Can a Warm Ocean Feature Cause a Typhoon to Intensify Rapidly?

The hypothesis that a warm ocean feature (WOF) such as a warm eddy may cause a passing typhoon to undergo rapid intensification (RI), that is, the storm’s maximum 1-min wind speed at 10-m height increases by more than 15.4 m/s in 1 day, is of interest to forecasters. Testing the hypothesis is a challenge, however. Besides the storm’s internal dynamics, typhoon intensity depends on other environmental factors such as vertical wind shear and storm translation. Here we designed numerical experiments that exclude these other factors, retaining only the WOF’s influence on the storm’s intensity change. We use a storm’s translation speed Uh = 5 m/s when surface cooling is predominantly due to 1D vertical mixing. Observations have shown that the vast majority (70%) of RI events occur in storms that translate between 3 to 7 m/s. We conducted a large ensemble of twin experiments with and without ocean feedback and with and without the WOF to estimate model uncertainty due to internal variability. The results show that the WOF increases surface enthalpy flux and moisture convergence in the storm’s core, resulting in stronger updrafts and intensity. However, the intensification rate is, in general, insufficiently rapid. Consequently, the number of RIs is not statistically significantly different between simulations with and without the WOF. An analytical coupled model supports the numerical findings. Furthermore, it shows that WOF-induced RI can develop only over eddies and ambient waters that are a few °C warmer than presently observed in the ocean.

A Numerical Study of Typhoon Megi (2010). Part II: Eyewall Evolution Crossing the Luzon Island

Typhoon Megi (2010) experienced drastic eyewall structure changes when it crossed the Luzon Island and entered the South China Sea (SCS), including the contraction and breakdown of the eyewall after landfall over the Luzon Island, the formation of a new large outer eyewall accompanied by re-intensification of the storm after it entered the SCS, and the appearance of a short-lived small inner eyewall. These features were reproduced reasonably well in a control simulation using the Advanced Weather Research and Forecasting (ARW–WRF) model. In this study, the eyewall processes of the simulated Megi during and after landfall have been analyzed.Results show that the presence of the landmass of Luzon Island increased surface friction and reduced surface enthalpy flux, leading to the original eyewall to contract and break down and the weakening of the storm. The formation of the new large eyewall results mainly from the axisymmetrization of outer spiral rainbands after the storm core moved across the Luzon Island and entered the SCS. The appearance of the small inner eyewall over the SCS was due to the increased surface enthalpy flux and the revival of convection in the central region of the storm core. In a sensitivity experiment with the mesoscale-mountain replaced by flat surface over the Luzon Island, a new large outer eyewall formed over the western Luzon Island with its size about one third smaller after the storm entered the SCS than that in the control experiment with the terrain over the Luzon Island unchanged.

Surface Thermodynamics, Viscosity, Activation Energy of N-Methyldiethanolamine Aqueous Solutions Promoted by Tetramethylammonium Arginate

The surface tension and viscosity values of N-methyldiethanolamine (MDEA) aqueous solutions promoted by tetramethylammonium arginate ([N1111][Arg]) were measured and modeled. The experimental temperatures were 303.2 to 323.2 K. The mass fractions of MDEA (wMDEA) and [N1111][Arg] (w[N1111][Arg]) were 0.300 to 0.500 and 0.025 to 0.075, respectively. The measured surface tension and viscosity values were satisfactorily fitted to thermodynamic models. With the aid of experimentally viscosity data, the activation energy (Ea) and H2S diffusion coefficient (DH2S) of MDEA-[N1111][Arg] aqueous solution were deduced. The surface entropy and surface enthalpy of the solutions were calculated using the fitted model of the surface tension. The quantitative relationship between the calculated values (surface tension, surface entropy, surface enthalpy, viscosity, activation energy, and H2S diffusion coefficient) and the operation conditions (mass fraction and temperature) was demonstrated.

Slow Modes of the Equatorial Waveguide

A recently developed linear model of eastward-propagating disturbances has two separate unstable modes: convectively coupled Kelvin waves destabilized by the wind dependence of the surface enthalpy flux, and slow, MJO-like modes destabilized by cloud–radiation interaction and driven eastward by surface enthalpy fluxes. This latter mode survives the weak temperature gradient (WTG) approximation and has a time scale dictated by the time it takes for surface fluxes to moisten tropospheric columns. Here we extend that model to include higher-order modes and show that planetary-scale low-frequency waves with more complex structures can also be amplified by cloud–radiation interactions. While most of these waves survive the WTG approximation, their frequencies and growth rates are seriously compromised by that approximation. Applying instead the assumption of zonal geostrophy results in a better approximation to the full spectrum of modes. For small cloud–radiation and surface flux feedbacks, Kelvin waves and equatorial Rossby waves are destabilized, but when these feedbacks are strong enough, the frequencies do not lie close to classical equatorial dispersion curves except in the case of higher-frequency Kelvin and Yanai waves. An eastward-propagating n = 1 mode, in particular, has a structure resembling the observed structure of the MJO.

Linking surface enthalpy fluxes to the forces driving the secondary circulation: towards a causal theory of tropical cyclone intensification

<p>While it is well accepted that tropical cyclones (TCs) derive their energy from surface enthalpy fluxes over the ocean, there is still little understanding of the precise causes and effects by which the latter ends up as TC vortex kinetic energy. For example, Potential Intensity (PI) theory, which has been so far the main framework for predicting TC intensities, assumes a balance between the Carnot power input and the kinetic energy dissipated by surface friction, but says nothing of the detailed physical processes linking the two. A similar criticism pertains to the WISHE (Wind Induced Surface Heat Exchange) theory. To achieve a causal theory of TC intensification, the main difficulty is in linking the power input to kinetic energy production, rather than kinetic energy dissipation. Because kinetic energy is produced at the expense of available potential energy (APE), APE theory is arguably the most promising candidate framework for achieving a causal theory of TC intensification. However, in its current form, the usefulness of APE theory appears to be limited in a number of ways because of its reliance on a notional reference state of rest. First, APE production associated with standard reference states (i.e., horizontally averaged density field, density field of initial sounding, adiabatically sorted states, ...) is usually found to systematically overestimate the kinetic energy actually produced in ideal TC simulations, similarly as the Carnot theory of heat engines; moreover, the standard APE is only connected to vertical buoyancy forces, but says nothing of the radial forces required to drive the secondary circulation. To address these shortcomings, this work presents a new theory of available energy (AE) that is based on the use of an axisymmetric vortex reference state in gradient wind balance. This theory possesses the following advantages over previous frameworks:</p><p> </p><ul><li>The available energy (AE) thus constructed possesses both a mechanical and thermodynamic component. The thermodynamic component is analogous to the well-known Slantwise Convective Available Potential Energy (SCAPE), whereas the mechanical component is proportional to the anomalous azimuthal kinetic energy;</li> <li>The rate of AE production by surface enthalpy fluxes is found to be a very accurate predictor of the amount of potential energy actually converted into kinetic energy in idealised TC simulations based on the Rotunno and Emanuel (1986) axisymmetric model, although a few exceptions are found for cold SSTs;</li> <li>In addition to the expected thermodynamic efficiencies, the production term for AE also involves mechanical efficiencies predicting the fraction of the sinks/sources of angular momentum creating/destroying AE;</li> <li>The AE is related to a generalised buoyancy/inertial force that has both vertical and horizontal components; at low levels, such a generalised force has radially inward and vertically upward components, as required to drive the expected secondary circulation.</li> </ul><p>The new theory, therefore, appears to possess all the ingredients to form the basis for a causal theory of TC intensification.</p>

Assessing the Influence of Convective Downdrafts and Surface Enthalpy Fluxes on Tropical Cyclone Intensity Change in Moderate Vertical Wind Shear

The thermodynamic impacts of downdraft-induced cooling/drying and downstream recovery via surface enthalpy fluxes within tropical cyclones (TCs) were investigated using dropsonde observations collected from 1996 to 2017. This study focused on relatively weak TCs (tropical depression, tropical storm, category 1 hurricane) that were subjected to moderate (4.5–11.0 m s−1) levels of environmental vertical wind shear. The dropsonde data were analyzed in a shear-relative framework and binned according to TC intensity change in the 24 h following the dropsonde observation time, allowing for comparison between storms that underwent different intensity changes. Moisture and temperature asymmetries in the lower troposphere yielded a relative maximum in lower-tropospheric conditional instability in the downshear quadrants and a relative minimum in instability in the upshear quadrants, regardless of intensity change. However, the instability increased as the intensification rate increased, particularly in the downshear quadrants. This was due to increased boundary layer moist entropy relative to the temperature profile above the boundary layer. Additionally, significantly larger surface enthalpy fluxes were observed as the intensification rate increased, particularly in the upshear quadrants. These results suggest that in intensifying storms, enhanced surface enthalpy fluxes in the upshear quadrants allow downdraft-modified boundary layer air to recover moisture and heat more effectively as it is advected cyclonically around the storm. By the time the air reaches the downshear quadrants, the lower-tropospheric conditional instability is enhanced, which is speculated to be more favorable for updraft growth and deep convection.

Quasigeostrophic Controls on Precipitating Ascent in Monsoon Depressions

South Asian monsoon depressions are convectively coupled cyclonic vortices that form and intensify in a region of easterly vertical shear of the horizontal wind. Observations of maximum precipitation downshear of the cyclonic center have led to prior theories of quasigeostrophic (QG) control of moist convection in these storms. This study examines the interaction between adiabatic QG lifting and moist convection in monsoon depressions using an atmospheric reanalysis and idealized model. Inversion of the QG omega equation in the reanalysis shows that in the downshear, heavily precipitating region, adiabatic QG ascent, due to advection of vorticity and temperature, is comparable to diabatic ascent in the lower troposphere, while diabatic ascent dominates in the middle and upper troposphere. The causal influence of adiabatic QG lifting on precipitating ascent in monsoon depressions is then examined in the column QG modeling framework, where moist convection evolves in the presence of vorticity and temperature advection. The heavy observed precipitation rates are only simulated when moist convective heating amplifies QG ascent, with this interaction accounting for roughly 40% of the increase in precipitation relative to the basic state. Another 40% of this increase is produced by enhanced surface wind speed in the surface enthalpy flux parameterization, which represents the effect of cyclonic winds in the monsoon depression. Horizontal advection of the mean-state poleward moisture gradient accounts for the remaining 20% of the precipitation increase. In the upshear region, adiabatic QG subsidence and horizontal moisture advection both suppress precipitation, and are opposed by wind-enhanced surface enthalpy fluxes.