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Rosa M. Vargas Martes
,
Ángel F. Adames Corraliza
, and
Víctor C. Mayta

Abstract

The thermodynamic processes associated with convection in tropical African and northeastern Pacific easterly waves (AEWs and PEWs, respectively) are examined on the basis of empirical orthogonal functions (EOFs) and a plume buoyancy framework. Linear regression analysis reveals the relationship between temperature, moisture, buoyancy, and precipitation in EWs. Plume buoyancy is found to be highly correlated with rainfall in both AEWs and PEWs, and a near 1:1 relationship is found between a buoyancy-based diagnostic of rainfall and rainfall rates from ERA5. Close inspection of the contribution of moisture and temperature to plume buoyancy reveals that temperature and moisture contribute roughly equally to the buoyancy in AEWs, while moisture dominates the distribution of buoyancy in PEWs. A scale analysis is performed in order to understand the relative amplitudes of temperature and moisture in easterly waves. It is found that the smaller contribution of temperature to the thermodynamics of PEWs relative to AEWs is related to their slower propagation speed, which allows PEWs to more robustly adjust to weak temperature gradient (WTG) balance. The consistency of the buoyancy analysis and the scale analysis indicates that PEWs are moisture modes: waves in which water vapor plays a dominant role in their thermodynamics. AEWs, on the other hand, are mixed waves in which temperature and moisture play similar roles in their thermodynamics.

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Jaime Fernando António
and
José Antonio Aravéquia

Abstract

Here we used meteorological datasets from ERA5 to study the dynamic and thermodynamic characteristics of a SACZ event that occurred between 12 and 26 December 2013. This is an atypical SACZ episode with considerable variations in cloudiness band positioning and high rainfall amounts, causing enormous problems for society. We study this case through the Lorenz energy cycle (LEC), focusing mainly on the role of diabatic heating in energy generation and consequently in circulation aspects, analyzing the event in three stages (formation, development, and dissipation), and discussing it according to the convection localization pattern. The diabatic heat rate has a large impact on the energy generation of SACZ events at midlevels south of 24°S and below 900 hPa in the tropics. In LEC, the generation terms in the SACZ area were larger at the beginning (12–15 December) and smaller at the ending periods (23–26 December), with means of 21.23 and −7.62 W m−2, respectively. The conversion terms follow the LEC directions, except for barotropic instability [C(KE , KM ) < 0] that dominates throughout the analyzed periods. The convection area expansion to the north between 16 and 22 December was reflected by the most intense heating in the tropics and weaker barotropic instability. The friction term did not favor the event decay; therefore, we concluded that the cooling through a negative covariance between Q and T contributed to the event decay. We find that these results were largely influenced by a midlatitude wave train configuration that acted to favor the persistence, expansion, and decay of the event.

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Kamal Kant Chandrakar
,
Hugh Morrison
, and
Raymond A. Shaw

Abstract

Turbulent fluctuations of scalar and velocity fields are critical for cloud microphysical processes, e.g., droplet activation and size distribution evolution, and can therefore influence cloud radiative forcing and precipitation formation. Lagrangian and Eulerian water vapor, temperature, and supersaturation statistics are investigated in direct numerical simulations (DNS) of turbulent Rayleigh–Bénard convection in the Pi Convection Cloud Chamber to provide a foundation for parameterizing subgrid-scale fluctuations in atmospheric models. A subgrid model for water vapor and temperature variances and covariance and supersaturation variance is proposed, valid for both clear and cloudy conditions. Evaluation of phase change contributions through an a priori test using DNS data shows good performance of the model. Supersaturation is a nonlinear function of temperature and water vapor, and relative external fluxes of water vapor and heat (e.g., during entrainment-mixing and phase change) influence turbulent supersaturation fluctuations. Although supersaturation has autocorrelation and structure functions similar to the independent scalars (temperature and water vapor), the autocorrelation time scale of supersaturation differs. Relative scalar fluxes in DNS without cloud make supersaturation PDFs less skewed than the adiabatic case, where they are highly negatively skewed. However, droplet condensation changes the PDF shape response: it becomes positively skewed for the adiabatic case and negatively skewed when the sidewall relative fluxes are large. Condensation also increases correlations between water vapor and temperature in the presence of relative scalar fluxes but decreases correlations for the adiabatic case. These changes in correlation suppress supersaturation variability for the nonadiabatic cases and increase it for the adiabatic case. Implications of this work for subgrid microphysics modeling using a Lagrangian stochastic scheme are also discussed.

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Chaehyeon C. Nam
,
Michael M. Bell
, and
Dandan Tao

Abstract

The combination of moderate vertical wind shear (VWS) and dry environments can produce the most uncertain scenarios for tropical cyclone (TC) genesis and intensification. We investigated the sources of increased uncertainty of TC development under moderate VWS and dry environments using a set of Weather Research and Forecasting (WRF) ensemble simulations. Statistical analysis of ensemble members for precursor events and time-lagged correlations indicates that successful TC development is dependent on a specific set of precursor events. A deficiency in any of these precursor events leads to a failure of TC intensification. The uncertainty of TC intensification can be largely attributed to the probabilistic characteristics of precursor events lining up together before TC intensification. The critical bifurcation point between successful and failed trials in these idealized simulations is the sustained vortex alignment process. Even for the failed intensification cases, most simulations showed deep organized convection, which reformed a midlevel vortex. However, for the failed cycles, the new midlevel vortex could not sustain vertical alignment with the low-level center and was carried away by VWS shortly. Under the most uncertain setup (VWS = 7.5 m s−1 and 50% moisture), the latest-developing ensemble member had seven events of tilt decreasing and increasing again that occurred during the 8 days before genesis. Some unsuccessful precursor events looked very close to the successful ones, implying limits on the intrinsic predictability for TC genesis and intensification in moderately sheared and dry environments.

Significance Statement

The aim of this study is to identify a critical bifurcation point that determines whether tropical disturbances in moderately sheared and dry environments will develop into intense storms or dissipate. When it comes to predicting the formation and strength of tropical cyclones, vertical wind shear, where the environmental wind changes with height, presents a challenging scenario. When the shear is neither too weak nor too strong, some systems manage to develop into cyclones, while others get torn apart under similar shear conditions. Understanding the differences between these outcomes remains a puzzle. Through extensive computer simulations, we have discovered a key factor that contributes to the uncertainty surrounding the alignment of the midlevel vortex with the center of the low-level vortex. These results reveal the complexity and multiple sources of uncertainty involved in forecasting tropical cyclone intensification, providing valuable insights into why moderate shear is a particularly challenging regime to predict tropical genesis and intensification.

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Antoine Michel
,
Ahmad Ababaei
, and
Bogdan Rosa

Abstract

The collision–coalescence of cloud droplets in atmospheric turbulent flow is analyzed numerically using direct numerical simulation coupled to a Lagrangian particle tracking. The droplet aerodynamic interactions (AI) are represented for employing two complementary approaches. For large separations, the interaction forces are evaluated by the superposition of Stokes disturbance velocities generated by moving particles. When the distance between droplets is comparable to their mean radii, lubrication forces are additionally considered. Simulation results show that without gravitational acceleration, aerodynamic interactions decrease the kinetics of the coalescence process but do not significantly impact the size spectrum broadening. The influence of AI on the coalescence kinetics is more complex in the presence of gravity and depends on the mass loading and on droplet inertia. Long-range aerodynamic interactions reduce the coalescences in dilute suspensions but increase the collision rate in dense suspensions of high-inertia droplets. In contrast, lubrication forces decrease the collision rate regardless of the mass loading. The collision efficiency induced by aerodynamic interactions additionally is influenced by the radius ratio of colliding droplets and the mechanisms leading to raindrops formation and growth. In cloud-like conditions, both long- and short-range AI decrease the fraction of raindrops created by collisions between droplets (autoconversion) while promoting raindrops growth by accretion (collection by settling drops). In turn, aerodynamic interactions favor the growth of a limited number of droplets and promote the broadening of the droplet size spectrum. This effect is stronger in dilute suspensions of weakly inertial droplets, corresponding to the flow properties encountered in developing precipitation.

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Martin Velez-Pardo
and
Timothy W. Cronin

Abstract

The organization of convection into relatively long-lived patterns of large spatial scales, like tropical cyclones, is a common feature of Earth’s atmosphere. However, many key aspects of convective aggregation and its relationship with tropical cyclone formation remain elusive. In this work, we simulate highly idealized setups of dry convection, inspired by the Rayleigh–Bénard system, to probe the effects of different thermal boundary conditions on the scale of organization of rotating convection, and on the formation of tropical cyclone–like structures. We find that in domains with sufficiently high aspect ratios, moderately turbulent ( Ra f 10 9 ), moderately rotating ( Ro c 1 ) convection organizes more persistently and at larger scales when thermal boundary conditions constrain heat fluxes rather than temperatures. Furthermore, for some thermal boundary conditions with asymmetric heat fluxes, convection organizes into persistent vortices with the essential properties of mature tropical cyclones: a warm core, high axisymmetry, a strong azimuthal circulation, and substantially larger size than individual buoyant plumes. We argue that flux asymmetry results in a persistent and localized input of buoyancy, which allows spatially aggregated convection to sustain a warm core in a developing large-scale vortex. Crucially, the most intense and axisymmetric cyclone forms for setups where the bottom heat flux is enhanced by the nearby flow and the top boundary is insulating, as long as the convective Rossby number is higher than about 1. Our results demonstrate the great potential for dialogue between classical turbulence research and the study of convective aggregation and tropical cyclones.

Significance Statement

On Earth, atmospheric convection frequently organizes into large spatial patterns that persist for several days, like tropical cyclones. However, many aspects of this process of organization and its link to tropical cyclone formation are not fully understood. In this work, we use numerical simulations of simple setups of rotating convection without moisture to study the minimal conditions that produce large-scale convective organization, and the spontaneous formation of tropical cyclone–like structures. We find that the latter form more readily for a particular set of controlling parameters and thermal boundary conditions. Our approach seeks to narrow the disciplinary gap between tropical cyclone physics and traditional turbulence research, by bringing together methods, questions, and results that are of potential interest to both.

Open access
Jacob D. Carstens
and
Allison A. Wing

Abstract

The spontaneous self-aggregation (SA) of convection in idealized model experiments highlights the importance of interactions between tropical convection and the surrounding environment. The authors have shown that SA fundamentally changes with the background rotation in previous f-plane simulations, in terms of both the resulting forms of organized convection and the relative roles of the physical feedbacks driving them. This study considers the dependence of SA on rotation in one large domain on the β plane, introducing an additional layer of complexity. Simulations are performed with uniform thermal forcing and explicit convection. Focuses include statistical and structural analysis of the convective modes, process-oriented diagnostics of how they develop, and resulting mean states. Two regimes of SA emerge within the first 15 days, separated by a critical zone where f is analogous to 10°–15° latitude. Organized convection at near-equatorial values of f primarily consists of convectively coupled Kelvin waves. Wind speed–surface enthalpy flux feedbacks are the dominant process driving moisture variability early on, then clear-sky shortwave radiative feedbacks are strongest in wave maintenance. In contrast, at higher f, numerous tropical cyclones develop and coexist, dominated by surface flux and longwave processes. Tropical cyclogenesis is most pronounced at intermediate f (analogous to 25°–40°), but are longer-lived at higher f. The resulting modes of SA at low f differ between these β-plane simulations (convectively coupled waves) and prior f-plane simulations (weak tropical cyclones or nonrotating clusters). Otherwise, these results provide further evidence for the changing roles of radiative, surface flux, and advective processes in influencing SA as f changes, as found in our previous study.

Significance Statement

In model simulations, convection often self-organizes due to interactions with its surrounding environment. These interactions are relevant in the real-world organization of rainfall and clouds, and may thus be useful to understand for improved prediction of tropical weather and climate. Previous work using a set of simple model experiments with constant Coriolis force showed that at different latitudes, different processes dominate, and different types of organized convection result. This study verifies that finding using a more complex and realistic model, where the Coriolis force varies within the domain to resemble different latitudes. Specifically, the convection here self-organizes into atmospheric waves (periodic disturbances) at low latitudes, and tropical cyclones at high latitudes.

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John M. Peters
,
Daniel R. Chavas
,
Chun-Yian Su
,
Hugh Morrison
, and
Brice E. Coffer

Abstract

This article introduces an analytic formula for entraining convective available potential energy (ECAPE) with an entrainment rate that is determined directly from an environmental sounding, rather than prescribed by the formula user. Entrainment is connected to the background environment using an eddy diffusivity approximation for lateral mixing, updraft geometry assumptions, and mass continuity. These approximations result in a direct correspondence between the storm-relative flow and the updraft radius and an inverse scaling between the updraft radius squared and entrainment rate. The aforementioned concepts, combined with the assumption of adiabatic conservation of moist static energy, yield an explicit analytic equation for ECAPE that depends entirely on state variables in an atmospheric profile and a few constant parameters with values that are established in past literature. Using a simplified Bernoulli-like equation, the ECAPE formula is modified to account for updraft enhancement via kinetic energy extracted from the cloud’s background environment. CAPE and ECAPE can be viewed as predictors of the maximum vertical velocity w max in an updraft. Hence, these formulas are evaluated using w max from past numerical modeling studies. Both of the new formulas improve predictions of w max substantially over commonly used diagnostic parameters, including undiluted CAPE and ECAPE with a constant prescribed entrainment rate. The formula that incorporates environmental kinetic energy contribution to the updraft correctly predicts instances of exceedance of 2 CAPE by w max, and provides a conceptual explanation for why such exceedance is rare among past simulations. These formulas are potentially useful in nowcasting and forecasting thunderstorms and as thunderstorm proxies in climate change studies.

Significance Statement

Substantial mixing occurs between the upward-moving air currents in thunderstorms (updrafts) and the surrounding comparatively dry environmental air, through a process called entrainment. Entrainment controls thunderstorm intensity via its diluting effect on the buoyancy of air within updrafts. A challenge to representing entrainment in forecasting and predictions of the intensity of updrafts in future climates is to determine how much entrainment will occur in a given thunderstorm environment without a computationally expensive high-resolution simulation. To address this gap, this article derives a new formula that computes entrainment from the properties of a single environmental profile. This formula is shown to predict updraft vertical velocity more accurately than past diagnostics, and can be used in forecasting and climate prediction to improve predictions of thunderstorm behavior and impacts.

Open access
Peter Brechner
,
Greg M. McFarquhar
,
Alfons Schwarzenboeck
, and
Alexei V. Korolev

Abstract

Total ice water content (IWC) derived from an isokinetic evaporator probe and ice crystal particle size distributions (PSDs) measured by a two-dimensional stereo probe and precipitation imaging probe installed on an aircraft during the 2014 European High Altitude Ice Crystals–North American High IWC field campaign (HAIC/HIWC) were used to characterize regions of high IWC consisting mainly of small ice crystals (HIWC_S) with IWC ≥ 1.0 g m−3 and median mass diameter (MMD) < 0.5 mm. A novel fitting routine developed to automatically determine whether a unimodal, bimodal, or trimodal gamma distribution best fits a PSD was used to compare characteristics of HIWC_S and other PSDs (e.g., multimodality, gamma fit parameters) for HIWC_S simulations. The variation of these characteristics and bulk properties (MMD, IWC) was regressed with temperature, IWC, and vertical velocity. HIWC_S regions were most pronounced in updraft cores. The three modes of the PSD reveal different dominant processes contributing to ice growth: nucleation for maximum dimension D < 0.15 mm, diffusion for 0.15 < D < 1.0 mm, and aggregation for D > 1.0 mm. The frequency of trimodal distributions increased with temperature. The volumes of equally plausible parameters derived in the phase space of gamma fit parameters increased with temperature for unimodal distributions and, for temperatures less than −27°C, for multimodal distributions. Bimodal distributions with 0.4 mm in the larger mode were most common in updraft cores and HIWC_S regions; bimodal distributions with 0.4 mm in the smaller mode were least common in convective cores.

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Ming Cai
,
Jie Sun
,
Feng Ding
,
Wanying Kang
, and
Xiaoming Hu

Abstract

The slope of the quasi-linear relation between planetary outgoing longwave radiation (OLR) and surface temperature (TS ) is an important parameter measuring the sensitivity of Earth’s climate system. The primary objective of this study is to seek a general explanation for the quasi-linear OLR–TS relation that remains valid regardless of the strength of the atmospheric window’s narrowing effect on planetary thermal emission at higher temperatures. The physical understanding of the quasi-linear OLR–TS relation and its slope is gained from observation analysis, climate simulations with radiative–convective equilibrium and general circulation models, and a series of online feedback suppression experiments. The observed quasi-linear OLR–TS relation manifests a climate footprint of radiative (such as the greenhouse effect) and nonradiative processes (poleward energy transport). The former acts to increase the meridional gradient of surface temperature and the latter decreases the meridional gradient of atmospheric temperatures, causing the flattening of the meridional profile of the OLR. Radiative processes alone can lead to a quasi-linear OLR–TS relation that is more steeply sloped. The atmospheric poleward energy transport alone can also lead to a quasi-linear OLR–TS relation by rerouting part of the OLR to be emitted from a warmer place to a colder place. The combined effects of radiative and nonradiative processes make the quasi-linear OLR–TS relation less sloped with a higher degree of linearity. In response to anthropogenic radiative forcing, the slope of the quasi-linear OLR–TS relation is further reduced via stronger water vapor feedback and enhanced poleward energy transport.

Significance Statement

The slope of the quasi-linear relation between planetary outgoing longwave radiation (OLR) and surface temperature (TS ) is an important parameter measuring the sensitivity of Earth’s climate system. The observed quasi-linear OLR–TS relation manifests a climate footprint of radiative (greenhouse effect) and nonradiative processes (poleward energy transport). Radiative processes alone can lead to a quasi-linear OLR–TS relation that is more steeply sloped. The atmospheric poleward energy transport alone can also lead to a quasi-linear OLR–TS relation by rerouting part of the OLR to be emitted from a warmer place to a colder place. The combined effects of radiative and nonradiative processes make the quasi-linear OLR–TS relation less sloped with a higher degree of linearity.

Open access