Low field electron transport in a GaAs/GaAlAs superlattice

This thesis presents some results of low field mobility and Hall factor calculations in a GaAs/Gao.7Alo.3As superlattice. There is much experimental evidence that for a superlattice with a large enough miniband width, the electron transport proceeds by extended Bloch states and consequently the Boltzmann transport formalism is used. For these calculations polar optical phonon scattering and interface roughness scattering are considered. Early calculations assumed the vibrational modes of a superlattice to be unperturbed by the superlattice structure. However the optical vibrational modes of a thin layer structure deviate strongly from those of the corresponding bulk materials. This effect is included in these calculations by using the dielectric continuum model and is found to increase the predicted mobility by up to a factor of two. This predicted polar optical phonon limited mobility is however, much larger than the experimentally measured mobility. By including interface roughness scattering agreement with experiment is significantly improved. For a superlattice grown without growth interruptions at the interfaces the interface roughness scattering dominates electron transport in the growth direction and is of similar importance to polar optical phonon scattering for transport parallel to the layers. The effect of growth interruptions is investigated and is found to reduce interface roughness scattering. Finally the Hall factors are calculated, the superlattice has two independent Hall factors both of which remain close to one

Data Assimilation Enhancements to Air Force Weathers Land Information System

The United States Air Force (USAF) has a proud and storied tradition of enabling significant advancements in the area of characterizing and modeling land state information. 557th Weather Wing (557 WW; DoDs Executive Agent for Land Information) provides routine geospatial intelligence information to warfighters, planners, and decision makers at all echelons and services of the U.S. military, government and intelligence community. 557 WW and its predecessors have been home to the DoDs only operational regional and global land data analysis systems since January 1958. As a trusted partner since 2005, Air Force Weather (AFW) has relied on the Hydrological Sciences Laboratory at NASA/GSFC to lead the interagency scientific collaboration known as the Land Information System (LIS). LIS is an advanced software framework for high performance land surface modeling and data assimilation of geospatial intelligence (GEOINT) information

The 2019-2020 Australian drought and bushfires altered the partitioning of hydrological fluxes

Though coarse in spatial resolution, the nearly all weather measurements from passive microwave sensors can help in improving the spatio‐temporal coverage of optical and thermal infrared sensors for monitoring vegetation changes on the land surface. This study demonstrates the use of vegetation optical depth (VOD) retrievals from the Soil Moisture Active Passive mission for capturing the vegetation alterations from the recent 2019 to 2020 Australian bushfires and drought. The impact of vegetation disturbances on terrestrial water budget is examined by assimilating the VOD retrievals into a dynamic phenology model. The results demonstrate that assimilating VOD observations lead to improved simulation of evapotranspiration, runoff, and soil moisture states. The study also demonstrates that the vegetation changes from the 2019 to 2020 Australian drought and fires led to significant modifications in the partitioning of evaporative and runoff fluxes, resulting in increased bare soil evaporation, reduced transpiration, and higher runoff