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Bethan L. Harris, Rémi Tailleux, Christopher E. Holloway, and Pier Luigi Vidale

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.

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Hai Lin, Bin Yu, and Nicholas M. J. Hall

Abstract

The warm Arctic–cold continent pattern (WACC) of near-surface air temperature variability has often been associated with the connection between Arctic sea ice reduction and cold weather over the midlatitude continents. Whether the existence of this pattern is due to variability of sea ice or is caused by atmospheric internal dynamics is subject to debate. Based on a long integration of a primitive equation atmospheric model (SGCM), this study examines the origin of the warm Arctic–cold North American pattern (WACNA), which is characterized by a pair of opposite surface air temperature anomalies over the high-latitude Chukchi–Bering Sea region and the North American continent, in boreal winter on the intraseasonal time scale. The model atmosphere is maintained by a time-independent forcing, so that atmospheric internal dynamics is the only source of variability. It is found that the SGCM model simulates well the behavior of the observed WACNA pattern. The WACNA pattern develops by interacting with the time-mean flow and synoptic-scale transient eddies. Two pathways of Rossby wave propagation are associated with WACNA. The northern pathway originates from eastern Siberia moving eastward across the Bering Strait to Canada, and the southern pathway is rooted in the subtropical waveguide propagating across the eastern North Pacific. Our simulation of this pattern implies that tropospheric dynamics alone can generate the WACNA, and the predictability associated with this pattern is likely limited by its internal dynamics nature.

Significance Statement

The warm Arctic–cold continent pattern of temperature variability has often been associated with the connection between Arctic sea ice reduction and cold weather over the midlatitude continents, which implies possible impacts of polar warming on midlatitude climate. There has been debate on whether the existence of this pattern depends on variability of sea ice or can be caused by processes within the atmosphere. In this study, we use a simple atmospheric model, which has a constant forcing; thus, atmospheric internal dynamics is the only source of variability. We show that atmospheric internal dynamics alone can generate the warm Arctic–cold North American pattern. The result has implications for our understanding of the impact of global warming.

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Nathan Sparks and Ralf Toumi

Abstract

We derive a simple physically based analytic model that describes the pressure filling of a tropical cyclone (TC) over land. Starting from the axisymmetric mass continuity equation in cylindrical coordinates, we derive that the half-life of the decay of the pressure deficit between the environment and TC center is proportional to the initial radius of maximum surface wind speed. The initial pressure deficit and column-mean radial inflow speed into the core are the other key variables. The assumptions made in deriving the model are validated against idealized numerical simulations of TC decay over land. Decay half-lives predicted from a range of initial TC states are tested against the idealized simulations and are in good agreement. Dry idealized TC decay simulations show that without latent convective heating, the boundary layer decouples from the vortex above leading to a fast decay of surface winds while a midlevel vortex persists.

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G. Alexander Sokolowsky, Sean W. Freeman, and Susan C. van den Heever

Abstract

A trimodal convective cloud distribution is commonly observed within the tropics due to the tropical-mean thermodynamic environment. The goal of this research has been to examine the integrated impacts of thermodynamic and aerosol properties on both the convective environment and the properties of the cloud modes themselves. This has been achieved by using LES experiments in which various thermodynamic and aerosol environments were independently and simultaneously perturbed. The key conclusions from this study are 1) large amounts of aerosol loading and low-level static stability suppress the bulk environment and the intensity and coverage of convective clouds; 2) cloud and environmental responses to aerosol loading tend to be stronger than those from static stability; 3) the effects of aerosol and stability perturbations modulate each other substantially; 4) the deepest convection and highest dynamical intensity occur at moderate aerosol loading, rather than at low or high loading; and 5) most of the strongest feedbacks due to aerosol and stability perturbations are seen in the boundary layer, though some are stronger above the freezing level. These results underscore the importance of considering the thermodynamic environment’s impact on aerosol-induced convective invigoration while highlighting the dominance of aerosol impacts on the trimodal distribution and revealing synergies between thermodynamics and aerosols.

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David J. Lorenz

Abstract

The annular mode, the leading pattern of low-frequency variability in the extratropics, owes its temporal persistence to a positive feedback between eddy momentum fluxes and the background zonal wind anomalies associated with the annular mode itself. The mechanisms by which the zonal wind anomalies impact the eddy momentum fluxes fall into two families: 1) baroclinic mechanisms: changes in the amount and location of wave activity generated via baroclinic instability cause the changes in eddy momentum fluxes and 2) barotropic mechanisms: the zonal wind anomalies impact the eddy momentum fluxes directly via critical levels, turning latitudes, and the refraction of meridionally propagating waves. This paper takes a critical look at various methodologies that conclude that baroclinic feedbacks are dominant by developing multiple independent estimates of the relative role of baroclinic versus barotropic processes. All methods conclude that barotropic mechanisms are most important; however, baroclinic mechanisms are not negligible. Additional experiments with the baroclinic feedback turned off (via manipulations to the vertical friction profile) also suggest that barotropic feedbacks are dominant. The methods for estimating the feedbacks are 1) Rossby wave chromatography, 2) forced manipulations of the vertical structure of EOF1 using linear response functions, and 3) quantitatively inferring the meridional wave propagation from the mean wave activity budget and then using this to analyze the wave activity response to anomalies. The last method is also applied to both Northern and Southern Hemisphere reanalysis, and similar conclusions about the feedbacks are reached.

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Spencer A. Hill, Simona Bordoni, and Jonathan L. Mitchell

Abstract

We present a theory for the latitudinal extents of both Hadley cells throughout the annual cycle by combining our recent scaling for the ascending edge latitude based on low-latitude supercriticality with the theory for the poleward, descending edge latitudes of Kang and Lu based on baroclinic instability and a uniform Rossby number (Ro) within each cell’s upper branch. The resulting expressions for all three Hadley cell edges are predictive except for diagnosed values of Ro and two proportionality constants. Thermal inertia—which damps and lags the ascent latitude relative to the insolation—is accounted for semianalytically through the Mitchell et al. model of an “effective” seasonal forcing cycle. Our theory, given empirically an additional ∼1-month lag for the descending edge, captures the climatological annual cycle of the ascending and descending edges in an Earthlike simulation in an idealized aquaplanet general circulation model (GCM). In simulations in this and two other idealized GCMs with varied planetary rotation rate (Ω), the winter, descending edge of the solsticial, cross-equatorial Hadley cell scales approximately as Ω−1/2 and the summer, ascending edge as Ω−2/3, both in accordance with our theory. Possible future refinements and tests of the theory are discussed.

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Allison A. Wing

Abstract

A complete understanding of the development of tropical cyclones (TC) remains elusive and forecasting TC intensification remains challenging. This motivates further research into the physical processes that govern TC development. One process that has, until recently, been under-investigated is the role of radiation. Here, the importance of radiative feedbacks in TC development and the mechanisms underlying their influence is investigated in a set of idealized convection-permitting simulations. A TC is allowed to form after initialization from a mesoscale warm, saturated bubble on an f plane, in an otherwise quiescent and moist neutral environment. Tropical storm formation is delayed by a factor of 2 or 3 when radiative feedbacks are removed by prescribing a fixed cooling profile or spatially homogenizing the model-calculated cooling profiles. The TC’s intensification rate is also greater when longwave radiative feedbacks are stronger. Radiative feedbacks in the context of a TC arise from interactions between spatially and temporally varying radiative heating and cooling (driven by the dependence of radiative heating and cooling rate on clouds and water vapor) and the developing TC (the circulation of which shapes the structure of clouds and water vapor). Further analysis and additional mechanism denial experiments pinpoint the longwave radiative feedback contributed by ice clouds as the strongest influence. Improving the representation of cloud-radiative feedbacks in forecast models, therefore, has the potential to yield critical advancements in TC prediction.

Significance Statement

Our understanding of the development of tropical cyclones, hurricanes, and typhoons is incomplete, and, thus, forecasting tropical cyclone formation and intensification remains challenging. This study investigates the importance of interactions between clouds and solar and infrared radiation for tropical cyclone development. I find that in idealized convection-permitting simulations, tropical cyclone development is accelerated by a factor of 2 or more with the inclusion of these cloud–radiation feedbacks. The interaction of ice clouds associated with strong thunderstorms with infrared radiation has the biggest effect. These results indicate that improving the representation of ice clouds and their radiative feedbacks in forecast models has the potential to yield critical advancements in tropical cyclone prediction.

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Jiming Sun, Jun Zhang, Wei Deng, Wenhao Hu, and Yongqing Wang

Abstract

Cloud droplet nucleation is classically defined as a droplet growing to a size such that its ambient supersaturation exceeds its surface equilibrium water vapor pressure. Unactivated particles are always in equilibrium with the ambient vapor pressure. Further studies showed that such an equilibrium assumption leads to many more cloud droplets being nucleated due to neglecting kinetic growth limitations, including the inertial mechanism, evaporation mechanism, and deactivation mechanism. Moreover, the inertial mechanism results in great discrepancy between the actual size and the critical size of nucleation for large aerosol particles. These issues complicate cloud droplet nucleation parameterization for applications in cloud modeling. To establish a physically based nucleation scheme, we established a highly size-resolved Lagrangian parcel model. Vapor diffusion and heat conduction were calculated according to Maxwell theory, and the surface vapor density and temperature were explicitly simulated. The surface temperature variation of a droplet with its size was considered. The surface supersaturation of a droplet, taking into account the surface temperature variation, is different from its equilibrium supersaturation at its large sizes. The nucleation simulation showed that the inertial and deactivation mechanisms can impact droplet nucleation. Moreover, very large nuclei can trigger rain embryo formation in a short time period. Even though there are kinetic limitations, the classical equilibrium assumption can be applied to determine the primary nucleation number of cloud droplets. Meanwhile, a regression formula for the size of a nucleated droplet and its dry aerosol size was established.

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M. Z. Sheikh, K. Gustavsson, E. Lévêque, B. Mehlig, A. Pumir, and A. Naso

Abstract

Collisions, resulting in aggregation of ice crystals in clouds, is an important step in the formation of snow aggregates. Here, we study the collision process by simulating spheroid-shaped particles settling in turbulent flows and by determining the probability of collision. We focus on platelike ice crystals (oblate ellipsoids), subject to gravity, and to the Stokes force and torque generated by the surrounding fluid. We also take into account the contributions to the drag and torque due to fluid inertia, which are essential to understand the tendency of crystals to settle with their largest dimension oriented horizontally. We determine the collision rate between identical crystals, of diameter 300 μm, with aspect ratios in the range 0.005 ≤ β ≤ 0.05, and over a range of energy dissipation per unit mass, ε, 1 ≤ ε ≤ 250 cm2 s−3. For all values of β studied, the collision rate increases with the turbulence intensity. The dependence on β is more subtle. Increasing β at low turbulence intensity (ε16cm2s3) diminishes the collision rate, but increases it at higher ε ≈ 250 cm2 s−3. The observed behaviors can be understood as resulting from three main physical effects. First, the velocity gradients in a turbulent flow tend to bring particles together. In addition, differential settling plays a role at small ε when the particles are thin enough (β small), whereas the prevalence of particle inertia at higher ε leads to a strong enhancement of the collision rate.

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Dario Lucente, Corentin Herbert, and Freddy Bouchet

Abstract

Many atmosphere and climate phenomena lie in the gray zone between weather and climate: they are not amenable to deterministic forecast, but they still depend on the initial condition. A natural example is medium-range forecasting, which is inherently probabilistic because it lies beyond the deterministic predictability time of the atmosphere, but for which statistically significant prediction can be made, which depends on the current state of the system. Similarly, one may ask the probability of occurrence of an El Niño event several months ahead of time. We introduce a quantity that corresponds precisely to this type of prediction problem: the committor function is the probability that an event takes place within a given time window, as a function of the initial condition. We compute it in the case of a low-dimensional stochastic model for El Niño, the Jin and Timmermann model. In this context, we show that the ability to predict the probability of occurrence of the event of interest may differ strongly depending on the initial state. The main result is the new distinction between probabilistic predictability (when the committor function is smooth and probability can be computed, which does not depend sensitively on the initial condition) and probabilistic unpredictability (when the committor function depends sensitively on the initial condition). We also demonstrate that the Jin and Timmermann model might be the first example of a stochastic differential equation with weak noise for which transition between attractors does not follow the Arrhenius law, which is expected based on large deviation theory and generic hypothesis.

Significance Statement

A key problem for atmospheric and climate phenomena is to predict events beyond the time scale over which deterministic weather forecast is possible. In a simple model of El Niño, we demonstrate the existence of two regimes, depending on initial conditions. For initial conditions in the “probabilistic predictability” regime, the system is unpredictable deterministically because of chaos, but the probability of occurrence of the event can still be predicted because it depends only weakly on the initial condition. In the “probabilistic unpredictability” regime, even predicting probabilities is difficult, because the probability depends strongly on initial conditions. These new concepts of probabilistic predictability and unpredictability should be key in understanding the predictability potential for rare events in climate problems, as well as in other complex dynamics.

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