B. J. Choudhury

Bio: B. J. Choudhury is an academic researcher from Goddard Space Flight Center. The author has contributed to research in topics: Desert climate & Normalized Difference Vegetation Index. The author has an hindex of 5, co-authored 5 publications receiving 134 citations.

Simulated and observed 37 GHz emission over Africa

Abstract: A model for simulating polarized 37-GHz microwave emissions from vegetated surfaces is proposed. An equation expressing the radiative transfer is solved using a two-point Gauss quadrature method. Surface temperature, reflectance, normalized difference, and polarized brightness temperatures are obtained from processed multispectral satellite data. The model simulations are evaluated against satellite data collected between January 1982 and December 1983 along a transect going from rain forest to hot desert over Africa (0 deg to 20 deg N, 11 deg E). It is found that the temporal variation of the average brightness temperature matches closely the surface temperature. The seasonal variation of the polarization difference is confined within the latitude band of 8 deg to 18 deg N, but the normalized difference continues to show seasonal variation south of 8 deg N. The relations between the polarization difference and the normal difference and between the polarization difference and the reflectance are considered to be nonlinear and almost linear, respectively.

Atmospheric effects on SMMR and SSM/I 37 GHz polarization difference over the Sahel

Abstract: The atmospheric effects on the difference of vertically and horizontally polarized brightness temperatures, Delta(T) observed at 37 GHz frequency of the SMMR on board the Nimbus-7 satellite and SSM/I on board the DMSP-F8 satellite are studied over two 2.5 by 2.5 deg regions within the Sahel and Sudan zones of Africa from January 1985 to December 1986 through radiative transfer analysis using surface temperature, atmospheric water vapor, and cloud optical thickness. It is found that atmospheric effects alone cannot explain the observed temporal variation of Delta(T), although the atmosphere introduces important modulations on the observed seasonal variations of Delta(T) due to rather significant seasonal variation of precipitable water vapor. These Delta(T) data should be corrected for atmospheric effects before any quantitative analysis of land surface change over the Sahel and Sudan zones.

Relating Nimbus-7 37 GHz data to global land-surface evaporation, primary productivity and the atmospheric CO2 concentration

Abstract: Global observations at 37 GHz by the Nimbus-7 SMMR are related to zonal variations of land surface evaporation and primary productivity, as well as to temporal variations of atmospheric CO2 concentration. The temporal variation of CO2 concentration and the zonal variations of evaporation and primary productivity are shown to be highly correlated with the satellite sensor data. The potential usefulness of the 37-GHz data for global biospheric and climate studies is noted.

Monitoring arid lands using AVHRR-observed visible reflectance and SMMR-37 GHz polarization difference

Abstract: Visible reflectance along a transect through the Sahel and Sudan zones of Africa has been derived from observations by the AVHRR on the NOAA-7 and NOAA-9 satellites and compared with concurrent observations of the 37-GHz polarization difference by the SMMR on the Nimbus-7 satellite. The study period was January 1982 to December 1986, which included an unprecedented drought during 1984 over the Sahel zone. While spatial and temporal patterns of these two data sets are found to be highly correlated, there are also quantitative differences which need to be understood.

Potential Net Primary Productivity in South America: Application of a Global Model

Abstract: We use a mechanistically based ecosystem simulation model to describe and analyze the spatial and temporal patterns of terrestrial net primary productivity (NPP) in South America. The Terrestrial Ecosystem Model (TEM) is designed to predict major carbon and nitrogen fluxes and pool sizes in terrestrial ecosystems at continental to global scales. Information from intensively studies field sites is used in combination with continental-scale information on climate, soils, and vegetation to estimate NPP in each of 5888 non-wetland, 0.5° latitude °0.5° longitude grid cells in South America, at monthly time steps. Preliminary analyses are presented for the scenario of natural vegetation throughout the continent, as a prelude to evaluating human impacts on terrestrial NPP. The potential annual NPP of South America is estimated to be 12.5 Pg/yr of carbon (26.3 Pg/yr of organic matter) in a non-wetland area of 17.0 ° 106 km2 . More than 50% of this production occurs in the tropical and subtropical evergreen forest region. Six independent model runs, each based on an independently derived set of model parameters, generated mean annual NPP estimates for the tropical evergreen forest region ranging from 900 to 1510 g°m-2 °yr-1 of carbon, with an overall mean of 1170 g°m-2 °yr-1 . Coefficients of variation in estimated annual NPP averaged 20% for any specific location in the evergreen forests, which is probably within the confidence limits of extant NPP measurements. Predicted rates of mean annual NPP in other types of vegetation ranged from 95 g°m-2 °yr-1 in arid shrublands to 930 g°m@ ?yr-1 in savannas, and were within the ranges measured in empirical studies. The spatial distribution of predicted NPP was directly compared with estimates made using the Miami mode of Lieth (1975). Overall, TEM predictions were °10% lower than those of the Miami model, but the two models agreed closely on the spatial patterns of NPP in south America. Unlike previous models, however, TEM estimates NPP monthly, allowing for the evaluation of seasonal phenomena. This is an important step toward integration of ecosystem models with remotely sensed information, global climate models, and atmospheric transport models, all of which are evaluated at comparable spatial and temporal scales. Seasonal patterns of NPP in South America are correlated with moisture availability in most vegetation types, but are strongly influenced by seasonal differences in cloudiness in the tropical evergreen forests. On an annual basis, moisture availability was the factor that was correlated most strongly with annual NPP in South America, but differences were again observed among vegetation types. These results allow for the investigation and analysis of climatic controls over NPP at continental scales, within and among vegetation types, and within years. Further model validation is needed. Nevertheless, the ability to investigate NPP-environment interactions with a high spatial and temporal resolution at continental scales should prove useful if not essential for rigorous analysis of the potential effects of global climate changes on terrestrial ecosystems.

Retrieval of land surface parameters using passive microwave measurements at 6-18 GHz

Abstract: An approach is evaluated for retrieval of land surface parameters (soil moisture, vegetation water content, and surface temperature) using satellite microwave radiometer data in the 6-18 GHz frequency range. The approach is applicable to data that will be acquired by the Advanced Microwave Scanning Radiometer (AMSR), planned for launch on the Japanese Advanced Earth Observing Satellite (ADEOS)-II and Earth Observing System (EOS) PM-1 platforms in 1999 and 2000, respectively. The retrieval method is based on a radiative transfer (RT) model for land-surface and atmospheric emission, with model coefficients that can be tuned over specific calibration regions and applied globally. The method uses an iterative, least-squares algorithm, based on six channels of radiometric data. Simulations using this algorithm indicate that, for an assumed sensor noise of 0.3 K in all channels, soil moisture and vegetation water content retrieval accuracies of 0.06 g cm/sup -3/ and 0.15 kg m/sup -2/, respectively, should be achievable in regions of vegetation water content less than approximately 1.5 kg m/sup -2/. A surface temperature accuracy of 2 C should be achievable, except for bare soils, where discrimination between moisture and temperature variability is difficult using this algorithm. These accuracies are for retrievals averaged over the sensor footprint, and they exclude conditions of precipitation, open water, snow cover, frozen ground, or high topographic relief within the footprint. The algorithm has been tested using data from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) for the years 1982-1985, over the African Sahel, and the retrieval results compared to output from an operational numerical weather prediction model.

Continental-scale models of water balance and fluvial transport: An application to South America

Abstract: A coupled water balance and water transport model (WBM / WTM) was constructed as part of a larger study of global biogeochemistry The WBM / WTM provides critical hydrologic information to models of terrestrial primary production, organic matter decay, riverine nutrient flux and trace gas exchanges with the troposphere Specifically, it creates high-resolution data sets for monthly soil moisture, evapotranspiration, runoff, river discharge and floodplain inundation As a first step toward eventual global coverage, the WBM / WTM was applied to South America, represented by more than 5700 1/2° (latitude / longitude) grid cells The WBM transforms spatially complex data on long-term climate, vegetation, soils and topography into predictions of soil moisture (SM), evapotranspiration (ET) and runoff (RO) For South America, field capacity in soils ranged from 27 to 582 mm of water, and computed values for mean annual SM, ET and RO were 284 mm, 1059 mm/yr and 619 mm/yr, respectively There were large differences regionally and over the year The transport model uses WBM-derived runoff, information on fluvial topology, linear transfer through river channels and a simple representation of floodplain inundation to generate monthly discharge estimates for any cell within a simulated catchment The WTM successfully determined the timing and magnitude of discharge at selected locations within the Amazon / Tocantins basin It also demonstrated the importance of floodplain inundation in defining flow regime on the mainstem Amazon Estimated mean annual discharge was 207,000 m3/s for the Amazon River and 17,000 m3/s for the Tocantins In these basins, 45% of the incident precipitation emerges as river flow; 55% is lost to ET The model described in this paper will be expanded to include the dynamics of carbon, major nutrients and sediments It will serve as a semimechanistic tool to quantify the transport of materials from the landscape to the world's oceans Such a capability becomes increasingly important as we seek to understand the impacts of climate and land use change on major river systems of the globe

Micrometeorology of Temperate and Tropical Forest

Abstract: The literature on the micrometeorology of temperate and tropical forests is reviewed to determine whether structural or species difference between these biomes alters their interaction with the atmosphere. Considerable consistency is found in the value of those whole-canopy features of most importance to this interaction, namely solarreflection coefficient, through-canopy radiation absorption, aerodynamic roughness, the symptoms of near-surface K-theory failure, the canopy store for rainfall interception and the magnitude and environmental response of their bulk stomatal (surface) resistance. Typical values of these parameters and functions are given with a view to their potential use in climate simulation models. Attention is drawn to the fact that this similar micrometeorological response can generate different timeaverage surface-energy partitions when interacting with different climates and, in particular, alters between the edge and the middle of continents. This is of considerable significance, implying tropical deforestation is likely to have most effect on river flow (though not climate) at continental edge and island locations. The similar micrometeorological response of forests is interpreted as the necessary consequence of energy and mass (water) conservation acting as an area average on vegetation that is, by definition, dense and extensive, to reconcile a characteristically tall growth habit with a perennial nature.