In an effort to attain a quantitative understanding of the temporal and spatial variability of the tropical radiation budgets and the cloud-radiative forcing, a complete annual cycle of measurements by the Earth Radiation Budget Satellite scanning radiometer is analyzed. / Results show the existence of strong diurnal variation in the longwave emission over land and desert scenes and in the reflected shortwave radiation for all scenes. A bias in the Earth Radiation Budget Experiment scene identification algorithm against clear tropical scenes at night is evident in the longwave diurnal variation results. Substantial east-west variation is exhibited in the radiation and the cloud forcing components for some latitude zones. The largest seasonal changes in the radiation and the cloud forcing components are associated with the Asian summer monsoon and the migration of the Intertropical Convergence Zone. Cloud forcing results show a strong dependence of net forcing on cloud type, with small values for convective clouds and large negative values for stratiform clouds.

The importance of the role of halogen chemistry in regulating tropospheric ozone budgets was revealed in the Arctic Marine Boundary Layer (MBL) during Polar Sunrise Studies where episodic destructions of ozone were accompanied by increases in bromine containing compounds. Chlorine chemistry becomes especially relevant in the tropospheric MBL and in coastal regions due to the existence of elevated concentrations of chlorine generated from sea-salt aerosols. In this thesis, the kinetic study of the reactions of seven tropospherically important hydrocarbons with chlorine atoms was done at atmospheric pressure and 298 < T < 358 K using a Gas Chromatograph with Flame Ionization Detection. This data set not only compares well with available literature values but also provides a first report on several systems for which no temperature dependent kinetic data existed. Some of these reactions were found to be fast enough to compete with HO radical chemistry in the MBL. Mass spectral product studies of Cl-initiated reactions of CHBr2Cl and CHBr3 indicated that halogen atoms could be recycled back to the atmosphere. The importance of these reactions on the chemistry of the troposphere is discussed.

A climatology of high wind events (HWEs) affecting Sable Island (44°N, 60°W) and Halifax (44.5°N, 63.5°W) was constructed for the period 1979--1995. We then focussed on HWEs at Sable Island in more detail because of their high frequency relative to Halifax. Events were stratified into four groups based on the direction of the peak speed: NE (1°--90°), SE (91°--180°), SW (181°--270°), and NW (271°--360°). Synoptic structures and statistically significant atmospheric anomalies were identified in composites for each group. More detailed structures were found in composites constructed with the aid of a cluster analysis. / NW HWEs were associated with rapidly deepening marine cyclones and lacked clear atmospheric predecessors. NE HWEs were linked to a slow moving offshore cyclone and prominent anticyclone near Labrador. An anomalously weak Icelandic Low was a dominant precursor signal and an important feature found in each of the groups. SE HWEs were related to the strong pressure gradient between a cyclone-anticyclone couplet. The anticyclone was evident four days before the HWE and played a crucial role in the development of the cyclone. SW HWEs were related to a variety of cyclone types, but were typically related to a low-frequency cold surge over eastern North America. / The storms most difficult to predict may be NW HWEs, since they lack clear atmospheric precursor signals. They are also among the most dangerous storms affecting Atlantic Canada, since they move and develop rapidly, and have some of the highest wind speeds.

The Canadian Regional Climate Model (CRCM) has been used to investigate the effects of Gulf of Mexico sea surface temperature (SST) anomalies on the regional climate. Three sets of experiments have been performed, each consisting of a control run and with two different imposed Gulf SST anomalies. The first is a uniform increase or decrease of the SST by 5K over the entire Gulf. The second and third experiments use 5K SST anomalies of smaller extent, characteristic of warm core rings shed off the Loop Current in the Gulf. The experiments are carried out for either 15 or 30 days, and statistics are computed after discarding the first 5 days of the integration. / The response of the SST anomalies are qualitatively similar in the three cases, except the response to the anomaly over the entire Gulf is stronger due to the much larger extent of the anomaly. For a positive SST anomaly, precipitation and moisture over the Gulf and southeastern U.S. both increase. The 1000mb temperature field shows a clear warming over the Gulf and adjacent areas, delineating the imposed SST anomaly. A low level cyclonic circulation forms over the Gulf and southeastern U.S. region, while an anticyclonic circulation develops at the upper levels. The negative SST anomaly experiments show a qualitatively similar response, except it is of opposite sign with a smaller magnitude.

The relationship between Arctic sea ice concentration anomalies, particularly those associated with the "Great Salinity Anomaly" of 1968-1982, and atmospheric circulation anomalies is investigated. Empirical orthogonal function (EOF) analyses are performed on winter and summer sea ice concentrations, sea-level pressure, 500 hPa heights and 850 hPa temperatures: these data cover the Northern Hemisphere north of 45$ sp circ$N during the post-World War II era. Spatial maps of temporal correlation coefficients between EOF 1 of winter sea ice concentration and the atmospheric anomaly fields are calculated. Significant correlations (at 95 and 99% levels) were found to exist between EOF 1 of winter sea ice and the atmospheric anomaly fields at zero lag, and with ice leading by one and one-and-a-half years, and ice lagging by one year. The main emphasis of the thesis is to identify connections between Arctic sea ice and atmospheric circulation anomalies at interannual timescales.

The various modes of atmospheric mass redistribution characterize the principal variations of the general circulation of the atmosphere. Interhemispheric exchanges of atmospheric mass occur with considerable regularity on intraseasonal time-scales. Observational evidence from previous studies indicates that anomalous and persistent regional atmospheric mass distributions (e.g., atmospheric blocking) may often be related to interhemispheric atmospheric mass exchange. / Using the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis, we identify significant events when the northern hemisphere (NH) loses dry atmospheric mass on subseasonal time-scales during the boreal winter from 1968 to 1997. A total of 25 events is found, with a preferred time-scale of 9 days. The linear correlation coefficient between the dry atmospheric mass anomalies for the NH and southern hemisphere (SH) is -0.91 for the 25 significant events, indicating very strong interhemispheric compensation and increasing our confidence in the suitability of this dataset for the study of interhemispheric dry atmospheric mass exchange. / Positive sea-level pressure anomalies are found over northern Eurasia, the North Pacific and the North Atlantic prior to the onset of the composite NH dry atmospheric mass collapse event. Over northern Eurasia the positive atmospheric mass anomaly associated with the building of the Siberian high is found to be a statistically significant precursor to the events. The breakdown of NH dry atmospheric mass occurs in association with the decay of the positive atmospheric mass anomaly in the North Pacific as a cyclone deepens explosively in the Gulf of Alaska. Pressure surges over Southeast Asia and North America, associated with statistically significant positive atmospheric mass anomalies, are mechanisms that act to channel the atmospheric mass equatorward on a rapid time-scale (~4 days). The dry atmospheric mass increase in the SH is manifested as enhanced surface ridging over the South Pacific and South Indian Oceans. / Preferential interhemispheric interaction is found in the region between 100°E and 130°E, and over the central Pacific in the vicinity of the dateline. A prominent channel of southeastward dry atmospheric mass flux, emanating from the Australian continent, combined with a southward channel from the equatorial central Pacific, contribute to the atmospheric mass buildup over the South Pacific. / The role of a Southeast Asian pressure surge was examined for a representative event. A large evacuation of atmospheric mass from northern Eurasia occurs as the atmospheric mass surges equatorward and into the SH. Along the west coast of Australia, a southerly pressure surge extends equatorward and converges with the northerly surge to create a pronounced near equatorial zonal pressure gradient. A low-level westerly wind burst develops in response to this enhanced zonal pressure gradient as part of the onset of an active phase of the Australian summer monsoon. We show that three prominent anticyclonic circulations intensify in the southern hemisphere extratropics, stretching from the South Indian Ocean to the South Pacific, beneath regions of upper tropospheric dry atmospheric mass convergence, originating from the monsoon convection outflow. These anticyclonic circulations are largely responsible for the dry atmospheric mass increase in the SH.

To evaluate the precision of a three dimensional cloud chemistry model, the ion concentrations of simulated clouds are compared with the chemical analysis of cloud water samples collected in the Muskoka, Ontario area. Five summer case studies are presented. / In general, the simulated concentrations of NO$ sbsp{3}{-},$ NH$ sbsp{4}{+},$H$ sp+$, H$ sb2O sb2$ and the concentrations of SO$ sbsp{4}{2-}$ calculated with the perfect nucleation condition concur with the observed concentrations. For the four polluted cases, more than 80% of the cloud water SO$ sbsp{4}{2-}$ comes from nucleation, whereas for the clean case, it is mostly created by SO$ sb2$ oxidation. The relative importance of $ rm H sb2O sb2$ and O$ sb3$ in SO$ sb2$ oxidation, as well as the relative importance of nucleation and NH$ sb3$ absorption on the cloud water NH$ sbsp{4}{+}$ budget vary according to the chemical state of the atmosphere.

No description available.

In this study, the synoptic and mesoscale environment of a case of severe convective outbreak over Alberta on 11-12 July 1985 are investigated using the state-of-the-art Mesoscale Compressible Community Model (MC2). The results of the control experiment confirm the conceptual model and the conclusion of Smith and Yau (1993a,b). A series of sensitivity experiments were conducted to study the effects of surface evapotranspiration, topography, shortwave heating and longwave cooling. The results of the sensitivity experiments are: (1) While increasing horizontal resolution of mesoscale models will likely improve the timing and location of the severe convective outbreak, the improvement in mesoscale forecast cannot be achieved by simply reducing the grid size. Realistic treatment of topography, surface processes, and model physics are essential to predict severe convective outbreak. (2) Local surface evapotranspiration was shown to be an important source of moisture. Without surface evapotranspiration, not enough moisture is transported to the foothills to feed the convection. As a result, the mountain-plain circulation became ineffective in initiating severe convection. (3) An often observed condition for severe convection, the presence of a strong capping lid, was found to be formed from two processes. One factor is the presence of radiative cooling at the surface. Another factor is the advection from the downslope flow and the accompanying diabatic warming. Strong surface heating is essential in eroding the lid over the foothills.

We have developed a C-grid primitive equation ocean general circulation model with Cartesian and $ beta$-plane geometry. Temperature is the only state variable. The C-grid gives better results than the B-grid in reproducing the growth rates of linear unstable modes of the Raileigh-Benard equations. A semi-implicit scheme is used to treat the Coriolis term, which is important for efficient integration in coarse resolution large scale modelling studies. A new viscosity term which has a damping effect only on the divergence field associated with gravity waves is also introduced. The model can reproduce sucessfully most of the coarse resolution model results of other studies. The biharmonic and Smagorinsky frictional parameterizations are not as efficient as our scheme in eliminating noise in the vertical velocity field. / This model is used to study the effects of no slip or free slip boundary conditions on the energetics and northward heat transport in the eddy resolving regime. The divergence dissipation term is used only in the subpolar gyre region, where the Rossby radius of deformation is not well resolved. This term has little effect elsewhere in the model domain. The eddy energetics is sensitive to the lateral boundary conditions used. Increasing vertical resolution can increase the basin average and midlatitude free jet energetics, but its effect is much less than that due to different lateral boundary conditions. The northward heat transports by eddies and mean flow are also examined. / The effect of a restoring condition is compared to a zero heat capacity atmospheric model as a surface boundary condition for the eddy resolving model. Two significant differences are found with the use of the zero heat capacity atmospheric model. First, both eddy and mean kinetic energy near the midlatitude free jet are increased. Second, the vertical profiles of standard temperature deviation (eddy available potential energy) become more realistic. / An analysis of the mean advection and eddy convergence terms in the mean momentum equations shows that both enhanced horizontal resolution and the zero heat capacity atmospheric model can increase the midlatitude jets in the surface and deep layers. The eddy momentum convergence in midlatitudes is the dominant ageostrophic contribution to both the mean zonal flow and its variation. The mean advection is consistently less important. The effects of eddies have been further investigated by using the mean vorticity equation. The results again show that the eddy convergence term is the most important ageostrophic term, and can be as important as the geostrophic effect. The mean vorticity equation budget shows a similar sensitivity to the horizontal resolution and zero heat capacity atmospheric model as for the momentum equations.