Haroon Stephen

Microwave backscatter modeling of erg surfaces in the Sahara desert

The Sahara desert includes large expanses of sand dunes called ergs. These dunes are formed and constantly reshaped by prevailing winds. Previous study shows that Saharan ergs exhibit significant radar backscatter (/spl sigma//spl deg/) modulation with azimuth angle (f). We use /spl sigma//spl deg/ measurements observed at various incidence angles and f from the NASA Scatterometer (NSCAT), the SeaWinds scatterometer, the ERS scatterometer (ESCAT), and the Tropical Rainfall Measuring Missions Precipitation Radar to model the /spl sigma//spl deg/ response from sand dunes. Observations reveal a characteristic relationship between the backscatter modulation and the dune type, i.e., the number and orientation of the dune slopes. Sand dunes are modeled as a composite of tilted rough facets, which are characterized by a probability distribution of tilt with a mean value, and small ripples on the facet surface. The small ripples are modeled as cosinusoidal surface waves that contribute to the return signal at Bragg angles only. Longitudinal and transverse dunes are modeled with rough facets having Gaussian tilt distributions. The model results in a /spl sigma//spl deg/ response similar to NSCAT and ESCAT observations over areas of known dune types in the Sahara. The response is high at look angles equal to the mean tilts of the rough facets and is lower elsewhere. This analysis provides a unique insight into scattering by large-scale sand bedforms.

A Framework for Environmental Monitoring with Arduino-Based Sensors Using Restful Web Service

The Nevada Solar Energy-Water-Environment Nexus project generates a large amount of environmental monitoring data from variety of sensors. This data is valuable for all related research areas, such as soil, atmosphere, biology, and ecology. An important aspect of this project is promoting data sharing and analysis using a common platform. To support this effort, we developed a comprehensive architecture that can efficiently collect the data from various sensors, store them in a database, and offer an intuitive user interface for data retrieval. We employed Arduino-based sensors due to their flexibility and cost-effectiveness. Restful Web Service is used for communication with the Arduino-based sensors, and Google Charts service has been used for data visualization. This framework for sensor data monitoring with Web Service is expected to allow the Nevada Nexus project to seamlessly integrate all types of sensor data and to provide a common platform for researchers to easily share the data.

Multi-spectral analysis of the Amazon basin using SeaWinds, ERS, Seasat scatterometers, TRMM-PR and SSM/I

The Amazon basin represents a vast geographical zone containing large proportion of global biomass. We use the SeaWinds scatterometer (QSCAT), ERS-1/-2 scatterometer (ESCAT), NASA scatterometer (NSCAT) Seasat scatterometer (SASS), Tropical Rain Measuring Mission Precipitation Radar (TRMM-PR) and Special Sensor Microwave/Imager (SSM/I) data to study the multi-spectral microwave response of Amazon vegetation. Incidence angle signatures of combined backscatter measurements (/spl sigma//sup o/) from the scatterometers and precipitation radar indicate a good inter-calibration of the sensors. The multi-frequency signatures of both /spl sigma//sup o/ and radiometric temperature measurements (T/sub b/) from SSM/I are also studied. Temporal variability of the Amazon basin is studied using C-band ERS data and a Ku-band time series formed by SASS, NSCAT and QSCAT data. ESCAT data reveals a possible mismatch in the calibration of scatterometers between ERS-1 and ERS-2. Although the central Amazon forest represents an area of very stable radar backscatter measurements, portions of the southern region exhibit backscatter changes over the past two decades.

Relating temperature trends to the normalized difference vegetation index in Las Vegas

This research examines the relation between temperature trends and vegetation change in Las Vegas. A temperature time series is modeled as a superposition of a linear trend and an annual cycle. The model is used to estimate the multi-annual temperature rate of change, which is related to changes in vegetation cover. The change in vegetation cover is estimated using an annual average normalized difference vegetation index (NDVI). The model reveals a general trend of decreasing temperature in Las Vegas from 2000 to 2010. This decrease is less than 0.2 Kelvin per year (K/yr) in older housing developments; however, areas developed during the past decade (North Las Vegas and Southern Highlands) exhibit a decrease of greater than 0.2 K/yr. This temperature change has a correlation of −0.655 with changes in the annual average NDVI. Results reveal that in an arid environment, new housing developments that have an increase in vegetation have a cooling affect. The long-term trend reveals a warming trend. This research provides a useful insight into the effect of NDVI on temperature trends in Las Vegas.

Isoscapes of δ18O and δ2H reveal climatic forcings on Alaska and Yukon precipitation

Spatially-extensive Arctic stable isotope data are sparse, inhibiting the climatic understanding required to interpret paleoclimate proxy records. To fill this need, we constrained the climatic and physiographic controls on δ18O and δD values of stream waters across Alaska and the Yukon to derive interpolated isoscape maps. δ18O is strongly correlated to winter temperature parameters and similarity of the surface water line (δ2H = 8.0 × δ18O + 6.4) to the Global Meteoric Water Line suggests stream waters are a proxy for meteoric precipitation. We observe extreme orographic δ18O decreases and a trans-Alaskan continental gradient of -8.3 ‰ 1000 km−1. Continental gradients are high in coastal zones and low in the interior. Localized δ18O increases indicate inland air mass penetration via topographic lows. Using observed δ18O/temperature gradients, we show that δ18O decreases in a ∼24 ka permafrost ice wedge relative to the late Holocene indicate mean annual and coldest quarter temperature reductions of 8.9 ± 1.7°C and 17.2 ± 3.2°C, respectively. This article is protected by copyright. All rights reserved.

Modeling microwave emissions of erg surfaces in the Sahara desert

Sand seas (ergs) of the Sahara are the most dynamic parts of the desert. Aeolian erosion, transportation, and deposition continue to reshape the surface of the ergs. The large-scale features (dunes) of these bedforms reflect the characteristics of the sand and the long-term wind. Radiometric emissions from the ergs have strong dependence on the surface geometry. We model the erg surface as composed of tilted rough facets. Each facet is characterized by a tilt distribution dependent upon the surface roughness of the facet. The radiometric temperature (T/sub b/) of ergs is then the weighted sum of the T/sub b/ from all the facets. We use dual-polarization T/sub b/ measurements at 19 and 37 GHz from the Special Sensor Microwave Imager aboard the Defense Meteorological Satellite Program and the Tropical Rainfall Measuring Mission Microwave Imager to analyze the radiometric response of erg surfaces and compare them to the model results. The azimuth angle (/spl phi/) modulation of T/sub b/ is caused by the surface geometrical characteristics. It is found that longitudinal and transverse dune fields are differentiable based on their polarization difference (/spl Delta/T/sub b/) /spl phi/-modulation, which reflects type and orientation of dune facets. /spl Delta/T/sub b/ measurements at 19 and 37 GHz provide consistent results. The magnitude of /spl Delta/T/sub b/ at 37 GHz is lower than at 19 GHz due to higher attenuation. The analysis of /spl Delta/T/sub b/ over dry sand provides a unique insight into radiometric emission over ergs.

Study of iceberg B10A using scatterometer data

The Antarctic continent continually releases glacial ice into the ocean in the form of icebergs calving from glaciers and ice shelves. Microwave scatterometers can provide useful information about the spatial and temporal behavior of large icebergs. In this paper, results from the observation of B10A during 1992-2000, using ERS-1/2 AMI Scatterometer (ESCAT), NASA Scatterometer (NSCAT) and SeaWinds on QuikScat (QSCAT) data are presented. Multi-sensor analysis shows a general consistency of C-band /spl sigma//sup 0/ measurements from ESCAT and Ku-band /spl sigma//sup 0/ measurements from NSCAT and SeaWinds. Certain subtle differences are observed which reflect the frequency dependence of iceberg /spl sigma//sup 0/ measurements.

Climatic Change and Desert Vegetation Distribution: Assessing Thirty Years of Change in Southern Nevada's Mojave Desert

A major theme in physical geography and biogeography is understanding how vegetation changes across geographic gradients during climate change. We assess shifts in distributions of fifteen Mojave Desert plant species based on a 2008 resurvey of 103 vegetation transects that were established in 1979. We model changes in species distributions using Maximum Entropy (Maxent) with environmental and climate variables to predict probability of species’ occurrences. Climate during the ten-year period preceding the 2008 vegetation survey was 1.5°C warmer and 3 cm per year of precipitation drier than the ten years preceding 1979. Species inhabiting the highest elevations and strongly correlated with precipitation displayed areal reductions from 1979 through 2008.

Analysis of scatterometer observations of Saharan ergs using a simple rough facet model

The Sahara desert includes large expanses of sand dunes called ergs. These dunes are formed and constantly reshaped by prevailing winds. Previous study shows that Saharan ergs exhibit significant radar backscatter (sigmadeg) modulation with azimuth angle (Phi). We use sigmadeg measurements observed at various incidence angles (thetas) and Phi from the NASA scatterometer (NSCAT), the Seawinds scatterometer aboard QuikSCAT (QS-CAT), the ERS scatterometer (ESCAT) and the Tropical Rain Monitoring Missions Precipitation Radar (TRMM-PR) to model the sigmadeg response from sand dunes. Sand dunes are modeled as a composite of tilted rough facets and small ripples. The dune fields are modeled as composed of many simple dunes. The sigmadeg measured by the scatterometer from (thetas, Phi) look direction is the sum of the returns from all the rough facets in the footprint. The model is applied to linear and transverse dunes with rough facets and Gaussian tilt distributions. The model results in a sigmadeg response similar to the NSCAT and ESCAT observations over areas of known dune types in the Sahara. This analysis gives a unique insight into scattering by large scale sand bedforms

Spatial and Temporal Behavior of Microwave Backscatter Directional Modulation Over the Saharan Ergs

Radar backscatter (sigmadeg) from ergs is modulated with view direction [incidence (thetas) and azimuth (phi) angles], where the modulation characteristics reflect the surface geometry. sigmadeg also varies spatially and reflects the spatial inhomogeneity of the sand surface. We use sigmadeg measurements at different thetas and phi angles from the NASA, European Remote Sensing satellite, and SeaWinds scatterometers to understand the relationship between wind and erg bedforms. A model incorporating the sigmadeg phi-modulation and spatial inhomogeneity is proposed. Surface slope variations are related to the sigmadeg spatial inhomogeneity. We compare the backscatter model results with numerically predicted wind direction data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) over the erg surfaces. We use the maxima of the phi-modulation at thetas=33deg to infer the orientation of the dominant slip-sides on the sand surface. These orientations are consistent with the ECMWF wind directions spatially and temporally