Noel E. Davidson

Influence of Environmental Vertical Wind Shear on the Intensity of Hurricane-Strength Tropical Cyclones in the Australian Region

Abstract NCEP–NCAR reanalyses have been used to investigate the impact of environmental wind shear on the intensity change of hurricane-strength tropical cyclones in the Australian region. A method of removing a symmetric vortex from objective analyses is used to isolate the environmental flow. A relationship between wind shear and intensity change is documented. Correlations between wind shear and intensity change to 36 h are of the order of 0.4. Typically a critical wind shear value of ∼10 m s−1 represents a change from intensification to dissipation. Wind shear values of less than ∼10 m s−1 favor intensification, with values between ∼2 and 4 m s−1 favoring rapid intensification. Shear values greater than ∼10 m s−1 are associated with weakening, with values greater than 12 m s−1 favoring rapid weakening. There appears to be a time lag between the onset of increased vertical wind shear and the onset of weakening, typically between 12 and 36 h. A review of synoptic patterns during intensification-weakenin...

Tropical prediction using dynamical nudging, satellite-defined convective heat sources, and a cyclone bogus

Abstract Some notable problems in tropical prediction have been (i) the sensitivity to, and inaccuracies in, the four-dimensional structure of parameterized convective heating, (ii) the inability of conventional data networks to adequately define tropical cyclone structures, and (iii) the so-called spinup problem of numerical models. To help overcome some of these deficiencies, a diabatic nudging scheme has been developed for the Bureau of Meteorology Research Centre (BMRC) limited-area tropical prediction system. A target analysis for the nudging is first obtained from statistical interpolation of all observational data, using, as first-guess field, output from a global assimilation and prediction system. Tropical cyclones are optionally inserted via bogus wind observations. From 12 or 24 h prior to the base time of the forecast, the prediction model is nudged toward the target analysis. During nudging the “observationally reliable” rotational wind component is preserved and the heating from the Kuo sche...

The BMRC High-Resolution Tropical Cyclone Prediction System: TC-LAPS

Abstract A new Tropical Cyclone Limited Area Prediction System has been developed at the Australian Bureau of Meteorology Research Centre. The features of the new prediction system can be summarized as follows: first, a 12-h, coarse-resolution data assimilation is used to define the outer structure and environment of the storm and to provide initial conditions for coarse mesh prediction. Then, synthetic data are generated to define a storm’s circulation, consistent with observed location, size, intensity, and past motion. The method involves a definition of the environmental flow by filtering of the misplaced tropical cyclone circulation in the (old) objective analysis, the generation of a new, correctly located and intense symmetric vortex, and the construction of vortex asymmetries by requiring that the observed motion be the vector sum of the environmental flow and the asymmetric flow. The subsequent initialization for fine-mesh prediction is carried out using 24 h of diabatic, dynamical nudging throug...

Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part I: The Critical Role of Vortex Enhancement in Deep Convection

This is the first of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS), an operational numerical weather prediction (NWP) forecast model. The primary TC-LAPS vortex enhancement mechanism is presented in Part I, the entire genesis process is illustrated in Part II using a single TC-LAPS simulation, and in Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. The primary vortex enhancement mechanism in TC-LAPS is found to be convergence/stretching and vertical advection of absolute vorticity in deep intense updrafts, which result in deep vortex cores of 60–100 km in diameter (the minimum resolvable scale is limited by the 0.15° horizontal grid spacing). On the basis of the results presented, it is hypothesized that updrafts of this scale adequately represent mean vertical motions in real TC genesis convective regions, and perhaps that explicitly resolving the individual convective processes may not be necessary for qualitative TC genesis forecasts. Although observations of sufficient spatial and temporal resolution do not currently exist to support or refute this proposition, relatively large-scale (30 km and greater), lower- to midlevel tropospheric convergent regions have been observed in tropical oceanic environments during the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE), the Equatorial Mesoscale Experiment (EMEX), and the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), and regions of extreme convection of the order of 50 km are often (remotely) observed in TC genesis environments. These vortex cores are fundamental for genesis in TC-LAPS. They interact to form larger cores, and provide net heating that drives the system-scale secondary circulation, which enhances vorticity on the system scale akin to the classical Eliassen problem of a balanced vortex driven by heat sources. These secondary vortex enhancement mechanisms are documented in Part II. In some recent TC genesis theories featured in the literature, vortex enhancement in deep convective regions of mesoscale convective systems (MCSs) has largely been ignored. Instead, they focus on the stratiform regions. While it is recognized that vortex enhancement through midlevel convergence into the stratiform precipitation deck can greatly enhance midtropospheric cyclonic vorticity, it is suggested here that this mechanism only increases the potential for genesis, whereas vortex enhancement through low- to midlevel convergence into deep convective regions is necessary for genesis.

Characteristics of Tropical Cyclones in the Australian Region

Abstract Characteristics of 500 tropical cyclones (TCs) in the Australian region and its three individual basins are examined based on 40 yr of satellite-supported observations. While tropical cyclones exhibit highly individual behaviors resulting in significant standard deviations, there are some systematic behaviors, which are documented. Most TCs in the Australian region originate from December to April. About 13 are observed each season, with half occurring in the western basin. Generally, the lifetime of a TC is about 7½ days, during which time it covers over 2500 km at a mean speed of 15 km h−1. Around half of the storms reach a maximum intensity corresponding to category 3 or higher (<970 hPa), as classified using a modified Saffir–Simpson scale. Tropical cyclones in the western and eastern basins have around 25% chance of making landfall, while those in the northern basin have an 80% chance. There appear to be preferred locations for TC genesis, close to the Australian coastline at around 120°, 13...

Tropical Cyclone Contribution to Rainfall over Australia

AbstractTropical cyclone (TC) rainfall over the Australian continent is studied using observations from 41 TC seasons 1969/70 to 2009/10. A total of 318 storms, whose centers either crossed the coastline or were located within 500 km of the coast, are considered in this study. Mean seasonal (November/April) contributions by TCs to the total rainfall are largest along the northern coastline from 120°–150°E. However, the percentage contributions by TCs are greatest west of 125°E, with mean coastal values of 20%–40% and inland values of approximately 20%. Farther east, percentages near the coast are only around 10%, and even lower inland. Inland penetration by TC rainfall is generally greatest over western portions of the continent, associated with greater inland penetration of TC tracks. During the peak of the TC season (January–March), TCs contribute around 40% to the rainfall total of coastal regions west of 120°E, while during December, TCs contribute approximately 60%–70% to the total rainfall west of 1...

Downstream Development in the Southern Hemisphere Monsoon during FGGE/WMONEX

Abstract Evidence is presented of a downstream development mechanism operating across the entire longitudinal span of the 1978/79 Southern Hemisphere monsoon. Observationally it is seen as progressive cyclonic and anticyclonic vorticity increases that develop eastward in the monsoon trough at a speed of approximately 5 m s−1. The process results in many tropical cyclone and tropical depression formations over northern Australia and the South Pacific. It is shown that the downstream development process is generally consistent with linearized barotropic dynamics, and that the Southern Hemisphere monsoon, because of an intrinsic westerly basic state, is a particularly suitable region for downstream events. It is also shown that some apparent contradictions in previous observational studies can be rationalized by the theory. The interactions between the regional components of the monsoon (Indonesian, Australian and South Pacific sectors) can also he better understood. We further suggest that the process has i...

The Flow during TOGA COARE as Diagnosed by the BMRC Tropical Analysis and Prediction System

Abstract The evolution of the large-scale flow through the four-month intensive observing period of TOGA COARE is documented from large-scale numerical analyses and GMS cloud imagery produced by the Australian Bureau of Meteorology and transmitted to the field stations during the experiment. The evolution of the flow is dominated by the following phenomena: 1) the normal seasonal evolution of the tropical flow over this region, including a southward and eastward progression of the tropical convective heat source as the Southern Hemisphere monsoon developed and matured; 2) a more eastward than normal progression of this monsoon circulation, associated with a warm event of the ENSO phenomenon; 3) the existence of a major westerly–easterly–westerly cycle of the Madden–Julian low-frequency wave occurring during the latter half of the experimental period, and 4) the development and subsequent movement of tropical cyclones in both (northern and southern) hemispheres. The Madden–Julian event consisted of two eas...

Prediction and Diagnosis of Tropical Cyclone Formation in an NWP System. Part II: A Diagnosis of Tropical Cyclone Chris Formation

This is the second of a three-part investigation into tropical cyclone (TC) genesis in the Australian Bureau of Meteorology’s Tropical Cyclone Limited Area Prediction System (TC-LAPS). The primary TC-LAPS vortex enhancement mechanism (convergence/stretching and vertical advection of absolute vorticity in convective updraft regions) was presented in Part I. In this paper (Part II) results from a numerical simulation of TC Chris (western Australia, February 2002) are used to illustrate the primary and two secondary vortex enhancement mechanisms that led to TC genesis. In Part III a number of simulations are presented exploring the sensitivity and variability of genesis forecasts in TC-LAPS. During the first 18 h of the simulation, a mature vortex of TC intensity developed in a monsoon low from a relatively benign initial state. Deep upright vortex cores developed from convergence/stretching and vertical advection of absolute vorticity within the updrafts of intense bursts of cumulus convection. Individual convective bursts lasted for 6–12 h, with a new burst developing as the previous one weakened. The modeled bursts appear as single updrafts, and represent the mean vertical motion in convective regions because the 0.15° grid spacing imposes a minimum updraft scale of about 60 km. This relatively large scale may be unrealistic in the earlier genesis period when multiple smaller-scale, shorter-lived convective regions are often observed, but observational evidence suggests that such scales can be expected later in the process. The large scale may limit the convection to only one or two active bursts at a time, and may have contributed to a more rapid model intensification than that observed. The monsoon low was tilted to the northwest, with convection initiating about 100–200 km west of the low-level center. The convective bursts and associated upright potential vorticity (PV) anomalies were advected cyclonically around the low, weakening as they passed to the north of the circulation center, leaving remnant cyclonic PV anomalies. Strong convergence into the updrafts led to rapid ingestion of nearby cyclonic PV anomalies, including remnant PV cores from decaying convective bursts. Thus convective intensity, rather than the initial vortex size and intensity, determined dominance in vortex interactions. This scavenging of PV by the active convective region, termed diabatic upscale vortex cascade, ensured that PV cores grew successively and contributed to the construction of an upright central monolithic PV core. The system-scale intensification (SSI) process active on the broader scale (300–500-km radius) also contributed. Latent heating slightly dominated adiabatic cooling within the bursts, which enhanced the system-scale secondary circulation. Convergence of low- to midlevel tropospheric absolute vorticity by this enhanced circulation intensified the system-scale vortex. The diabatic upscale vortex cascade and SSI are secondary processes dependent on the locally enhanced vorticity and heat respectively, generated by the primary mechanism.

Evaluation of TMPA 3B42 daily precipitation estimates of tropical cyclone rainfall over Australia

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 3B42s 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 3B42s 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. Key Points Good agreement between 3B42 and AWAP TC intensity, location, terrain and season affect TMPA 3B42s detection skill TMPA 3B42s performance varies from case to case ©2013. American Geophysical Union. All Rights Reserved.