Catherine Prigent

Microwave land surface emissivities estimated from SSM/I observations

Microwave emissivities of land surfaces are estimated from special sensor microwave/imager (SSM/I) observations by removing the contributions from the atmosphere, clouds, and rain using ancillary satellite data (International Satellite Cloud Climatology Project (ISCCP) and TIROS Operational Vertical Sounder (TOVS) products). In the first step, cloud-free SSM/I observations are isolated with the help of collocated visible/infrared satellite observations (ISCCP data). The cloud-free atmospheric contribution is then calculated, from an estimate of the local atmospheric temperature-humidity profile (TOVS retrieval). Finally, with the surface skin temperature derived from IR observations (ISCCP estimate), the surface emissivity is calculated for all the SSM/I channels. As an exploration the method is applied to the SSM/I data for four months in 1991 within the Meteosat observation area. The magnitude and fluctuations of the atmospheric contributions are estimated along with the effect of surface temperature variations. Correspondences between geographical and seasonal patterns of the emissivities and topography, vegetation, flooding, and snow cover are analyzed. The potential for using microwave emissivities to monitor vegetation phenology and surface properties at regional and continental scale is investigated, and the possibility of retrieving atmospheric parameters (water vapor content, cloud liquid water path, and precipitation) over land is discussed.

Remote sensing of global wetland dynamics with multiple satellite data sets

Thiss tudy isthe firs t global effort to quantify seasonality and extent of inundation with a suite of satellite observations, including passive and active microwave along with visible and infrared measurements. A clustering tech- nique which merges the satellite observations is used to de- tect inundation. Monthly flooded areasare then calculated by estimating pixel fractional coverage of flooding using the passive microwave signal and a linear mixture model with end-memberscalibrated with radar obs ervationsto account for vegetation cover. The global results, comprising natural wetlands, irrigated rice, and lakes/rivers, indicate a min- imum inundated area for the July 1992-June 1993 period of 2.16x10 6 km 2 , about 38% of the maximum 5.75x10 6 km 2 , to be compared to maximum areasof 5.83x10 6 km 2 and 5.70x10 6 km 2 from independent data sets. Comprehensive evaluation requires substantial additions to the sparse ob- servational record now available.

Interannual variations of river water storage from a multiple satellite approach: A case study for the Rio Negro River basin

Spatiotemporal variations of water volume over inundated areas located in a large river basin have been determined using combined observations from a multisatellite inundation data set, the TOPEX/POSEIDON (T/P) altimetry satellite, and in situ hydrographic stations for the water levels over rivers and floodplains. We computed maps of monthly surface water volume change over the period of common availability of T/P and the multisatellite data (1993–2000). The basin of the Negro River, the largest tributary in terms of discharge to the Amazon River, was selected as a test site. A strong seasonal signal is observed with minima in October and maxima in June. A strong interannual component is also present, particularly important during ENSO years. The surface water change was estimated to be 167 ± 39 km3 between October 1995 (low water) and June 1996 (high water). This result is consistent with previous estimates obtained for the 1995–1996 hydrological cycle over the same area using the JERS mosaic data. The surface water volume change is then compared to the total water volume change inferred from the GRACE satellite for an average annual cycle. The difference between the surface storage change and the total storage change derived from GRACE was computed to estimate the contribution of the soil moisture and groundwater to the total storage change. Our study supports the hypothesis that total water storage is almost equally partitioned between surface water and the combination of soil moisture and groundwater for the Negro River basin. The water volume changes are also evaluated using in situ discharge measurements and the GPCP precipitation product (correlation of 0.61). The results show the high potential for the new technique to provide valuable information to improve our understanding of large river basin hydrologic processes.

Microwave land emissivity calculations using AMSU measurements

Atmospheric parameter retrievals over land from Advanced Microwave Sounding Unit (AMSU) measurements, such as atmospheric temperature and moisture profiles, could be possible using a reliable estimate of the land emissivity. The land surface emissivities have been calculated using six months of data, for 30 beam positions (observation zenith angles from -58/spl deg/ to +58/spl deg/) and the 23.8-, 31.4-, 50.3-, 89-, and 150-GHz channels. The emissivity calculation covers a large area including Africa, Eurasia, and Eastern South America. The day-to-day variability of the emissivity is less than 2% in these channels. The angular and spectral dependence of the emissivity is studied. The obtained AMSU emissivities are in good agreement with the previously derived SSMI ones. The scan asymmetry problem has been evidenced for AMSU-A channels. And possible extrapolation of the emissivity from window channels to sounding ones has been successfully tested.

Modeling regional to global CH4 emissions of boreal and arctic wetlands

Methane (CH4) emission from boreal, arctic and subarctic wetlands constitutes a potentially positive feedback to global climate warming. Many process-based models have been developed, but high uncertainties remain in estimating the amount of CH4 released from wetlands at the global scale. This study tries to improve estimates of CH4 emissions by up-scaling a wetland CH4 emission model, PEATLAND-VU, to the global scale with a spatial resolution of 0.5 degrees for the period 2001-2006. This up-scaling was based on the global circum-arctic distribution of wetlands with hydrological conditions being specified by a global hydrological model, PCR-GLOBWB. In addition to the daily hydrological output from PCR-GLOBWB, comprising water table depths and snow thickness, the parameterization included air temperature as obtained from the ECMWF Operational Archive. To establish the uncertainty in the representations of the circum-arctic distribution of wetlands on the CH4 emission, several existing products were used to aggregate the emissions. Using the description of potential peatlands from the FAO Digital Soil Map of the World and the representation of floodplains by PCR-GLOBWB, the average annual flux over the period 2001-2006 was estimated to be 78 Tg yr(-1). In comparison, the six-year average CH4 fluxes were 37.7, 89.4, 145.6, and 157.3 Tg yr(-1) for different estimates of wetland extends based on the studies by Matthews and Fung, Prigent et al., Lehner and Doll, and Kaplan, respectively. This study shows the feasibility to estimate interannual variations in CH4 emissions by coupling hydrological and CH4 emission process models. It highlights the importance of an adequate understanding of hydrology in quantifying the total emissions from northern hemispheric wetlands and shows how knowledge of the sub-grid variability in wetland extent helps to prescribe relevant hydrological conditions to the emission model as well as to identify the uncertainty associated with existing wetland distributions. (Less)

Joint characterization of vegetation by satellite observations from visible to microwave wavelengths: A sensitivity analysis

This study presents an evaluation and comparison of visible, near-infrared, passive and active microwave observations for vegetation characterization, on a global basis, for a year, with spatial resolution compatible with climatological studies. Visible and near-infrared observations along with the Normalized Difference Vegetation Index come from the Advanced Very High Resolution Radiometer. An atlas of monthly mean microwave land surface emissivities from 19 to 85 GHz has been calculated from the Special Sensor Microwave/Imager for a year, suppressing the atmospheric problems encountered with the use of simple channel combinations. The active microwave measurements are provided by the ERS-1 scatterometer at 5.25 GHz. The capacity to discriminate between vegetation types and to detect the vegetation phenology is assessed in the context of a vegetation classification obtained from in situ observations. A clustering technique derived from the Kohonen topological maps is used to merge the three data sets and interpret their relative variations. NDVI varies with vegetation density but is not very sensitive in semiarid environments and in forested areas. Spurious seasonal cycles and large spatial variability in several areas suggest that atmospheric contamination and/or solar zenith angle drift still affect the NDVI. Passive and active microwave observations are sensitive to overall vegetation structure: they respond to absorption, emission, and scattering by vegetation elements, including woody parts. Backscattering coefficients from ERS-1 are not sensitive to atmospheric variations and exhibit good potential for vegetation discrimination with ∼10 dB dynamic range between rain forest to arid grassland. Passive microwave measurements also show some ability to characterize vegetation but are less sensitive than active measurements. However, passive observations show sensitivity to the underlying surface wetness that enables detection of wetlands even in densely vegetated areas. Merging the data sets using clustering techniques capitalizes on the complementary strengths of the instruments for vegetation discrimination and shows promising potential for land cover characterization on a global basis.

Estimation of the aerodynamic roughness length in arid and semi‐arid regions over the globe with the ERS scatterometer

Estimates of the aerodynamic roughness lengths z0 in arid and semi-arid regions are for the first time provided for the whole globe, using satellite ERS scatterometer observations. A statistical relationship is derived between the ERS scatterometer backscattering coefficients and quality in situ and geomorphological z0 estimates. It is a practical solution to provide realistic roughness maps with large-scale spatial patterns that are consistent with independent surface characterization. In addition, it makes it possible to analyze the seasonal and interannual variations of this parameter on a global basis. The satellite-derived surface roughness parameters have been implemented into the dust emission scheme of a global dust cycle model. Dust emission computations are improved when using the ERS derived z0 as input parameter. The spatial correlation between the TOMS observation frequency and the model dust events increases from 0.34 when using fixed z0 to 0.52 when using the satellite-derived estimates.

Surface freshwater storage and dynamics in the Amazon basin during the 2005 exceptional drought

The Amazon river basin has been recently affected by extreme climatic events, such as the exceptional drought of 2005, with significant impacts on human activities and ecosystems. In spite of the importance of monitoring freshwater stored and moving in such large river basins, only scarce measurements of river stages and discharges are available and the signatures of extreme drought conditions on surface freshwater dynamics at the basin scale are still poorly known. Here we use continuous multisatellite observations of inundation extent and water levels between 2003 and 2007 to monitor monthly variations of surface water storage at the basin scale. During the 2005 drought, the amount of water stored in the river and floodplains of the Amazon basin was 130 km 3 ( 70%) below its 2003‐7 average. This represents almost a half of the anomaly of minimum terrestrial water stored in the basin as estimated using the Gravity Recovery and Climate Experiment (GRACE) data.

Variations of surface water extent and water storage in large river basins: A comparison of different global data sources

[1] For the period 2003–2004 and for six large river basins, the present study compares monthly time series of multi-satellite-derived surface water extent with other independent global data sets related to land water dynamics, such as water mass variations monitored by GRACE, simulated surface and total water storage from WGHM, water levels from altimetry, and GPCP precipitation estimates. In general, the datasets show a strong agreement with each other at seasonal timescale. In particular, over the Amazon and the Ganges basins, analysis of seasonal phase differences and hysteresis behavior between surface water extent, water level and storage reveal the complex relations between water extent and storage variations and the different effects of water transport processes within large river basins. The results highlight the value of combining multi-satellite techniques for retrieving surface water

Microwave Radiometric Signatures of Different Surface Types in Deserts

In arid environments, specific microwave signatures have been observed with the Special Sensor Microwave /Imager (SSM/I). For a given diurnal change in surface skin temperature, the corresponding change in the microwave brightness temperature is smaller than expected. With the help of a one-dimensional, time-dependent heat conduction model, this behavior is explained by microwave radiation coming from different depths in the soil, depending on the soil type and on the microwave radiation frequency. Using the 8-times daily estimates of the surface skin temperature by the International Satellite Cloud Climatology Project (ISCCP) and a simple Fresnel model, collocated month-long time series of the SSM/I brightness temperatures and the surface skin temperatures give a consistent estimate of the effective microwave emissivity and penetration depth parameters. Results are presented and analyzed for the Sahara and the Arabian Peninsula, for July and November 1992. The case of the Australian desert is also briefly mentioned. Assuming a reasonable thermal diffusivity for the soil in desert areas, the microwave radiation is estimated to come from soil layers down to depths of at least five wavelengths in some locations. Regions where the microwave radiation comes from deeper soil layers also have large microwave emissivity polarization differences and large visible reflectances, suggesting that these areas correspond to sand dune fields. Two soil classification data sets show good correspondence of sand dunes and the microwave signature of significant penetration. This suggests that this analysis of microwave observations, along with other remote sensing technics, can be used to map the sand dunes in large, poorly surveyed deserts; a map of the sand dune fields in the Sahara and Saudi Arabia is derived from SSM/I observations.