The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the 1880–2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere–ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation of stratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

The origin of the tropical lower-stratospheric response to 11-yr solar forcing and its possible coupling to a troposphere–ocean response is investigated using multiple linear regression (MLR) analyses of stratospheric ozone and temperature data over the 1979–2009 period and tropospheric sea level pressure (SLP) data over the 1880–2009 period. Stratospheric MLR results, comparisons with simulations from a chemistry–climate model, and analyses of decadal variations of meridional eddy heat flux indicate that the tropical lower-stratospheric response is produced mainly by a solar-induced modulation of the Brewer–Dobson circulation (BDC), with a secondary contribution from the Hadley circulation in the lowermost stratosphere. MLR analyses of long-term SLP data confirm previous results indicating a distinct positive response, on average, during the northern winter season in the North Pacific. The mean response in the Northern Hemisphere resembles a positive Arctic Oscillation mode and can also be characterized as “La Niña–like,” implying a reduction of Rossby wave forcing, a weakening of the BDC, and an increase in tropical lower-stratospheric ozone and temperature near solar maxima. However, MLR analyses of different time periods show that the Pacific SLP response is not always present during every cycle; it was most clearly detected mainly during the ~1938–93 period when 11-yr solar variability was especially strong. During the 1979–93 period, the SLP response was strongly present when the lower-stratospheric responses were large. But during the 1994–2009 period, the SLP response was much less significant and the lower-stratospheric responses were weak, supporting the hypothesis that the lower-stratospheric and surface climate responses are dynamically coupled.

The general circulation model COMMA-LIM solves the primitive equations on a sphere using gridpoints. The relative large interval between adjacent gridpoints (5° × 5.6° latitude versus longitude) causes an incorrect meridional temperature gradient in the coarsly resolved troposphere that leads to too weak winds there, particularly in the lower winter stratosphere above the polar region. By using the technique of nudging 11-year averaged NCEP zonal mean temperature data were assimilated into COMMA-LIM. This means that longitudinal dependent processes as calculated by the model still influence the atmosphere. The nudging method has improved not only the lower atmosphere, but also the middle atmospheric jets show a more realistic behaviour. A numerical experiment by forcing the 16-day wave was carried out in order to investigate the influence of an improved background circulation on the vertical propagation of planetary waves. / Das globale Zirkulationsmodell COMMA-LIM berechnet die primitiven Gleichungen auf einem Kugelgitter. Der relativ große Gitterabstand von 5°× 5.6° in Breite und Länge und die grobe vertikale Auflösung führen zu einem inkorrekten meridionalen Temperaturgradienten in der Troposphäre, so dass die troposphärischen Jets und der polare Winterwirbel zu schwach ausgeprägt sind. Mit Hilfe der Methode des Nudging wurden in den unteren 30 km der Atmosphäre 11-Jahres gemittelte NCEP Reanalysedaten des Temperaturfeldes assimiliert. Dabei wurde nur der zonale Mittelwert der berechneten Temperatur an die Reanalysedaten relaxiert, so dass die Antriebsterme, die von COMMA-LIM berechnet werden, erhalten bleiben. Durch diese Methode wurden Wind- und Temperaturfeld sowohl in der Troposphäre als auch in der mittleren Atmosphäre verbessert. Ein Experiment zur Ausbreitung der 16-Tage Welle wurde unter den neuen Bedingungen durchgeführt, und der Einfluß der veränderten Atmosphäre auf die vertikale Wellenausbreitung wurde untersucht.

M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / Biogenic volatile organic compounds (BVOCs), in the presence of nitrogen oxide gases (NOx), play a role in the production of tropospheric ozone (O3) which is an effective greenhouse gas and is hazardous to human health (Haagen-Smit, 1952, Chameides et al, 1988, Atkinson, 2000, Kanakidou et al, 2004). Isoprene is a single BVOC that accounts for over 50% of all emitted BVOCs. Isoprene emissions are species specific and vary according to temperature, light and leaf area index. Climate change studies predict that the geographic distribution of species, temperature ranges, light intensity and leaf area index will shift, thus altering future isoprene emissions. Several attempts to model BVOC emissions have been undertaken in an effort to quantify BVOC emission rates and the impact on ozone formation. The most widely used and empirically tested emission algorithms to date were developed by Guenther et al (1993) and are incorporated into the emission model Model of Emissions of Gases and Aerosols from Nature (MEGAN). MEGAN is used in this study to model isoprene emission rates over southern Africa under current and future climate conditions. Current and future climate conditions are taken from the regional climate model, Conformal-Cubic Atmospheric Model (C-CAM), which has been shown to simulate current climate well for the region. Emissions were modelled for January and July only, to represent summer and winter conditions. January isoprene emission rates for the current climate range from 0 to 1.41 gm-2month-1 and total 0.938 Tg of isoprene for the study domain. The highest emission rates are caused by combinations of driving variables which are: high temperature only; high temperature and high leaf area index; high emission factor and high leaf area index. Emission rates effectively shut down in July due to low temperatures and low leaf area index. July emission rates range from 0 to 0.61 gm-2month-1 and total 0.208 Tg of isoprene. Temperature is shown to cause the greatest variation in isoprene emission rates, and thus future scenarios represent an increase in temperature only. The spatial distribution of future emission rates does not shift when compared to current emission rates, but does show an increase in magnitude. Future emission totals for January increase iv by 34% to 1.259 Tg of isoprene and the July emission total increases by 38% to 0.289 Tg of isoprene. Future emission rates responded to temperature as expected, increasing in magnitude, rate of change and range of temperature over which the greatest rate of change occurs. Three areas demonstrating the highest increase in emission rates and highest future emission rates were identified. As temperature was the only variable altered in future scenarios, these areas can be deemed as areas most sensitive to changes in temperature. These areas are situated near the Angola-Namibia border, the Northern Interior of South Africa and the low-lying areas of Mozambique.

Atmospheric mass is transported in and out of the stratospheric polar cap region by a wave-driven meridional circulation. Using composites of polar cap pressure anomalies, defined as deviations from the average annual cycle, it is shown that this stratospheric mass flux is accompanied by a similar mass flux near the surface. This 'tropospheric amplification' of the stratospheric signal is introduced as a new way to quantify stratosphere-troposphere coupling. Regression analysis is used to create a vertical profile of atmospheric pressure during a tropospheric amplification event, and the regression slope profile is used as a tool to quantify the amplification. Using data from 5 reanalysis datasets and 11 climate models, it is shown that high-top models, with a model lid of above 1 hPa, are significantly better at reproducing tropospheric amplification than low-top models, due to having more detailed parameterisations of stratospheric processes. However, the regression slope profiles of all models, bar one, are significantly different to the profile of reanalysis data at a 95% confidence level. Tropospheric amplification is also investigated in historical and future simulations from these models, and it is concluded that there is not expected to be a large change in the phenomenon over the next 100 years. The processes needed to reproduce tropospheric amplification can be identified by comparing idealised models of different complexity. A simple dry-core model is not able to reproduce tropospheric amplification, while a model with a comprehensive radiation scheme does produce the basic regression slope profile under certain configurations. The associations between pressure change and mass flux are further investigated using primitive equations. It is found that vertical and horizontal contributions to mass flux act to mostly cancel each other out, leaving a poorly-conditioned residual, and that the horizontal mass flux across the polar cap boundary has both geostrophic and ageostrophic components.

<p>The study presented in this thesis was motivated by the large uncertainty on the concentration of atmospheric electrical discharges to the global nitrogen budget. This uncertainty is partly due to the fact that information concerning the <i>NO</i><i>x</i> production efficiency of electrical discharges having current signatures similar to those of lightning flashes is not available in the literature. Another reason for this uncertainty is the fact that energy is used as a figure of merit in evaluating <i>NO</i><i>x</i> production from lightning flashes even though insufficient knowledge is available concerning the energy dissipation in lightning flashes. The third reason for this uncertainty is the lack of knowledge concerning the contribution of discharge processes other than return strokes to the <i>NO</i><i>x</i> production in the atmosphere. Lightning is not the only process in the atmosphere that causes ionisation and dissociation of atmospheric air. Cosmic rays continuously bombard the Earth with high energetic particles and radiation causing ionization and dissociation of air leading to the production of <i>NO</i><i>x</i> in the atmosphere. The work carried out in this thesis is an attempt to improve the current knowledge on the way in which these processes contribute to the global <i>NO</i><i>x</i> production. Experiments have been conducted in this thesis to estimate the <i>NO</i><i>x</i> production efficiency of streamer discharges, laser-induced plasma, laboratory sparks having current signatures similar to those of lightning flashes, alpha particle impact in air and finally with the lightning flash itself. The results obtained from laboratory electrical discharges show the following: (a) The <i>NO</i><i>x</i> production efficiency, in terms of energy, of positive streamer discharges is more or less similar to those of hot discharges. (b) The <i>NO</i><i>x</i> production efficiency of an electrical discharge depends not only on the energy but also on the peak and the shape of the current waveform. (c) The current signature is a better figure of merit in evaluating the <i>NO</i><i>x</i> yield of electrical discharges. As a part of this thesis work a direct measurement of <i>NO</i><i>x</i> generated by lightning flashes was conducted and the results show that slow discharge processes such as continuing currents could be the main source of <i>NO</i><i>x</i> in lightning flashes. Concerning <i>NO</i><i>x</i> production by other ionisation processes such as alpha particle impacts in the atmosphere, the data gathered in this thesis show that each ionising event in air leads to the creation of one <i>NO</i><i>x</i> molecule. In terms of energy the <i>NO</i><i>x</i> production efficiency of alpha particles is similar to that of electrical discharges. The theoretical studies conducted within this thesis indicate that M-components contribute more than the return strokes to the <i>NO</i><i>x</i> production. The calculations also show that the contribution to the global <i>NO</i><i>x</i> budget by return stroke is not as high as that assumed in the current literature.</p>

The classical problem of predicting the atmospheric circulation response to extratropical surface forcing is revisited in the context of the observed connection between autumn snow cover anomalies over Eurasia and the wintertime Northern Annular Mode (NAM). In general circulation model (GCM) simulations with prescribed autumn Siberian snow forcing, a vertically propagating Rossby wave train is generated, driving dynamical stratospheric warming and a negative NAM response that couples to the troposphere. It is shown that unexplained aspects of the evolution of this response can be clarified by examining the time evolution of the phasing, and hence the linear interference, between the wave response and the background climatological wave. When the wave response and background wave are in phase (out of phase), wave activity into the stratosphere is amplified (attenuated) and the zonal mean stratosphere-troposphere NAM response displays a negative (positive) tendency. This effect is probed further in a simplified GCM with imposed lower tropospheric cooling. As in the comprehensive GCM, linear interference strongly influences the NAM response. The transition from linear to nonlinear behaviour is shown to depend on forcing strength. Linear interference also plays a key role in the observed October Eurasian snow cover-NAM connection. It is shown that the time lag between October Eurasian snow anomalies and the peak wave activity flux arises because the Rossby wave train associated with the snow is out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November, the associated wave anomaly migrates into phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes. Current generation climate models do not capture this behaviour. Linear interference is not only associated with stratospheric warming due to Eurasian snow cover anomalies but is a general feature of both Northern and Southern Hemisphere stratosphere-troposphere interactions, and in particular dominated the negative NAM events of the fall-winter of 2009-2010. The interannual variability in upward wave activity flux during the season of strongest stratosphere-troposphere interactions is primarily determined by linear interference of quasi-stationary waves. The persistence of the linear interference component of this flux may help improve wintertime extratropical predictability.

The classical problem of predicting the atmospheric circulation response to extratropical surface forcing is revisited in the context of the observed connection between autumn snow cover anomalies over Eurasia and the wintertime Northern Annular Mode (NAM). In general circulation model (GCM) simulations with prescribed autumn Siberian snow forcing, a vertically propagating Rossby wave train is generated, driving dynamical stratospheric warming and a negative NAM response that couples to the troposphere. It is shown that unexplained aspects of the evolution of this response can be clarified by examining the time evolution of the phasing, and hence the linear interference, between the wave response and the background climatological wave. When the wave response and background wave are in phase (out of phase), wave activity into the stratosphere is amplified (attenuated) and the zonal mean stratosphere-troposphere NAM response displays a negative (positive) tendency. This effect is probed further in a simplified GCM with imposed lower tropospheric cooling. As in the comprehensive GCM, linear interference strongly influences the NAM response. The transition from linear to nonlinear behaviour is shown to depend on forcing strength. Linear interference also plays a key role in the observed October Eurasian snow cover-NAM connection. It is shown that the time lag between October Eurasian snow anomalies and the peak wave activity flux arises because the Rossby wave train associated with the snow is out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November, the associated wave anomaly migrates into phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes. Current generation climate models do not capture this behaviour. Linear interference is not only associated with stratospheric warming due to Eurasian snow cover anomalies but is a general feature of both Northern and Southern Hemisphere stratosphere-troposphere interactions, and in particular dominated the negative NAM events of the fall-winter of 2009-2010. The interannual variability in upward wave activity flux during the season of strongest stratosphere-troposphere interactions is primarily determined by linear interference of quasi-stationary waves. The persistence of the linear interference component of this flux may help improve wintertime extratropical predictability.

Regional patterns in tropospheric and sea surface temperature (SST) trends are examined for the period 1979 ??01 using MSU, NCEP-NCAR, ECMWF ERA-40 reanalyses, NOAA OI SST, and the CARDS radiosonde data set. Trends are estimated using a nonparametric Mann-Kendall approach. Substantial regional variability in temperature trends is seen in all of these data sets, with the magnitude of the variability (including substantial regions with cooling trends) far exceeding the average warming trend. The global analyses from MSU and the NCAR/NCEP and ECMWF reanalyses are used to identify sampling problems in using the radiosonde network to infer global trends. Analysis of the trends in tropospheric temperature concurrent with trends in SST shows regions where the signs disagree for both surface cooling and warming. Interpretation of these differing trends using the reanalyses suggest that the models used for the reanalyses are simulating the necessary dynamics/thermodynamics thatcould lead to a tropospheric cooling in contrast to a surface warming (and vice versa).

This thesis uses a method called "parabolic equation approach." This method can treat both the variations of the terrain and the refractive index simultaneously. This method makes it possible to predict the radio propagation more precisely. We can discuss the effects of the variations of the refractive index to radio signal and demo effects by using parabolic equation method. The Effective Earth Radius Factor is 4/3 suggested by CCIR so-called "Standard Atmosphere Model". But we try to find more suitable K in the Southern Taiwan area. We adopt Parabolic Equation Propagation Model to simulate real situation of radio propagation in the Southern Taiwan area and the prediction is compared with the measurement obtained previously. We can get the best K in the Southern Taiwan area is 1.8 and 1.9. The best K is greater than the value of 4/3 suggested by CCIR. Recently the government on Taiwan release more radio broadcasting licenses to the general public. As the number of radio stations increases, the interference between stations becomes more likely. There have been reports about the poor quality of broadcasting from stations. In this paper, we will study the interference using FM radio stations as an example. Given the characteristics of the transmitting antenna including location, frequency, pattern, height and power, the field strength can be computed with the equivalent earth radius factor K as a parameter. The difference in interference level is obtained under the standard atmosphere (K=4/3) and a case of K=1.55 which has been reported to be more suitable in Taiwan. Finally an extreme case that a ducting exists will be studied. Our results can be used to find more suitable separation distances free from interferences between co-channel and adjacent channel stations. By including a realistic tropospheric term, the more accurate field strength predictions can give the Spectrum Authority a better spectrum assignment tool. This has the potential to increase the number of available stations that can be made available or to reduce the interference stations may experience.