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.

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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 ...