Richard Rotunno

• Senior Scientist at National Agricultural Research Center - NARC

No-slip boundary conditions are often employed in idealized axisymmetric models to study tornadoes. These boundary conditions result in a poor representation of near-ground winds, which are of crucial importance for tornadoes. In this study, the boundary layer of tornadoes is investigated using more realistic semislip conditions as the lower bounda...

Improved representation of turbulent processes in numerical models of tropical cyclones (TCs) is expected to improve intensity forecasts. To this end, the authors use a large-eddy simulation (with 31-m horizontal grid spacing) of an idealized category 5 TC to understand the role of turbulent processes in the inner core of TCs and their role on the...

This study presents a simple 2D linear analytical model aimed at investigating gravity waves forced by temporally periodic convection near a coastline. This investigation encompasses two distinct convective heating scenarios: deep convective heating and stratiform heating/cooling. Our model explores the intricate behavior of gravity waves in proxim...

This study investigates the effects of lakes in mountainous terrain on the evolution of supercell thunderstorms. With a newly developed radar-based, mesocyclone-detection algorithm, a recent study has characterized the occurrence and evolution of supercell thunderstorms in the Swiss Alpine region. That study highlights the influence of orography on...

On the evening of November 12, 2019, an exceptional high tide – the second‐highest in the ranking since sea‐level data have been recorded – hit the city of Venice in northern Italy and its entire lagoon, damaging a large part of its historical center. A small warm‐core mesoscale cyclone, which formed in the central Adriatic Sea and intensified duri...

Atmospheric predictability is measured by the average difference (or ‘error’) within an ensemble of forecasts starting from slightly different initial conditions. The spatial scale of the error field is a fundamental quantity; for meteorological applications, the error field typically varies with latitude and longitude and so requires a two-dimensi...

Sea-breeze fronts (SBFs) are frequently found to trigger deep convection. The convective updrafts near the SBF are critical in this triggering process. Here, the size and strength of the updrafts near an idealized SBF are investigated with large-eddy simulations. A central focus of this study is to compare the updrafts near the SBF, which are subst...

This study examines the characteristics of tropical‐cyclone (T‐C) turbulence and its related predictability implications. Using the Fourier‐Bessel spectral decomposition for convection‐permitting simulations, it is shown that T‐C turbulence possesses different spectral properties in the azimuthal and radial directions, with a steeper power law in t...

The data includes 2 subfolders data_brgd: contains the 2D output of (u,v,w) for the HWRF control simulation during the fully-developed TC turbulence at the mature stage from t = 72-120h at an interval of 30 mn and z = 950 mb, which is used for Figures 1 and 2. The data has 201x201 grid points in (x,y) directions at resolution of 900 m. data_diff...

Sea-breeze fronts (SBFs) are frequently found to trigger deep convection. The convective updrafts near the SBF are critical in this triggering process. Here, the size and strength of the updrafts near an idealized SBF are investigated with large-eddy simulations. A central focus of this study is to compare the updrafts near the SBF, which are subst...

This study examines the characteristics of tropical-cyclone (T-C) turbulence and its related predictability implications. Using the Fourier-Bessel spectral decomposition for convective-permitting simulations, it is shown that T-C turbulence possesses different spectral properties in the azimuthal and radial directions, with a steeper power law in t...

Deep convection plays important roles in producing severe weather and regulating the large-scale circulation. However, deep-convection initiation (DCI), which determines when and where deep convection develops, has not yet been fully understood. Here, large-eddy simulations are performed to investigate the detailed processes of DCI, which occurs th...

Large-eddy simulations are performed to investigate the process of deep-convection initiation (DCI) over a peninsula. In each simulation, two sea-breeze circulations develop over the two coasts. The two sea-breeze fronts move inland and collide, producing strong instability and strong updrafts near the centerline of the domain, and consequently lea...

Two tropical‐like cyclones in the Mediterranean Sea, aka medicanes, are analysed herein by means of numerical simulations. The cyclones, which were recently investigated in Miglietta and Rotunno, Quarterly Journal of the Royal Meteorological Society, 2019, 145, are reconsidered in the present study, in which we focus on their respective preconditio...

Theories for the maximum intensity of tropical cyclones (TCs) assume steady state. However, many TCs in simulations that run for tens of days tend to decay considerably from an early steady state in the core (CS), before stabilizing at a final equilibrium steady state (ES). This decay raises the question of whether CS or ES should be used as a comp...

An analysis of the influence and sensitivity of moisture in an idealized two-dimensional moist semigeostrophic frontogenesis model is presented. A comparison between a dry (relative humidity, RH=0%) and moist (RH=80%) version of the model demonstrates that the impact of moisture is to increase frontogenesis, strengthen the transverse circulation (...

This study examines the role of frictional feedback in the atmospheric boundary layer during tropical cyclone (TC) development. Using a reduced model of TC dynamics, it is shown that a feedback between frictional convergence and convective heating in the absence of slantwise moist neutrality is capable of producing a stable maximum intensity limit,...

This study revisits the axisymmetric tropical cyclone (TC) theory from D. K. Lilly’s unpublished manuscript (Lilly model) and compares it to axisymmetric TC simulations from a nonhydrostatic cloud model. Analytic solutions of the Lilly model are presented through simplifying assumptions. Sensitivity experiments varying the sea-surface, boundary-lay...

Several previous studies have demonstrated the significant sensitivity of simulated tropical cyclone structure and intensity to variations in surface-exchange coefficients for enthalpy (Ck) and momentum (Cd), respectively. In this study we investigate the consistency of the estimated peak intensity, intensification rate and steady-state structure b...

The rapid technological development of the past few decades has allowed for an unprecedented wealth of data about ourselves and our planet. The cost reduction of space platforms, the microelectronic revolution and the nearly exponential increase in computer power have been generating novel opportunities to explore and understand the world around us...

Plain Language Summary According to the maximum potential intensity theory, the maximum intensities for tropical cyclones should be the same given the same environmental conditions, which means the radius of maximum wind (rm) at the boundary layer top should be linearly proportional to the absolute angular momentum such that rm~aMm. In model simula...

This study considers a two-layer fluid with constant density in each layer connected by a layer of continuously varying density for flows past topography in which hydraulic jumps with lee vortices are expected based on shallow water theory. Numerical integrations of the Navier-Stokes equations at a Reynolds number high enough for a direct numerical...

The impact of vertical wind shear on the land-sea breeze circulation at the equator is explored using idealized 2D numerical simulations and a simple 2D linear analytical model. Both the idealized and linear analytical models indicate Doppler shifting and attenuation effects co-exist under the effect of vertical wind shear for the propagation of gr...

In orographic precipitation events, there are times when subsaturated low-level layers are observed to be below saturated, nearly moist-neutral, upper-level layers. By performing a series of idealized two-dimensional simulations, this study investigates the response of orographic precipitation to subsaturated low-level layers. When the nondimension...

The criteria and assumptions that were used to derive the steady-state tropical cyclone intensity and structure theory of Emanuel and Rotunno are assessed using three-dimensional convection-allowing simulations using the Weather Research and Forecasting Model. One real-data case of Hurricane Patricia (2015) and two idealized simulations with and wi...

In a previous study, the authors showed that the intensification process of a numerically simulated axisymmetric tropical cyclone (TC) can be divided into two periods denoted by “phase I” and “phase II.” The intensification process in phase II can be qualitatively described by Emanuel’s intensification theory in which the angular momentum ( M) and...

Midlatitude cyclones with characteristics similar to tropical cyclones (also known as Tropical‐Like Cyclones, TLCs, or medicanes) are sometimes observed in the Mediterranean region. The Wind Induced Surface Heat Exchange (WISHE) mechanism has been considered responsible for their development, in analogy with tropical‐cyclone theory. However, some r...

Gli intensi vortici a piccola scala che hanno interessato recentemente la nostra penisola hanno catalizzato l'attenzione dei mezzi di comunicazione tradizionali e dei social media, che spesso hanno usato indistintamente i termini tromba d'aria e tornado per identificarli. Questo articolo considera due problemi: (i) la correttezza del lessico nelle...

A recent study found that surface hodographs over the Great Plains of the United States turn in a counterclockwise direction with time. This observed turning is opposite of the clockwise turning observed (and expected, based on theory) at higher altitudes. Using a mesoscale forecast model, the same study shows that it has the same hodograph behavio...

Axisymmetric and three-dimensional simulations are used to evaluate the theory of tropical cyclone (TC) intensification proposed by K. A. Emanuel, which is based on gradient wind balance and moist-neutral ascent along angular momentum (M) surfaces. According to the numerical model results, the intensification of the TC can be divided into two perio...

The characteristics and mechanisms of diurnal rainfall and winds near the south coast of China are explored using satellite data (CMORPH), long-term hourly WRF Model data (Du model data), a simple 2D linear model, and 2D idealized simulations. Both the CMORPH and Du model data indicate that the diurnal cycle of rainfall has two propagation modes ne...

Laboratory observations of the leeside hydraulic jump indicate it consists of a statistically stationary turbulent motion in an overturning wave. From the point of view of the shallow-water equations (SWE), the hydraulic jump is a discontinuity in fluid-layer depth and velocity at which kinetic energy is dissipated. To provide a deeper understandin...

Vertical shear in the boundary layer affects the mode of convective storms that can exist if they are triggered. In western portions of the southern Great Plains of the United States, vertical shear, in the absence of any transient features, changes diurnally in a systematic way, thus leading to a preferred time of day for the more intense modes of...

Two Intensive Observation Periods (IOPs) of the Hydrological cycle in the Mediterranean eXperiment (HyMeX) are examined in this study. IOP6 and IOP13 were characterized by troughs with associated cold fronts entering the western Mediterranean and, in both cases, organized frontal convection in the Po Valley was observed. These similarities notwiths...

An analytical model of maximum potential intensity (PI) for tropical cyclones (TCs) incorporating wind-induced ocean cooling is developed on the basis of Emanuel's PI theory. The model consists of a one-dimensional ocean and an axisymmetric TC vortex that is translating at a constant speed. The model advances upon previous approaches by accounting...

On 28 November 2012, a multivortex EF3 tornado occurred in southeastern Italy causing one fatality and estimated damage of 60 million euros. At approximately 1050 LT (0950 UTC), this tornado, which initially formed in association with a supercell thunderstorm over the Ionian Sea, moved inland. The environment where the tornadic supercell developed...

A large-eddy simulation (LES) framework with an “eddy injection” technique has been developed that ensures a majority of turbulent kinetic energy in numerically simulated tornado-like vortices is represented by resolved eddies. This framework is used to explore the relationships between environmental forcing mechanisms, surface boundary conditions,...

Numerical models of supercell thunderstorms produce near-ground rotation about a vertical axis (i.e., vertical vorticity) after the development of rain-cooled outflows and downdrafts. The physical processes involved in the production of near-ground vertical vorticity in simulated supercells have been a subject of discussion in the literature for ov...

The structure and intensity of tornado-like vortices are examined using large-eddy simulations (LES) in an idealized framework. The analysis focuses on whether the simulated boundary layer contains resolved turbulent eddies, and whether most of the vertical component of turbulent momentum flux is resolved rather than parameterized. Initial conditio...

Large-eddy simulations are used to produce realistic, high-resolution depictions of near-surface winds in translating tornadoes. The translation speed, swirl ratio, and vertical forcing are varied to provide a range of vortex intensities and structural types. Observation experiments are then performed in which the tornadoes are passed over groups o...

With high-resolution mesoscale model simulations, the authors have confirmed a recent study demonstrating that convective systems, triggered in a horizontally homogeneous environment, are able to generate a background mesoscale kinetic energy spectrum with a slope close to -5/3, which is the observed value for the kinetic energy spectrum at mesosca...

Observations and models of nocturnal katabatic winds indicate strong low-level stability with much weaker stability aloft. When such winds encounter an embedded depression in an otherwise smooth sloping plane, the flow responds in a manner that is largely describable by the inviscid fluid dynamics of stratified flow. Building on earlier work, the p...

Idealized two-dimensional model simulations are performed to study the frequent nocturnal occurrence of downslope-windstorm-type flows in Arizona's Meteor Crater. The model topography is a simplified representation of the Meteor Crater and its surroundings, with an approximately 1° mesoscale slope upstream and downstream of the crater basin. A stro...

An analysis is presented here of intense convection affecting the Friuli Venezia Giulia region (FVG, northeastern Italy) during the Intensive Observation Period 2b (IOP2b) in the first Special Observation Period (SOP1) of the HyMeX (HYdrological cycle in Mediterranean EXperiment). The present study focuses on the first of three severe-convection ep...

This study is the first in a series that investigates the effects of turbulence in the boundary layer of a tornado vortex. In this part, axisymmetric simulations with constant viscosity are used to explore the relationships between vortex structure, intensity, and unsteadiness as functions of diffusion (measured by a Reynolds number Rer) and rotati...

The possibility offered by the internet to share pictures of tornadoes and the storm-report archiving in the European Storm Weather Database, have made it apparent that their occurrence over Europe has been underestimated. Together with weak waterspouts and tornadoes, large and intense vortices are occasionally observed. Among these, an EF3 multi-v...

AbstractThe second Meteor Crater Experiment (METCRAX II) was conducted in October 2013 at Arizona's Meteor Crater. The experiment was designed to investigate nighttime downslope-windstorm-type flows that form regularly above the inner southwest sidewall of the 1.2-km-diameter crater as a southwesterly mesoscale katabatic flow cascades over the crat...

The quasi-steady-state limit of the diurnal valley wind system is investigated over idealized three-dimensional topography. Although this limit is rarely attained in reality due to ever-changing forcings, the investigation of this limit can provide valuable insight, in particular on the mass and heat fluxes associated with the along-valley wind. We...

A holepunch cloud is a curious phenomenon where a disturbance in a thin cloud layer initiates an expanding circular hole of clear air. Usually triggered by the passage of aircraft, observations of these holes in clouds date back to the earliest days of aviation, but only recently has a holepunch cloud been simulated within a full-physics numerical...

Recent idealized studies examined the sensitivity of topographically forced rain and snowfall to changes in mountain geometry and upwind sounding in moist stable and neutral environments. These studies were restricted by necessity to small ensembles of carefully chosen simulations. Research presented here extends earlier studies by utilizing a Baye...

Talk given at the 9th International Conference on Urban Climate in Toulouse, July 2015

The characteristics of thermally driven diurnally periodic wind signals off the east coast of China are studied using hourly model data for the period June 2006-2011 simulated with a mesoscale model. Analysis of this model data indicates low-level diurnally periodic wind signals propagate eastward off the southeast coast whereas diurnal wind variat...

A simple analytical model including both diurnal thermal forcing over sloping terrain (the “Holton” mechanism) and diurnally varying boundary layer friction (the “Blackadar” mechanism) is developed to account for the observed amplitude and phase of the low-level jet (LLJ) over the Great Plains and to understand better the role of each mechanism. Th...

As part of the Meteor Crater Experiment (METCRAX II) at the Barringer Meteor Crater in Arizona we installed five infrared time lapse cameras (VarioCAM hr research & VarioCAM High Definition, InfraTec) at the crater rim to observe surface temperature dynamics. Measuring the crater's surface infrared (IR) temperatures from different views into the cr...

The Second Meteor Crater Experiment (METCRAX II) was designed to study downslope-windstorm-type flows (DWF) that occur above the south and west inner sidewalls of the 1.2 km diameter Barringer Meteorite Crater in Arizona. These DWFs occur intermittently within the crater basin on clear, undisturbed nights in connection with a mesoscale drainage flo...

The Mediterranean countries are experiencing important challenges. related to the water cycle, including water shortages and floods, extreme winds, and ice/snow storms, that impact critically the socioeconomic vitality in the area (causing damage to property, threatening lives, affecting the energy and transportation sectors, etc.). There are gaps...

During the first Hymex campaign (5 September-6 November 2012) referred to as Special Observation Period (SOP-1), dedicated to heavy precipitation events and flash floods in Western Mediterranean, three Italian hydro-meteorological monitoring sites were activated: Liguria-Tuscany, North-Eastern Italy and Central Italy. The extraordinary deployment o...

Mesoscale numerical weather prediction models using fine-grid [0(1) km] meshes for weather forecasting, environmental assessment, and other applications capture aspects of larger-than-grid-mesh size, convectively induced secondary circulations (CISCs) such as cells and rolls that occur in the convective planetary boundary layer (PBL). However, 1-km...

The rotating-flow boundary layer is a special case of the more general three-dimensional boundary layer in which the pressure gradient imposed by the outer flow (above the boundary layer) is not in the same direction as the outer flow. The rotating-flow boundary layer thus has motion that is transverse to the streamlines of the outer flow, that is,...

This study examines properties of gravity currents in confined channels with sheared environmental flow. Under the assumptions of steady and inviscid flow, two-dimensional analytic solutions are obtained for a wide range of shear values. The slope of a gravity current interface just above the surface increases as environmental shear increases, whic...

The low-level diurnal winds for different locations of eastern China are documented using the June 2006-2011 hourly model data simulated with the Weather Research and Forecasting (WRF) mesoscale model with a 9-km horizontal resolution. A simple 1-D model, including both diurnal thermal forcing and diurnally varying boundary-layer friction, is found...

This study examines the lifting of sheared environmental air by gravity currents, focusing primarily on the theoretical optimal state in which near-surface flow is turned into a vertically oriented jet. Theoretical models are presented from multiple perspectives, including the vorticity perspective that was first presented by Rotunno, Klemp, and We...

This work is motivated by an unusual feature associated with the start-up of a moist nearly neutral atmospheric flow over a mountain ridge that was previously observed in a full-physics numerical model. In that study, the upstream propagation of a wave of subsidence precluded the establishment of upward-displaced and saturated flow that might be ex...

During the first Hymex campaign (5 September–6 November 2012) referred to as Special Observation Period (SOP-1), dedicated to heavy precipitation events and flash floods in Western Mediterranean, three Italian hydro-meteorological monitoring sites were activated: Liguria-Tuscany, North-Eastern Italy and Central Italy. The extraordinary deployment o...

Because of the difficulty in making measurements under controlled conditions, most of what is known about the fluid dynamics of tornadoes comes from laboratory experiments that produce vortices with features similar to those observed in a tornado. Numerical simulation of laboratory experiments has become a valuable analytical tool owing to the grea...

During autumn 2011, three dramatic floods occurred in Italy causing deaths and terrible injuries: in Rome on October 20, in Liguria and Tuscany on October 25, and in Genoa on November 4. One of the hardest-hit areas was the Cinque Terre, a national park with a marine protected area; its landscape was completely changed by the landslide caused by th...

Previous studies have observed upstream-propagating modes in two-dimensional numerical simulations of idealized flow over topography with moist, nearly neutral conditions in the troposphere, topped by a stable stratosphere. The generation and propagation mechanisms for these modes were attributed to localized and dramatic changes in stability induc...

In a series of papers, Miglietta and Rotunno reported on numerical simulations of conditionally unstable flows past an idealized mesoscale mountain ridge. These idealized simulations, which were performed with a three- dimensional, explicitly cloud-resolving model, allowed the investigation of simulated-precipitation characteristics as a function o...

In this study the authors analyze and interpret the effects of parameterized diffusion on the nearly steady axisymmetric numerical simulations of hurricanes presented in a recent study. In that study it was concluded that horizontal diffusion was the most important control factor for the maximum simulated hurricane intensity. Through budget analysi...

In a recent study, the authors investigated the mechanisms leading to the formation of diurnal along-valley winds in a valley formed by two isolated mountain ridges on a horizontal plain. The main focus was on the relation between the valley heat budget and the valley plain pressure difference. The present work investigates the influence of the val...

Tropical cyclones intensify and are maintained by surface enthalpy fluxes that result from the thermodynamics disequilibrium that exists between the tropical oceans and atmosphere. While this general result has been known for at least a half century, the detailed nature of feedbacks between thermodynamic and dynamic processes in tropical cyclones r...

This article investigates the linear dynamics of the sea breeze in an along-shore thermal wind shear. The present analysis shows that the sea-breeze circulation is tilted towards the slanted isentropes associated with the thermal wind. At a critical value of the thermal wind shear, the tilt of the sea-breeze circulation becomes equal to the slope o...

An unusually intense subsynoptic-scale cyclone has been documented in southeastern Italy on 26 September 2006. Numerical simulations are performed with the Weather Research and Forecasting model (WRF). Model results show that the model is able to realistically capture the timing and intensity of the cyclone, identifying in this small-scale cyclone...

Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. Since the latter approximation fails at the leading edges of the mutually intruding fluids of lock-exchange flow, solutions to the shallow-water equations can be obtained only through the specification of front conditions. In the present paper, analytic...

Nearly all analytical models of lock-exchange flow are based on the shallow-water approximation. Since the latter approximation fails at the leading edges of the mutually intruding fluids of lock-exchange flow, solutions to the shallow-water equations can be obtained only through the specification of front conditions. In the present paper, analytic...

The physical mechanisms leading to the formation of diurnal along-valley winds are investigated over idealized three-dimensional topography. A diagnostic equation for the along-valley pressure gradient is developed and used in combination with numerical model simulations to clarify the relative role of various forcing mechanisms such as the valley...

In a recent study, the authors performed numerical simulations of conditionally unstable flows past a mesoscale mountain ridge in order to investigate the statistically stationary features of the solution precipitation characteristics for intermediate-to-high values of convective available potential energy (CAPE). That study proposed a functional d...

Large-eddy simulation (LES) of a tropical cyclone (TC) was carried out with with the Advanced Research Weather research and forecasting (ARW) model. It was demonstrated the use of a horizontal grid size < 100 m in a TC simulation produced three-dimensional turbulent eddies. These three-dimensional turbulent eddies had the effect of decreasing the m...

Several studies have shown that the intensity of numerically simulated tropical cyclones can exceed (by 50%) a theoretical upper limit. To investigate the cause, this study evaluates the underlying components of Emanuel's commonly cited analytic theory for potential intensity (herein referred to as E-PI). A review of the derivation of E-PI highligh...

Results from homogeneous, isotropic turbulence suggest that predictability behavior is linked to the slope of a flow's kinetic energy spectrum. Such a link has potential implications for the predictability behavior of atmospheric models. This article investigates these topics in an intermediate context: a multilevel quasi- geostrophic model with a...

The presence of a subsynoptic-scale vortex over the Mediterranean Sea has been documented in southeastern Italy on 26 September 2006. The transit of the cyclone over land allowed an accurate diagnosis of the structure of the vortex, based on radar and surface station data, showing that the cyclone had features similar to those observed in tropical...

This presentation describes numerical simulations of conditionally unstable flows impinging on an idealized mesoscale mountain ridge. These idealized simulations, which were performed with an explicitly resolving cloud model, allow the investigation of the solution precipitation characteristics as a function of the prescribed environment. The numer...

An axisymmetric numerical model is used to evaluate the maximum possible intensity of tropical cyclones. As compared with traditionally formulated nonhydrostatic models, this new model has improved mass and energy conservation in saturated conditions. In comparison with the axisymmetric model developed by Rotunno and Emanuel, the new model produces...

A forecasting system has been implemented for operational weather prediction over southern Italy in the framework of the RISKMED (INTERREG IIIB) project, which aims at weather-risk reduction in the central and eastern Mediterranean area. The system comprises two different regional forecasting chains, one based on BOLAM and MOLOCH models, the second...

Using a time-dependent axisymmetric numerical model, the authors evaluate whether high-entropy air near the surface in hurricane eyes can substantially increase hurricanes' maximum intensity. This local high-entropy anomaly is ultimately created by surface entropy fluxes in the eye. Therefore, simulations are conducted in which these surface fluxes...

This presentation will show results from resolution sensitivity studies of severe convective storms (including midlatitude squall lines and tropical cyclones). The highest resolution simulations use 62 m grid spacing, and the coarsest resolution simulations use 8 km horizontal grid spacing. Hence, these experiments span the resolutions appropriate...

The presence of a subsynoptic-scale vortex over the Mediterranean Sea in southeastern Italy on 26 September 2006 has been recently documented by the authors. The transit of the cyclone over land allowed an accurate diagnosis of the structure of the vortex, based on radar and surface station data, showing that the cyclone had features similar to tho...

The effects of upstream relative humidity (RH) on low-level wind and precipitation patterns for low- speed, statically stable flows over a mountain are investigated using idealized two- and three-dimensional numerical-simulation experiments in which RH is increased from 0% to 100%. For RH less than some critical threshold, the flow upstream becomes...

A case-study of the 26 July 2005 Mumbai heavy-rain event that recorded 944 mm rainfall in 24 h with significant spatial variability was carried out using the Weather Research and Forecasting (WRF) model. The event was poorly forecasted by operational models and resulted in large human and economic losses. The present results indicate that the WRF s...