1) (2) (3) (4)
(5)}
Some Features of the Model: Soil
• Dielectric mixing models:1. Wang and Schmugge, 1980 validated
for 1.4 and 5 GHz
2. Dobson et al., 1985
frequency domain: 4-18 GHz
• Radiative transfer (smooth surface):1. Wilheit, 1978: Multi-layer soil model
2. Simple one-layer reflection and emission
• Effects of rough surface1. Wang and Choudhury, 1981
validated for 1.4 GHz
2. Wegmüller and Mätzler, 1999
frequency domain: 1-100 GHz
Some features of the model:Vegetation and atmosphere
• Effects of vegetation:1. -model from effective medium theory for
frequencies below 5 GHz (Kirdyashev et al.,1979)
2. Modified -model from geometrical optics for frequencies below 40 GHz (Wegmüller et al., 1995)
• Atmospheric effects:• Radiative transfer after Liebe.• Absorption by oxygen and water vapor (no liquid water).• No scattering.
Data from the Southern Great Plains Hydrology Experiment (SGP 97)
• Observation period: June 18 – July 17 in 1997.
• Nearly daily measurements of brightness temperatures from an 1.4 GHz radiometer (ESTAR) flown on aircraft at 7.5 km altitude.
– Observation area: more than 10.000 km².
• 3 intense measurement areas: ARM CART Central Facility, USDA ARS Grazinglands Research Lab at El Reno and USDA ARS Little Washita Watershed.
• Daily soil moisture measurements for more than 40 sites (partly profiles).
• Soil and vegetation properties for all sites.
• Surface flux measurements for selected sites.
from Jackson,T.: SGP97
Experiment Plan
Example of ESTAR 1.4 GHz brightness temperature image
Principle Investigator: Thomas Jackson, USDA-ARS Hydrology Lab
(http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/SGP97/estar.html)
Selected SGP97 sites for single column experiments
• Site LW02 within the Little Washita watershed.
(NOAA/ATDD long term flux site)
• Site CF01 within the ARM CART area.
(Central Facility)
Available Data at Site LW02
• Half-hourly measurements from NOAA/ATDD long term flux site (Tilden Meyers´ dataset):– latent, sensible and ground heat flux– incoming solar radiation, net radiation– 2m temperature, relative humidity and wind– precipitation, surface pressure, surface temperature– soil temperatures at 6 depths and soil water content at 10 cm
• Soil matric potential data for 6 depths.
• Daily gravimetric soil water content (0–5 cm).
• ESTAR brightness temperatures.
• Additional soil and vegetation properties.
• Meteorological data from surrounding Micronet sites.
Soil Moisture Measurements: Site LW02
Available Data at Site CF01
• ARM Surface Meteorological Observation System:5-min meteorological data.
• Energy Balance Bowen Ratio station: Surface fluxes and additional meteorological
measurements every 30 minutes.• Baseline Surface Radiation Network station:
Upward and downward shortwave and longwave radiation every minute.
• Soil Water and Temperature System: Hourly soil temperature and water content
profiles.• Daily gravimetric soil water content (0-5 cm).• ESTAR brightness temperatures.• Additional soil and vegetation properties.
Soil Moisture Measurements: Site CF01
Coupling of Single Column Model and Microwave Radiative Transfer Model
Model InputSoil Parameters for LW02
• Soil temperature and soil water content:
Values of first soil layer (0-7cm) from TESSEL.
• Salinity of soil water: 3.0 psuSalinity domain ranges from ≈ 0 psu (non-saline) to ≈ 20 psu (extremely saline).
• Fractions of sand and clay: 35% and 20%• Soil specific density: 2.65 g/cm³• rms height (as roughness parameter): 0.5 cm
– Range of realistic values: 0 - ≈1.2 cm
Model InputVegetation Parameters for LW02
• Vegetation temperature: Skin temperature from TESSEL• Vegetation water content: 0.34 kg/m²
(as determined from measurements and NDVI values, Jackson et al., 1999)
• Vegetation cover: taken from TESSEL(TESSEL calculates it from vegetation type which we set to 97% tall grass and 3% interrupted forest)
• Structure parameter: 0.003• Single scattering albedo: 0.1 (H- and V-polarization)
Literature: 0.03 – 0.13 for various vegetation types
• Salinity of vegetation water: 6.0 psu
Model InputAtmospheric Parameters
• Profiles of temperature, water vapor and air pressure taken from ECMWF ERA40 reanalysis.
(60 layers as in single column model.)
• Microwave frequency: 1.4 GHz(Measument frequency of ESTAR.)
• Incidence angle: 0°(Brightness temperature measurements of ESTAR were normalized to nadir.)
Coupled Modeling for LW02
Outlook (MIUB)
• Sensitivity studies with stand-alone microwave radiative transfer model and with ECMWF brightness temperature assimilation scheme: – Use of a synthetic 1.4 GHz brightness temperature
dataset for different incidence angles, temperature and soil moisture profiles.
• Integration of a snow module into the microwave radiative transfer model.
• Verification of ELDAS soil moisture fields with SSM/I (AMSR) in low vegetated areas.
(Details by M. Drusch on Friday ...)
And now to assimilation ...