The research described in this dissertation is predicted on the hypothesis that remotely sensed information on vegetation cover classes can be used to improve the representation of heterogeneous continental surfaces in global climate models. The problem it addressed was that current understanding of soil-vegetation-atmosphere interactions is considered only to be relevant to small plots of uniform vegetation with dimensions of the order 10-1000 m but, in order to provide realistic simulation of climate, General Circulation Models require description of such interactions for large areas of mixed vegetation with dimensions of the order 100-1000 km. The methods used to investigate this issue was to create and apply a coupled model that provided realistic representation of both surface and atmospheric boundary layer processes, and to use this model to simulate surface-atmosphere interactions with explicit representation of patches of vegetation on the one hand, and with a single, area-average representation of exchanges on the other. These modeling studies were given credibility by initiating and validating the coupled model using appropriate data from the FIFE site in Kansas and the ABRACOS site in Brazil. The results showed that when quite simple aggregation rules are used to derive the effective area-average values of the vegetation-related parameters, these parameters give adequate simulation of surface-atmosphere interactions. These aggregation rules were then applied using remotely sensed maps of land cover to derive parameter values. Significant differences were found in the resulting parameters, and in the surface energy fluxes and modeled climate calculated using those parameters. Thus, it has been shown that remotely sensed data can indeed be used to improve the representation of heterogeneous land surfaces in global climate models using the methods developed in this research, and that using these data significantly alters the simulated global climate.

A Snow-Atmosphere-Soil Transfer (SAST) model has been developed to extend the point snowmelt model to vegetated areas using the parameterization concepts of the Biosphere-Atmosphere Transfer Scheme (Dickinson et al. 1993). The model applications for short-grass and forest fields show that the simulated surface temperature, albedo, and snow depth have close agreement with observations. In addition, because of biases in simulated runoff in the high-latitudes, a Shuffled Complex Evolution (Sorooshian et al. 1993) scheme for automatic calibration has been connected with the SAST model to determine the realistic distribution of runoff components from different soil layers and search the optimized parameter set. The calibrated runoff closely matches observations. Because the Community Climate Model version 3 (CCM3) coupled with the SAST model overestimates snow depth and precipitation and underestimates surface temperature over the Rocky Mountains, remotely sensed snow depth data have been assimilated in the model to alleviate model discrepancies based on energy and mass balances. The improved surface temperature simulations result from the decreased snowmelt and albedo in winter and spring and from the weakened evaporation in summer due to drier soil. Meanwhile, modeled summer precipitation over the Rocky Mountains has a minor improvement. The relationship between the variations of tropical Pacific SST and snowpack anomalies in the western United States (U.S.) has been studied by comparing observations and CCM3 output. The results indicate that in the northwestern U.S., the warm tropical Pacific phase of the El Nino-Southern Oscillation (ENSO) is associated with diminished snowpack while its cool phase is related to enhanced snowpack. This relationship is largely determined by winter precipitation variability for the observations; however, it relies heavily on the variations of temperature due to the biases in atmospheric patterns for the model output. In the southwestern U.S., positive snowpack anomalies for both observations and simulations result from the strong warm phase of the ENSO and negative ones are connected with exaggerated local precipitation in fall.

The North American Monsoon System (NAMS) is an important climatological feature of much of southwestern North America because it is responsible for large portions of the annual rainfall in many otherwise arid and semi-arid environments. This dissertation explores issues related to numerical simulation of the North American Monsoon climate. Simulation studies using both an atmospheric general circulation model (AGCM) and a regional climate model (RCM), forced by model analyzed boundary conditions, are presented. The RCM was run for a single season with three different convective parameterization schemes for a single season to assess the sensitivity to convective representation. The main conclusion from these simulations was that substantial differences in both the time-integrated thermodynamic and circulation structures of the simulated July 1999 NAM atmosphere evolve in the simulations when different convective parameterization schemes (CPSs) are used. All simulations reproduced the maximum of precipitation along the western slope of the Sierra Madre Occidental. However, root mean squared errors and model biases in precipitation and surface climate variables were substantial, and showed strong regional dependencies between each of the simulations. There are large differences in the modeled monthly-total surface runoff between simulations. These differences appear to be more closely related to differences in local, precipitation intensity than to time-average or basin-average intensity. It was found that many features of the North American Monsoon were poorly simulated by the AGCM used in its current configuration when using a yearly repeating cycle of sea-surface temperatures. In particular, the model is unable to simulate the regional patterns of monsoon circulation and rainfall. Modeled rainfall over the southwest U.S. and Mexico is much too low, while tropical precipitation is overestimated. Anomalous sea-surface temperature forcing in the Pacific Ocean also induced model responses that resemble observed responses suggesting that sea-surface temperatures may play a modest role in establishing the monsoon circulation and hence in the generation of monsoon rainfall.

Optical depths at visible and infrared wavelengths obtained in Tucson, Arizona before and after the Pinatubo eruption in June 1991 have been used to investigate the characteristics of the stratospheric aerosols due to the Pinatubo eruption. The intrusion of the Pinatubo aerosols over Tucson first occurred on July 26, 1991 when the spectral optical depth values rose to two to four times their normal values. In general, there was a pattern of increase between June 1991 and April 1992, and a gradual decrease after April 1992. The stratospheric Pinatubo aerosol in April 1992 was characterized by a typical columnar total number density on the order of 8.78 x 106 in the size range of 0.2-0.7 μm. The total number density decreased to the order of 9.28 x 105 by April 1994. Simulations of the size distribution using a simple polydisperse coagulation and fallout model showed that both of the processes played a very important role in the evolution and transport of the particles in the interval from April 1992 to March 1993. A strong seasonal variation was observed in the aerosol optical depth data. The values are higher in the winter and spring and lower in the summer and fall. This variation is explained by more effective transport of particles from the tropics poleward in the winter and spring than in the summer and fall. We also observed that there was a reduction in stratospheric ozone associated with the Pinatubo aerosols, possibly because of the extra sites available for heterogeneous chemical reactions. The reduction was more noticeable in the spring and summer than in other seasons. The magnitude of the ozone reduction was in a good agreement with other studies.

Thirteen years of satellite-based total ozone measurements, extending from January 1979 through December 1991, are analyzed with a multiple regression statistical model to isolate the components of interannual variability associated with (1) linear trends and (2) the 11-year variation in solar ultraviolet irradiance. Lower stratospheric temperature and geopotential height data obtained from satellite- and ground-based sources are analyzed in similarly, providing a comprehensive assessment of the interannual variability in the lower stratosphere over the 1979-1991 period. The results of the statistical analyses indicate coherent variations in ozone, temperature, and geopotential height at extratropical latitudes in NH winter which are related to both the trend and solar-cycle components; the amplitudes of these variations exhibit pronounced spatial dependences. A three-dimensional mechanistic ozone transport model is used to describe the spatial distribution of total ozone in NH winter using observed lower stratospheric temperature and geopotential height fields. Application of this model on a year-to-year basis demonstrates that a large percentage of the observed interannual variability in the spatial distribution of total ozone is directly associated with changes in the dynamical structure of the lower stratosphere. The influence of dynamical variability on zonal mean total ozone is also investigated using an empirical approach. From the results of the observational and modeling studies, it is concluded that changes in the dynamics of the lower stratosphere over the 1979-1991 period have contributed significantly to the observed total ozone trends in the Northern Hemisphere. In contrast, the observed variability in total ozone associated with the 11-year solar cycle could not be explained in terms of a systematic variation in the dynamical forcing of the lower stratosphere in-phase with the 11-year cycle.

This research focuses on the numerical simulation of a rare, high-latitude hail storm observed during the CAGES (Canadian GEWEX Enhanced Study) field experiment. On 11 May 1999, a shortwave trough moved northward from British Columbia and continued its passage over the Northwest Territories. A hail storm developed in an environment of small convective available potential energy. To understand the processes responsible for the formation of the storm, the Canadian Mesoscale Compressible Community Model (MC2) is used to simulate the case. The addition of a second soil type to the lower boundary of the model allows for a realistic simulation of the location and time of the storm. The results indicate that the dynamics associated with the shortwave trough, coupled with diurnal heating effects over the more realistic soil type, produced the weakly-forced hail storm that passed directly over Fort Simpson, Northwest Territories.

The precipitations on meteorological scanning radar may comes from different altitudes and different process. The challenge for operational meteorology is to assess the part of this precipitation which will reach the ground and at what place. Many factors influence the difference between radar data and ground data: partial beam filling, attenuation and beam blocking, bright band enhancement, wind transport of the precipitation, growth or decay of the drops/flakes below the lowest elevation angle of the radar. An important case for operational meteorology is that of light snow aloft whose base has an horizontal slope toward the ground: "snow virgas". I will use the output of two vertical pointing radar in this thesis to find what happens in those trails and try to explain the influencing mechanisms. I will also describe an algorithm that attempts to predict the place where the snow will reach the ground using McGill University scanning radar. My study shows that the slope of the snow virgas is essentially due to the transport by winds in saturated airmasses and evaporation of flakes in unsaturated ones. Finally, finding the slope of the virgas toward the ground, by an automatic algorithm, is extremely difficult on a scanning meteorological radar due to its coarse resolution.

A parameterization that describes in-cloud oxidation of S(IV) by hydrogen peroxide and ozone, has been developed for use in large scale models. This parameterization, which is based on the reaction rate equations and basic cloud characteristics, is an explicit function of the concentration of ambient chemical species and some gross cloud parameters. Comparisons of the parameterization scheme with a well-established three-dimensional cloud chemistry model, and also with the cloud chemistry module of a regional model have been used to formulate and test this parameterization scheme. Results show that the parameterization agrees with the 3-D chemistry model very well and that the parameterization holds considerable potential for application in large-scale models. / Preliminary application in a regional climate model confirms that the parameterization is able to improve the agreement of the mass budget and spectrum distribution of sulphate aerosol with observations.

Bistatic Doppler radar networks have become in the last five years a viable and inexpensive alternative to multiple-Doppler networks. Operational experience with a bistatic network at McGill University showed many cases in which data quality seemed heavily affected. Study of those situations suggested sidelobe contamination from the transmitter antenna pattern to be the principal cause. / To confirm these findings a sidelobe simulation model (SISI model) was constructed. Comparison between simulations and actual data showed a good reproduction of the observed effect. It is also shown that this effect may have damaging consequences in Doppler fields in both convective and stratiform precipitation events. An index of contamination that can be obtained either with the SISI model or directly using the reflectivity bistatic data is introduced to detect areas of low quality data. Recommendations for the effective use of bistatic data are presented. / These findings are taken into account when the optimization of the layout of a bistatic network is analyzed. Sidelobe contamination was found to be a serious problem irrespective of the receiver's location. More than one passive receiver increases the extent of the dual Doppler area but unfortunately does not significantly reduce the problem of sidelobe contamination within a predetermined area. A rule-of-thumb for the deployment of a bistatic network is presented. Some suggestions for improvements of the network are given.

The purpose of this research is to further our understanding of the hydrostatic adjustment process due to diabatic heating. This is achieved by the use of an atmospheric model based on Euler's non-hydrostatic, fully elastic set of equations resolved by the semi-Lagrangian, semi-implicit marching scheme. Such a model thus allows a tri-dimensional, numerical approach of a problem which could, until now, only be addressed within a simplified context (one-dimensional and analytical; bi-dimensional and numerical). / The hydrostatic adjustment is the mechanism by which the atmosphere tends to reach a new hydrostatic balance when this one has been upset. This can be the case through a diabatic forcing, such as the sudden latent heat release during the formation of a thunderstorm cell. This is actually the kind of forcing we chose for this study. The problem of the hydrostatic adjustment is tackled through two different contexts: an idealized and a real case. Both are simulated by three different model's versions: fully compressible (PE), quasi-hydrostatic (QH) and quasi-anelastic (QA). These versions differ from each other only by the formulation of the diabatic heating terms. / The idealized case makes use of a prescribed heating source. A high resolution, in time and space, allows to show (PE version) how the mass, pressure and energy redistribution process is accomplished by the elastic, Lamb and gravity disturbances. The QH version, which consists in a spreading of the heating over the whole column above the source, inhibits vertically propagating elastic perturbations. The QA version, which considers the omission of the diabatic term in the pressure prognostic equation, shows that only the gravity mode is maintained. / The real case simulates a highly convective summer situation. The choice of a long time step brings out only the gravity mode; the hydrostatic adjustment by the fast modes (elastic and Lamb) is taking place quasi instantaneously. Thus this large-scale case doesn't exhibit significant differences between the PE, QH and QA versions.