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Zi-Qi Liu
and
Zhe-Min Tan

Abstract

This study analyzes the variations in the thermodynamic cycle and energy of a tropical cyclone (TC) under the influence of vertical wind shear (VWS), exploring the possible thermodynamic pathways through which VWS affects TC intensity. The maximum energy harnessed by the TC diminishes alongside a decrease in storm intensity in the presence of VWS. In the sheared TC, the ascending branch of the thermodynamic cycles of TC shifts toward lower entropy, which is related to the reduction of entropy in the eyewall and/or the increase of entropy and enhanced upward motion outside the eyewall. Moreover, the descending leg to shift toward higher entropy due to the increase in entropy and weakening of downward motion in both the ambient environment and upper troposphere. These changes in the ascending and descending branches could reduce the work done by the heat engine cycle, with the former playing a primary role in the presence of VWS.

Given that the ascending branch is influenced by the eyewall and the rainbands outside the eyewall under VWS, the thermodynamic pathways could be categorized into inner ventilation and outer ventilation based on the location of their roles. The pathways associated with inner ventilation primarily reduce the entropy in the eyewall. In addition to the conventional low- and mid-level ventilation, the inner ventilation also encompasses new pathways entering the mid-level eyewall after descending from the upper level and ascending from the boundary layer. Conversely, the pathways of outer ventilation are related to the increase the entropy outside the eyewall. These include the ascent of high-entropy air to the middle and upper troposphere related to the inner and outer rainbands, the outward advection of high-entropy air from the eyewall in the mid- and upper-levels, and air warming by the descending draft from the upper to the mid-level troposphere. These insights contribute to a nuanced understanding of the sophisticated interactions within TCs and their response to VWS.

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Cunbo Han
,
Corinna Hoose
, and
Viktoria Dürlich

Abstract

Multiple mechanisms have been proposed to explain secondary ice production (SIP), and SIP has been recognized to play a vital role in forming cloud ice crystals. However, most weather and climate models do not consider SIP in their cloud microphysical schemes. In this study, in addition to the default rime splintering (RS) process, two SIP processes, namely, shattering/fragmentation during freezing of supercooled rain/drizzle drops (DS) and breakup upon ice–ice collisions (BR), were implemented into a two-moment cloud microphysics scheme. Besides, two different parameterization schemes for BR were introduced. A series of sensitivity experiments were performed to investigate how SIP impacts cloud microphysics and cloud phase distributions in warm-based deep convective clouds developed in the central part of Europe. Simulation results revealed that cloud microphysical properties were significantly influenced by the SIP processes. Ice crystal number concentrations (ICNCs) increased up to more than 20 times and surface precipitation was reduced by up to 20% with the consideration of SIP processes. Interestingly, BR was found to dominate SIP, and the BR process rate was larger than the RS and DS process rates by four and three orders of magnitude, respectively. Liquid pixel number fractions inside clouds and at the cloud top decreased when implementing all three SIP processes, but the decrease depended on the BR scheme. Peak values of ice enhancement factors (IEFs) in the simulated deep convective clouds were 102–104 and located at −24°C with the consideration of all three SIP processes, while the temperature dependency of IEF was sensitive to the BR scheme. However, if only RS or RS and DS processes were included, the IEFs were comparable, with peak values of about 6, located at −7°C. Moreover, switching off the cascade effect led to a remarkable reduction in ICNCs and ice crystal mass mixing ratios.

Significance Statement

The cloud phase is found to have a significant impact on cloud evolution, radiative properties, and precipitation formation. However, the simulation of the cloud phase is a big challenge for cloud research because multiple processes are not well described or missing in numerical models. In this study, we implemented two secondary ice production (SIP) processes, namely, shattering/fragmentation during the freezing of supercooled rain/drizzle drops and breakup upon ice–ice collisions, which are missing in most numerical models. Sensitivity experiments were conducted to investigate how SIP impacts cloud microphysics and cloud phase in deep convective clouds. We found that SIP significantly impacts in-cloud and cloud-top phase distribution. We also identified that the collisional breakup of ice particles is the dominant SIP process in the simulated deep convective clouds.

Open access
Weixuan Xu
,
Baylor Fox-Kemper
,
Jung-Eun Lee
,
J. B. Marston
, and
Ziyan Zhu

Abstract

The rotation of Earth breaks time-reversal and reflection symmetries in an opposite sense north and south of the equator, leading to a topological origin for certain atmospheric and oceanic equatorial waves. Away from the equator, the rotating shallow-water and stably stratified primitive equations exhibit Poincaré inertia–gravity waves that have nontrivial topology as evidenced by their strict superinertial time scale and a phase singularity in frequency–wavevector space. This nontrivial topology then predicts, via the principle of bulk-interface correspondence, the existence of two equatorial waves along the equatorial interface, the Kelvin and Yanai waves. To directly test the nontrivial topology of Poincaré-gravity waves in observations, we examine ERA5 data and study cross correlations between the wind velocity and geopotential height of the midlatitude stratosphere at the 50 hPa height. We find the predicted vortex and antivortex in the relative phase of the geopotential height and velocity at the high frequencies of the waves. By contrast, lower-frequency planetary waves are found to have trivial topology also as expected from theory. These results demonstrate a new way to understand stratospheric waves and provide a new qualitative tool to investigate waves in other components of the climate system.

Open access
Fan Wu
and
Kelly Lombardo

Abstract

This study employs 3D idealized numerical experiments to investigate the physical processes associated with coastal convection initiation (CI) as an offshore-moving squall line traverses a mountainous coastal region. A squall line can propagate discretely as convection initiates over the lee slope downstream of the primary storm as the cold pool collides with a sea breeze. Intensity of the initiating convection, thus the downstream squall line, is sensitive to the sea breeze numerical initialization method, since it influences sea breeze and cold pool characteristics, instability and vertical wind shear in the sea breeze environment, and ultimately the vertical acceleration of air parcels during CI. Here, sea breezes are generated through four commonly used numerical methods: a cold-block marine atmospheric boundary layer (MABL), prescribed surface sensible heat flux function, prescribed surface sensible plus latent heat flux functions, and radiation plus surface-layer parameterization schemes. For MABL-initialized sea breezes, shallow weak sea breeze flow in a relatively low instability environment results in weak CI. For the remainder, deeper stronger sea breeze flow in an environment of enhanced instability supports more robust CI. In a subset of experiments, however, the vertical trajectory of air parcels is suppressed leading to weaker convection. Downward acceleration forms due to the horizontal rotation of the sea breeze flow. Accurate simulations of coastal convective storms rely on an accurate representation of sea breezes. For idealized experiments such as the present simulations, a combination of initialization methods likely produces a more realistic representation of sea breeze and the associated physical processes.

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Yi Li
and
Craig Epifanio

Abstract

The physics of the surface drag (or surface stress) boundary condition is explored in the context of semi-idealized flows past realistic terrain. Numerical experiments are presented to explore the impact of the drag condition on flows past a region of complex topography, with a particular focus on the dependence on terrain geometry. Arguments are presented to show that the drag condition depends on the geometry of the terrain in two respects: (i) a dependence on terrain slope, as represented by a normal gradient term; and (ii) a dependence on the curvature, which appears in the drag condition as a Dirichlet term. The dependence on the geometry is illustrated through a series of numerical experiments in which simulations using the full form of the drag condition are compared to companion simulations using one of two widely used approximations: (a) the normal gradient condition, which accounts for the terrain slope but neglects curvature; and (b) the flat boundary assumption, which neglects both slope and curvature. The results show that the role of the terrain geometry in the drag condition is strongly dependent on grid spacing, with more highly resolved topography leading to a stronger dependence on the slope and curvature. For sufficiently high resolutions, the dependence on the geometry becomes significant, to the extent that simulations using the approximate drag conditions fail to capture important aspects of the flow. Some basic implications of these results for the problem of high resolution wind energy forecasting are discussed.

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Sofia Menemenlis
,
Gabriel A. Vecchi
,
Kun Gao
,
James A. Smith
, and
Kai-Yuan Cheng

Abstract

The extratropical stage of Hurricane Ida (2021) brought extreme sub-daily rainfall and devastating flooding to parts of eastern Pennsylvania, New Jersey, and New York. We investigate the predictability and character of this event using 31-member ensembles of perturbed-initial condition hindcasts with T-SHiELD, a ∼13 km global weather forecast model with a ∼3 km nested grid. At lead times of up to four days, the ensembles are able to capture the most extreme observed hourly and daily rainfall accumulations, but are negatively biased in the spatial extent of heavy precipitation. Large intra-ensemble differences in the magnitudes and locations of simulated extremes suggest that although impacts were highly localized, risks were widespread. In Ida’s tropical stage, inter-ensemble spread in extreme hourly rainfall is well predicted by large-scale moisture convergence; by contrast, in Ida’s extratropical stage, the most extreme rainfall is governed by mesoscale processes that exhibit chaotic and diverse forms across the ensembles. Our results are relevant to forecasting and communication in advance of extratropical transition, and imply that flood preparedness efforts should account for the widespread possibility of severe localized impacts.

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Zhibo Zhang
,
David B. Mechem
,
J. Christine Chiu
, and
Justin A. Covert

Abstract

Because of the coarse grid size of Earth system models (ESM), representing warm-rain processes in ESMs is a challenging task involving multiple sources of uncertainty. Previous studies evaluated warm-rain parameterizations mainly according to their performance in emulating collision-coalescence rates for local droplet populations over a short period of a few seconds. The representativeness of these local process rates comes into question when applied in ESMs for grid sizes on the order of 100 kilometers and time steps on the order of 20-30 minutes. We evaluate several widely used warm-rain parameterizations in ESM application scenarios. In the comparison of local and instantaneous autoconversion rates, the two parameterization schemes based on numerical fitting to stochastic collection equation (SCE) results perform best. However, because of Jessen’s inequality, their performance deteriorates when grid-mean, instead of locally-resolved, cloud properties are used in their simulations. In contrast, the effect of Jessen’s inequality partly cancels the overestimation problem of two semi-analytical schemes, leading to an improvement in the ESM-like comparison. In the assessment of uncertainty due to the large time step of ESMs, it is found that the rain-water tendency simulated by the SCE is roughly linear for time steps smaller than 10 minutes, but the nonlinearity effect becomes significant for larger time steps, leading to errors up to a factor of 4 for a time step of 20 minutes. After considering all uncertainties, the grid-mean and time-averaged rain-water tendency based on the parameterization schemes are mostly within a factor of 4 of the local benchmark results simulated by SCE.

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

Abstract

In mixed-phase clouds, graupel forms by riming, a process whereby ice crystals and supercooled water droplets settling through a turbulent flow collide and aggregate. We consider here the early stage of the collision process of small ice crystals with water droplets and determine numerically the geometric collision kernel in turbulent flows (therefore neglecting all interactions between the particles and assuming a collision efficiency equal to unity), over a range of energy dissipation rate 1–250 cm2 s−3 relevant to cloud microphysics. We take into account the effect of small, but nonzero fluid inertia, which is essential since it favors a biased orientation of the crystals with their broad side down. Since water droplets and ice crystals have different masses and shapes, they generally settle with different velocities. Turbulence does not play any significant role on the collision kernel when the difference between the settling velocities of the two sets of particles is larger than a few millimeters per second. The situation is completely different when the settling speeds of droplets and crystals are comparable, in which case turbulence is the main cause of collisions. Our results are compatible with those of recent experiments according to which turbulence does not clearly increase the growth rate of tethered graupel in a flow transporting water droplets.

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David C. Fritts
,
Ling Wang
,
Tom Lund
, and
Marvin A. Geller

Abstract

A companion paper by Fritts et al. (2023) reviews extensive evidence for Kelvin-Helmholtz instability (KHI) “tube” and “knot” (T&K) dynamics at multiple altitudes in the atmosphere and in the oceans that reveal these dynamics to be widespread. A second companion paper by Fritts and Wang (2023) reveals KHI T&K events at larger and smaller scales to arise on multiple highly-stratified sheets in a direct numerical simulation (DNS) of idealized, multi-scale gravity wave – fine structure interactions. These studies reveal the diverse environments in which KHI T&K dynamics arise and suggest their potentially ubiquitous occurrence throughout the atmosphere and oceans. This paper describes DNS of multiple KHI evolutions in wide and narrow domains enabling and excluding T&K dynamics. These DNS employ common initial conditions, but are performed for decreasing Reynolds numbers, Re, to explore whether T&K dynamics enable enhanced KHI-induced turbulence where it would be weaker or not otherwise occur. The major results are that KHI T&K dynamics extend elevated turbulence intensities and energy dissipation rates, ε, to smaller Re. We expect these results to have important implications for improving parameterizations of KHI-induced turbulence in the atmosphere and oceans.

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Lakshmi Kantha

Abstract

In 1977, S. A. Thorpe proposed a method to estimate the dissipation rate ε of turbulence kinetic energy (TKE) in an overturning turbulent layer in a lake, by sorting the observed (unstable) density profile to render it stable and thus deriving a length scale LT named after him, from the resulting vertical displacements of water parcels. By further proposing that this purely empirical scale (with no a priori physical basis, unlike many other turbulence length scales) is proportional to the Ozmidov scale LO , definable only for stably (not unstably or neutrally) stratified flows, he was able to extract ε. The simplicity of the approach that requires nothing but CTD (Conductivity, Temperature and Depth) casts in water bodies, including lakes and oceans, made it attractive, until microstructure profilers were developed and perfected in later decades to actually make in-situ measurements of ε. Since equivalent microstructure devices are not available for the atmosphere, Thorpe technique has been resurrected in recent years for application to the atmosphere, using potential temperature profiles obtained from high vertical resolution radiosondes. Its popularity and utility have increased lately, in spite of unresolved issues related to the validity of assuming LT is proportional to LO . In this study, we touch upon these issues and offer an alternative interpretation of the Thorpe length scale as indicative of the turbulence velocity scale σ K , which allows Thorpe sorting technique to be applied to all turbulent flows, including those generated by convection.

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