[1] 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.

A simulation of Hurricane Katrina (2005) using the Australian Bureau of Meteorology's operational model for tropical-cyclone prediction (TCLAPS) shows that the simulated vortex vacillates between almost symmetric and highly asymmetric phases. During the symmetric phase, the eyewall comprises elongated convective bands and both the low-level potential vorticity (PV) and pseudo-equivalent potential temperature θ e fields exhibit a ring structure, with the maximum at some radius from the vortex centre. During this phase the mean flow intensifies comparatively rapidly, as the maximum acceleration of the mean tangential wind occurs near the radius of maximum mean tangential wind (RMW). In contrast, during the asymmetric phase the eyewall is more polygonal, with vortical hot towers (VHTs) located at the vertices. The low-level PV and θ e fields have monopole structures with the maximum at the centre. The intensification rate is lower than during the symmetric phase because the mean tangential wind accelerates most rapidly well within the RMW.The symmetric-to-asymmetric transition is accompanied by the development of VHTs within the eyewall. The VHTs are shown to be initiated by barotropic-convective instability associated with the ring-like structure of PV in the eyewall where the convective instability is large. During the reverse asymmetric-tosymmetric transition, the VHTs weaken as the local vertical wind shear increases and the convective available potential energy is consumed by convection. The weakened VHTs move outwards, similar to vortex Rossby waves, and are stretched by the angular shear of the mean vortex. Simultaneously, the rapid filamentation zone outside the RMW weakens, becoming more favourable for the development of convection. The next symmetric phase emerges as the convection reorganizes into a more symmetric eyewall. It is proposed that vacillation cycles occur in young tropical cyclones and are distinct from the eyewall replacement cycles that tend to occur in strong and mature tropical cyclones.

Tropical 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 115°E. Rain from TCs varies sharply between TC seasons, with some longitude bands receiving no TC rain during some seasons. For the 110°–115°E longitude band the TC rain contribution is quite inconsistent, varying interannually from 0%–86%. This has an impact on water supplies, with storage dams falling to low levels during some years, while filling to capacity during TC-related flood events in other years. These large interannual variations and their impacts underline why it is important to understand TC rainfall characteristics over the Australian continent.

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 ϳ10 m s Ϫ1 represents a change from intensification to dissipation. Wind shear values of less than ϳ10 m s Ϫ1 favor intensification, with values between ϳ2 and 4 m s Ϫ1 favoring rapid intensification. Shear values greater than ϳ10 m s Ϫ1 are associated with weakening, with values greater than 12 m s Ϫ1 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-weakening cycles revealed the juxtaposition of a low-level anticyclone on the poleward side of the storm and an approaching 200-hPa trough to the west. In most cases, intensification commences under weak shear with the approach of the trough, but just prior to the onset of high shear. Further, based on described cases when wind shear was weak but no intensification occurred, it is suggested that weak shear is a necessary but not a sufficient condition for intensification. It is illustrated here that the remote dynamical influence of upper-level potential vorticity anomalies may offset the negative effects of environmental shear.

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.

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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, generat...

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