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The article discusses the importance of peer reviewers in scholarly publishing and expresses gratitude towards those who volunteer their time to review manuscripts. The American Meteorological Society (AMS) analyzed data on peer reviews and found that there is no trend of declining reviewer acceptance rates or longer review times. The article also emphasizes the responsibility of authors to participate in the peer-review process by reviewing papers themselves. The authors of the article acknowledge the time and effort involved in reviewing and encourage those who have not yet embraced their responsibilities as reviewers to volunteer. [Extracted from the article] more...

Numerical simulations of turbulent moist Rayleigh–Bénard convection driving CCN activation and droplet growth in the laboratory Pi chamber are discussed. Supersaturation fluctuations come from isobaric mixing of warm and humid air rising from the lower boundary with colder air featuring lower water vapor concentrations descending from the upper boundary. Lagrangian particle–based microphysics is used to represent the growth of haze CCN and cloud droplets with kinetic, solute, and surface tension effects included. Dry CCN spectra in the range between 2- and 200-nm radii from field observations are considered. Increasing the total CCN concentration from pristine to polluted conditions results in an increase in the droplet concentration and reduction in the mean droplet radius and spectral width. These are in agreement with Pi chamber observations and numerical simulations, as well as with numerous past studies of CCN cloud-base activation in natural clouds. The key result is that a relatively small fraction of the available CCN is activated in the Pi chamber fluctuating supersaturations, from about a half in the pristine case to only a 10th in the polluted case. The activation fraction as a function of the dry CCN radius is similar in all simulations, close to zero at the CCN small end, increasing to a maximum at CCN radius around 50 nm, and decreasing to close to zero at the large CCN end. This is explained as too small supersaturations to activate small CCN as in natural clouds and insufficient time to allow large CCN reaching the critical radius. Significance Statement: Impact of turbulence on the formation and growth of cloud droplets is an important cloud physics problem. Laboratory experiments in the Michigan Technological University cloud chamber provide key insights into this problem. Numerical simulations of cloud chamber processes discussed in this paper complement laboratory experiments by providing insights difficult or impossible to obtain in the laboratory. The study contrasts the formation and growth of cloud droplets in the laboratory cloud chamber with processes taking place in natural clouds. The differences documented in this paper pose questions concerning the impact of turbulence on the formation and growth of cloud droplets as a result of interactions of clouds with their environment. [ABSTRACT FROM AUTHOR] more...

We derive, at leading order in the thin-shell parameter, a consistent set of nonlinear governing equations for the dynamics of flows confined to a zonal cloud band such as those on Jupiter, in a thin layer near the top of the planetary troposphere. Some exact solutions are provided in the material (Lagrangian) framework. The explicit specification of the individual particle paths enables a detailed study of these flows that model oscillations superimposed on a mean current. This approach is applied to Jupiter's Great Red Spot and to the filamentary zonal flow at its southern boundary. Significance Statement: We propose a new approach to the study of some flow patterns visible in zonal cloud bands on Jupiter. Motivated by observations showing that the dominant motions in the cloud bands on Jupiter are zonal and rotational, we provide some exact solutions to the governing equations for the leading-order dynamics. These solutions model rotating particle paths interacting with a straight-line flow. The approach offers detailed insight into basic features of the flow, highlighting the interplay between density variations and wind forcing. The exact solutions presented here are a useful starting point for a perturbation analysis. Current advances in computing methods enhance the feasibility of numerical simulations of perturbed flows that are designed to capture a wider range of effects, whose relevance can be ascertained by comparison with the exact solutions presented in this paper. [ABSTRACT FROM AUTHOR] more...

Eastward-moving moist deep convection and atmospheric circulation signals associated with the tropical Madden–Julian oscillation (MJO) sometimes break down as they cross the Maritime Continent region, but other times, the signal propagates across the region maintaining amplitude or regaining it over the west Pacific basin. This paper assesses the hypothesis that upper-tropospheric zonal diffluence of the background wind over the Maritime Continent causes much of this Maritime Continent barrier effect and its variation over time, through two mechanisms: 1) by slowing down the MJO as stronger-than-average background upper-tropospheric zonal wind over the Indian Ocean advects the MJO circulation signal westward, slowing its eastward advance, and 2) through the zonal advection of the background wind by subseasonal zonal wind across a region of zonal diffluence of the background wind, which advects the background wind of the opposite sign to the MJO wind. Advection of the opposite-signed background wind counteracts the MJO wind and reduces its associated upper-tropospheric mass divergence, weakening the mechanisms of the upper-tropospheric Kelvin wave component of the MJO circulation. Composites of MJO-associated zonal wind and outgoing longwave radiation signals diminish as they cross the Maritime Continent region when the region's background zonal winds are diffluent, and composites of data reconstructing the relevant advection terms reveal the direct action of the advection mechanisms. Significance Statement: The Madden–Julian oscillation (MJO) is the leading subseasonal variation of the tropical atmosphere. This project addresses how diffluence of the upper-tropospheric background zonal wind can break down MJO events through advection of and by the background wind. [ABSTRACT FROM AUTHOR] more...

Boundary layer cloud transformations at high latitudes play a key role for the Arctic climate and are partially controlled by large-scale dynamics such as subsidence. While measuring large-scale and mesoscale divergence on spatial scales on the order of 100 km has proven notoriously difficult, recent airborne campaigns in the subtropics have successfully applied measurement techniques using multiple dropsonde releases in circular flight patterns. In this paper, it is shown that this method can also be effectively applied at high latitudes, in spite of the considerable differences in atmospheric dynamics compared to the subtropics. To show the applicability, data collected during the airborne High Altitude and Long Range Research Aircraft–Transregional Collaborative Research Center TRR 172-Arctic Amplification: Climate Relevant Atmospheric and Surface Processes and Feedback Mechanisms [HALO–(AC)3] field campaign near Svalbard in spring 2022 were analyzed, where several flight patterns involving multiple dropsonde launches were realized by two aircraft. This study presents a first overview of the results. We find that the method indeed yields reliable estimates of mesoscale gradients in the Arctic, producing robust vertical profiles of horizontal divergence and, consequently, subsidence. Sensitivity to aspects of the method is investigated, including dependence on sampling area and the divergence calculation. Significance Statement: The aim of this work is to report encouraging results with a recently proposed aircraft-based method for measuring mesoscale vertical motions at high latitudes. Dropsonde data from the recent High Altitude and Long Range Research Aircraft–Transregional Collaborative Research Center TRR 172-Arctic Amplification: Climate Relevant Atmospheric and Surface Processes and Feedback Mechanisms [HALO–(AC)3] campaign are used for this purpose. The method has so far mainly been applied at subtropical to midlatitudes, but not in the Arctic, where the weather and climate conditions are very different. Gaining insight is significant because vertical winds play a key role in the Arctic climate system, including airmass transformations and the behavior of clouds and precipitation. The results motivate the use of the measured vertical winds in follow-up studies with process models. [ABSTRACT FROM AUTHOR] more...

The supersaturation equation for a vertically moving adiabatic cloud parcel is analyzed. The effects of turbulent updrafts are incorporated in the shape of a stochastic Lagrangian model, with spatial and time correlations expressed in terms of turbulent kinetic energy. Using the Fokker–Planck equation, the steady-state probability distributions of supersaturation are analytically computed for a number of approximations involving the time-scale separation between updraft fluctuations and phase relaxation, and droplet or ice particle size fluctuations. While the analytical results are presented in general for single-phase clouds, the calculated distributions are used to compute mixed-phase cloud properties—mixed fraction and mean liquid water content in an initially icy cloud—and are argued to be useful for generalizing and constructing new parameterization schemes. Significance Statement: Supersaturation is the fuel for the development of clouds in the atmosphere. In this paper, our goal is to better understand the supersaturation budget of clouds embedded in a turbulent environment by analyzing the basic equations of cloud microphysics. It is found that the turbulent characteristics of an air parcel substantially affect the cloud's supersaturation budget and hence its life cycle. This is also shown in the context of mixed-phase clouds where, depending on the turbulent regime, different liquid-to-ice ratios are found. Consequently, the theoretical approach of this paper is crucial to develop tools to parameterize small-scale atmospheric features, like clouds, into global circulation models to improve climate projections for the future. [ABSTRACT FROM AUTHOR] more...

The author offers information on efforts made by the organization American Meteorological Society (AMS) to develop Meteorological Monographs, a collections of review papers, and distinguish them from special collections of the journal articles. It discusses the procedure for sending monograph papers to the journal and also discuss peer-review process for each paper. more...

Presents papers about atmospheric sciences scheduled for publication in the upcoming issues of the periodical 'Journal of the Atmospheric Sciences.' Range of topics; Names of authors.

The spectrum of kinetic energy of vertical motions (VKE) is less well understood compared to the kinetic energy spectrum of horizontal motions (HKE). One challenge that has limited progress in describing the VKE spectrum is a lack of a unified approach to the decomposition of vertical velocities associated with the Rossby motions and inertia–gravity (IG) wave flows. This paper presents such a unified approach using a linear Rossby–IG vertical velocity normal-mode decomposition appropriate for a spherical, hydrostatic atmosphere. New theoretical developments show that for every zonal wavenumber k, the limit VKE is proportional to the total mechanical energy and to the square of the frequency of the normal mode. The theory predicts a VKE ∝ k−5 and a VKE ∝ k1/3 power law for the Rossby and IG waves, assuming a k−3 and a k−5/3 power law for the Rossby and IG HKE spectra, respectively. The Kelvin and mixed Rossby–gravity wave VKE spectra are predicted to follow k−1 and k−5 power laws, respectively. The VKE spectra for ERA5 data from August 2018 show that the Rossby VKE spectra approximately follow the predicted a k−5 power law. The expected k1/3 power law for the gravity wave VKE spectrum is found only in the SH midlatitude stratosphere for k ≈ 10–60. The inertial range IG VKE spectra in the tropical and midlatitude troposphere reflect a mixture of ageostrophic and convection-coupled dynamics and have slopes between −1 and −1/3, likely associated with too steep IG HKE spectra. The forcing by quasigeostrophic ageostrophic motions is seen as an IG VKE peak at synoptic scales in the SH upper troposphere, which gradually moves to planetary scales in the stratosphere. Significance Statement: The spectrum of kinetic energy of vertical motions (VKE) is less well understood compared to the kinetic energy spectrum of horizontal motions. One challenge is a lack of a unified approach to the decomposition of vertical velocities associated with the Rossby motions and inertia–gravity (IG) wave flows. This paper presents such a unified approach using a linear Rossby–IG vertical velocity normal-mode decomposition appropriate for a spherical, hydrostatic atmosphere. It is shown that for every zonal wavenumber, the limit VKE is proportional to the total mechanical energy and to the square of the frequency of the normal mode. The theory is successfully applied to the ERA5 data. It leads the way for a more accurate computation of momentum fluxes. [ABSTRACT FROM AUTHOR] more...

A companion paper by Fritts et al. reviews evidence for Kelvin-Helmholtz instability (KHI) "tube" and "knot" (T&K) dynamics that appear to be widespread throughout the atmosphere. Here we describe the results of an idealized direct numerical simulation of multiscale gravity wave dynamics that reveals multiple larger- and smaller-scale KHI T&K events. The results enable assessments of the environments in which these dynamics arise and their competition with concurrent gravity wave breaking in driving turbulence and energy dissipation. A larger-scale event is diagnosed in detail and reveals diverse and intense T&K dynamics driving more intense turbulence than occurs due to gravity wave breaking in the same environment. Smaller-scale events reveal that KHI T&K dynamics readily extend to weaker, smaller-scale, and increasingly viscous shear flows. Our results suggest that KHI T&K dynamics should be widespread, perhaps ubiquitous, wherever superposed gravity waves induce intensifying shear layers, because such layers are virtually always present. A second companion paper demonstrates that KHI T&K dynamics exhibit elevated turbulence generation and energy dissipation rates extending to smaller Reynolds numbers for relevant KHI scales wherever they arise. These dynamics are suggested to be significant sources of turbulence and mixing throughout the atmosphere that are currently ignored or underrepresented in turbulence parameterizations in regional and global models. [ABSTRACT FROM AUTHOR] more...

Gradient wind PI The main derivation and assumptions of the gradient wind PI theory are by no means new ([2]) and all the assumptions are detailed in the paper that introduced it. Mixing with high angular momentum environmental air allows the air in the outflow of the hurricane to regain angular momentum while the environment loses some. This does not change the fact that eddies can move angular momentum radially since, as the authors of the comment point out, angular momentum is conserved (not angular velocity). Clearly, the angular velocity at a given radius cannot change without also changing angular momentum. c. Response to comment on energetics Heat energy does not offset the loss of angular momentum, but rather the dissipation of kinetic energy. [Extracted from the article] more...

Tropical cyclone (TC) structure and intensity are strongly modulated by interactions with deep-layer vertical wind shear (VWS)—the vector difference between horizontal winds at 200 and 850 hPa. This paper presents a comprehensive review of more than a century of research on TC–VWS interactions. The literature broadly agrees that a TC vortex becomes vertically tilted, precipitation organizes into a wavenumber-1 asymmetric pattern, and thermal and kinematic asymmetries emerge when a TC encounters an environmental sheared flow. However, these responses depend on other factors, including the magnitude and direction of horizontal winds at other vertical levels between 200 and 850 hPa, the amount and location of dry environmental air, and the underlying sea surface temperature. While early studies investigated how VWS weakens TCs, an emerging line of research has focused on understanding how TCs intensify under moderate and strong VWS (i.e., shear magnitudes greater than 5 m s−1). Modeling and observational studies have identified four pathways to intensification: vortex tilt reduction, vortex reformation, axisymmetrization of precipitation, and outflow blocking. These pathways may not be uniquely different because convection and vortex asymmetries are strongly coupled to each other. In addition to discussing these topics, this review presents open questions and recommendations for future research on TC–VWS interactions. [ABSTRACT FROM AUTHOR] more...

On 7 February 2020, precipitation within the comma-head region of an extratropical cyclone was sampled remotely and in situ by two research aircraft, providing a vertical cross section of microphysical observations and fine-scale radar measurements. The sampled region was stratified vertically by distinct temperature layers and horizontally into a stratiform region on the west side, and a region of elevated convection on the east side. In the stratiform region, precipitation formed near cloud top as side-plane, polycrystalline, and platelike particles. These habits occurred through cloud depth, implying that the cloud-top region was the primary source of particles. Almost no supercooled water was present. The ice water content within the stratiform region showed an overall increase with depth between the aircraft flight levels, while the total number concentration slightly decreased, consistent with growth by vapor deposition and aggregation. In the convective region, new particle habits were observed within each temperature-defined layer along with detectable amounts of supercooled water, implying that ice particle formation occurred in several layers. Total number concentration decreased from cloud top to the −8°C level, consistent with particle aggregation. At temperatures > −8°C, ice particle concentrations in some regions increased to >100 L−1, suggesting secondary ice production occurred at lower altitudes. WSR-88D reflectivity composites during the sampling period showed a weak, loosely organized banded feature. The band, evident on earlier flight legs, was consistent with enhanced vertical motion associated with frontogenesis, and at least partial melting of ice particles near the surface. A conceptual model of precipitation growth processes within the comma head is presented. Significance Statement: Snowstorms over the northeast United States have major impacts on travel, power availability, and commerce. The processes by which snow forms in winter storms over this region are complex and their snowfall totals are hard to forecast accurately because of a poor understanding of the microphysical processes within the clouds composing the storms. This paper presents a case study from the NASA IMPACTS field campaign that involved two aircraft sampling the storm simultaneously with radars, and probes that measure the microphysical properties within the storm. The paper examines how variations in stability and frontal structure influence the microphysical evolution of ice particles as they fall from cloud top to the surface within the storm. [ABSTRACT FROM AUTHOR] more...

Mixed-phase clouds contain both supercooled cloud liquid and ice crystals. In principle, precipitation may be initiated either by the liquid phase or by the ice phase. Ice crystals may grow by vapor diffusion to become snow ("ice crystal process"), forming "cold" precipitation. Equally, cloud droplets, when large enough, coalesce to form "warm" precipitation by the "warm rain process." Warm rain could be supercooled and freeze as "warm" graupel. In the present paper, a new simplified theoretical analysis is provided to examine the microphysical system consisting of three species of hydrometeor, namely, cloud liquid, "cold ice" (crystals, snow), and "warm rain" (frozen or supercooled). This is obtained by nondimensionalizing and simplifying the evolution equations for the mass of each species. Analytical formulas are given for equilibria. Feedback analysis shows that the sign of the feedback is linked to the abundance of precipitation, with a neutral surface in the 3D phase space. The system's precipitation amount explodes while in the initial unstable regime, crossing the neutral surface and approaching the equilibrium point that is a stable attractor. Positive and negative feedbacks are elucidated. In a standard case, the cold ice mass is about 1000 times larger than the warm rain mass. To illustrate the physical behavior of the theory, sensitivity tests are performed with respect to environmental conditions (e.g., aerosol, updraft speed) and microphysical parameters (e.g., riming and sedimentation rates for cold ice). Cold ice prevails, especially in fast ascent, due to its low bulk density, favoring slow sedimentation and a wide cross-sectional area for riming. Significance Statement: The theory elucidates how the ice phase can prevail in the precipitation from any mixed-phase clouds with supercooled cloud liquid and crystals. The ice phase radically suppresses cloud liquid by riming when active and "wins" the competition against coalescence. This prevalence of ice is shown to arise from the low bulk density of snow. The cloud is viewed as a system of negative and positive feedbacks that prevail in realms of stability and instability in a 3D phase space. [ABSTRACT FROM AUTHOR] more...

The results of airborne measurements and statistical characteristics of mesoscale fluctuations of wind velocity, temperature, and concentrations of gas constituents at different heights of a stably stratified troposphere are presented. The measurements were carried out in September 2022 in the Arctic region of Russia with the aircraft laboratory Tu-134 "Optik." The obtained spectra and structure functions of the fluctuations are interpreted with the theoretical model of formation of the spectrum of mesoscale wind velocity and temperature fluctuations described in the paper. The presence at high wavenumbers of a steep section in the obtained horizontal wavenumber spectra of the fluctuations of wind velocity and greenhouse gas concentration with a slope close to −3 is discussed. The fluctuation spectra along different slanted tracks of the aircraft crossing the tropospheric layer between altitudes of 1 and 9 km are also obtained and analyzed with the theoretical model. [ABSTRACT FROM AUTHOR] more...

Sudden stratospheric warmings (SSWs) are impressive events that occur in the winter hemisphere's polar stratosphere and are capable of producing temperature anomalies upward of +50 K within a matter of days. While much work has been dedicated toward determining how SSWs occur and their ability to interact with the underlying troposphere, one underexplored aspect is the role of radiation, especially during the recovery phase of SSWs. Using a radiative transfer model and a heating rate analysis for distinct layers of the stratosphere averaged over the 60°–90°N polar region, this paper accounts for the radiative contribution to the removal of the anomalous temperatures associated with SSWs. In total 17 events are investigated over the 1979–2016 period. This paper reveals that in the absence of dynamical heating following major SSWs, longwave radiative cooling dominates and often results in a strong negative temperature anomaly. The polar winter stratospheric temperature change driven by the radiative cooling is characterized by an exponential decay of temperature with an increasing e-folding time of 5.7 ± 2.0 to 14.6 ± 4.4 days from the upper to middle stratosphere. The variability of the radiative relaxation rates among the SSWs was determined to be most impacted by the initial temperature of the stratosphere and the combined dynamic and solar heating rates following the onset of the events. We also found that trace-gas anomalies have little impact on the radiative heating rates and the temperature evolution during the SSWs in the mid- to upper stratosphere. [ABSTRACT FROM AUTHOR] more...

In a previous paper a formula was derived for the maximum potential intensity of the tangential wind in a tropical cyclone called PI+. The formula, PI+2 = EPI2 + αrmwmηm, where EPI is the maximum potential intensity of the gradient wind and αrmwmηm represents the supergradient winds. The latter term is the product of the radius rm, the vertical velocity wm, the azimuthal vorticity ηm at the radius and height of the maximum tangential wind (rm, zm), and the (nearly constant) α. Examination of a series of simulations of idealized tropical cyclones indicate an increasing contribution from the supergradient-wind term to PI+ as the radius of maximum wind increases. In the present paper, the physical content of the supergradient-wind term is developed showing how it is directly related to tropical cyclone boundary layer dynamics. It is found that r m w m η m ∝ u min 2 z m (r m) / l υ (z m) ∝ r m , where −umin is the maximum boundary layer radial inflow velocity and lυ(z) is the vertical mixing length. [ABSTRACT FROM AUTHOR] more...

This paper explores whether particles within uniformly spaced generating cells falling at terminal velocity within observed 2D wind fields and idealized deformation flow beneath cloud top can be reorganized consistent with the presence of single and multibanded structures present on WSR-88D radars. In the first experiment, two-dimensional wind fields, calculated along cross sections normal to the long axis of snowbands observed during three northeast U.S. winter storms, were taken from the initialization of the High-Resolution Rapid Refresh model. This experiment demonstrated that the greater the residence time of the particles in each of the three storms, the greater particle reorganization occurred. For experiments with longer residence times, increases in particle concentrations were nearly or directly collocated with reflectivity bands. For experiments with shorter residence times, particle reorganization still conformed to the band features but with less concentration enhancement. This experiment demonstrates that the combination of long particle residence time and net convergent cross-sectional flow through the cloud depth is sufficient to reorganize particles into locations consistent with precipitation bands. Increased concentrations of ice particles can then contribute, along with any dynamic forcing, to the low-level reflectivity bands seen on WSR-88D radars. In a second experiment, the impact of flow deformation on the reorganization of falling ice particles was investigated using an idealized kinematic model with stretching deformation flow of different depths and magnitudes. These experiments showed that deformation flow provides for little particle reorganization given typical deformation layer depths and magnitudes within the comma head of such storms. Significance Statement: Past research with vertically pointing and scanning radars presents two different perspectives regarding snowfall organization in winter storms. Vertically pointing radars often observe cloud-top generating cells with precipitation fallstreaks descending into a broad stratiform echo at lower altitudes. In contrast, scanning radars often observe snowfall organized in quasi-linear bands. This work attempts to provide a connection between these two perspectives by examining how two-dimensional convergent and deformation flow occurring in winter storms can contribute to the reorganization of snowfall between cloud top and the ground. [ABSTRACT FROM AUTHOR] more...

The variational method for vortex flow (VF) analyses, called VF-Var (formulated in Part I), is applied to the 20 May 2013 Newcastle–Moore tornadic mesocyclone observed from the operational KTLX radar and an experimental phased-array radar. The dual-Doppler-analyzed VF field reveals the following features: The axisymmetric part of the VF is a well-defined slantwise two-cell vortex in which the maximum tangential velocity is nearly 40 m s−1 at the edge of the vortex core (0.6 km from the vortex center), the central downdraft velocity reaches −35 m s−1 at 3-km height, and the surrounding-updraft velocity reaches 26 m s−1 at 5-km height. The total VF field is a loosely defined slantwise two-cell vortex consisting of a nearly axisymmetric vortex core (in which the ground-relative surface wind speed reaches 50 m s−1 on the southeast edge), a strong nonaxisymmetric slantwise downdraft in the vortex core, and a main updraft in a banana-shaped area southeast of the vortex core, which extends slantwise upward and spirals cyclonically around the vortex core. The single-Doppler analysis with observations from the KTLX radar only exhibits roughly the same features as the dual-Doppler analysis but contains spurious vertical-motion structures in and around the vortex core. Analysis errors are assessed by leveraging the findings from Parts II and III, which indicate that the dual-Doppler-analyzed VF is accurate enough to represent the true VF but the single-Doppler-analyzed VF is not (especially for nonaxisymmetric vertical motions in and around the vortex core), so the dual-Doppler-analyzed VF should be useful for initializing/verifying high-resolution tornado simulations. Significance Statement: After the variational method for vortex flow (VF) analyses, called VF-Var (formulated in Part I of this paper series), was tested successfully with simulated radar observations in Part II and its sensitivity to vortex center location error was examined in Part III, the method is now applied to the 20 May 2013 Newcastle–Moore tornadic mesocyclone observed from the operational KTLX radar and an experimental phased-array radar. Analysis errors are assessed by leveraging the findings from Parts II and III. The results indicate that the dual-Doppler-analyzed VF is accurate enough to represent the true VF (although the single-Doppler-analyzed VF is not especially for nonaxisymmetric vertical motions in and around the vortex core) and thus should be useful for initializing/verifying high-resolution tornado simulations. [ABSTRACT FROM AUTHOR] more...

When the vortex center location is estimated from a radar-scanned tornadic mesocyclone, the estimated location is not error-free. This raises an important issue concerning the sensitivities of analyzed vortex flow (VF) fields by the VF-Var (formulated in Part I of this paper series and tested in Part II) to vortex center location errors, denoted by Δxc. Numerical experiments are performed to address this issue with the following findings: The increase of |Δxc| from zero to a half of vortex core radius causes large analysis error increases in the vortex core but the increased analysis errors decrease rapidly away from the vortex core especially for dual-Doppler analyses. The increased horizontal-velocity errors in the vortex core are mainly in the Δxc-normal component, because this component varies much more rapidly than the other component along the Δxc direction in the vortex core. The vertical variations of Δxc distort the vertical correlation structure of Δxc-dislocated VF-dependent background error covariance, which can increase the analysis errors in the vortex core. The dual-Doppler analyses have adequate accuracies outside the vortex core even when |Δxc| increases to a half of vortex core radius, while single-Doppler analyses can also have adequate accuracies outside the vortex core mainly for the single-Doppler-observed velocity component. The sensitivities to Δxc are largely unaffected by the vortex slanting. The above findings are important and useful for assessing the accuracies of analyzed VFs for real radar-observed tornadic mesocyclones. Significance Statement: When the vortex center location is estimated from a radar-scanned tornadic mesocyclone, the estimated location is not error-free. This raises an issue concerning the sensitivity of analyzed vortex flow (VF) by the VF-Var (formulated in Part I of this paper series and tested with simulated radar observations in Part II) to vortex center location error. This issue and its required investigations are very important for the VF-Var to be applied to real radar-observed tornadic mesocyclones, especially in an operational setting with the WSR-88Ds. Numerical experiments are performed to address this issue. The findings from these experiments are important and useful for assessing the accuracies of VF-Var analyzed VF fields for real radar-observed tornadic mesocyclones. [ABSTRACT FROM AUTHOR] more...

It is typically interpreted that more moisture in the atmosphere leads to more intense rains. This notion may be supported, for example, by taking a scatterplot between rain and column precipitable water. The present paper suggests, however, that the main consequence of intense rains with more moisture in the atmosphere is that there is a higher chance of occurrence rather than an increase in the expected magnitude. This tendency equally applies to any rains above 1 mm (6 h)−1 to a lesser extent. The result is derived from an analysis of 33 local rain gauge station data and a shared sounding over Friuli Venezia Giulia, northeast Italy. Significance Statement: Moisture is the source of clouds. Clouds, in turn, are source of rain. So we may expect that more moisture in the atmosphere causes more intense rains. We may further speculate that with more moisture in the atmosphere as a consequence of the global warning, we must face more catastrophic rain events and floods. However, this paper, by analyzing data over Friuli Venezia Giulia, northeast Italy, suggests otherwise: more moisture indeed increases frequencies of intense rains, but not their magnitudes as much. [ABSTRACT FROM AUTHOR] more...

In this Part II paper we present a fully consistent analytical derivation of the "dry" isentropic 1½-layer shallow-water model described and used in Part I of this study, with no convection and precipitation. The mathematical derivation presented here is based on a combined asymptotic and slaved Hamiltonian analysis, which is used to resolve an apparent inconsistency arising from the application of a rigid-lid approximation to an isentropic two-layer shallow-water model. Real observations based on radiosonde data are used to justify the scaling assumptions used throughout the paper, as well as in Part I. Eventually, a fully consistent isentropic 1½-layer model emerges from imposing fluid at rest (v1 = 0) and zero Montgomery potential (M1 = 0) in the upper layer of an isentropic two-layer model. [ABSTRACT FROM AUTHOR] more...

Climate models and observations robustly agree that Earth's clear-sky longwave feedback has a value of about −2 W m−2 K−1, suggesting that this feedback can be estimated from first principles. In this study, we derive an analytic model for Earth's clear-sky longwave feedback. Our approach uses a novel spectral decomposition that splits the feedback into four components: a surface Planck feedback and three atmospheric feedbacks from CO2, H2O, and the H2O continuum. We obtain analytic expressions for each of these terms, and the model can also be framed in terms of Simpson's law and deviations therefrom. We validate the model by comparing it against line-by-line radiative transfer calculations across a wide range of climates. Additionally, the model qualitatively matches the spatial feedback maps of a comprehensive climate model. For present-day Earth, our analysis shows that the clear-sky longwave feedback is dominated by the surface in the global mean and in the dry subtropics; meanwhile, atmospheric feedbacks from CO2 and H2O become important in the inner tropics. Together, these results show that a spectral view of Earth's clear-sky longwave feedback elucidates not only its global-mean magnitude, but also its spatial pattern and its state dependence across past and future climates. Significance Statement: The climate feedback determines how much our planet warms due to changes in radiative forcing. For more than 50 years scientists have been predicting this feedback using complex numerical models. Except for cloud effects the numerical models largely agree, lending confidence to global warming predictions, but nobody has yet derived the feedback from simpler considerations. We show that Earth's clear-sky longwave feedback can be estimated using only pen and paper. Our results confirm that numerical climate models get the right number for the right reasons, and allow us to explain regional and state variations of Earth's climate feedback. These variations are difficult to understand solely from numerical models but are crucial for past and future climates. [ABSTRACT FROM AUTHOR] more...

In Saltzman's seminal paper from 1962, the author developed a framework based on the spectral method for the analysis of the solution to the classical Rayleigh–Bénard convection problem using low-order models (LOMs), LOM (n) with n ≤ 52. By way of illustrating the power of these models, he singled out an LOM (7) and presented a very preliminary account of its numerical solution starting from one initial condition and for two values of the Rayleigh number, λ = 2 and 5. This paper provides a complete mathematical characterization of the solution of this LOM (7), herein called the Saltzman LOM (7) [S-LOM (7)]. Historically, Saltzman's examination of the numerical solution of this low-order model contained two salient characteristics: 1) the periodic solution (in the physical 3D space and time) that expand on Rayleigh's classical study and 2) a nonperiodic solution (in the temporal space dealing with the evolution of Fourier amplitude) that served Lorenz in his fundamental study of chaos in the early 1960s. Interestingly, the presence of this nonperiodic solution was left unmentioned in Saltzman's study in 1962 but explained in detail in Lorenz's scientific biography in 1993. Both of these fundamental aspects of Saltzman's study are fully explored in this paper and bring a sense of completeness to the work. [ABSTRACT FROM AUTHOR] more...

Turbulence is ever produced in the low-viscosity/large-scale fluid flows by velocity shears and, in unstable stratification, by buoyancy forces. It is commonly believed that both mechanisms produce the same type of chaotic motions, namely, the eddies breaking down into smaller ones and producing direct cascade of turbulent kinetic energy and other properties from large to small scales toward viscous dissipation. The conventional theory based on this vision yields a plausible picture of vertical mixing and has remained in use since the middle of the twentieth century in spite of increasing evidence of the fallacy of almost all other predictions. This paper reveals that in fact buoyancy produces chaotic vertical plumes, merging into larger ones and producing an inverse cascade toward their conversion into the self-organized regular motions. Herein, the velocity shears produce usual eddies spreading in all directions and making the direct cascade. This new paradigm is demonstrated and proved empirically; so, the paper launches a comprehensive revision of the theory of unstably stratified turbulence and its numerous geophysical or astrophysical applications. [ABSTRACT FROM AUTHOR] more...

This paper examines the impact of the meridional and vertical structures of a preexisting upstream storm track (PUST) organized by preexisting synoptic-scale eddies on eddy-driven blocking in a nonlinear multiscale interaction model. In this model, the blocking is assumed, based on observations, to be comprised of barotropic and first baroclinic modes, whereas the PUST consists of barotropic, first baroclinic, and second baroclinic modes. It is found that the nonlinearity (dispersion) of blocking is intensified (weakened) with increasing amplitude of the first baroclinic mode of the blocking itself. The blocking tends to be long lived in this case. The lifetime and strength of blocking are significantly influenced by the amplitude of the first baroclinic mode of blocking for given basic westerly winds (BWWs), whereas its spatial pattern and evolution are also affected by the meridional and vertical structures of the PUST. It is shown that the blocking mainly results from the transient eddy forcing induced by the barotropic and first baroclinic modes of PUST, whereas its second baroclinic mode contributes little to the transient eddy forcing. When the PUST shifts northward, eddy-driven blocking shows an asymmetric dipole structure with a strong anticyclone–weak cyclone in a uniform BWW, which induces northward-intensified westerly jet and storm-track anomalies mainly on the north side of blocking. However, when the PUST has no meridional shift and is mainly located in the upper troposphere, a north–south antisymmetric dipole blocking and an intensified split jet with maximum amplitude in the upper troposphere form easily for vertically varying BWWs without meridional shear. Significance Statement: Atmospheric blocking is an important weather phenomenon that leads to a wide range of cold (warm) extremes in winter (summer). Thus, the physical mechanism of the changes in the evolution and structure of blocking has emerged as an important research topic over the past several decades. This paper examines the impact of the meridional and vertical structures of storm tracks on the spatial structure and evolution of blocking by extending a nonlinear multiscale interaction model. The results obtained provide new understanding of how the structure and evolution of blocking change with the structure of storm tracks. [ABSTRACT FROM AUTHOR] more...

A companion paper by Lund et al. employed a compressible model to describe the evolution of mountain waves arising due to increasing flow with time over the southern Andes, their breaking, secondary gravity waves and acoustic waves arising from these dynamics, and their local responses. This paper describes the mountain wave, secondary gravity wave, and acoustic wave vertical fluxes of horizontal momentum, and the local and large-scale three-dimensional responses to gravity breaking and wave–mean-flow interactions accompanying this event. Mountain wave and secondary gravity wave momentum fluxes and deposition vary strongly in space and time due to variable large-scale winds and spatially localized mountain wave and secondary gravity wave responses. Mountain wave instabilities accompanying breaking induce strong, local, largely zonal forcing. Secondary gravity waves arising from mountain wave breaking also interact strongly with large-scale winds at altitudes above ~80 km. Together, these mountain wave and secondary gravity wave interactions reveal systematic gravity wave–mean-flow interactions having implications for both mean and tidal forcing and feedbacks. Acoustic waves likewise achieve large momentum fluxes, but typically imply significant responses only at much higher altitudes. [ABSTRACT FROM AUTHOR] more...

The stability of ice crystal orientation is studied by modeling the airflow around ice crystals at moderate Reynolds number, where an ice crystal is approximated by a cylinder with three parameters: diameter D, length L, and zenith angle of the axis θ. In this paper, the torque acting on ice crystals is simulated at different θ first, and then a special θ with zero horizontal torque, denoted as θe, is sought as an equilibrium of ice crystal orientation. The equilibrium is classified into two kinds: stable and unstable. Ice crystals rotate to θe of stable equilibriums while deviating from θe of unstable ones once they are released into quiet air. Multiple equilibriums of ice crystal orientation are found via numerical simulations. A cylinder with D/L close to one has three equilibriums, two of which are stable (i.e., θe = 0° and 90°). A cylinder with D/L away from one has only two equilibriums, one of which is stable (i.e., either θe = 0° or 90°). In addition, an asymmetric cylinder has two, three, or five equilibriums, and their θe is sensitive to the distance between its geometrical center and its center of gravity. The sensitivity of θe to crystal asymmetry suggests large symmetric ice crystals tend to become asymmetric (or irregular) and subsequently oriented randomly. Significance Statement: Ice crystal orientation impacts high-cloud reflectance and satellite-based observations of high clouds significantly. However, its laboratory and field observations look dissimilar: the percentage of horizontally oriented ice crystals (HOICs) observed in the laboratory is quite high, while in the field it is often low and varies greatly in space and time. The motivation for this study is to elucidate what causes the difference between the laboratory and field observations. The torque acting on ice crystals are computed by modeling the airflow around ice crystals, revealing the conditions for nonhorizontal orientations of ice crystals. In quiet air, an ice crystal is oriented either horizontally or vertically when its shape is close to sphere. When its shape is elongated in one direction, its orientation depends on its asymmetry in density and shape. The sensitivity of ice crystal orientation to ice crystal asymmetry explains the low percentage of HOICs in the field, because asymmetric ice crystals are common in clouds. As an application, this sensitivity together with the observed percentage of HOICs can be used to infer the processes of ice crystal growth in clouds, providing clues to better representation of ice crystals in weather and climate models. [ABSTRACT FROM AUTHOR] more...

A recent paper by Mak et al. grants the opportunity to discuss two different definitions of the frontogenetical function proposed in the literature to study the formation and evolution of upper-level fronts. This comment exposes some problems that, in this author's opinion, are related to the use of the Lagrangian tendency of the 3D (in place of horizontal) gradient of potential temperature, as adopted in the Mak et al. paper. [ABSTRACT FROM AUTHOR] more...

Intense tropical cyclones (TCs) often experience secondary eyewall formations and the ensuing eyewall replacement cycles. Better understanding of the underlying dynamics is crucial to make improvements to the TC intensity and structure forecasting. Radar imagery of some double-eyewall TCs and a real-case simulation study indicated that the barotropic instability (BI) across the moat (aka type-2 BI) may play a role in inner eyewall decay. A three-dimensional numerical study accompanying this paper pointed out that type-2 BI is able to withdraw the inner eyewall absolute angular momentum (AAM) and increase the outer eyewall AAM through the eddy radial transport of eddy AAM. This paper explores the reason why the eddy radial transport of eddyAAMis intrinsically nonzero. Linear and nonlinear shallow water experiments are performed and they produce expected evolutions under type-2 BI. It will be shown that only nonlinear experiments have changes inAAMover the inner and outer eyewalls, and the changes solely originate from the eddy radial transport of eddy AAM. This result highlights the importance of nonlinearity of type-2 BI. Based on the distribution of vorticity perturbations and the balanced-waves arguments, it will be demonstrated that the nonzero eddy radial transport of eddyAAMis an essential outcome from the intrinsic interaction between the mutually growing vortex Rossby waves across the moat under type-2 BI. The analyses of the most unstable mode support the findings and will further attribute the inner eyewall decay and outer eyewall intensification to the divergence and convergence of the eddy angular momentum flux, respectively. [ABSTRACT FROM AUTHOR] more...

In the last years, ensemble methods have been widely popular in atmospheric, climate, and ocean dynamics investigations and forecasts as convenient methods to obtain statistical information on these systems. In many cases, ensembles have been used as an approximation to the probability distribution that has acquired more and more a central role, as the importance of a single trajectory, or member, was recognized as less informative. This paper shows that using results from the dynamical systems and more recent results from the machine learning and AI communities, we can arrive at a direct estimation of the probability distribution evolution and also at the formulation of predictor systems based on a nonlinear formulation. The paper introduces the theory and demonstrates its application to two examples. The first is a one-dimensional system based on the Niño-3 index; the second is a multidimensional case based on time series of monthly mean SST in the Pacific. We show that we can construct the probability distribution and set up a system to forecast its evolution and derive various quantities from it. The objective of the paper is not strict realism, but the introduction of these methods and the demonstration that they can be used also in the complex, multidimensional environment typical of atmosphere and ocean applications. [ABSTRACT FROM AUTHOR] more...

In a companion paper, the Multiscale Gravity Wave Model (MS-GWaM) has been introduced and its application to a global model as a transient subgrid-scale parameterization has been described. This paper focuses on the examination of intermittency of gravity waves (GWs) modeled by MS-GWaM. To introduce the variability and intermittency in wave sources, convective GW sources are formulated, using diabatic heating diagnosed by the convection parameterization, and they are coupled to MS-GWaM in addition to a flow-independent source in the extratropics accounting for GWs due neither to convection nor to orography. The probability density function (PDF) and Gini index for GW pseudomomentum fluxes are assessed to investigate the intermittency. Both are similar to those from observations in the lower stratosphere. The intermittency of GWs over tropical convection is quite high and is found not to change much in the vertical direction. In the extratropics, where nonconvective GWs dominate, the intermittency is lower than that in the tropics in the stratosphere and comparable to that in the mesosphere, exhibiting a gradual increase with altitude. The PDFs in these latitudes seem to be close to the lognormal distributions. Effects of transient GW–mean-flow interactions on the simulated GW intermittency are assessed by performing additional simulations using the steady-state assumption in the GW parameterization. The intermittency of parameterized GWs over tropical convection is found to be overestimated by the assumption, whereas in the extratropics it is largely underrepresented. Explanation and discussion of these effects are included. [ABSTRACT FROM AUTHOR] more...

A bin (or spectral) model is developed to investigate the sensitivity of warm rain initiation to cloud condensation nuclei (CCN). It explicitly represents CCN with a formula whose parameters come from the Twomey relationship (or CCN measurements). By seamlessly integrating CCN activation and drop collection with thousands of bins, the model can replicate the effect of CCN on rain initiation, providing a benchmark to test the process parameterizations in rain initiation. The model is used to simulate two extreme cases with CCN parameters of maritime and continental clouds, respectively, where other actual cases usually lie between these two extreme cases. Its simulations show that rain can initiate within half an hour or less as observed in cumulus clouds. The fast rain initiation modeled is attributed mainly to a new process: the condensational conversion of cloud drops to raindrops via collision–coalescence initiators (or drops with radius between 28 and 100 μm). Since the new process is more important in rain initiation than the autoconversion of cloud drops to raindrops when large CCN exist, it is suggested that the process be parameterized into the weather and climate models to better represent CCN and subsequently remove the common bias of "too dense clouds." Significance Statement: The current weather and climate models represent aerosols via implicit parameterizations and have a bias of "too dense clouds." Their implicit parameterizations of aerosols usually overlook (or misrepresent) some cloud processes. In this paper and its preceding part (Zeng and Li 2020) we proposed a new framework to explicitly parameterize one subset of aerosols: cloud condensation nuclei (CCN). To embody an explicit parameterization of CCN, we still need quantitative information to connect CCN activation and rain initiation, which motivates this study. In the study we developed an accurate microphysical model to simulate the growth of small CCN to large raindrops, providing information on the sensitivity of rain initiation to CCN. We performed many sensitivity simulations and found the condensational conversion of cloud drops to raindrops via collision–coalescence initiators is a vital process in warm rain initiation. Since the process has been overlooked by all the weather and climate models, the study suggests that the process be introduced in the weather and climate models to properly represent the fast warm rain initiation observed and subsequently remove the bias of "too dense clouds." [ABSTRACT FROM AUTHOR] more...

Monin–Obukhov similarity theory (MOST) is a well-tested approach for specifying the fluxes when the roughness surfaces are homogeneous. For flow over waves (inhomogeneous surfaces), phase-averaged roughness length scales are often prescribed through models based on the wave characteristics and the wind speed. However, such approaches lack generalizability over different wave ages and steepnesses due to the reliance on model coefficients tuned to specific datasets. In this paper, a sea surface–based hydrodynamic drag model applicable to moving surfaces is developed to model the pressure-based surface drag felt by the wind due to the waves. The model is based on the surface gradient approach of Anderson and Meneveau applicable to stationary obstacles and extended here to the wind–wave problem. The wave drag model proposed specifies the hydrodynamic force based on the incoming momentum flux, wave phase speed, and the surface frontal area. The drag coefficient associated with the wind–wave momentum exchange is determined based on the wave steepness. The wave drag model is used to simulate turbulent airflow above a monochromatic wave train with different wave ages and wave steepnesses. The mean velocity profiles and model form stresses are validated with available laboratory-scale experimental data and show good agreement across a wide range of wave steepnesses and wave ages. The drag force is correlated with the wave surface gradient and out-of-phase with the wave height distribution by a factor of π/2 for the sinusoidal wave train considered. These results demonstrate that the current approach is sufficiently general over a wide parameter space compared to wave phase-averaged models with a minimal increase in computational cost. Significance Statement: Understanding the physics of wind waves plays an important role in the context of numerous geophysical and engineering applications. A drag-based model is developed that characterizes the effect of the sea surface waves on the wind above. The model is validated with existing experimental datasets and is shown to be effective in predicting the average wind velocity and stress over waves with varied steepnesses and phase speeds. The ease of implementation and low computational cost of the model make it useful for studying turbulent atmospheric-scale flows over the sea surface important in offshore wind energy research as well as for modeling air–sea fluxes of momentum, heat, and mass. [ABSTRACT FROM AUTHOR] more...

Turbulence and winds below 328 m were measured on 5 successive nights in a program to study tracer transport in the nocturnal boundary layer at a site with moderately complex terrain and mixed land use. The instruments included sonic anemometers and CO2/H2O analyzers at four levels on a 328 m tall tower, a minisodar/RASS system, a midrange sodar, a ceilometer, and an array of 61 m towers. Preliminary simulations indicated satisfactory perfluorocarbon mixing to 68 m but insufficient transport to the 328 m level on both weakly stable and stable nights, possibly due to insufficient turbulence kinetic energy and/or small vertical mixing lengths, or the presence of meso-β fronts, e.g., sea-breeze fronts, that could transport trace chemicals efficiently to 328 m. To examine the problem further, time–height distributions of turbulence kinetic energy (TKE), mixing length, Richardson number, potential temperature, and winds were derived from the observations of mean winds and temperature and the TKE budget equation, interpolated to fit the observations, under the flux/gradient and z-less scaling assumptions, and displayed with aerosol profiles. The results indicated higher and more variable levels of TKE and mixing lengths above a typical turbulence maximum at 30–50 m. Oscillations with periods of ∼2 h were common and occasional meso-β fronts and shear zones between 75 and 150 m were seen, which increased TKE aloft and in some cases led to a poorly defined boundary layer top. Significance Statement: The atmosphere's boundary layer is the interface between the free atmosphere and natural and human activity near Earth's surface. The daytime boundary layer has been studied extensively and, because of vigorous sun-driven mixing, is well understood and readily parameterized in forecast and global climate models. In contrast, the nocturnal boundary layer is less well understood or predictable because turbulence is weak and tends to decouple it from the surface and the free atmosphere above. This paper focuses on the least-studied upper part of the nocturnal boundary layer over the southeastern United States where topography and land–sea contrast affect winds, turbulence, and chemical transport. [ABSTRACT FROM AUTHOR] more...

This is a reply to a set of criticisms regarding a previously published work. It briefly addresses the main criticisms. In particular, it explains why some papers identified as having some fundamental flaws were referenced in the original work without detailed exposition of those flaws. It also explains why parts of the conclusions criticized as being contradictory are, in fact, not. It further highlights the need for more publishing of scientific criticisms. [ABSTRACT FROM AUTHOR] more...

This paper addresses the compressible nonlinear dynamics accompanying increasing mountain wave (MW) forcing over the southern Andes and propagation into the mesosphere and lower thermosphere (MLT) under winter conditions. A stretched grid provides very high resolution of the MW dynamics in a large computational domain. A slow increase of cross-mountain winds enables MWs to initially break in the mesosphere and extend to lower and higher altitudes thereafter. MW structure and breaking is strongly modulated by static mean and semidiurnal tide fields exhibiting a critical level at ~114 km for zonal MW propagation. Varying vertical group velocities for different zonal wavelengths λx yield initial breaking in the lee of the major Andes peaks for λx ~ 50 km, and extending significantly upstream for larger λx approaching the critical level at later times. The localized extent of the Andes terrain in latitude leads to “ship wave” responses above the individual peaks at earlier times, and a much larger ship-wave response at 100 km and above as the larger-scale MWs achieve large amplitudes. Other responses above regions of MW breaking include large-scale secondary gravity waves and acoustic waves that achieve very large amplitudes extending well into the thermosphere. MW breaking also causes momentum deposition that yields local decelerations initially, which merge and extend horizontally thereafter and persist throughout the event. Companion papers examine the associated momentum fluxes, mean-flow evolution, gravity wave–tidal interactions, and the MW instability dynamics and sources of secondary gravity waves and acoustic waves. [ABSTRACT FROM AUTHOR] more...

In this two-part paper, influences from environmental factors on lightning in a convective storm are assessed with a model. In Part I, an electrical component is described and applied in the Aerosol–Cloud model (AC). AC treats many types of secondary (e.g., breakup in ice–ice collisions, raindrop-freezing fragmentation, rime splintering) and primary (heterogeneous, homogeneous freezing) ice initiation. AC represents lightning flashes with a statistical treatment of branching from a fractal law constrained by video imagery. The storm simulated is from the Severe Thunderstorm Electrification and Precipitation Study (STEPS; 19/20 June 2000). The simulation was validated microphysically [e.g., ice/droplet concentrations and mean sizes, liquid water content (LWC), reflectivity, surface precipitation] and dynamically (e.g., ascent) in our 2017 paper. Predicted ice concentrations (~10 L−1) agreed—to within a factor of about 2—with aircraft data at flight levels (−10° to −15°C). Here, electrical statistics of the same simulation are compared with observations. Flash rates (to within a factor of 2), triggering altitudes and polarity of flashes, and electric fields, all agree with the coincident STEPS observations. The “normal” tripole of charge structure observed during an electrical balloon sounding is reproduced by AC. It is related to reversal of polarity of noninductive charging in ice–ice collisions seen in laboratory experiments when temperature or LWC are varied. Positively charged graupel and negatively charged snow at most midlevels, charged away from the fastest updrafts, is predicted to cause the normal tripole. Total charge separated in the simulated storm is dominated by collisions involving secondary ice from fragmentation in graupel–snow collisions. [ABSTRACT FROM AUTHOR] more...

Atmospheric flows are often decomposed into balanced (low frequency) and unbalanced (high frequency) components. For a dry atmosphere, it is known that a single mode, the potential vorticity (PV), is enough to describe the balanced flow and determine its evolution. For a moist atmosphere with phase changes, on the other hand, balanced-unbalanced decompositions involve additional complexity. In this paper, we illustrate that additional balanced modes, beyond PV, arise from the moisture. To support and motivate the discussion, we consider balanced-unbalanced decompositions arising from a simplified Boussinesq numerical simulation and a hemispheric-sized channel simulation using theWeather Research and Forecasting (WRF) Model. One important role of the balanced moist modes is in the inversion principle that is used to recover the moist balanced flow: rather than traditional PV inversion that involves only the PV variable, it is PV-and-Minversion that is needed, involvingMvariables that describe the moist balanced modes. In examples of PV-and-M inversion, we show that one can decompose all significant atmospheric variables, including total water or water vapor, into balanced (vortical mode) and unbalanced (inertio-gravity wave) components. The moist inversion, thus, extends the traditional dry PV inversion to allow for moisture and phase changes. In addition, we illustrate that the moist balanced modes are essentially conserved quantities of the flow, and they act qualitatively as additional PV-like modes of the system that track balanced moisture. [ABSTRACT FROM AUTHOR] more...

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. [ABSTRACT FROM AUTHOR] more...

A correction to the article "Dynamics of Convectively Driven Banded Jets in the Laboratory," by Peter L. Read and colleagues that was published in the November 2007 issue is presented.

Atmospheric radiative cooling is a fundamental aspect of Earth's greenhouse effect, and is intrinsically connected to atmospheric motions. At the same time, basic aspects of longwave radiative cooling, such as its characteristic value of 2 K day−1, its sharp decline (or "kink") in the upper troposphere, and the large values of CO2 cooling in the stratosphere, are difficult to understand intuitively or estimate with pencil and paper. Here we pursue such understanding by building simple spectral (rather than gray) models for clear-sky radiative cooling. We construct these models by combining the cooling-to-space approximation with simplified greenhouse gas spectroscopy and analytical expressions for optical depth, and we validate these simple models with line-by-line calculations. We find that cooling rates can be expressed as a product of the Planck function, a vertical emissivity gradient, and a characteristic spectral width derived from our simplified spectroscopy. This expression allows for a pencil-and-paper estimate of the 2 K day−1 tropospheric cooling rate, as well as an explanation of enhanced CO2 cooling rates in the stratosphere. We also link the upper-tropospheric kink in radiative cooling to the distribution of H2O absorption coefficients, and from this derive an analytical expression for the kink temperature Tkink ≈ 220 K. A further, ancillary result is that gray models fail to reproduce basic features of atmospheric radiative cooling. [ABSTRACT FROM AUTHOR] more...

This paper investigates the coupling between the rotational and divergent circulations aiming to explain the observations that show that the tropical eddy momentum flux is due to correlations between divergent eddy meridional velocities and rotational eddy zonal velocities. A simple linear model in which the observed eddy divergence field is used to force the vorticity equation can reproduce quite well the observed tropical eddy momentum flux. The eddy momentum flux in the model shows little sensitivity to the basic-state winds and is mainly determined by the characteristics of the divergent forcing. Vortex stretching and divergent advection of planetary vorticity produce eddy momentum flux contributions with the same sign but the former forcing dominates. It is shown that the main factor affecting the direction of the eddy momentum flux response to both forcings is the meridional tilt of the divergence phase lines, albeit with an opposite sign to the classical relation between rotational momentum flux and streamfunction phase tilt. How this divergent structure is determined remains an open question. [ABSTRACT FROM AUTHOR] more...

Time series of stratospheric climate variables are used to derive master equations in the discretized phase space spanned by three variables. The empirical master equation (EME) predicts the probability density function (PDF) in this phase space. The numerical properties of EMEs have been investigated in the first part of this paper using synthetic time series. In this part of the paper, the time series consist of normalized and deseasonalized daily and zonally averaged meteorologically relevant quantities obtained from the 40-yr ECMWF Re-Analysis (ERA-40) and observations. One EME reproduces the climatological features of the quasi-biennial oscillation (QBO) of stratospheric equatorial zonal wind including the probabilistic character of transitions between phases. Also, the Arctic stratosphere at 10 hPa is about 2 K warmer during the easterly phase of the QBO than during the westerly phase. Another EME including a time series of the solar radio flux at 10.7 cm hints that the relationship between the QBO and the temperature in the Arctic stratosphere is shifted toward warmer (colder) states by about 1 K during periods of high (low) solar activity. Finally, an EME is derived from time series of variables highly correlated with the northern annular mode (NAM). The EME shows that NAM anomalies in the middle stratosphere propagate into the lower stratosphere and then into the lower troposphere with a time scale of about two and four weeks, respectively. The influence of strong tropospheric NAM anomalies is confined to the lower stratosphere. [ABSTRACT FROM AUTHOR] more...

In subgrid-scale condensation schemes of cloud models, the majority of previous authors have relied on results presented in a paper by Bougeault. In the present paper, second-order liquid water correlations are restated that differ from the former paper but are coherent with the corrigendum of Mellor. These differences are explained here through two different underlying definitions of cloud water content fluctuations; they can be summarized by whether or not unsaturated air within a grid box contributes to the eddy flux of the cloud water content. Taking into account the issue mentioned above, the “buoyancy flux” is also derived. Although the full impact of these changes has not been evaluated here, it may become important for future global cloud-resolving climate models. [ABSTRACT FROM AUTHOR] more...

Previous studies have shown that Kelvin–Rossby instability is a viable mechanism for producing equatorial superrotation in small and/or slowly rotating planets. It is shown in this paper that this mechanism can also produce superrotation with terrestrial parameters when the baroclinic forcing moves to low latitudes, explaining previous results by Williams. The transition between superrotating and subrotating flow occurs abruptly as the baroclinic forcing moves poleward. Although Kelvin–Rossby instability weakens when the baroclinic zone moves away from the equator, the key factor explaining the abrupt transition is the change in the baroclinic eddies. When differential heating is contained within the tropics, baroclinic eddies do not decelerate the subtropical jet and the upper-tropospheric flow approximately conserves angular momentum, providing conditions favorable for Kelvin–Rossby instability. In contrast, when baroclinic eddies are generated in the extratropics, they decelerate the subtropical jet and prevent the Kelvin–Rossby coupling. Due to this sensitivity to baroclinic eddies, the system exhibits hysteresis: near the transition parameter, extratropical eddies can prevent superrotation when they are initially present. [ABSTRACT FROM AUTHOR] more...

In a prior paper, insights into tornadogenesis in supercell storms were gained by discovering analytical formulas for vorticity variations along streamlines in idealized, steady, frictionless, isentropic inflows of dry air imported from a horizontally uniform environment. This work is simplified and extended to the evolution of parcel vorticity in unsteady, nonisentropic flows by integrating the vorticity equation using nonorthogonal Lagrangian coordinates. The covariant basis vectors e ¯ 1 , e ¯ 2 , and e ¯ 3 are material line elements attached to each parcel. Initially they form an orthonormal set with e ¯ 1 in the direction of and e ¯ 2 left normal to the storm-relative wind at each level in the environment, and e ¯ 3 upward. The surface containing all parcels with the same initial height constitutes a material surface, within which initially streamwise and transverse material lines are reoriented and stretched or shrunk. The basis vectors propagate a parcel's barotropic vorticity through time by factoring in the "frozen-field" effect. With a horizontally uniform environment, the barotropic vorticity of a parcel depends on its initial streamwise vorticity times its current e ¯ 1 plus its initial crosswise vorticity times its current e ¯ 2 . For baroclinic and frictional vorticity, each contravariant component is the integral from initial to current time of the corresponding contravariant component of the generation vector. The "river-bend" effect acting on all parts (baroclinic, frictional, and barotropic) of transverse vorticity produces streamwise vorticity (parallel to 3D wind). In left-turning steady flow, it arises from e ¯ 2 rotating toward e ¯ 1 . For steady, frictionless, dry isentropic flow, previous vorticity formulas are recovered. Significance Statement: Air parcels rising in a tornado spin rapidly about their direction of motion. Theory herein describes the processes that can produce this streamwise spin in supercells. Cyclonic updraft rotation originates from strong low-level environmental storm-relative winds that turn clockwise with height. Parcels flowing into the updraft have initially large streamwise spins that are amplified by streamwise stretching. Rain curtains falling through the cyclonic updraft cause other parcels to descend and turn leftward. Buoyancy and frictional torques give them horizontal spin. Even if these spins are transverse to the flow initially, they are turned streamwise by secondary flow that develops in left-hand bends. As the parcels reach the ground and converge into the tornado, streamwise stretching greatly magnifies their streamwise spins. [ABSTRACT FROM AUTHOR] more...

Intermittent transitions between turbulent and nonturbulent states are ubiquitous in the stable atmospheric surface layer (ASL). Data from two field experiments in Utqiaġvik, Alaska, and from direct numerical simulations are used to probe these state transitions so as to (i) identify statistical metrics for the detection of intermittency, (ii) probe the physical origin of turbulent bursts, and (iii) quantify intermittency effects on overall fluxes and their representation in closure models. The analyses reveal three turbulence regimes, two of which correspond to weakly turbulent periods accompanied by intermittent behavior (regime 1: intermittent; regime 2: transitional), while the third is associated with a fully turbulent flow. Based on time series of the turbulence kinetic energy (TKE), two nondimensional parameters are proposed to diagnostically categorize the ASL state into these regimes; the first characterizes the weakest turbulence state, while the second describes the range of turbulence variability. The origins of intermittent turbulence activity are then investigated based on the TKE budget over the identified bursts. While the quantitative results depend on the height, the analyses indicate that these bursts are predominantly advected by the mean flow, produced locally by mechanical shear, or lofted from lower levels by turbulent ejections. Finally, a new flux model is proposed using the vertical velocity variance in combination with different mixing length scales. The model provides improved representation (correlation coefficients with observations of 0.61 for sensible heat and 0.94 for momentum) compared to Monin–Obukhov similarity (correlation coefficients of 0.0047 for sensible heat and 0.49 for momentum), thus opening new pathways for improved parameterizations in coarse atmospheric models. Significance Statement: Airflow in the lowest layer of the atmosphere is often modulated by a strong gradient of temperature when the surface is much cooler than the air. Such a regime results in weak turbulence and mixing, and is ubiquitous during nighttime and in polar regions. Understanding and modeling atmospheric flow and turbulence under such conditions are further complicated by "turbulence intermittency," which manifests as periods of strong turbulent activity interspersed in a more quiescent airflow. The turbulent periods dominate the air–surface exchanges even when they occur over a small fraction of the time. This paper develops approaches to detect and classify such intermittent regimes, examines how the turbulent bursts are generated and advected, and offers guidance on representing such regimes in geophysical models. The findings have the potential to advance weather forecasting and climate modeling, particularly in the all-important polar regions. [ABSTRACT FROM AUTHOR] more...

Recent field observations suggest that cloud-top precipitation generating cells (GCs) are ubiquitous in the warm-frontal and comma-head regions of midlatitude winter cyclones. The presence of fallstreaks emanating from the GCs and their persistence either to the surface or until merging into precipitation bands suggests that GCs are a critical component of the precipitation process in these cyclones. This paper is the second part of a three-part series that investigates the dynamics of GCs through very-high-resolution idealized Weather Research and Forecasting (WRF) Model simulations. This paper assesses the role of cloud-top instability paired with nighttime, daytime, or no radiative forcing on the development and maintenance (or lack) of GCs. Under initially unstable conditions at cloud top, GCs develop regardless of radiative forcing but only persist clearly with radiative forcing. Cloud-top destabilization due to longwave cooling leads to development of GCs even under initially neutral and stable conditions, providing a physical explanation for the observed ubiquity of GCs atop winter cyclones. GCs do not develop in initially stable simulations with no radiation. Decreased range in vertical velocity spectra under daytime radiative forcing is consistent with offset of the destabilizing influence of longwave cooling by shortwave heating. [ABSTRACT FROM AUTHOR] more...

An editorial is presented which discusses plans of American Meteorological Society (AMS) to terminate use of Expedited Contributions (ECs) article type. Topics include increasing efficiency in publication process resulting in lack of author's interest in ECs; with introduction of Early Online Releases, papers appear online with a digital object identifier (DOI); and information on improvements in efficiency of peer review. more...