Hamish A. Ramsay

Interannual Variability of Tropical Cyclones in the Australian Region: Role of Large-Scale Environment

Abstract This study investigates the role of large-scale environmental factors, notably sea surface temperature (SST), low-level relative vorticity, and deep-tropospheric vertical wind shear, in the interannual variability of November–April tropical cyclone (TC) activity in the Australian region. Extensive correlation analyses were carried out between TC frequency and intensity and the aforementioned large-scale parameters, using TC data for 1970–2006 from the official Australian TC dataset. Large correlations were found between the seasonal number of TCs and SST in the Nino-3.4 and Nino-4 regions. These correlations were greatest (−0.73) during August–October, immediately preceding the Australian TC season. The correlations remain almost unchanged for the July–September period and therefore can be viewed as potential seasonal predictors of the forthcoming TC season. In contrast, only weak correlations (<+0.37) were found with the local SST in the region north of Australia where many TCs originate; these ...

Tropical Cyclone Climatology of the South Pacific Ocean and Its Relationship to El Nino-Southern Oscillation

AbstractClimatological features of tropical cyclones in the South Pacific Ocean have been analyzed based on a new archive for the Southern Hemisphere. A vortex tracking and statistics package is used to examine features such as climatological maps of system intensity and the change in intensity with time, average tropical cyclone system movement, and system density. An examination is presented of the spatial variability of these features, as well as changes in relation to phase changes of the El Nino–Southern Oscillation phenomenon. A critical line is defined in this study based on maps of cyclone intensity to describe the statistical geographic boundary for cyclone intensification. During El Nino events, the critical line shifts equatorward, while during La Nina events the critical line is generally displaced poleward. Regional variability in tropical cyclone activity associated with El Nino–Southern Oscillation phases is examined in relation to the variability of large-scale atmospheric or oceanic varia...

The Effects of Imposed Stratospheric Cooling on the Maximum Intensity of Tropical Cyclones in Axisymmetric Radiative–Convective Equilibrium

AbstractThe effects of stratospheric cooling and sea surface warming on tropical cyclone (TC) potential intensity (PI) are explored using an axisymmetric cloud-resolving model run to radiative–convective equilibrium (RCE). Almost all observationally constrained datasets show that the tropical lower stratosphere has cooled over the past few decades. Such cooling may affect PI by modifying the storms outflow temperature, which together with the sea surface temperature (SST) determines the thermal efficiency in PI theory. Results show that cooling near and above the model tropopause (∼90 hPa), with fixed SST, increases the PI at a rate of 1 m s−1 per degree of cooling. Most of this trend comes from a large increase in the thermal efficiency component of PI as the stratosphere cools. Sea surface warming (with fixed stratospheric temperature) increases the PI by roughly twice as much per degree, at a rate of about 2 m s−1 K−1. Under increasing SST, most of the PI trend comes from large changes in the air–sea ...

The Effects of Complex Terrain on Severe Landfalling Tropical Cyclone Larry (2006) over Northeast Australia

Abstract The interaction between complex terrain and a landfalling tropical cyclone (TC) over northeastern Australia is investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5). Severe TC Larry (in March 2006) made landfall over an area of steep coastal orography and caused extensive damage. The damage pattern suggested that the mountainous terrain had a large influence on the TC wind field, with highly variable damage across relatively small distances. The major aims in this study were to reproduce the observed features of TC Larry, including track, intensity, speed of movement, size, decay rate, and the three-dimensional wind field using realistic high-resolution terrain data and a nested grid with a horizontal spacing of 1 km for the finest domain (referred to as CTRL), and to assess how the above parameters change when the terrain height is set to zero (NOTOPOG). The TC track for CTRL, including the timing and locat...

Why is the tropical cyclone boundary layer not "well-mixed"?

AbstractPlausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer ad...

A High-Resolution Simulation of Asymmetries in Severe Southern Hemisphere Tropical Cyclone Larry (2006)

Abstract Advances in observations, theory, and modeling have revealed that inner-core asymmetries are a common feature of tropical cyclones (TCs). In this study, the inner-core asymmetries of a severe Southern Hemisphere tropical cyclone, TC Larry (2006), are investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the Kepert–Wang boundary layer model. The MM5-simulated TC exhibited significant asymmetries in the inner-core region, including rainfall distribution, surface convergence, and low-level vertical motion. The near-core environment was characterized by very low environmental vertical shear and consequently the TC vortex had almost no vertical tilt. It was found that, prior to landfall, the rainfall asymmetry was very pronounced with precipitation maxima consistently to the right of the westward direction of motion. Persistent maxima in low-level convergence and vertical motion formed ahead of the translating TC, resul...

Seasonal Tropical Cyclone Predictions Using Optimized Combinations of ENSO Regions: Application to the Coral Sea Basin

AbstractThis study examines combining ENSO sea surface temperature (SST) regions for seasonal prediction of Coral Sea tropical cyclone (TC) frequency. The Coral Sea averages ~4 TCs per season, but is characterized by strong interannual variability, with 1–9 TCs per season, over the period 1977–2012. A wavelet analysis confirms that ENSO is a key contributor to Coral Sea TC count (TCC) variability. Motivated by the impact of El Nino Modoki on regional climate anomalies, a suite of 38 linear models is constructed and assessed on its ability to predict Coral Sea seasonal TCC. Seasonal predictions of TCC are generated by a leave-one-out cross validation (LOOCV). An important finding is that models made up of multiple tropical Pacific SST regions, such as those that comprise the El Nino Modoki Index (EMI) or the Trans-Nino Index (TNI), perform considerably better than models comprising only single regions, such as Nino-3.4 or Nino-4. Moreover, enhanced (suppressed) TC activity is expected in the Coral Sea when...

Impact of variations in upper-level shear on simulated supercells

AbstractIt has previously been suggested, based on limited observations, that vertical wind shear in the upper troposphere is a key control on supercell morphology, with the low-precipitation, high-precipitation, and classic archetypes favored under strong, weak, and moderate shear, respectively. The idea is that, with increasing upper-level shear (ULS), hydrometeors are transported farther from the updraft by stronger storm-relative anvil-level winds, limiting their growth and thereby reducing precipitation intensity. The present study represents the first attempt to test this hypothesis, using idealized simulations of supercells performed across a range of 6–12-km shear profiles.Contrary to expectations, there is a significant increase in surface precipitation and an associated strengthening of outflow winds as ULS magnitude is increased from 0 to 20 m s−1. These changes result from an increase in storm motion, which drives stronger low-level inflow, a wider updraft, and enhanced condensation. A further...

The Modulating Influence of Indian Ocean Sea Surface Temperatures on Australian Region Seasonal Tropical Cyclone Counts

AbstractThe Australian region seasonal tropical cyclone count (TCC) maintained a robust statistical relationship with El Nino–Southern Oscillation (ENSO), with skillful forecasts of above (below) average TCC during La Nina (El Nino) years from 1969 until about 1998, weakening thereafter. The current study identifies an additional climate driver that mitigates the loss of predictive skill for Australian TCC after about 1998. It is found that the seasonal Australian TCC is strongly modulated by a southwest-to-northeast-oriented dipole in Indian Ocean sea surface temperature anomalies (SSTAs), referred to here as the transverse Indian Ocean dipole (TIOD). The TIOD emerges as the leading mode of detrended Indian Ocean SSTAs in the Southern Hemisphere during late winter and spring. Active (inactive) TC seasons are linked to positive (negative) TIOD phases, most notably during August–October immediately preceding the TC season, when SSTAs northwest of Australia, in the northeast pole of the TIOD, are positive (...