Potential Intensity Theory Research Articles

Comprehensive Comparison of Seven Widely-Used Planetary Boundary Layer Parameterizations in Typhoon Mangkhut Intensification Simulation

Numerical experiments using the WRF model were conducted to analyze the sensitivity of Typhoon Mangkhut intensification simulations to seven widely used planetary boundary layer (PBL) parameterization schemes, including YSU, MYJ, QNSE, MYNN2, MYNN3, ACM2, and BouLac. The results showed that all simulations generally reproduced the tropical cyclone (TC) track and intensity, with YSU, QNSE, and BouLac schemes better capturing intensification processes and closely matching observed TC intensity. In terms of surface layer parameterization, the QNSE scheme produced the highest Ck/Cd ratio, resulting in stronger TC intensity based on Emanuel’s potential intensity theory. In terms of PBL parameterization, the YSU and BouLac schemes, with the same revised MM5 surface layer scheme, simulated weaker turbulent diffusivity Km and shallower mixing height, leading to stronger TC intensity. During the intensification period, the BouLac, YSU, and QNSE PBL schemes exhibited stronger tangential wind, radial inflow within the boundary layer, and updraft around the eye wall, consistent with TC intensity results. Both PBL and surface layer parameterization significantly influenced simulated TC intensity. The QNSE scheme, with the largest Ck/Cd ratio, and the YSU and BouLac schemes, with weaker turbulent diffusivity, generated stronger radial inflow, updraft, and warm core structures, contributing to higher storm intensity.

Impact of Global Warming on Tropical Cyclone Track and Intensity: A Numerical Investigation

Despite numerous studies, the impact of global warming on the tropical cyclone (TC) track and intensity by reasons of data inhomogeneity in remote sensing and large natural variability over a relatively short period of observation is still controversial. Three carbon-emission sensitivity experiments are conducted to investigate how TC track and intensity respond to changes in the oceanic and atmospheric environment under global warming. The results show a high sensitivity of the simulated TC track and intensity to global warming. On one hand, with increase in carbon emissions, the western Pacific subtropical high expands notably, increasing the poleward steering flow and eventually leading to a poleward shift of TC. On the other hand, the underlying sea-surface temperature and surface-entropy flux increase and, thus, favor the convections near the eyewall. Moreover, the TC structure becomes more upright, which is closely related to the larger pressure gradient near the eyewall. As a result, TC intensity increases with carbon emissions. However, this increase is notably smaller than the maximum potential intensity theory as the TC intensity can reach a threshold if carbon emission still increases in the future. The involved mechanisms on the changes of TC track and intensity are also revealed.

Evaluation of Air–Sea Flux Parameterization for Typhoon Mangkhut Simulation during Intensification Period

Using the Advanced Research Weather Research and Forecasting (WRF) model, a series of numerical experiments are conducted to examine the sensitivity of the Typhoon Mangkhut intensification simulation to different air–sea flux parameterization schemes (isftcflx option), including option 0 (OPT0), option 1 (OPT1), and option 2 (OPT2). The results show that three schemes basically reproduce tropical cyclone (TC) track and intensity of observation, and the simulated exchange coefficient of three schemes is consistent with theoretical results. Using the same upper limit of Cd as OPT0 and OPT2, OPT1 has much larger Ck than the other two options, which leads to larger latent heat (and sensible heat) flux and produces stronger inflow (within boundary layer) and updrafts (around eyewall), and thus stronger TC intensity. Meanwhile, the results that larger Ck/Cd corresponds with stronger TC in the mature stage are consistent with Emanuel’s potential intensity theory. The fact that Ck in OPT1 is evidently larger than the Ck from previous studies leads to produce a better TC intensity simulation. Generally, we should use more reasonable air–sea flux parameterization based on observation to improve TC intensity simulation.

A Moist Available Potential Energy Budget for an Axisymmetric Tropical Cyclone

Abstract The main energy source for the intensification of a tropical cyclone (TC) is widely accepted to be the transfer of energy from the ocean to the atmosphere via surface fluxes. The pathway through which these surface fluxes lead to an increase in the kinetic energy of the cyclone has typically been interpreted either in terms of total potential energy or dry available potential energy (APE), or through the entropy-based heat engine viewpoint. Here, we use the local theory of APE to construct a budget of moist APE for an idealized axisymmetric simulation of a tropical cyclone. This is the first full budget of local moist APE budget for an atmospheric model. In the local moist APE framework, latent surface heat fluxes are the dominant generator of moist APE, which is then converted into kinetic energy via buoyancy fluxes. In the core region of the TC, the inward transport of APE by the secondary circulation is more important than its local production. The APE viewpoint describes spatially and temporally varying efficiencies; these may be useful in understanding how changes in efficiency influence TC development, and have a maximum that can be linked to the Carnot efficiency featuring in potential intensity theory.

Targeted artificial ocean cooling to weaken tropical cyclones would be futile

Proposals to use technology to cool sea surface temperatures have received attention for the potential application of weakening a tropical cyclone ahead of landfall. Here, application of an ocean-mixing aware maximum potential intensity theory finds that artificial ocean cooling could drastically weaken tropical cyclones over high sea surface temperature and deep ocean mixed layer environments, especially for fast storm motion speeds. In contrast, realistic mesoscale numerical simulations reveal that massive regions - the largest evaluated here contains a volume of 2.1 × 104 km3 and a surface area of 2.6 × 105 km2 - of artificially cooled ocean waters could weaken a tropical cyclone two days before landfall by 15% but only under the most ideal atmospheric and oceanic conditions. Thus, the fundamental theory provides an unreachable upper-bound that cannot be attained even by expending vast resources.

Can Existing Theory Predict the Response of Tropical Cyclone Intensity to Idealized Landfall?

AbstractTropical cyclones cause significant inland hazards, including wind damage and freshwater flooding, which depend strongly on how storm intensity evolves after landfall. Existing theoretical predictions for storm intensification and equilibrium storm intensity have been tested over the open ocean but have not yet been applied to storms after landfall. Recent work examined the transient response of the tropical cyclone low-level wind field to instantaneous surface roughening or drying in idealized axisymmetricf-plane simulations. Here, experiments testing combined surface roughening and drying with varying magnitudes of each are used to test theoretical predictions for the intensity response. The transient response to combined surface forcings can be reproduced by the product of their individual responses, in line with traditional potential intensity theory. Existing intensification theory is generalized to weakening and found capable of reproducing the time-dependent inland intensity decay. The initial (0–10 min) rapid decay of near-surface wind caused by surface roughening is not captured by existing theory but can be reproduced by a simple frictional spindown model, where the decay rate is a function of surface drag coefficient. Finally, the theory is shown to compare well with the prevailing empirical decay model for real-world storms. Overall, results indicate the potential for existing theory to predict how tropical cyclone intensity evolves after landfall.

Potential Role of Irreversible Moist Processes in Modulating Tropical Cyclone Surface Wind Structure

AbstractTropical cyclone (TC) wind structure is important for its intensity change and induced damage, but its modulating factors remain to be explored. A heat-engine-based surface wind structure parameter α, reflecting TC’s relative compactness, is introduced and derived based on an entropy budget framework. We found that α is modulated by three key parameters: the thermodynamic efficiency ϵPI in potential intensity theory, the Carnot efficiency ϵC of the system, and the degree of irreversibility αirr of the system. A higher αirr contributes to a larger α and a lower heat engine efficiency. An expression linking TC intensity and compactness also emerges under this framework. Idealized simulations of a typical moist TC (CTL), a dry (DRY) TC, and a moist reversible TC (REV; in which hydrometeors do not fall out) evinced that the significantly higher αirr in CTL, due to irreversible entropy productions from precipitation dissipation, water vapor diffusion, and irreversible phase changes, contributes to its much larger compactness compared to DRY and REV. The study illustrates the importance of irreversible entropy production processes in modulating TC surface wind field. Simple estimate suggests that α will increase due to a hypothesized increased αirr with warming because of increased water content. This indicates that TCs will become more compact in a warmer climate.

Influence of dust aerosols on eastern Pacific tropical cyclone intensity

The thermodynamic state of the tropical atmosphere plays an important role in the development of tropical cyclone (TC) intensity. This study reports new results that demonstrate a negative association between eastern Pacific TC intensity in offshore regions and dust aerosol optical depth (AOD) for the years 1980–2019. At the same time, a negative response of cloud water in the lower troposphere to dust AOD is reported by analyzing MERRA-2 reanalysis data and GCM simulations from CMIP6. This response can be explained by the dust semi-direct effect, in which dust aerosols absorb solar radiation, promoting the evaporation of clouds. In principle, this aerosol-driven vaporization modification could affect the enthalpy of the air surrounding a TC. Using potential intensity theory, the authors demonstrate that change in TC intensity related to dust AOD conditions is a consequence of the anomalous enthalpy of the air surrounding a TC caused by the dust semi-direct effect.摘要热带地区大气的热力学状态对台风的发展起着重要作用.本项研究结果表明, 东太平洋台风强度与沙尘AOD之间存在一定的负相关.同时, MERRA-2再分析数据和CMIP6的GCM模拟的分析结果, 都表明了低层大气中云对沙尘AOD的负响应.沙尘颗粒物的准直接辐射效应可以解释上述响应:沙尘颗粒物吸收太阳辐射, 并用此热量促进云层蒸发.理论上, 这种气溶胶驱动的汽化作用会影响台风周围空气的焓.根据台风的潜在强度理论, 与沙尘相关的台风强度变化, 实则是沙尘的准直接辐射效应引起的台风周围空气焓值变化的结果.

Precise Prediction of Hurricane Power vs Ocean Temperature

It has long been known that hurricanes, the strongest and most destructive atmospheric events, do not occur below a sea surface temperature near 26 C. The detailed dependence of hurricane power on ocean temperature is of increasing interest and concern in the prospect of continuing global warming. The hurricane power was usefully quantified by Emanuel in 2005 with the definition and tabulations of the power dissipation index, PDI. This is the integral over the relevant sea areas of the cube of the maximum windspeed, representiting the power dissipated over one year. In his important 2005 paper Emanuel found that the PDI for the North Atlantic increased strongly in recent decades and showed in plots a close correlation of PDI with sea surface temperature. A critical temperature Tc, and a linear T-Tc power law dependence, typical of a continuous phase transition, for hurricanes have prcviously (Wolf, 2020) been inferred from plots of the power dissipation index PDI vs sea surface temperature T. This implies that tropical cyclone formation can usefully be regarded as a second order phase transition of the warm ocean-atmosphere system, driven by disequilibrium in atmospheric water content. We here show that the theory of phase transitions allows a precise prediction of the temperature dependence of hurricane power and windspeed on ocean surface temperature. We find that the wind velocity transition of the hurricane is in the same universality class as the Ising Model, the uniaxial antiferromagenet and the vapor- liquid transition of simple fluids, and shares their critical exponent. An implication for the applicability of potential intensity theory is noted.

An Index to Better Estimate Tropical Cyclone Intensity Change in the Western North Pacific

AbstractA revised predictor called the net energy gain rate (NGR) is suggested by considering wind‐dependent drag coefficient based on the existing maximum potential intensity theory. A series of wind speed‐dependent NGR, known as NGR‐w, is calculated based on pretropical cyclone (TC) averaged ocean temperatures from the surface down to 120 m (at 10‐m intervals) to include the TC‐induced vertical mixing for 13 years (2004–2016) in the western North Pacific. It turns out that NGR50‐w (NGR‐w based on temperature averaged over top 50 m) has the highest correlation with 24‐hr TC intensity change compared with the commonly used sea surface temperature‐based intensification potential (POT), depth‐averaged temperature‐based POT (POTDAT), and constant drag coefficient in the NGR. To demonstrate the effectiveness of NGR50‐w, we designed and conducted experiments for training (2004–2014) and testing (2015–2016). The model with NGR50‐w shows greater skill than does the model with POTDAT or POT by reducing prediction errors by about 16%.

Dominant Role of the Ocean Mixed Layer Depth in the Increased Proportion of Intense Typhoons During 1980–2015

AbstractOver the past decade extensive studies have been undertaken to understand the increasing trend in the proportion of intense tropical cyclones (categories 4 and 5 on the Saffir‐Simpson scale). The trend has been found globally and in some individual basins since the late 1970s. This study quantifies the contributions of various factors that control the proportion of intense typhoons. It is demonstrated that the increase of the proportion of intense typhoons during 1980–2015 is consistent with the corresponding changes in the ocean/atmosphere environment. The proportion change resulted from the temporal variations of the environmental parameters (sea surface temperature, ocean mixed layer depth, outflow temperature, and vertical wind shear), as well as the shifts of tropical cyclone prevailing tracks. The nonuniform spatial distribution of environmental parameters makes the shifts of tropical cyclone prevailing tracks contribute at least half the increase of the proportion of intense typhoons. The deepening of the ocean mixed layer resulting from the temporal variations and track shifts plays a dominant role in the observed increase of the proportion of intense typhoons. Although the maximum potential intensity theory and numerical modeling project an increase of tropical cyclone intensity in a warming climate, the effects of the temporal change of the ocean mixed layer depth and the prevailing track change were not taken into account in the projection. This study suggests that the increase of the proportion of intense typhoons in the western North Pacific basin could be larger than the projection in previous studies.

Revised Parameterization of Air–Sea Exchanges in High Winds for Operational Numerical Prediction: Impact on Tropical Cyclone Track, Intensity, and Rapid Intensification

Abstract 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.

A Reformulation of Tropical Cyclone Potential Intensity Theory Incorporating Energy Production along a Radial Trajectory

AbstractA modified formula for calculating tropical cyclone (TC) potential intensity (PI) from a balance between energy production and frictional dissipation in the TC surface layer is developed. This modified formula accounts for energy production and frictional dissipation at multiple radii (and therefore at multiple wind speeds) along the TC inflow trajectory. The PI maximum wind speed values VMAX are calculated using this expanded formula for four canonical radial profiles of wind speed. These results are compared to PI VMAX values calculated using the standard assumption that all energy production and frictional dissipation relevant to maximum intensity occurs at the radius of maximum winds (RMW).The new PI formulation developed here results in PI VMAX values substantially higher than the standard PI VMAX for all four of the radial wind speed profiles examined; the difference is explained by the increase in the outer radial limit of energy production. This increase holds true even if outflow temperature increases with increasing radius, although the VMAX increases with increasing outer radius are somewhat more modest in this case. The extended PI formula yields VMAX values 3–17 m s−1 higher than the standard PI VMAX value when calculated with outer energy production–dissipation limits of 2.0–2.5 RMW, although it yields potentially unrealistic values when calculated with larger outer limits (e.g., 6 RMW). These results are presented as a potential explanation for why individual TCs can exceed their standard PI VMAX values in terms of the storm thermodynamics.

最大潜在強度理論による将来の温暖化した環境における台風強度の定量化

最大潜在強度理論による将来の温暖化した環境における台風強度の定量化

Cyclone‐cyclone interactions through the ocean pathway

AbstractThe intense sea surface temperature cooling caused by tropical cyclone‐induced mixing lasts several weeks and may thus influence a later cyclone passing over it. Using a 28 year analysis spanning the North Atlantic, eastern Pacific, and Northwest Pacific, we systematically demonstrate that, on average, when tropical cyclones encounter lingering wakes, they experience sea surface temperatures that are ∼0.25–0.5°C colder. Consequently, the intensification rates are lower for cyclones when they interact with wakes, consistent with the maximum potential intensity theory. The probability for cyclones to encounter lingering wakes varies positively with cyclone frequency, is ∼10% on average, and has been as high as 27%–37% in the past. These large interaction probabilities reduce the mean intensification rates for cyclones by 3%–6% on average and by ∼12%–15% during the most active years. “Cyclone‐cyclone interactions” may therefore represent a mechanism through which tropical cyclones self‐regulate their activity to an extent on intraseasonal time scales.

Future Changes in Structures of Extremely Intense Tropical Cyclones Using a 2-km Mesh Nonhydrostatic Model

Abstract Recent studies have projected that global warming may lead to an increase in the number of extremely intense tropical cyclones. However, how global warming affects the structure of extremely intense tropical cyclones has not been thoroughly examined. This study defines extremely intense tropical cyclones as having a minimum central pressure below 900 hPa and investigates structural changes in the inner core and thereby changes in the intensity in the future climate. A 2-km mesh nonhydrostatic model (NHM2) is used to downscale the 20-km mesh atmospheric general circulation model projection forced with a control scenario and a scenario of twenty-first-century climate change. The eyewall region of extremely intense tropical cyclones simulated by NHM2 becomes relatively smaller and taller in the future climate. The intense near-surface inflow intrudes more inward toward the eye. The heights and the radii of the maximum wind speed significantly decrease and an intense updraft area extends from the lower level around the leading edge of thinner near-surface inflows, where the equivalent potential temperature substantially increases in the future climate. Emanuel’s potential intensity theory suggests that about half of the intensification (increase in central pressure fall) is explained by the changes in the atmospheric environments and sea surface temperature, while the remaining half needs to be explained by other processes. It is suggested that the structural change projected by NHM2, which is significant within a radius of 50 km, is playing an important role in the intensification of extremely intense tropical cyclones in simulations of the future climate.

Effects of Parameterized Diffusion on Simulated Hurricanes

Abstract In this study the authors analyze and interpret the effects of parameterized diffusion on the nearly steady axisymmetric numerical simulations of hurricanes presented in a recent study. In that study it was concluded that horizontal diffusion was the most important control factor for the maximum simulated hurricane intensity. Through budget analysis it is shown here that horizontal diffusion is a major contributor to the angular momentum budget in the boundary layer of the numerically simulated storms. Moreover, a new scale analysis recognizing the anisotropic nature of the parameterized model diffusion shows why the horizontal diffusion plays such a dominant role. A simple analytical model is developed that captures the essence of the effect. The role of vertical diffusion in the boundary layer in the aforementioned numerical simulations is more closely examined here. It is shown that the boundary layer in these simulations is consistent with known analytical solutions in that boundary layer depth increases and the amount of “overshoot” (maximum wind in excess of the gradient wind) decreases with increasing vertical diffusion. However, the maximum wind itself depends mainly on horizontal diffusion and is relatively insensitive to vertical diffusion; the overshoot variation with vertical viscosity mainly comes from changes in the gradient wind with vertical viscosity. The present considerations of parameterized diffusion allow a new contribution to the dialog in the literature on the meaning and interpretation of the Emanuel potential intensity theory.

Observed Structure, Evolution, and Potential Intensity of Category 5 Hurricane Isabel (2003) from 12 to 14 September

AbstractUnprecedented observations of Hurricane Isabel (2003) at category 5 intensity were collected from 12 to 14 September. This study presents a detailed analysis of the inner-core structure, atmospheric boundary layer, sea surface temperature, and outflow layer of a superintense tropical cyclone using high-resolution in situ flight-level, NCAR GPS dropwindsonde, Doppler radar, and satellite measurements. The analysis of the dropwindsonde and in situ data includes a comprehensive discussion of the uncertainties associated with this observational dataset and provides an estimate of the storm-relative axisymmetric inner-core structure using Barnes objective analysis. An assessment of gradient and thermal wind balance in the inner core is also presented. The axisymmetric data composites presented in this study suggest that Isabel built a reservoir of high moist entropy air by sea-to-air latent heat flux inside the low-level eye that was utilized as an additional energy source to nearly maintain its extreme intensity even after crossing the cool wake of Hurricane Fabian. It is argued here that the combined mean and asymmetric eddy flux of high moist entropy air from the low-level eye into the eyewall represents an additional power source or “turbo boost” to the hurricane heat engine. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound for the given environmental conditions on all three days. This discrepancy between a priori potential intensity theory and observations may be as high as 35 m s−1 on 13 September.

A critique of Emanuel's hurricane model and potential intensity theory

AbstractWe present a critique of Emanuel's steady‐state hurricane model, which is a precursor to his theory for hurricane potential intensity (PI). We show that a major deficiency of the theory is the tacit assumption of gradient wind balance in the boundary layer, a layer that owes its existence to gradient wind imbalance in the radial momentum equation. If a more complete boundary‐layer formulation is included using the gradient wind profiles obtained from Emanuel's theory, the tangential wind speed in the boundary layer becomes supergradient, invalidating the assumption of gradient wind balance. We show that the degree to which the tangential wind is supergradient depends on the assumed boundary‐layer depth. The full boundary‐layer solutions require a knowledge of the tangential wind profile above the boundary layer in the outer region where there is subsidence into the layer and they depend on the breadth of this profile. This effect is not considered in Emanuel's theory. We argue that a more complete theory for the steady‐state hurricane would require the radial pressure gradient above the boundary layer to be prescribed or determined independently of the boundary layer.The issues raised herein highlight a fundamental problem with Emanuel's theory for PI, since that theory makes the same assumptions as in the steady‐state hurricane model. Our current findings together with recent studies examining intense hurricanes suggest a way forward towards a more consistent theory for hurricane PI. Copyright © 2008 Royal Meteorological Society