Microwave land emissivity and skin temperature for AMSU‐A and ‐B assimilation over land

Karbou, Fatima; Gérard, É.; Rabier, Florence

doi:10.1256/qj.05.216

Cited by 87 publications

(70 citation statements)

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“…Jones et al (1997Jones et al ( , 2004) also estimated land emissivity from AMSU observations. Overall consistency was found between the spatial distribution of the emissivity values obtained with SSM/I and AMSU and the global land cover-land use maps (Karbou et al, 2006). These findings were in line with those obtained from forward modeling of land emissivity.…”

supporting

confidence: 85%

The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties

Norouzi

Temimi

Rossow

et al. 2011

Hydrol. Earth Syst. Sci.

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Abstract. Microwave observations at low frequencies exhibit more sensitivity to surface and subsurface properties with little interference from the atmosphere. The objective of this study is to develop a global land emissivity product using passive microwave observations from the Advanced Microwave Scanning Radiometer -Earth Observing System (AMSR-E) and to investigate its sensitivity to land surface properties. The developed product complements existing land emissivity products from SSM/I and AMSU by adding land emissivity estimates at two lower frequencies, 6.9 and 10.65 GHz (C-and X-band, respectively). Observations at these low frequencies penetrate deeper into the soil layer. Ancillary data used in the analysis, such as surface skin temperature and cloud mask, are obtained from International Satellite Cloud Climatology Project (ISCCP). Atmospheric properties are obtained from the TIROS Operational Vertical Sounder (TOVS) observations to determine the small upwelling and downwelling atmospheric emissions as well as the atmospheric transmission. A sensitivity test confirms the small effect of the atmosphere but shows that skin temperature accuracy can significantly affect emissivity estimates. Retrieved emissivities at C-and X-bands and their polarization differences exhibit similar patterns of variation with changes in land cover type, soil moisture, and vegetation density as seen at SSM/I-like frequencies (Ka and Ku bands). The emissivity maps from AMSR-E at these higher frequencies agree reasonably well with the existing SSM/I-based product. The inherent discrepancy introduced by the difference between SSM/I and AMSR-E frequencies, incidence angles, and calibration has been assessed. SigCorrespondence to: H. Norouzi (hnorouzi@citytech.cuny.edu) nificantly greater standard deviation of estimated emissivities compared to SSM/I land emissivity product was found over desert regions. Large differences between emissivity estimates from ascending and descending overpasses were found at lower frequencies due to the inconsistency between thermal IR skin temperatures and passive microwave brightness temperatures which can originate from below the surface. The mismatch between day and night AMSR-E emissivities is greater than ascending and descending differences of SSM/I emissivity. This is because of unique orbit time of AMSR-E (01:30 a.m./p.m. LT) while other microwave sensors have orbit time of 06:00 to 09:00 (a.m./p.m.). This highlights the importance of considering the penetration depth of the microwave signal and diurnal variability of the temperature in emissivity retrieval. The effect of these factors is greater for AMSR-E observations than SSM/I observations, as AMSR-E observations exhibit a greater difference between day and night measures. This issue must be addressed in future studies to improve the accuracy of the emissivity estimates especially at AMSR-E lower frequencies.

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supporting

confidence: 85%

The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties

Norouzi

Temimi

Rossow

et al. 2011

Hydrol. Earth Syst. Sci.

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“…Instantaneous measures of microwave brightness temperature, Tb, have been used in a variety of applications to estimate column water vapor abundance, rainfall rate, surface ocean wind speed, ocean salinity, soil moisture, freeze/thaw state, land surface temperature, inundation fraction, and vegetation structure (Boukabara et al, 2007;Entekhabi et al, 2010;Fily et al, 2003;Karbou et al, 2006;McCollum and Ferraro, 2005;Min et al, 2010;Njoku et al, 2003;Papa et al, 2006;Tedesco and Kim, 2006;Wilheit et al, 2003;Zhang et al, 2010). Land surface properties can be inferred accurately if physical temperature and emissivity variations can be separated (Prigent et al, 1997).…”

Section: Introductionmentioning

confidence: 99%

Using microwave brightness temperature diurnal cycle to improve emissivity retrievals over land

Norouzi

Rossow

Temimi

et al. 2012

Remote Sensing of Environment

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Reza, "Using microwave brightness temperature diurnal cycle to improve emissivity retrievals over land" (2012 To retrieve microwave land emissivity, infrared surface skin temperatures have been used as surface physical temperature since there is no global information on physical vegetation/soil temperature profiles. However, passive microwave emissions originate from deeper layers with respect to the skin temperature. So, this inconsistency in sensitivity depths between skin temperatures and microwave temperatures may introduce a discrepancy in the determined emissivity. Previous studies showed that this inconsistency can lead to significant differences between day and night retrievals of land emissivity which can exceed 10%. This study proposes an approach to address this inconsistency and improve the retrieval of land emissivity using microwave observations from Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E). The diurnal cycle of the microwave brightness temperature (Tb) was constructed over the globe for different frequencies/polarizations using a constellation of satellites. Principal component analysis (PCA) was conducted to evaluate the spatial variation of the Tb diurnal cycle. The diurnal amplitudes of microwave temperatures observed in desert areas were not consistent with the larger amplitudes of the diurnal cycle of skin temperature. Densely vegetated areas with more moisture have shown smaller amplitudes. A lookup table of effective temperature (T eff ) anomalies is constructed based on the Tb diurnal cycle to resolve the inconsistencies between infrared and Tb diurnal variation. This lookup table of T eff anomalies is a weighted average over the layers contributing to the microwave signal, for each channel and month. The integration of this T eff in the retrieval of land emissivity reduced the differences between day and night retrieved emissivities to less than 0.01 for AMSR-E observations.

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“…The dynamical method emissivity can be derived using a simplified radiative transfer equation and it is described in Mathew et al (2008) and it has been applied for sounder data assimilation over land (Karbou et al, 2006). It is based on simulations of the brightness temperature using the atmospheric radiative transfer model RTTOV-8 with surface emissivities (e) at zero and at one, that is, Tb sim (e 00) and Tb sim (e 01) and AMSU channel 3 (50.3 GHz) brightness temperatures:…”

Section: Comparison Of the Sea Ice Emissivity Estimate Using Differenmentioning

confidence: 99%

The EUMETSAT OSI SAF near 50 GHz sea ice emissivity model

Tonboe¹,

Schyberg²,

Nielsen³

et al. 2013

Tellus A: Dynamic Meteorology and Oceanography

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A sea ice thermal microwave emission model for 50 GHz was developed under EUMETSAT's Ocean and Sea Ice Satellite Application Facility (OSI SAF) programme. The model is based on correlations between the surface brightness temperature at 18, 36 and 50 GHz. The model coefficients are estimated using simulated data from a combined thermodynamic and emission model. The intention with the model is to provide a first guess sea ice surface emissivity estimate for atmospheric temperature sounding applications in the troposphere in numerical weather prediction (NWP) models assimilating Advanced Microwave Sounding Unit (AMSU) and Special Sensor Microwave Imager/Sounder (SSMIS) data. The spectral gradient ratio is defined as the difference over the sum of the SSMIS brightness temperatures at 18 and 36 GHz vertical linear polarisation (GR1836). The GR1836 is related to the emissivity at the atmospheric temperature sounding channels at around 50 GHz. Furthermore, the brightness temperatures and the polarisation ratio (PR) at the neighbouring 18, 36 and 50 GHz channels are highly correlated. Both the gradient ratio at 18 and 36 GHz and the PR at 36 GHz measured by SSMIS are input into the model predicting the 50 GHz emissivity for horizontal and vertical linear polarisations and incidence angles between 0° and 60° The simulated emissivity is compared to the emissivity derived with alternative methods. The fit to real AMSU observations is investigated using the different emissivity estimates for simulating the observations with atmospheric data from a regional weather prediction model

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Microwave land emissivity and skin temperature for AMSU‐A and ‐B assimilation over land

Cited by 87 publications

References 50 publications

The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties

The sensitivity of land emissivity estimates from AMSR-E at C and X bands to surface properties

Using microwave brightness temperature diurnal cycle to improve emissivity retrievals over land

The EUMETSAT OSI SAF near 50 GHz sea ice emissivity model

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