After decades of exploration, the martian neutral hydrogen exosphere has remained poorly characterized. The first measurements of this layer by Mariner 6 and 7 revealed it to be optically thick in Lyman α emission, along with a characteristic temperature that was higher than the majority of the collisional atmosphere of Mars. Further exploration revealed that the hydrogen in the martian exosphere was formed from photodissociation of water vapor by solar UV light, and that its escape can be directly linked to the escape of water from Mars. Theoretical analysis of hydrogen transport in the martian atmosphere suggested a steady escape rate limited by diffusion of hydrogen through the martian atmosphere. Subsequent missions to Mars provided a wide range of values for the temperature and density of hydrogen at Mars. It is important to determine the properties of the martian hydrogen exosphere in order to constrain the escape flux, which can then be used to calculate the total amount of water lost by Mars during its evolutionary history. In this dissertation the characteristics of the martian hydrogen exosphere are constrained using data from the Hubble Space Telescope (HST). HST observations of this layer reveal short-term seasonal changes, thereby disproving the theory of constant escape rate for H from Mars. Analysis of these datasets using a radiative transfer model constructed at Boston University revealed a large seasonal variation in the hydrogen escape flux, making it difficult to easily backtrack the martian water loss history. Results also indicate the possible presence of a superthermal population created by non-thermal processes at Mars. Exploration of the latitudinal symmetry of the martian exosphere indicates that it is symmetric above 2.5 martian radii and asymmetric below this altitude, which could be due to temperature differences between the day and night side. Finally, there are large uncertainties in determining the characteristics of the martian exosphere after decades of exploration, due to various assumptions about the intrinsic characteristics of the martian exosphere in the modeling process, degeneracy in the two modeling parameters for hydrogen -i.e. its temperature and density, unaccounted seasonal effects and uncertainties introduced from spacecraft instrumentation and viewing geometry.

The effects of ontogeny and environmental oscillations on plant responses to oxygen deprivation were investigated for a wide spectrum of species, ranging from agricultural to forestry crops, and from temperate to tropical plants. The extent to which hypoxia- or anoxia-tolerance were affected by ontogeny and environmental oscillations, was assessed mainly through changes in survival and growth and physiological parameters such as respiratory activity, ethanol production and carbohydrate depletion. Anoxia-tolerance of germinating seeds of chickpea (Cicer arietlnum L.) was found to vary, even within the earlier stages of germination, according to the length of the aerobic imbibition period previous to the anoxic shock. The notable fact was that 6 minutes of seed aerobic imbibition prior to anoxic treatment was sufficient to significantly increase post-anoxic survival after 4 days anoxia, compared to seeds not allowed to previously imbibe aerobically. These survival results were mirrored by the significant increase in the oxygen uptake by the embryos of seeds which were allowed to imbibe aerobically for 2 hours prior to anoxia, compared to the embryos of anaerobically imbibed seeds. Germination stage also affected the response of barley (Hordeum vulgare L.) to washing under anoxia, a treatment used to investigate membrane stability and other factors associated with anoxic injury. Temperature, frequency of washing and nutrients present in the washing solution also affect the post-anoxic responses of barley. Daily washing of seedlings under anoxia was often detrimental to post-anoxic survival. One washing only, at the end of the anoxic period, often enhanced survival. This positive effect seemed to be more linked to protection against plasmamembrane leakage due to calcium ions present in the washing solution than to removal of anaerobically-produced potentially toxic volatiles. Two Brazilian tree species were also studied. The seeds of Parkia pendula (Willd.) Benth. ex. Walp., a species typically present in unflooded areas in the Amazon, were still able to germinate after seven months submergence. P.pendula one-month old seedlings, however, did not survive longer than one month flooding, which can be a considerable disadvantage in the ca. six-month long flooding period of the Amazon floodplains. However, adult trees can still be found, although rarely, in flooded areas. The possible strategies involved in an eventual establishment of P.pendula individuals in flooded areas of the Brazilian Amazon are discussed. A contrast is drawn between the responses to flooding of this species and flood-tolerant Parkia discolor. Enterolobium contortisiliquum (Veil.) Morong, is a tree species which is present in both the dry soils of the cerrados (neotropical savannas) in Central Brazil and in the flood-prone Gallery Forests. This species showed considerable tolerance to flooding and drought, as reflected by the various morphological and metabolic adaptations observed in response to these stresses. The role of the xylopodium, a rigid wood tuber, in such tolerance to flood and drought stresses is discussed. Water-stressed roots of some crop species studied presented higher ethanol levels than control plants. Additionally, these same drought-treated roots showed a considerable amount of shrinkage compared to control roots, as measured by root diameter. It is argued that drought causes root shrinkage, which possibly reduces ability of such roots to capture oxygen and results in hypoxia in the tissues and consequent increase in ethanol production. This hypothesis of drought-induced hypoxia is compared with several recent findings in the literature, and is discussed as a possible factor which allows drought, under specific circumstances, to acclimatise plants to a subsequent flooding. From preliminary experiments with alternation of flooding and drought in Eucalyptus species, it appeared that a previous stress affects a plant's response to a subsequent stress. This effect was not always negative, and in E.regnans a five-week drought allowed a subsequent 3-fold increase in flooding survival. This experiment, however, needs to be repeated in order to confirm these results. A common cause for anaerobic injury seems to be unlikely for the diverse plants studied, and anoxia survival often seemed to be related to a combination of morphological and metabolic adaptations. A critical reflection on the risks of labelling plants as tolerant or sensitive to oxygen deprivation is provided, as well as a discussion on the perspectives of applied research which may further the development of ecophysiological theory.

Supervisor: Professor Robert W. Stewart The process by which the wind makes sea waves grow is not well-understood, partly because of the lack of adequate observational information on the normal pressures which transfer energy to the waves. The principal object of this experiment has been to provide some of the missing data. A system for making simultaneous measurements of normal pressure and wave height was developed and tested in the laboratory and in the field. The system consisted of a disc-shaped buoy 23 cm in diameter (in which was embedded the pressure sensor) which rode up and down on a vertical rod, which was the wave sensor. Careful attention was paid to rejecting so-called "dynamic" pressures associated with the distortion of the air flow by the buoy. The results from the experiment are presented as power and cross-spectra of the pressure and wave signals. Spectra of Energy (E) and Momentum (τω) fluxes to the waves, and of ζ, the fractional energy increase of the waves per radian, are also presented. Wave power spectra are found to be normal for the site; the pressure power spectra consist of a "basic" spectrum similar to that observed over land, on which is superimposed a wave-induced "hump". The phase angle between the waves and the pressure at the frequency of the peak of the wave spectrum is found to be shifted from -180° (pressures high over wave troughs) by amounts which exceed the theoretical predictions of Miles (1957) by an average of 20 ± 5° over a wide range of conditions. The Ė and τω spectra are found to be sharply peaked at or above the frequency of the peak of the wave spectrum. The integrated energy fluxes Ē show large scatter, indicating that the wave generation process varies considerably in time (and space). The integrated momentum fluxes τω to the waves show no significant difference from total fluxes from air to water computed assuming a constant drag coefficient of 1.2 x 10⁻³; it appears that about 80% of the total drag of the water on the wind is caused by the wave generation process. The ζ spectra exceed the predictions of Miles' (1957) theory by factors of 5 to 8, indicating that his "inviscid laminar" model is not adequate to explain observed rates of wave growth. The present results fall close to an empirical curve suggested by Snyder and Cox (1966) except at high frequencies, where they are considerably lower. A dimensionless plot of ζ versus the ratio of wind speed to wave speed is presented; the observed data is fitted by the simple relation [formula omitted], where [formula omitted]is the ratio of the densities of air and water, U₂ is the mean wind speed at a height of two meters, and c is the phase velocity of the waves. This formula is only considered applicable for U₅/c < 6, where U₅ is the mean wind speed at 5 meters height. Also presented are the results of a dry-land comparison of the buoy pressure sensor with two other pressure sensors; besides indicating that the buoy sensor was adequate, this comparison produced some interesting preliminary information on the vertical and horizontal structure of the turbulent pressure field in the atmospheric boundary layer. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This dissertation presents the Imaging Wind and Rain Airborne Profiler (IWRAP), the first high-resolution dual-band airborne Doppler radar designed to study the inner core of Tropical Cyclones (TCs). IWRAP is usually operated from a National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft during missions through TCs and severe ocean storms. The system is designed to provide high-resolution dual-polarized C- and Ku-band reflectivity and Doppler velocity profiles of the atmospheric boundary layer (ABL) within the inner core precipitation bands of TCs and to study the effects precipitation has on ocean wind scatterometry as it applies to TCs. A summary of the principles of operation and the design of the instrument is given, with an emphasis on the unique digital receiver. Airborne ocean backscatter measurements at C- and Ku-band wavelengths and HH and VV polarizations obtained in moderate to very high wind speed conditions (25–65 m·s−1) are presented. The differences between these measurements and current geophysical model functions (GMFs) are reported. The impact of these results on satellite-based scatterometry is discussed, and their application is illustrated through the reprocessing of QuikSCAT passes with the new GMFs. The use of dual wavelength techniques to determine differential attenuation from IWRAP's dual band reflectivity measurements, as well as to derive rainfall rates and drop-size distribution parameters is also addressed. In addition, results on the use of the radar Doppler velocity measurements to derive the three-dimensional (north, east and vertical) components of the ABL winds within rainbands of TCs are presented. Validation of these results is performed by comparison against other available datasets. IWRAP's unique imaging capability provides, to our knowledge, the highest-resolution measurements of the ABL winds of a hurricane ever obtained.

As numerical models of turbulent flow in the Atmospheric Boundary Layer (ABL) improve in their temporal and spatial resolution, high-resolution remote sensing measurements become increasingly important for validation and improved understanding. The Turbulent Eddy Profiler, a recently developed UHF imaging radar, provides a tool both to observe the three dimensional structure of clear-air scattering from refractive index turbulence with fine spatial and temporal resolution and to explore the limits to this resolution. This dissertation focuses on the space-time statistics of the clear-air radar signal and their implications on measurements of winds, temperature, and spatial coherence. A theoretical analysis of the statistics of the radar echo is presented, resulting in a general expression of the space-time autocorrelation of the radar signal. This expression is later used in the analysis of wind profiling techniques, where Spaced Antennas (SA) algorithms are compared to Doppler Beam Swinging (DBS) methods. The relation between SA and DBS methods is studied, leading to the conclusion that DBS methods are better suited for use with imaging radars where beams may be electronically steered. The implementation of a TEP-based Radio Acoustic Sounding System (RASS) to retrieve temperature fields is described. Using TEP, the spatial structure of RASS echoes using a UHF radar is revealed for the first time. Three-dimensional RASS-measurements of virtual temperature are used to explicitly calculate structure functions, and to retrieve profiles of the structure function parameter of virtual temperature. The stream-wise spectral coherence of the vertical wind field in the convective boundary layer is studied using TEP data. Results are compared to theoretical curves that include the effect of volume averaging inherent to radar data, and to Large Eddy Simulation data. There is scant experimental validation of these relationships. Experimental results are found to be in good agreement with both simulations and theory under convective conditions. This promising result suggests multiple beam radars may study spatial coherence under neutral or stable conditions.

This dissertation presents the design and operation of a high-resolution frequency-modulated continuous-wave (FM-CW) radar system to study the structure and dynamics of clear-air turbulence in the atmospheric boundary layer (ABL). This sensitive radar can image the vertical structure of the ABL with both high spatial and temporal resolutions, and provide both qualitative information about the morphology of clear-air structures and quantitative information on the intensity of fluctuations in refractive-index of air. The principles of operation and the hardware and data acquisition characteristics of the radar are described in the dissertation. In October 1999, the radar participated in the Cooperative Atmosphere-Surface Exchange Study (CASES'99) Experiment to characterize the temporal structure and evolution of the boundary-layer features in both convective and stable conditions. The observed structures include clear-air convection, boundary layer evolution, gravity waves, Kelvin-Helmholtz instabilities, stably stratified layers, and clear-air turbulence. Many of the S-band radar images also show high-reflectivity returns from Rayleigh scatterers such as insects. An adaptive median filtering technique based on local statistics has, therefore, been developed to discriminate between Bragg and Rayleigh scattering in clear-air radar observations. The filter is tested on radar observations of clear air convection with comparison to two commonly used image processing techniques. The dissertation also examines the statistical mean of the radar-measured [special characters omitted] for clear-air convection, and compares it with the theoretical predictions. The study also shows that the inversion height, local thickness of the inversion layer, and the height of the elevated atmospheric layers can be estimated from the radar reflectivity measurements. In addition, comparisons to the radiosonde-based height estimates are made. To examine the temporal and spatial structure of [special characters omitted], the dissertation presents two case studies with the measurements of remote (the FM-CW radar and Doppler lidar) and in-situ (research aircraft, kite, and radiosonde) sensors from the stable nighttime boundary layer. It also presents a unique observation of evolution of the convective and nocturnal boundary layers by the S-band radar, and provides description of the observed boundary layer characteristics with the aid of in-situ measurements by the 55m instrumented tower and radiosonde.

This dissertation analyzes the retrieval of water vapor profiles via microwave radiometry; in particular it determines the information content of spectral data and identifies optimal measurement frequencies using an information content technique. The vertical resolution and estimate variance of water vapor profiles derived from the linear inversion of atmospheric data is examined and the effects of measurement noise on these quantities is considered. The Microwave Remote Sensing Laboratory at the University of Massachusetts has developed a unique K-Band Spectral Radiometer (KSR) system that simultaneously monitors eighteen frequencies near the 22.235 GHz water vapor absorption line and is designed to retrieve atmospheric water vapor density profiles by inverting spectral radiance measurements. This system is unique in its measurement speed and breadth. The dissertation discusses calibration techniques, system parameters, and the derivation of a statistical estimation algorithm is that is applied to KSR measurements taken during a field experiment in Lamont, Oklahoma. The resulting water vapor profiles are presented, along with a comparison of in-situ and independent observations.

The Microwave Remote Sensing Laboratory (MIRSL) at the University of Massachusetts, in collaboration with The Jet Propulsion Laboratory, has developed a new radar designed to facilitate measurements of the radiative properties of clouds. Details of this new design are described with particular emphasis on improvements from previous systems. This radar system was used to collect cloud data during three experiment campaigns. During these experiments, reflectivity data from all prevalent cloud types were collected over a wide range of geographical locations. The observations were then used to examine the reflectivity vs. altitude and temperature characteristics and layer structure for various types of cloud complexes. To increase the representation of tropical cirrus clouds in the composite data set, the airborne data was supplemented with data from MCTEX collected by the UMass CPRS radar. All observations were classified into four classes of clouds and histograms of altitude and temperature vs. reflectivity were used to demonstrate the reflectivity characteristics of various clouds types. Statistics of layer base and top altitudes, thickness and number of layers were also computed. Also, the relationship between cirrus cloud thickness, reflectivity and ice water path (IWP) is examined. The data sets from the four experiments were then used to address performance issues for a spaceborne radar. The problem of cloud detectability is discussed and an analysis of the ice water content (IWC) estimation error resulting from spatial inhomogeneity is presented. The fraction of clouds thinner than one range gate of the CloudSat radar was found to be 14% for all data sets combined. The data sets are used to simulate satellite radar pixels and the distributions of errors in IWC estimates due to inhomogeneity are calculated. On average, 40% of the pixels were partially filled and the relative IWC error was 24%. The distribution of the relative errors vs. IWC values indicated that the largest relative error occurred at vary small values of IWC and the mean error for all experiments was only 15% for IWC values larger than 10 –3 gm3.

Explosive volcanism can release large amounts of particles and gases into the atmosphere. Sulfuric acid droplets in the lower stratosphere are the primary substance interacting with the radiative fluxes over many months and possibly years. Because of their sub-micron size, they are more efficient at scattering incoming shortwave radiation from the sun back into space than absorbing and trapping longwave radiation from the earth. This results in a negative impact on the earth energy balance causing a general cooling below the aerosol layer. The magnitude of the cooling depends mostly on the amount of radiatively active aerosol particles as well as the duration of the perturbation. The cooling signal is largest in the upper troposphere through feedbacks with a slowed hydrologic cycle. At the surface, heat release, mostly from the oceans, can buffer some of the cooling. A combined approach using both observations/proxy data and a state-of-the-art coupled General Circulation Model (GCM) to analyze the volcanic effects on climate can help in our understanding of the possible range of responses. Here, the most recent large eruption of Mt. Pinatubo (June 1991) was used to verify the implementation of the aerosol parameterization in the radiation code of the GCM. From there, an analysis of the volcanic contribution since 1870 A.D. was performed. A simple way of describing the spatial aerosol distribution is presented. In general, only a handful of eruptions were found large enough to significantly perturb the radiative balance of the earth. These few events caused a global climate signal, which is clearly detectable against the background noise of internal variability of the climate system. Next to the influence of isolated events, model simulations confirm earlier suggestions that temporally closely spaced large events can cause a further cooling in climate before the system can recover. Thus, explosive volcanism must be regarded as an important player in decadal to multi-decadal natural climate variations. In case of the 20th-Century, volcanic cooling in the last decades could have offset any possible warming due to increased solar irradiation. The potential volcanic role in other important time periods in the past must be studied, including the cooling in decades generally solely attributed to the sun. Proxy records, in particular tree rings, point to a potentially large role of explosive volcanism in the past. Additionally to the radiative effects, increased atmospheric flow at high latitudes, particularly in winter, is the result of dynamical responses to changes in meridional temperature gradients in the lower stratosphere through heating in the aerosol layer. This effect helps to orchestrate the spatial distribution of the climate signal for several years after the eruption. Currently, no clear influence on other internal modes of variability, such as El Niño, could be unanimously confirmed in both observations/proxy reconstructions and the model simulations. But more work is needed, as better proxy climate data for earlier large events get available.

Bibliography: pages 143-154. / The relationships between interannual sea surface temperature variability (SST) of the oceans surrounding southern Africa and summer rainfall variations over South Africa are investigated using statistical, observational and mechanistic approaches. Positive correlations are identified between summer rainfall and SSTs of the Mozambique/ Agulhas Current region, the Agulhas Retroflection region and the northern Benguela Current system. These relationships are stronger when rainfall anomalies associated with the Southern Oscillation are not considered. The observation of significant lag relationships involving temperatures of the Agulhas Current system suggests that future prediction efforts for summer rainfall will benefit from a consideration of SST anomaly patterns east and south of Africa, in combination with other atmospheric indices. Surface winds, heat fluxes and atmospheric boundary layer characteristics are investigated using compositing analyses to assess pertinent ocean-atmosphere coupling mechanisms. Easterly wind anomalies across the southwest Indian Ocean and over source regions of the Agulhas Current accompany and precede the local oceanic "warm events" which correspond with higher rainfall. Thus a class of event is identified in which warmer waters along the east coast and stronger easterly wind forcing accompany wetter seasons over South Africa. The atmospheric boundary layer is considerably warmer and moister in association with positive SST anomalies along the east coast and increased tropical airflow. Consequently, moisture convergence and tropical convection are increased over the eastern interior near 20° to 25°S. South of Africa, positive SST anomalies generate surface heat flux anomalies, increasing instability and moisture levels within the boundary layer. Horizontal heat flux gradients are strengthened across the Agulhas/Subtropical Convergence SST front and the conditions necessary for cyclogenesis and westerly wave amplification are optimized. Tropical-temperate troughs account for most of the abnormal rainfall during local Agulhas "warm events". The presence of positive SST anomalies east and south of southern Africa increases the likelihood of their formation by intensifying tropical and temperate components. The contribution provided by each component is influenced by the position and magnitude of the SST anomaly as well as the season of occurrence. Conceptual models are presented which summarize the most important ocean-atmosphere coupling mechanisms associated with rainfall variations of southern Africa.