Single Column Experiments with a Microwave Radiative Transfer Model Henning Wilker, Meteorological Institute of the University of Bonn (MIUB) Gisela Seuffert, ECMWF Matthias 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.

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.

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 surface 1.Wang and Choudhury, 1981 validated for 1.4 GHz 2.Wegmüller and Mätzler, 1999 frequency domain: 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 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 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 (

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 Input Soil Parameters for LW02 Soil temperature and soil water content: Values of first soil layer (0-7cm) from TESSEL. Salinity of soil water: 3.0 psu Salinity 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 Input Vegetation 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: 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 Input Atmospheric 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...