ORCID Profile
0000-0003-3207-3733
Current Organisation
University of Fiji
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Publisher: CSIRO Publishing
Date: 02-02-2017
DOI: 10.22499/3.6604.005
Publisher: Copernicus GmbH
Date: 30-07-2007
DOI: 10.5194/ANGEO-25-1509-2007
Abstract: 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 s ling 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 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.
Publisher: American Meteorological Society
Date: 12-2018
DOI: 10.1175/JTECH-D-18-0065.1
Abstract: This study cross validates the radar reflectivity Z the rainfall drop size distribution parameter (median volume diameter D o ) and the rainfall rate R estimated from the Tropical Rainfall Measuring Mission (TRMM) satellite Precipitation Radar (PR), a combined PR and TRMM Microwave Imager (TMI) algorithm (COM), and a C-band dual-polarized ground radar (GR) for TRMM overpasses during the passage of tropical cyclone (TC) and non-TC events over Darwin, Australia. Two overpass events during the passage of TC Carlos and 11 non-TC overpass events are used in this study, and the GR is taken as the reference. It is shown that the correspondence is dependent on the precipitation type whereby events with more (less) stratiform rainfall usually have a positive (negative) bias in the reflectivity and the rainfall rate, whereas in the D o the bias is generally positive but small (large). The COM reflectivity estimates are similar to the PR, but it has a smaller bias in the D o for most of the greater stratiform events. This suggests that combining the TMI with the PR adjusts the D o toward the “correct” direction if the GR is taken as the reference. Moreover, the association between the TRMM estimates and the GR for the two TC events, which are highly stratiform in nature, is similar to that observed for the highly stratiform non-TC events (there is no significant difference), but it differs considerably from that observed for the majority of the highly convective non-TC events.
Publisher: Elsevier BV
Date: 11-2016
Publisher: CSIRO
Date: 2022
DOI: 10.25919/SR2H-8282
Publisher: Springer Science and Business Media LLC
Date: 25-04-2016
Publisher: CSIRO Publishing
Date: 2016
DOI: 10.1071/SP16002
Abstract: Tweek and whistler atmospherics (sferics) recorded at a low latitude station, Suva (18.2oS, 178.3oE, geomag. lat. 22.2oS, L = 1.17) Fiji, in the South Pacific region, during September 2003–July 2004 and March–December 2006 were analyzed to probe the nighttime D-region ionosphere and plasmasphere. From the first harmonic analysis of the selected 1063 tweeks, the path integrated ionospheric reflection height was estimated mostly between 75–97 km with median and standard deviation of 83.6 and 4.1 km, respectively. The path integrated mean electron density was estimated to vary from 24.4–31.5 cm–3 over the reflection heights. The whistler recorded on 20 September 2006, a magnetically quite day with maximum three hourly Kp value of 2o, most likely propagated in the ducted mode to this station. The whistler spectrum was used to calculate the equatorial magnetic field and the equatorial electron density at L = 1.3 which are in agreement with low latitude whistler observations.
Publisher: Elsevier BV
Date: 08-2007
Publisher: Springer Science and Business Media LLC
Date: 18-03-2021
DOI: 10.1007/S00382-021-05680-5
Abstract: 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 in idual 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 ex le, 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.
Publisher: Springer Science and Business Media LLC
Date: 07-2009
DOI: 10.1186/BF03353201
Abstract: 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 nem 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.
Publisher: Springer Science and Business Media LLC
Date: 06-2022
DOI: 10.1007/S10584-022-03391-2
Abstract: Tropical cyclones (TCs) are amongst the costliest natural hazards for southwest Pacific (SWP) Island nations. Extreme winds coupled with heavy rainfall and related coastal hazards, such as large waves and high seas, can have devastating consequences for life and property. Effects of anthropogenic climate change are likely to make TCs even more destructive in the SWP (as that observed particularly over Fiji) and elsewhere around the globe, yet TCs may occur less often. However, the underpinning science of quantifying future TC projections amid multiple uncertainties can be complex. The challenge for scientists is how to turn such technical knowledge framed around uncertainties into tangible products to inform decision-making in the disaster risk management (DRM) and disaster risk reduction (DRR) sector. Drawing on experiences from past TC events as analogies to what may happen in a warming climate can be useful. The role of science-based climate services tailored to the needs of the DRM and DRR sector is critical in this context. In the first part of this paper, we examine cases of historically severe TCs in the SWP and quantify their socio-economic impacts. The second part of this paper discusses a decision-support framework developed in collaboration with a number of agencies in the SWP, featuring science-based climate services that inform different stages of planning in national-level risk management strategies.
Publisher: American Meteorological Society
Date: 06-2022
Abstract: Reconstructed weather datasets, such as reanalyses based on model output with data assimilation, often show systematic biases in magnitude when compared with observations. Postprocessing approaches can help adjust the distribution so that the reconstructed data resemble the observed data as closely as possible. In this study, we have compared various statistical bias-correction approaches based on quantile–quantile matching to correct the data from the Twentieth Century Reanalysis, version 2c (20CRv2c), with observation-based data. Methods included in the comparison utilize a suite of different approaches: a linear model, a median-based approach, a nonparametric linear method, a spline-based method, and approaches that are based on the lognormal and Weibull distributions. These methods were applied to daily data in the Australian region for rainfall, maximum temperature, relative humidity, and wind speed. Note that these are the variables required to compute the forest fire danger index (FFDI), widely used in Australia to examine dangerous fire weather conditions. We have compared the relative errors and performances of each method across various locations in Australia and applied the approach with the lowest mean-absolute error across multiple variables to produce a reliable long-term bias-corrected FFDI dataset across Australia. The spline-based data correction was found to have some benefits relative to the other methods in better representing the mean FFDI values and the extremes from the observed records for many of the cases examined here. It is intended that this statistical bias-correction approach applied to long-term reanalysis data will help enable new insight on climatological variations in hazardous phenomena, including dangerous wildfires in Australia extending over the past century.
No related grants have been discovered for Anil Deo.