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The indirect effects of anthropogenic aerosols—through their interactions with clouds—are currently one of the most uncertain perturbations to the radiative energy balance at the top of the atmosphere. A crucial link between aerosol and cloud is that aerosols can act as cloud condensation nuclei (CCN). This microphysical process must be parameterised if the large-scale effects are to be represented in a general circulation model (GCM). Theoretical work presented in this thesis highlights the importance of incorporating the kinetic limitations on droplet formation in aerosol activation parameterisations. HadGEM-UKCA is a GCM, capable of representing the chemical and microphysical aerosol processes required to model CCN accurately. The author has incorporated a Köhler theory based parameterisation of aerosol activation into HadGEM-UKCA, to facilitate quantitative predictions of the indirect aerosol effects. This thesis presents an estimate of the range of uncertainty in such predictions attributable to the choice of parameterisation of the sub-grid-scale variability of vertical velocity. Results of simulations demonstrate that the use of a characteristic updraught velocity cannot replicate results derived with a distribution of vertical velocities, and is to be discouraged in GCMs. Consequently, work focuses on the effect of the variance (<var>σ</var>_w²) of a Gaussian pdf of vertical velocity. Fixed values of <var>σ</var>_w and a configuration in which <var>σ</var>_w depends on turbulent kinetic energy are tested. Results from the mid-range fixed <var>σ</var>_w and TKE-based configurations both compare well with vertical velocity distributions and cloud droplet number concentrations measured in situ. However, the sparse set of available measurements does not provide enough of a constraint to recommend one or the other as the best configuration globally. The radiative flux perturbation (RFP) due to the total effects of anthropogenic aerosol is estimated at −1.7Wm⁻² for the TKE-based configuration. To the extent that it is valid to decouple the individual aerosol effects, the direct effect accounts for approximately −0.6Wm⁻² of the total, the cloud albedo effect −0.8Wm⁻² and the cloud lifetime effect −0.3Wm⁻², indicating that these effects are additive within HadGEM-UKCA. Total aerosol RFP ranges from −1.4Wm⁻² from simulations with <var>σ</var>_w=0.1ms⁻¹, up to −2.0Wm⁻² for <var>σ</var>_w=0.7ms⁻¹. This range of 0.6Wm⁻² corresponds to almost a third of the total estimate of −1.9Wm⁻², obtained with the mid-range value of <var>σ</var>_w=0.4ms⁻¹. Reducing the uncertainty in the parameterisation of <var>σ</var>_w is therefore an important step towards reducing the uncertainty in estimates of the indirect aerosol effects.

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The detection of specific temporal patterns in communication signals may be of vital importance for certain organisms. In crickets, for instance, a female will move towards a singing male only if she can recognize the appropriate pulse rate characteristic to its own species' song. Additionally, in order to evade predatory insectivorous bats, flying crickets must be able to track the predator's ultrasonic echolocation signals, which are emitted at a variety of pulse rates. In this thesis, the temporal processing, or the integration of stimulus through time, in the peripheral1 auditory system of the cricket will be investigated. / The ON1 interneuron temporal processing was first examined and compared at high (bat-like) and low carrier (cricket-like) frequencies in three different experimental paradigms. First, integration time, which corresponds to the time it takes for a neuron to reach threshold when stimulated at the minimum effective intensity, was found to be significantly shorter at high carrier frequency than at low carrier frequency. Second, phase locking to sinusoidally amplitude modulated (SAM) signals was more efficient at high frequency, especially at high modulation rates and low modulation depths. Finally, we examined the efficiency with which ON1 detects gaps in a constant tone. As reflected by the decrease in firing rate in the vicinity of the gap, ON1 is better at detecting gaps at low carrier frequency. Following a gap, firing rate increases beyond the pre-gap level. This "rebound" phenomenon is similar for low and high carrier frequencies. / To determine the source of this differential temporal processing, the sensory afferents making synapses with ON1 were investigated. Low frequency (MT-type) and ultrasound auditory receptors were compared on the basis of latency, maximum firing rate, adaptation, information transmission, bursting and feature detection. Ultrasound receptors (HFs) were found to have a shorter latency, a higher maximum firing rate and stronger adaptation than low-frequency receptors (LFs). Individual HFs transmitted more linear (lower-bound) information than LFs. However, HFs' responses were more correlated than LFs' (i.e. they had larger mutual information), so that when superposing the spike trains of LFs, information transmission in the lowest amplitude modulation rates was greatly improved, and, in some cases, reached the level of HFs. Feature detection by spike in HFs was better than in LFs. Feature detection by bursts was better than for spikes, but equivalent in both types of receptors. The level of bursting in HFs, however, was much higher than in LFs, making them better feature detectors in general. / 1Because it lies in the prothoracic ganglion, ON1 is technically part of the central nervous system. For the purpose of this thesis, however, because ON1 receives direct input from the receptors, it will be considered to be part of the peripheral auditory systems.

We used the auditory system of crickets as a model system to examine the importance of temporal coding in sensory processing. The bilaterally paired Ascending Neurons 1 and 2 (AN1 and AN2) of crickets receive inputs from the auditory receptors on one side and carry the information to the brain. We used stimuli with either conspecific-like or predator-like (i.e. bats) carrier frequency to quantify the accuracy with which the interneurons code the information contained within the amplitude modulation (AM) envelope of the stimulus. AN1, which is tuned to the dominant carrier frequency of cricket songs, selectively codes the limited range of amplitude-modulation frequencies that occur in these signals. AN2, which is most sensitive to ultrasound, serves as a "bat-detector" and codes a broader range of AM frequencies, as occur in bat calls. / A striking characteristic in AN2's responses to ultrasound is the presence of bursts of high-frequency spiking separated by relatively sparse spikes. We examined the relative importance of isolated spikes and bursts in the processing of ultrasound. We showed that bursts reliably signal the occurrence of salient amplitude increases. Furthermore, we showed that burst, but not isolated spikes, reliably predict behavioural responses. We suggest AN2 encodes behaviourally important information with bursts. / The Omega Neuron 1 (ON1) responds to conspecific signals and to the ultrasonic echolocation sounds. ON1's temporal coding properties vary with carrier frequency, allowing it to encode both of these behaviourally important signals. Furthermore, the temporal coding properties of ON1 in response to cricket-like sound and bat-like sound match those of AN1 and AN2 respectively. / ON1 is a source of contralateral inhibition to AN1 and AN2, enhancing binaural contrast and facilitating sound localization. We used dichotic stimulation to examine the importance of the temporal structure of contralateral inhibition for enhancing binaural contrast. Contralateral inhibition degrades the accuracy with which amplitude modulation is encoded by AN 1 and AN2, but only if the temporal pattern of inhibitory input matches that of excitation. Our results show that the CF-specific coding properties of ON1 allow this single neuron to enhance localization cues most effectively for both cricket-like and bat-like acoustic signals.