In this study the rainfall drop size distribution (DSD) during the passage of seven tropical cyclones (TCs) over Darwin is compared and contrasted with that associated with nontropical cyclone (non-TC) events, using the impact disdrometer data at the Darwin Atmospheric Radiation and Measurement (ARM) site. The disparity of the DSD with respect to rainfall types (between TC and non-TC conditions) and distance from TC centre is also examined. It is shown that TC DSDs are statistically different from the non-TC DSDs, the former encompassing a larger concentration of small to moderate drop sizes. The TC massweighted mean diameter (D m ) is lower than the non-TC values at all rain rates and also for the different precipitation types (convective, transition and stratiform). The TC DSD varies with distance from the TC centre, as rainfall near the TC centre (< 60 km) comprises of relatively smaller drops which are strongly evident at small to moderate rain rates (< 30 mm hr -1 ). Such variations in the DSD have implications for the parameters used in the algorithm that A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTP a g e | 2converts radar reflectivity to rainfall rate in TCs, as well as for the analytical expressions used in describing the observed DSD employed in cloud modelling parameterizations.

Atmospherics or sferics that originate from lightning discharges on propagating large distances in the Earthionosphere waveguide, particularly at the night, form dispersed sferics called tweeks. Tweeks are novel diagnostic tool to monitor the nighttime D-region ionosphere. Mean equivalent electron density n em at mean tweek reflection heights h m and electron density profile have been estimated using the higher harmonic tweeks recorded in the time between 21-03 hrs LT at Suva (18.2• S, 178.3• E), Fiji, during a period March-December 2006. The values of n em vary from 29-170 cm −3 in the altitude range of about 3.5 km at h m of about 83 km. In terms of usually used exponential electron density profile, the ionospheric reference height and the exponential sharpness factor are calculated to be 83.1 km and 0.64 km −1 , respectively. The scale height is calculated to be 1.9 km. Equivalent electron density profile of the nighttime lower ionosphere, using tweek method, shows lower values of electron density by about 20-45% than those obtained from the IRI-2001 model.

Abstract. A simple technique to estimate the distance of the lightning strikes d with a single VLF electromagnetic wave receiver at a single station is described. The technique is based on the recording of oscillatory waveforms of the electric fields of sferics. Even though the process of estimating d using the waveform is a rather classical one, a novel and simple procedure for finding d is proposed in this paper. The procedure adopted provides two independent estimates of the distance of the stroke. The accuracy of measurements has been improved by employing high speed (333 ns sampling rate) signal processing techniques. GPS time is used as the reference time, which enables us to compare the calculated distances of the lightning strikes, by both methods, with those calculated from the data obtained by the World-Wide Lightning Location Network (WWLLN), which uses a multi-station technique. The estimated distances of the lightning strikes (77), whose times correlated, ranged from ~3000–16 250 km. When d<3500 km, the average deviation in d compared with those calculated with the multi-station lightning location system is ~4.7%, while for all the strokes it was ~8.8%. One of the lightnings which was recorded by WWLLN, whose field pattern was recorded and the spectrogram of the sferic was also recorded at the site, is analyzed in detail. The deviations in d calculated from the field pattern and from the arrival time of the sferic were 3.2% and 1.5%, respectively, compared to d calculated from the WWLLN location. FFT analysis of the waveform showed that only a narrow band of frequencies is received at the site, which is confirmed by the intensity of the corresponding sferic in the spectrogram.

Southwest Pacific nations are among some of the worst impacted and most vulnerable globally in terms of tropical cyclone (TC)-induced flooding and accompanying risks. This study objectively quantifies the fractional contribution of TCs to extreme rainfall (hereafter, TC contributions) in the context of climate variability and change. We show that TC contributions to extreme rainfall are substantially enhanced during active phases of the Madden–Julian Oscillation and by El Niño conditions (particularly over the eastern southwest Pacific region); this enhancement is primarily attributed to increased TC activity during these event periods. There are also indications of increasing intensities of TC-induced extreme rainfall events over the past few decades. A key part of this work involves development of sophisticated Bayesian regression models for individual island nations in order to better understand the synergistic relationships between TC-induced extreme rainfall and combinations of various climatic drivers that modulate the relationship. Such models are found to be very useful for not only assessing probabilities of TC- and non-TC induced extreme rainfall events but also evaluating probabilities of extreme rainfall for cases with different underlying climatic conditions. For example, TC-induced extreme rainfall probability over Samoa can vary from ~ 95 to ~ 75% during a La Niña period, if it coincides with an active or inactive phase of the MJO, and can be reduced to ~ 30% during a combination of El Niño period and inactive phase of the MJO. Several other such cases have been assessed for different island nations, providing information that have potentially important implications for planning and preparing for TC risks in vulnerable Pacific Island nations.

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