Comparing ERA-40-Based L-Band Brightness Temperatures with Skylab Observations: A Calibration/Validation Study Using the Community Microwave Emission Model

Abstract

The Community Microwave Emission Model (CMEM) has been used to compute global L-band brightness temperatures at the top of the atmosphere. The input data comprise surface fields from the 40-yr ECMWF Re-Analysis (ERA-40), vegetation data from the ECOCLIMAP dataset, and the Food and Agriculture Organization's (FAO) soil database. Modeled brightness temperatures have been compared against (historic) observations from the S-194 passive microwave radiometer onboard the Skylab space station. Different parameterizations for surface roughness and the vegetation optical depth have been used to calibrate the model. The best results have been obtained for rather simple approaches proposed by Wigneron et al. and Kirdyashev et al. The rms errors after calibration are 10.7 and 9.8 K for North and South America, respectively. Comparing the ERA-40 soil moisture product against the corresponding in situ observations suggests that the uncertainty in the modeled soil moisture is the predominant contributor to these rms errors. Although the bias between model and observed brightness temperatures are reduced after the calibration, systematic differences in the dynamic range remain. For NWP analysis applications, bias correction schemes should be applied prior to data assimilation. The calibrated model has been used to compute a 10-yr brightness temperature climatology based on ERA-40 data.

Evaluation of the Community Microwave Emission Model Coupled with the Community Land Model over East Asia

“…CMEM was developed by the ECMWF for numerical weather prediction applications and is used to simulate passive microwave brightness temperatures of the surface at low frequencies (from 1 GHz to 20 GHz) (Holmes et al, 2008;Drusch et al, 2009). CMEM consists of the physics and parameterizations used in the Land Surface Microwave Emission Model (LSMEM; Drusch et al, 2001) and the L-band Microwave Emission of the Biosphere (L-MEB; Wigneron et al, 2007).…”

“…For polarization (p), the brightness temperatures over snow-free areas at the top of the atmosphere (TOA) T Btoa,p , which result from the contributions of three dielectric layers (soil, vegetation, and atmosphere), can be expressed as follows: CMEM includes a modular choice regarding the physical parameterizations for the soil, vegetation, and atmosphere dielectric layers. For soil, as described in Drusch et al (2009), three parameterizations are considered for the soil dielectric constant, four for the effective temperature, two for the smooth emissivity model, and five for soil roughness. The vegetation optical depth can be represented through a choice among four different parameterizations, while the atmospheric opacity can be represented by three parameterizations.…”

“…Consequently, there are a total of 1440 different configurations provided by CMEM. More detailed descriptions of CMEM can be found in Holmes et al (2008) and Drusch et al (2009). However, we could not test all of these possible configurations.…”

“…The Community Microwave Emission Model (CMEM) was developed by the European Centre for Medium-Range Weather Forecasts (ECMWF) as the forward operator for low frequency passive microwave brightness temperatures (from 1 GHz to 20 GHz) of the surface (Holmes et al, 2008;Drusch et al, 2009). It combines many choices regarding the physical parameterizations for soil, vegetation, and atmospheric dielectric layers.…”

SMRT: an active–passive microwave radiative transfer model for snow with multiple microstructure and scattering formulations (v1.0)

“…In addition, several models were tailored to low frequencies (i.e., up to a few gigahertz), such as 2S (Schwank et al, 2014), CMES (Drusch et al, 2009), WALOMIS (Leduc-Leballeur et al, 2015), and others (Tan et al, 2015a), triggered by the inception of spaceborne L-band radiometry (Barre et al, 2008). Early models for active microwave observations include only single scattering mechanisms (Bingham and Drinkwater, 2000;Flach et al, 2005;Longepe et al, 2009;Lacroix et al, 2008), which is generally sufficient at low frequencies at which scattering is weak compared to absorption.…”