Tropical Cyclone (TC) Oswald (2013) significantly impacted Australia with extensive rainfall and prolonged circulation over land, largely influenced by two mid‐latitude troughs. Unlike other documented studies, Oswald's interaction with the two troughs occurred in the mid‐troposphere, not the upper troposphere. Under the high vertical wind shear, the upper TC circulation was greatly weakened. However, in the middle levels between 400 and 600 hPa, high cyclonic potential vorticity (PV) air, was transported from the troughs to Oswald's mid‐layer circulation, replenishing its outer circulation. With the inner circulation, PV redistribution between the inner core and outer core was observed over the southeastern quadrant. This process enhanced mid‐to‐lower updrafts and boundary‐layer convergence, supporting the downshear reformation of mesovortices. Hence, despite sustained unfavorable strong shear and the absence of a warm ocean surface, the lower half of Oswald's circulation persisted and reorganized over land, significantly extending its impact after landfall.

The Australian tropical cyclone (TC) best track database (BT) maintained by the Bureau of Meteorology has records since 1909 of varying quality and completeness. Since 2005 a series of efforts to improve the database have included: removing internal inconsistencies, adding fixes, and identifying errors using comparisons with other datasets; upgrading intensity information since 1973 including adding maximum winds (Vm) prior to 1984–85, rederiving Dvorak Current Intensity numbers from archived material and accounting for different wind–pressure relationships used; a partial reanalysis of satellite imagery including microwave imagery using the HURSAT dataset since 1987; and considering an objective intensity dataset. The BT homogeneity is reviewed in the context of improvements in satellite technology, observational coverage, scientific developments, BT procedures and the subjective variation between analysts across time and offices. The scale of these variances is greatest in the early stages prior to 1981 in the absence of geostationary satellite imagery until 1978, satellite calibration issues from 1978–80 and prior to the introduction of the enhanced infra-red Dvorak technique in 1981. The current era since 2003 is considered to be the most accurate, comprehensive and homogeneous corresponding to the expansion of the TC database to include the current suite of fields; the application of microwave and scatterometry imagery; greater standardisation of BT practices and slight changes in the application of the Dvorak technique. These improvements have generated a more consistent dataset that could be used for weather and climate research and other TC-related work.

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of Typhoon Rammasun. We show that the basic difference stems from subtle differences in initializations of (a) 500-850-hPa environmental winds, and (b) midlevel moisture and ventilation. We then describe how these differences impact on the evolving convective organization, storm structure, and the timing of RI. As expected, ascent, diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI. The evolution of vortex cloudiness from this member is similar to the actual imagery, with the development of an inner cloud band wrapping inwards to form the eyewall. We present evidence that this structure, and hence the enhanced diabatic heating, is related to the tilt and associated dynamics of the developing inner-core in shear. For the most accurate ensemble member: (a) inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air, which is transported to the down-shear sector to feed a developing convective asymmetry; (b) with minimal ventilation, undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; and (c) this process seems related to a critical down-shear tilt of the vortex from midlevels, and the vertical phase-locking of the circulation over up-shear quadrants. For the member that forecasts a much-delayed RI, these processes are inhibited by stronger vertical wind shear, initially resulting in poor vertical coherence of the circulation, lesser moisture and larger ventilation. Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear, moisture and vortex inner-structure.

AbstractResults from object‐based verification of rainfall forecasts for landfalling tropical cyclones (TCs) over China during the period 2012–2015 are presented. The sample consists of 25 landfall events and 133 operational numerical forecasts from the TC version of the Australian Community Climate and Earth System Simulator. Mean equitable threat scores, probabilities of detection and false alarm ratios for the 30 mm isohyet for the unadjusted forecasts at 0–6 hr (essentially the initialization) are (0.23, 0.55, 0.65), while the performance measures of 24 hr forecast accumulations are the best for the 0–24 hr forecast (0.37, 0.67, 0.40) and then worsen to (0.16, 0.38, 0.66) for the 48–72 hr forecast. Forecast ability also decreases with the increase in rainfall amount. The contiguous rain area (CRA) verification method is used to diagnose the source of systematic errors from the displacement, rotation, volume and pattern of the forecasted rain fields. Results show that the errors are mostly from rainfall patterns, followed by displacement errors, particularly for very heavy rain. After application of the displacement and rotation adjustments of the CRA method, averaged errors improve by about 15%. Results suggest that rainfall prediction will continue to improve with improved track prediction, but more work is needed on model initialization and the prediction of TC structure. The study has uncertainty related to the limited sample size, which could cause large variability, particularly for heavy rainfall at 6 and 72 hr. However, the results still represent a useful benchmark for future verification of landfalling TCs.

Tropical cyclone (TC) Oswald made landfall over north-east Australia as a minimal or Category 1 TC on the Australian scale on 21 January 2013. As it moved southward, it intensified over land and produced extreme rainfall for nearly 7 days. Tornadoes were reported and confirmed. Tragically, seven people died and insurance estimates were ~$1 billion. It is demonstrated that the event was associated with an interaction between the ex-Oswald circulation and an amplifying Rossby wave, which propagated north-eastward from high latitudes. Diagnoses showed that as the wave amplified and broke, a potential vorticity (PV) anomaly (PVA) extended to mid-levels, moved equatorward, merged with or axisymmetrised the ex-Oswald circulation through mid-levels. Backward trajectories from locations regularly scattered within the mid-level circulation illustrated that the storm transitioned from an isolated vortex into a circulation which was strongly influenced by its environment for at least 5 days. During this interaction, PV was advected from the environment towards the storm through mid-levels. The heavy rain coincided with the commencement and maintenance of this PV injection. The PV injection is quantified and shown to be consistent with PV advection by the mean radial flow. In addition, eddy angular momentum convergence in the mid- to upper levels coincided with an intensification of the circulation through this region. This was first related to outward transport of anticyclonic momentum by the asymmetric outflow at upper levels, followed by inward transport of cyclonic momentum by the asymmetric inflow. It is shown that the environmental interaction had an impact on vortex structure changes, rainfall and tornado development. We propose that the environmental processes influenced the ascent within the storm (1) via differential vorticity advection and baroclinic forcing, as the mid- to upper level PVA approached the circulation and (2) by low- to mid-level warm air advection.

This study demonstrates the useful information that can be derived from contiguous rain area (CRA) evaluation, such as systematic errors in tropical cyclone (TC) rainfall location and components of rainfall error due to incorrect predictions of location, rain volume, and rain pattern. CRA verification uses pattern matching techniques to determine the location error, as well as errors in area, mean and maximum intensity, and spatial pattern. In this study, CRA verification was applied to evaluate Australian Community Climate and Earth System Simulator (ACCESS)‐TC, the TC version of ACCESS, daily rainfall forecasts over 15 TCs in the north west Pacific ocean during 2012–2013, by comparing with Tropical Rainfall Measuring Mission (TRMM) 3B42 satellite estimates. The results showed that pattern error was the major contributor to the total TC rainfall forecast error, followed by volume and displacement. ACCESS‐TC forecasts tended to predict more rainfall closer to the TC center compared to Tropical Rainfall Measuring Mission (TRMM) 3B42 estimates. This bias occurred for different CRA rainfall thresholds, verification grid resolutions and forecast lead times. Furthermore, rain event verification showed that for short lead time (24 hr) forecasts, overestimation of rain volume was a major problem for ACCESS‐TC forecasts, while displacement error was more significant in longer lead time (72 hr) forecasts. Finally, we compared empirical probability distribution functions and radial probability distributions of rainfall in the forecasts and observations to further characterise the rain volume error. This confirmed that ACCESS‐TC tended to produce more extreme rain in the locations closer to the TC center (eyewall).

Using a previously documented successful simulation of secondary eyewall formation (SEF), the coupled dynamic and thermodynamic processes during SEF are investigated from an axisymmetric mean perspective. The budgets of momentum, density potential temperature, and radial pressure gradient force (PGF) with very small residuals reveal that rather than a single process dominating, the SEF is the consequence of tight coupling processes between the hurricane tangential wind, transverse circulation, diabatic heating, mean and eddy fluxes of momentum and heat, and the hurricane boundary layer. The collective actions of these coupling processes generate the following conditions for the SEF: (i) along with slow eyewall contraction, the azimuthal‐mean tangential wind increases on the inner side of the eyewall; (ii) while the regions in the eye and beyond the eyewall are warming, a lesser warming or cooling zone sandwiched between these two warming regions appears near the axis of maximum vertical velocity; (iii) low‐level pressure decreases (increases) with the column‐integrated warming (cooling); the increment in the PGF beyond the eyewall brings about a significant low‐level radial and tangential wind increment at SEF radii, and (iv) the hurricane boundary layer radial flow corresponding to the PGF increment enhances the convergence of moisture and angular momentum within the SEF radii. Eventually, these processes result in the SEF.

In high-wind conditions, sea spray, in conjunction with a generally decreasing drag coefficient for increasing winds, greatly modulates surface heat and momentum fluxes. It has been suggested that the process can be particularly important for the prediction of tropical cyclones (TCs), yet its robust application in operational forecast systems has remained elusive. A sea spray inclusion scheme and a modified algorithm for momentum exchange have been implemented in the Australian Bureau of Meteorology's current operational TC model. Forecasts for a limited sample of TCs demonstrate that the revised parameterizations improve initialized and forecast intensities, while mostly maintaining track prediction skill. TC Yasi (2011) has been studied for impacts of the revised parameterization on rapid intensification (RI). Compared with the conventional bulk air-sea exchange parameterization, the revised version simulates a cooler and moister region near the surface in the eyewall/eye region, adjusts the RI evolution by an earlier and stronger subsidence in the eye, and simulates a stronger radial pulsating of the eye and eyewall convection on relatively short time scales. The inclusion of the new scheme enhances RI features characterized by eyewall ascent, radial convergence, and inertial stability inside the radius of azimuthal-mean maximum wind over low-to midlevels, and by a ringlike radial distribution of relative vorticity above the boundary layer. In addition, it allows a higher maximum intensity wind speed based on Emanuel's maximum potential intensity theory. It is shown that, as expected, this is mainly because of a larger ratio of enthalpy and momentum exchange coefficients.

TRMM satellite 3B42 rainfall estimates for 133 landfalling tropical cyclones (TCs) over China during 2001-15 are used to examine the relationship between TC intensity and rainfall distribution. The rain rate of each TC is decomposed into axisymmetric and asymmetric components. The results reveal that, on average, axisymmetric rainfall is closely related to TC intensity. Stronger TCs have higher averaged peak axisymmetric rain rates, more averaged total rain, larger averaged rain areas, higher averaged rain rates, higher averaged amplitudes of the axisymmetric rainfall, and lower amplitudes of wavenumbers 1-4 relative to the total rainfall. Among different TC intensity change categories, rapidly decaying TCs show the most rapid decrease in both the total rainfall and the axisymmetric rainfall relative to the total rain. However, the maximum total rain, maximum rain area, and maximum rain rate are not absolutely dependent on TC intensity, suggesting that stronger TCs do not have systematically higher maximum rain rates than weaker storms. Results also show that the translational speed of TCs has little effect on the asymmetric rainfall distribution in landfalling TCs. The maximum rainfall of both the weaker and stronger TCs is generally located downshear to downshear left. However, when environmental vertical wind shear (VWS) is less than 5 m s(-1), the asymmetric rainfall maxima are more frequently located upshear and onshore, suggesting that in weak VWS environments the coastline could have a significant effect on the rainfall asymmetry in landfalling TCs.

Tropical Cyclone (TC) Bilis made landfall on the China coast at 0500 UTC 14 July 2006. Following the landfall, sudden and unforecast torrential rain commenced some 400 km southwest of the weakening circulation center at around 1200 UTC 14 July 2006. At least 843 people were killed and the direct economic loss was estimated at up to $5 billion (U.S. dollars) in this event.Prior to the rain event, as the environmental fields evolved, the vertical vorticity weakened and deformation increased around Bilis's circulation. It is illustrated that a strong gradient wind imbalance (GWI) through midlevels became established over the northwestern quadrant of Bilis, from which a large quantity of air with high potential vorticity (PV) was redistributed from the inner circulation to the outer radii. Both backward and forward Lagrangian trajectories show this redistribution as an outward bulge of midlevel PV toward the rainfall areas. The transport of midlevel PV from inner to outer radii provides a dynamical reason for the rapid decline in rainfall around Bilis's center. It is also associated with large differential horizontal PV advection below 400 hPa over the rainfall area. Diagnostic analysis further demonstrates that the redistribution of high PV to over the rainfall areas is associated with a raising of the local isentropic surfaces and the formation of a cold dome in the mid-to lower troposphere. This is not only a direct lifting mechanism but also establishes favorable conditions for warm advection and ascent on the raised isentropic surfaces. These adiabatic ascent mechanisms are considered to have released conditional instability, resulting in broadscale convection and heavy rainfall.

To this end, Figure 1 depicts the correlation coefficients between forecast and observed inter-diurnal sets of changes in minimum temperature for lead times from Day-1 (the column for Day-1 represents the correlation coefficient between the two sets of changes from Day-2 to Day-1) to Day-31 (the column for Day-31 represents the correlation coefficient between the two sets of changes from Day-32 to Day-31). Figures 2, 3 and 4 respectively depict correlation coefficients between forecast and observed inter-diurnal changes in maximum temperature, precipitation amount and precipitation probability. _________________________________ School of Earth Sciences, University of Melbourne, Parkville, Australia. hstern@unimelb.edu.au Centre for Australian Weather and Climate Research, Bureau of Meteorology, Melbourne, Australia. n.davidson@bom.gov.au Positive values of the correlation coefficient suggest that the associated predictions possess skill at forecasting day-to-day changes. Figures 1, 2, 3 and 4 show that some skill is evident at predicting day-to-day fluctuations in each of the four weather elements out to at least Day-10. However, little skill is evident in regard to predictions of any of the weather elements beyond Day-14. Figure 5, which depicts a set of averages of the correlation coefficients shown in Figures 1, 2, 3 and 4, and represents, therefore, an attempt to illustrate ‘overall’ skill, underlines the aforementioned conclusion.

Typhoon Fitow made landfall south of Shanghai, China, on 6 October 2013. During the following two days, precipitation in excess of 300 mm day(-1) occurred 400 km to the north of the typhoon center. The rain-producing systems included (i) outward-spiraling rainbands, which developed in the storm's north sector in favorable environmental wind shear, and (ii) frontal cloud as a result of coastal frontogenesis. Over the rain area, in addition to enhanced ascent, there were increases in low-level moisture, convective instability, and midlevel relative vorticity. There is evidence of a preconditioning period prior to the rain when midlevel subsidence and boundary layer moistening occurred. From analysis of low-level equivalent potential temperature the following observations were made: (i) after landfall, a cold, dry airstream wrapped into Fitow's circulation from the north, limiting the inner-core rainfall and producing a cold-air boundary, and (ii) an extended warm, moist airstream from the east converged with the cold-air intrusion over the rain area. The heavy rain occurred as the large-scale flow reorganized. Major anticyclones developed over China and the North Pacific. At upper levels, a large-amplitude trough relocated over central China with the entrance to a southwesterly jet positioned near Shanghai. Back trajectories from the rain area indicate that four environmental interactions developed: (i) increasing midlevel injection of moist potential vorticity (PV) from Fitow's circulation; (ii) low-level warm, moist inflow from the east; (iii) midlevel inflow from nearby Typhoon Danas; and (iv) decreasing mid- to upper-level injection of PV from the midlatitude trough. The authors propose that the resultant PV structure change provided a very favorable environment for the development of rain systems.

Unique, multi‐year datasets of weather observations and official and experimental predictions are used to document trends in weather forecast accuracy and the current level of forecast skill specifically for Melbourne, Australia. The data are applied to quantify prediction skill out to Day‐14 for maximum and minimum temperature, and for precipitation amount and probability. An innovative statistical analysis is applied to the data. This analysis clearly demonstrates the need for long time series of forecasts in order to reliably assess long‐term trends. The accuracy of the current official Day 5–7 forecasts is found to be similar to that of Day‐1 forecasts from 50 years ago. The accuracy of experimental Day 8–10 forecasts is comparable to that of the Day 5–7 forecasts, when they were first officially provided 15 years ago. Some overall skill, albeit limited, is evident out to Day‐14 and significance testing indicates that it is unlikely that this apparent skill arose by chance. The results provide evidence of deterministic weather forecast skill out to the hypothesised 15 day limit on such predictions. However, in so doing, the results raise the possibility that the limit may be breached at some stage in the future.

Increased understanding of the importance of TC structure in dynamical, climatological and prediction studies makes determination of TC size important. A new algorithm for the objective estimation of the radius of outermost closed isobar (roci) has been developed. The new method uses storm position and global analyses of mean sea level pressure to compute a mean (axisymmetric) roci. This radius can be used, together with the central pressure, for the construction of a synthetic vortex that is initialized in a numerical prediction model. The method also has important applications in dynamical and climatological studies of TC intensity, size and structure. The algorithm is robust and capable of estimating roci, even in the case of a weak system that may not have a closed isobar in the global analysis. The values produced by the new method are shown to be more consistent than the corresponding operational estimates which are subjective and produced under strong time constraints. Statistical comparison between subjective and objective estimates gives a mean absolute difference of 110 km, which given the difficulty in making a subjective estimate, is satisfactory. In addition, even though limitations exist with the estimates of vortex parameters like the radius to gales (r34), comparison with estimates from an extended best track data set provides independent evaluation of the scheme. Mean absolute difference for r34 for around 3200 cases is near 80 km, even though the best track estimates are subjective and the objective r34 is estimated only from storm central pressure and the objective roci. This validation suggests that the algorithm can be used to obtain useful size estimates of TCs.

As a follow-up to this work, it was considered that it would be interesting to assess what might be achieved using the output of other global NWP models, both by themselves and in the context of a multi-model ensemble framework. This is the main focus of the current paper, which presents an analysis of preliminary results from a seven-month trial (Jul-14 to Jan-15). The trial involved applying an algorithm to statistically interpret the output of the ECMWF (EC) NWP control models in terms of day-to-day local weather for Week 1 (Days 1-7) and Week 2 (Days 8-14), and also covering the Day 15-32 period.

This study evaluates the latest release TRMM 3B42 version 7 (V7) estimates of daily rainfall in tropical cyclones (TCs) using the Comprehensive Pacific Rainfall Database (PACRAIN) of 24 h rain gauge observations. The evaluation is performed on two different terrain types: low‐lying atoll sites (assumed to represent open ocean conditions) and coastal and island sites (over land). The results show that TRMM 3B42 has good skill at detecting intense TC rainfall, with good correlation and pattern matching with PACRAIN observations. However, it tends to overestimate heavy rain frequency on atoll sites, but tends to underestimate heavy rain frequency on coastal and island sites. Overall, TRMM 3B42 is better able to estimate the intensity of TC heavy rain over ocean than over land. It is least skillful at coastal and island sites with high elevation, where it significantly underestimates TC heavy rainfall, suggesting that TRMM 3B42 is unable to capture orographic enhancement during TC landfall. Finally, results from V7 were compared with results from its predecessor, Version 6, showing that Version 7 of TRMM 3B42 has higher values on average for TC rain.

The impact of initial structure on storm evolution is examined for the case of a tropical storm entering rapid intensification. At the onset of rapid intensification, satellite cloud signatures suggest that the structural organization of Typhoon Sinlaku (2008) was dominated by a primary band of convection present at outer radii. The development of the eyewall subsequently occurred within this band of deep convection.Numerical forecasts of Sinlaku are initialized at 15- and 5-km resolution using a broad range of vortex scales, at a time when the storm was still weak and its structure not clearly defined. Evidence is presented that beta propagation played a key role in changing the storm's motion under weak environmental steering. It is found that the track forecast improves over the period when beta propagation is prominent if the vortex is initialized with a large radius of maximum wind (RMW), corresponding with the primary outer cloud band. The initial vortex structure is also suggested to play a critical role in the pathway to rapid intensification, and in the formation of the eyewall for the defined environmental forcing. With an initially large RMW, the forecast captures the evolution of structure and intensity more skillfully. Eyewall formation inside the primary outer convective band for the weak storm is illustrated and some possible dynamical interpretations are discussed.

The Australian Community Climate and Earth System Simulator (ACCESS) has been adapted for operational and research applications on tropical cyclones. The base system runs at a resolution of 0.11 degrees and 50 levels. The domain is relocatable and nested in coarser-resolution ACCESS forecasts. Initialization consists of five cycles of four-dimensional variational data assimilation (4DVAR) over 24 h. Forecasts to 72 h are made. Without vortex specification, initial conditions usually contain a weak and misplaced circulation pattern. Significant effort has been devoted to building physically based, synthetic inner-core structures, validated using historical dropsonde data and surface analyses from the Atlantic. Based on estimates of central pressure and storm size, vortex specification is used to filter the analyzed circulation from the original analysis, construct an inner core of the storm, locate it to the observed position, and merge it with the large-scale analysis at outer radii.Using all available conventional observations and only synthetic surface pressure observations from the idealized vortex to correct the initial location and structure of the storm, the 4DVAR builds a balanced, intense 3D vortex with maximum wind at the radius of maximum wind and with a well-developed secondary circulation. Mean track and intensity errors for Australian region and northwest Pacific storms have been encouraging, as are recent real-time results from the Australian National Meteorological and Oceanographic Centre. The system became fully operational in November 2011. From preliminary diagnostics, some interesting structure change features are illustrated. Current limitations, future enhancements, and research applications are also discussed.

Heavy rain from tropical cyclone (TC) landfall has extensive impacts on human life and society. Its estimation is subject to considerable uncertainty, especially in Australian tropical regions. In this study we evaluate the Tropical Rainfall Measuring Mission (TRMM) Multi‐satellite Precipitation Analysis (TMPA) 3B42 rainfall estimates in landfalling TCs over Australia. A high‐quality gauge‐based gridded rainfall product from the Australian Water Availability Project (AWAP) is utilized as reference data. The overall characteristics of TMPA 3B42 estimates are measured by mean rain rate, correlation coefficient, relative bias, relative root‐mean‐square error, and empirical orthogonal function analysis on both AWAP and TMPA 3B42. These comparisons show good correspondence over space and time between TMPA 3B42 and AWAP analysis for rainfall at TC landfall over Australia. The results also show that TMPA 3B42 generally overestimates TC rain for low rain rate but underestimates TC rain at high rain rate. TC intensity, location, terrain, and TC seasons all have impacts on TMPA 3B42's detection skill. For TC heavy rain, TMPA 3B42 shows better agreement with AWAP during more intense TCs (CAT3–5), in the eyewall as opposed to the rain bands, in the tropics as opposed to the subtropics, and in late TC seasons as opposed to early and peak TC seasons. Finally, a case study for TC Yasi (2011) is chosen to illustrate TMPA 3B42's ability to estimate TC landfall rainfall over Australia. Even though the performance of TMPA 3B42 can vary from case to case, TMPA 3B42 has a high correlation coefficient with AWAP and achieves good skill scores in most cases.