Mid-level Vortex Research Articles

The impact of vertical shear on the sensitivity of tropical cyclogenesis to environmental rotation and thermodynamic state

AbstractThe impact of vertical wind shear on the sensitivity of tropical cyclogenesis to environmental rotation and thermodynamic state is investigated through idealized cloud‐resolving simulations of the intensification of an incipient vortex. With vertical shear, tropical cyclones intensify faster with a higher Coriolis parameter, f, irrespective of the environmental thermodynamic state. The vertical shear develops a vertically tilted vortex, which undergoes a precession process with the midlevel vortices rotating cyclonically around the surface center. With a higher f, the midlevel vortices are able to rotate continuously against the vertical shear, leading to the realignment of the tilted vortex and rapid intensification. With a lower f, the rotation is too slow such that the midlevel vortices are advected away from the surface center and the intensification is suppressed. The parameter, χb, measuring the effect from the low‐entropy downdraft air on the boundary layer entropy, is found to be a good indicator of the environmental thermodynamic favorability for tropical cyclogenesis in vertical shear. Without vertical shear, tropical cyclones are found to intensify faster with a lower f by previous studies. We show this dependency on f is sensitive to the environmental thermodynamic state. The thermodynamical favorability for convection can be measured by χm, which estimates the time it takes for surface fluxes to moisten the midtroposphere. A smaller χm not only leads to a faster intensification due to a shorter period for moist preconditioning of the inner region but also neutralizes the faster intensification with a lower f due to enhanced peripheral convection.

Impacts of a Storm Merger on the 24 May 2011 El Reno, Oklahoma, Tornadic Supercell

Abstract On 24 May 2011, a tornadic supercell (the El Reno, Oklahoma, storm) produced tornadoes rated as category 3 and 5 events on the enhanced Fujita scale (EF3 and EF5, respectively) during a severe weather outbreak. The transition (“handoff”) between the two tornadoes occurred as the El Reno storm merged with a weaker, ancillary storm. To examine the impacts of the merger on the dynamics of these storms, a series of three-dimensional cloud-scale analyses are created by assimilating 1-min volumetric observations from the National Weather Radar Testbed’s phased array radar into a numerical cloud model using the local ensemble transform Kalman filter technique. The El Reno storm, its updrafts, and vortices in the analyzed fields are objectively identified, and the changes in these objects before, during, and after the merger are examined. It is found that the merger did not cause the tornado handoff, which preceded the updraft merger by about 5 min. Instead, the handoff likely resulted from midlevel mesocyclone occlusion, in which the midlevel mesocyclone split and a portion is shed rearward with respect to storm motion. During the merger process, the midlevel mesocyclone and updraft structure in the El Reno storm became relatively disorganized. New updraft pulses that formed above colliding outflow boundaries between the two storms tilted environmental vorticity from low levels to generate an additional midlevel vortex that later merged with the El Reno storm’s midlevel mesocyclone. Once the ~10-min merger process was complete, the El Reno storm and its mesocyclone rapidly reintensified, as access to buoyant inflow sector air was restored.

Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes

Abstract The “bottom up” generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel “marsupial pouch” associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs’ merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to “escape” from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby’s genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is transformed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided.

An Investigation of the Tropical Cyclogenesis of Arlene (2005) Using ERA-Interim Reanalysis and the WRF Model Simulation

The process and development sequence of meso-beta-scale vortices at multiple levels and the synoptic-scale environmental conditions for the genesis of Tropical Storm Arlene (2005) were analyzed using ERA-Interim Reanalysis data and Weather Research and Forecasting (WRF) model simulations. Results from the two agree in that although the confluence of low-level westerly winds from the eastern North Pacific and low-level easterly winds from the North Atlantic preceded tropical cyclogenesis over the western Caribbean Sea, midlevel enhanced wind surges over Lake Nicaragua were also important in the pregenesis period. A closer examination of the WRF simulation revealed that the midlevel enhanced wind surges were produced by the interactions between the 500 hPa anticyclone over Mexico and a wind stream from northern South America. The 500 hPa surges augmented the vorticity of the incipient storm as it passed over Lake Nicaragua. Due to the subsequent influxes of strong westerly and easterly winds in the lower atmosphere, the midlevel vortex development was followed by a low-level vorticity increase, enhancing the vertical structure of the vortex motion of the storm from the 500 hPa level downward. This vortex development resulted in tropical cyclogenesis at 1800 UTC on 8 June and a tropical storm by 0600 UTC on 9 June.

Potential vorticity diagnosis of tropical cyclone Usagi (2001) genesis induced by a mid-level vortex over the South China Sea

In this study, we first show that tropical cyclone (TC) Usagi evolved from a mid-level vortex over the South China Sea (SCS) in August 2001. The initial disturbance of TC Usagi had a maximum potential vorticity (PV) near 500 hPa, and an anticyclonic circulation with a cold core near the surface. The cyclonic circulation and its warm core of the mid-level vortex developed gradually downward toward the surface when environmental easterly and dry air intruded from the upper troposphere; finally, the mid-level vortex evolved into TC Usagi under favorable environment conditions such as weak vertical wind shear, deep moist layer, etc. To investigate the dynamic and thermodynamic processes during TC Usagi genesis, the technique of piecewise PV inversion is employed. The results show that the actions of upper-layer PV and potential temperature anomalies were not important in TC Usagi genesis. Surface-layer thermal anomalies mainly produced negative disturbances of temperature at the vortex center below 800 hPa, which was unfavorable to the genesis of a cyclonic circulation near the surface. Middle-to-lower-layer latent heat played a key role in TC Usagi genesis and downward development of dynamic and thermodynamic processes. The actions of dry air intrusion from the upper troposphere, environmental westerly changing into easterly in the middle and lower troposphere, and baroclinic structure of the vortex were also important. The cyclonic circulation of the mid-level vortex could develop downward quickly from the middle troposphere toward the surface. However, whether the warm core of the vortex developed near the surface depended on the combined actions of surface-layer thermal anomaly and middle-to-lower-layer latent heat. Finally, we present a conceptual model of TC Usagi genesis induced by a mid-level vortex over the SCS.

Interaction between dynamics and thermodynamics during tropical cyclogenesis

Abstract. Observational data of tropical disturbances are analyzed in order to investigate tropical cyclogenesis. Data from 37 cases observed during three field campaigns are used to investigate possible correlations between various dynamic and thermodynamic variables. The results show that a strong mid-level vortex is necessary to promote spin up of the low-level vortex in a tropical cyclone. This paper presents a theory on the mechanism of this process. The mid-level vortex creates a thermodynamic environment conducive to convection with a more bottom-heavy mass flux profile that exhibits a strong positive vertical gradient in a shallow layer near the surface. Mass continuity then implies that the strongest horizontal mass and vorticity convergence occurs near the surface. This results in low-level vortex intensification. For two of the disturbances that were observed during several consecutive days, evolution of the dynamics and thermodynamics is documented. One of these disturbances, Karl, was observed in the period before it intensified into a tropical storm; the other one, Gaston, was observed after it unexpectedly decayed from a tropical storm to a tropical disturbance. A hypothesis on its decay is presented.

Tropical cyclogenesis and mid-level vorticity

This paper reviews and summarizes our work on the thermodynamics and vorticity dynamics of tropical cyclogenesis with special emphasis on the use of observations from two recent field programs, T-PARC/TCS08 (THORPEX Asian Regional Campaign/Tropical Cyclone Structure experiment) and PREDICT (PRE-Depression Investigation of Cloud systems in the Tropics). Dropsondes deployed from high altitude plus airborne Doppler radar data (in T-PARC/TCS08) allowed examination of candidates for tropical cyclogenesis in unprecedented detail. The subtle interplay of dynamics and thermodynamics, and in particular, the role of the mid-level vortex in enabling spin up of a surface cyclone are elucidated. The interplay between this process and other investigators’ ideas about cyclogenesis is examined also. Finally, those areas of cyclogenesis needing further work are enumerated.

Analysis of the Thermodynamic Properties of Developing and Nondeveloping Tropical Disturbances Using a Comprehensive Dropsonde Dataset

Abstract A dropsonde dataset is analyzed to quantify the necessary thermodynamic conditions for tropical cyclogenesis by evaluating the properties that distinguish developing tropical disturbances from nondeveloping disturbances, and by describing the temporal evolution of the developing inner core. The dataset consists of 2204 dropsonde observations from 12 developing disturbances and 245 from four nondeveloping disturbances. These disturbances are the cases with the best pregenesis sampling from field programs between 2005 and 2010, and include those investigated by three coincident field programs during 2010: the NASA Genesis and Rapid Intensification Processes (GRIP) and NCAR/NSF Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiments, as well as NOAA’s Intensity Forecast Experiment (IFEX). Composite analyses indicate clear differences between developing and nondeveloping disturbances: developing disturbances exhibit greater moisture and a higher humidity at midlevels (above 800 hPa) than nondeveloping, and while the developing inner core experiences some midlevel moistening and stabilization as genesis nears, nondeveloping disturbances become progressively drier and more convectively unstable. Developing disturbances also exhibit some important characteristics in their inner core within 2 days of genesis: the low to midtroposphere (below 500 hPa) approaches near-saturation, a mid- to upper-level warm temperature anomaly develops and progressively deepens toward the low levels, and a low-level (below 900 hPa) cool, dry anomaly develops and is removed by the day of genesis. Overall the results support one proposed pathway to tropical cyclone formation in which an initially stronger midlevel vortex, in a moist, humid environment, precedes primarily low-level intensification within a day of genesis.

Impacts of 4DVAR Assimilation of Airborne Doppler Radar Observations on Numerical Simulations of the Genesis of Typhoon Nuri (2008)

AbstractThe Weather Research and Forecasting Model and its four-dimensional variational data assimilation (4DVAR) system are employed to examine the impact of airborne Doppler radar observations on predicting the genesis of Typhoon Nuri (2008). Electra Doppler Radar (ELDORA) airborne radar data, collected during the Office of Naval Research–sponsored Tropical Cyclone Structure 2008 field experiment, are used for data assimilation experiments. Two assimilation methods are evaluated and compared, namely, the direct assimilation of radar-measured radial velocity and the assimilation of three-dimensional wind analysis derived from the radar radial velocity. Results show that direct assimilation of radar radial velocity leads to better intensity forecasts, as this process enhances the development of convective systems and improves the inner-core structure of Nuri, whereas assimilation of the radar-retrieved wind analysis is more beneficial for tracking forecasts, as it results in improved environmental flows. The assimilation of both the radar-retrieved wind and the radial velocity can lead to better forecasts in both intensity and tracking, if the radial velocity observations are assimilated first and the retrieved winds are then assimilated in the same data assimilation window. In addition, experiments with and without radar data assimilation led to developing and nondeveloping disturbances in numerical simulations of Nuri’s genesis. The improved initial conditions and forecasts from the data assimilation imply that the enhanced midlevel vortex and moisture conditions are favorable for the development of deep convection in the center of the pouch and eventually contribute to Nuri’s genesis. The improved simulations of the convection and associated environmental conditions produce enhanced upper-level warming in the core region and lead to the drop in sea level pressure.

Tropical Cyclogenesis in the Western North Pacific as Revealed by the 2008–09 YOTC Data*

Abstract The Year of Tropical Convection (YOTC) high-resolution global reanalysis dataset was analyzed to reveal precursor synoptic-scale disturbances related to tropical cyclone (TC) genesis in the western North Pacific (WNP) during the 2008–09 typhoon seasons. A time filtering is applied to the data to isolate synoptic (3–10 day), quasi-biweekly (10–20 day), and intraseasonal (20–90 day) time-scale components. The results show that four types of precursor synoptic disturbances associated with TC genesis can be identified in the YOTC data. They are 1) Rossby wave trains associated with preexisting TC energy dispersion (TCED) (24%), 2) synoptic wave trains (SWTs) unrelated to TCED (32%), 3) easterly waves (EWs) (16%), and 4) a combination of either TCED-EW or SWT-EW (24%). The percentage of identifiable genesis events is higher than has been found in previous analyses. Most of the genesis events occurred when atmospheric quasi-biweekly and intraseasonal oscillations are in an active phase, suggesting a large-scale control of low-frequency oscillations on TC formation in the WNP. For genesis events associated with SWT and EW, maximum vorticity was confined in the lower troposphere. During the formation of Jangmi (2008), maximum Rossby wave energy dispersion appeared in the middle troposphere. This differs from other TCED cases in which energy dispersion is strongest at low level. As a result, the midlevel vortex from Rossby wave energy dispersion grew faster during the initial development stage of Jangmi.

A Spatial Filter Approach to Evaluating the Role of Convection on the Evolution of a Mesoscale Vortex

Abstract A new spatial filter is proposed that exploits a spectral gap in power between the convective scale and the system (“vortex”) scale during tropical cyclone (TC) genesis simulations. Using this spatial separation, this study analyzes idealized three-dimensional numerical simulations of deep moist convection in the presence of a symmetric midlevel vortex to quantify and understand the energy cascade between the objectively defined convective scale and system scale during the early stages of tropical cyclogenesis. The simulations neglect surface momentum, heat, and moisture fluxes to focus on generation and enhancement of vorticity within the interior to more completely close off the energy budget and to be consistent for comparison with prior benchmark studies of modeled TC genesis. The primary contribution to system-scale intensification comes from the convergence of convective-scale vorticity that is supplied by vortical hot towers (VHTs). They contribute more than the convergence of system-scale vorticity to the spinup of vorticity in these simulations by an order of magnitude. Analysis of the change of circulation with time shows an initial strengthening of the surface vortex, closely followed by a growth of the mid- to upper-level circulation. This evolution precludes any possibility of a stratiform precipitation–induced top-down mechanism as the primary contributor to system-scale spinup in this simulation. Instead, an upscale cascade of rotational kinetic energy during vortex mergers is responsible for spinup of the simulated mesoscale vortex. The spatial filter employed herein offers an alternative approach to the traditional symmetry–asymmetry paradigm, acknowledges the highly asymmetric evolution of the system-scale vortex itself, and may prove useful to future studies on TC genesis.

An examination of two pathways to tropical cyclogenesis occurring in idealized simulations with a cloud-resolving numerical model

Abstract. Simulations are conducted with a cloud-resolving numerical model to examine the transformation of a weak incipient mid-level cyclonic vortex into a tropical cyclone. Results demonstrate that two distinct pathways are possible and that development along a particular pathway is sensitive to model physics and initial conditions. One pathway involves a steady increase of the surface winds to tropical cyclone strength as the radius of maximum winds gradually decreases. A notable feature of this evolution is the creation of small-scale lower tropospheric cyclonic vorticity anomalies by deep convective towers and subsequent merger and convergence by the low-level secondary circulation. The second pathway also begins with a strengthening low-level circulation, but eventually a significantly stronger mid-level circulation develops. Cyclogenesis occurs subsequently when a small-scale surface concentrated vortex forms abruptly near the center of the larger-scale circulation. The small-scale vortex is warm core throughout the troposphere and results in a fall in local surface pressure of a few millibars. It usually develops rapidly, undergoing a modest growth to form a small tropical cyclone. Many of the simulated systems approach or reach tropical cyclone strength prior to development of a prominent mid-level vortex so that the subsequent formation of a strong small-scale surface concentrated vortex in these cases could be considered intensification rather than genesis. Experiments are performed to investigate the dependence on the inclusion of the ice phase, radiation, the size and strength of the incipient mid-level vortex, the amount of moisture present in the initial vortex, and the sea surface temperature. Notably, as the sea surface temperature is raised, the likelihood of development along the second pathway is increased. This appears to be related to an increased production of ice. The sensitivity of the pathway taken to model physics and initial conditions revealed by these experiments raise the possibility that the solution to this initial value problem is near a bifurcation point. Future improvements to model parameterizations and more accurate observations of the transformation of disturbances to tropical cyclones should clarify the conditions that favor a particular pathway when starting from a mid-level vortex.

Balanced thermal structure of an intensifying tropical cyclone

ABSTRACTThis study tests the hypothesis that the formation of a virtual potential temperature dipole in a developing tropical cyclone is a balanced response to the growth of an associated mid-level vortex. The dipole is collocated with the vortex and consists of a warm anomaly in the upper troposphere and a cool anomaly in the lower troposphere. An axisymmetric approximation to the observed potential vorticity distribution is inverted subject to non-linear balance for two successive days during the formation of typhoon Nuri in 2008. Good agreement is found between the area-averaged actual and balanced virtual temperature dipoles in these two cases. Furthermore, a strong correlation exists between the degree of bottom-heaviness of convective mass flux profiles and the strength of the balanced virtual potential temperature dipole. Since the dipole is balanced, it cannot be an immediate artefact of the existing convection, but rather is an inherent feature of the developing cyclone. Cloud resolving numerical modelling suggests that the dipole temperature anomaly actually promotes more bottom-heavy convective mass flux profiles, as observed. Such profiles are associated with low-level mass and vorticity convergence via mass continuity and the circulation theorem, resulting in low-level spin-up. The present work thus supports the hypothesis that the low-level spin-up associated with tropical cyclogenesis is made possible by the thermodynamic environment created by a strong mid-level vortex.

A study of rotation in thunderstorms in a weakly- or moderately-sheared environment

This study investigates two cases of thunderstorms with rotating characteristics, which occurred in Hungary and formed in an environment of relatively low or moderate wind shear (well below 20m/s) in the lowest 6km layer of the troposphere. For the selected cases, the properties of the thunderstorms and their environment were examined both from observational and modeling perspectives. The observed storms showed both multicellular and supercellular features (e.g. multiple, fast developing maxima of radar reflectivity but also presence of Bounded Weak Echo Reflectivity or couplets of Doppler radar velocity extremes). Cloud-base rotation was observed by the Hungarian storm-chasers. Cloud-resolving and real-data numerical simulations with the WRF model produced generation of meso-γ-scale vortices in both mid- and low-tropospheric levels connected to convective storms. The simulated low-level vortices exhibited quasi-permanent behavior and their intensity seemed to be comparable to supercell mesocyclones. Vorticity equation terms were analyzed on the model fields in order to explain the origin of the rotation and its relation to the environmental wind shear and wind profile. The results indicated that the more transient midlevel vortices were generated via the tilting mechanism, whereas the evolution of the quasi-persistent low-level vortices was initiated by a relatively small tilting along a gust front, then they subsequently rapidly intensified by the stretching of the vertical vorticity. The storms analyzed in the model field exhibited a hybrid behavior, since the structure and evolution of the vorticity field resembled supercellular mesocyclones and mesovortices as well.

Modeled and Observed Variations in Storm Divergence and Stratiform Rain Production in Southeastern Texas

Abstract Storm divergence profiles observed by an S-band Doppler radar are compared to ensemble simulations of 10 disparate precipitating systems occurring in warm-season, weakly baroclinic, and strongly baroclinic environments in southeastern Texas. Eight triply nested mesoscale model simulations are conducted for each case using single- and double-moment microphysics with four convective treatments (i.e., two convective parameterizations and explicit versus parameterized convection at 9 km). Observed and simulated radar reflectivities are objectively separated into convective, stratiform, and nonprecipitating anvil columns and comparisons are made between ensemble mean echo coverages and levels of nondivergence (LNDs). In both the model and observations, storms occurring in less baroclinic environments have more convective rain area, less stratiform rain area, and more elevated divergence profiles. The model ensemble and observations agree best for well-organized leading-line trailing-stratiform systems. Excessive convective area fractions are simulated for several less-organized cases, especially those whose ensemble mean LNDs are about 1–2 km more elevated than observed. Simulations parameterizing convection on the intermediate grid produced less-elevated divergence profiles with smaller magnitudes compared to their explicit counterparts. In one warm-season case, utilizing double-moment microphysics when parameterizing convection on both outer grids generated lower LNDs associated with variations in convective intensity and depth, detraining less ice to anvil and stratiform regions at midlevels relative to its single-moment counterpart. Similarly, mesoscale convective vortex simulations employing an ensemble-based versus a single-closure convective parameterization on both outer grids produced the least-elevated heating structures (closer to observed), resulting in the weakest midlevel vortices.

Thermodynamics of tropical cyclogenesis in the northwest Pacific

[1] This paper presents analyses of five tropical disturbances of various types, derived from observations made over the northwest Pacific in August and September of 2008. Various dynamic and thermodynamic products were derived from dropsonde and airborne Doppler radar data, with the goal of increasing our understanding of tropical cyclogenesis. From these analyses we draw the following tentative conclusions: The formation of a strong midlevel circulation, with its associated cold core at low levels and warm core aloft, greatly aids the spin-up of a tropical cyclone by changing the vertical mass flux profile of deep convection from top heavy to bottom heavy. This has two effects: (1) the enhancement at low levels of the convergence of mass and hence vorticity, thus aiding the spin-up of a warm-core vortex and (2) the suppression of the lateral export of moist entropy by deep convective inflows and outflows from the core of the developing system. This allows the relative humidity to build up, resulting in more intense convection and further development. Our results also suggest that strong horizontal strain rate at middle levels, as measured by a form of the Okubo-Weiss parameter, is detrimental to tropical cyclogenesis. Not only can such flow tear apart the midlevel vortex, it can also import air with low moist entropy. In our small sample, the Okubo-Weiss parameter was the best indicator of the potential for development. Vertical shear appeared to play a less important role in the systems we investigated.

A numerical study of the role of the vertical structure of vorticity during tropical cyclone genesis

An eight-level axisymmetric model with simple parameterizations for clouds and the atmospheric boundary layer was developed to examine the evolution of vortices that are precursors to tropical cyclones. The effect of vertical distributions of vorticity, especially that arising from a merger of mid-level vortices, was studied by us to provide support for a new vortex-merger theory of tropical cyclone genesis. The basic model was validated with the analytical results available for the spin-down of axisymmetric vortices. With the inclusion of the cloud and boundary layer parameterizations, the evolution of deep vortices into hurricanes and the subsequent decay are simulated quite well. The effects of several parameters such as the initial vortex strength, radius of maximum winds, sea-surface temperature and latitude (Coriolis parameter) on the evolution were examined. A new finding is the manner in which mid-level vortices of the same strength decay and how, on simulated merger of these mid-level vortices, the resulting vortex amplifies to hurricane strength in a realistic time frame. The importance of sea-surface temperature on the evolution of full vortices was studied and explained. Also it was found that the strength of the surface vortex determines the time taken by the deep vortex to amplify to hurricane strength.

Patterns of Precipitation and Mesolow Evolution in Midlatitude Mesoscale Convective Vortices

Abstract Surface pressure manifestations of mesoscale convective vortices (MCVs) that traversed Oklahoma during the periods May–August 2002–05 are studied using the Weather Surveillance Radar-1988 Doppler (WSR-88D), the Oklahoma Mesonet, and the NOAA Profiler Network data. Forty-five MCVs that developed from mesoscale convective systems (MCSs) have been investigated, 28 (62%) of which exhibit mesolows detectable at the surface. Within this group, three distinct patterns of precipitation organization and associated mesolow evolution have been identified. The remaining 17 (38%) of the cases do not contain a surface mesolow. Two repeating patterns of precipitation organization are identified for the latter group. The three categories of MCVs possessing a surface mesolow are as follows. Nineteen are classified as “rear-inflow-jet MCVs,” and tend to form within large and intense asymmetric MCSs. Rear inflow into the MCS, enhanced by the development of an MCV on the left-hand side relative to system motion, produces a rear-inflow notch and a distinct surface wake low at the back edge of the stratiform region. Hence, the surface mesolow and MCV are displaced from one another. Eight are classified as “collapsing-stratiform-region MCVs.” These MCVs arise from small asymmetric MCSs. As the stratiform region of the MCS weakens, a large mesolow appears beneath its dissipating remnants due to broad subsidence warming, and at the same time the midlevel vortex spins up due to column stretching. One case, called a “vertically coherent MCV,” contains a well-defined surface mesolow and associated cyclonic circulation, apparently due to the strength of the midlevel warm core and the weakness of the low-level cold pool. In these latter two cases, the surface mesolow and MCV are approximately collocated. Within the group of MCVs without a surface mesolow, 14 are classified as “remnant-circulation MCVs” containing no significant precipitation or surface pressure effects. Finally, three are classified as “cold-pool-dominated MCVs;” these cases contain significant precipitation but no discernible surface mesolow. This study represents the first systematic analysis of the surface mesolows associated with MCVs. The pattern of surface pressure and winds accompanying MCVs can affect subsequent convective development in such systems. Extension of the findings herein to tropical oceans may have implications regarding tropical cyclogenesis.

Tropical cyclogenesis sensitivity to environmental parameters in radiative–convective equilibrium

AbstractIn this study, the relationship between the likelihood of tropical cyclogenesis and external environmental forcings is explored in the simplest idealized modelling framework possible: radiative‐convective equilibrium on a doubly periodic f‐plane. In such an environment, control of the equilibrium environmental sounding is reduced to three parameters: the sea‐surface temperature, the Coriolis parameter, and the imposed background surface wind speed. Cloud‐resolving mesoscale model simulations are used to generate environments of radiative‐convective equilibrium determined by these three factors. The favourability of these environments for tropical cyclogenesis is measured in three ways: in terms of the maximum potential intensity (MPI) of the sounding, based on the thermodynamic theory of Emanuel; in terms of the ‘genesis potential’ determined by an empirical genesis parameter; and in terms of the propensity of weak initial vortices in these environments to form into tropical cyclones.The simulated environments of radiative—convective equilibrium with no vertical wind shear are found to be very favourable for tropical cyclogenesis. Weak initial vortices always transition to a tropical cyclone, even for rather low sea‐surface temperatures. However, the time required for these vortices to make the transition from a weak, mid‐level vortex to a rapidly developing tropical cyclone decreases as the MPI increases, indicating the importance of MPI in enhancing the frequency of cyclogenesis. The relationship between this ‘time to genesis’ and the thermodynamic parameters is explored. The time to genesis is found to be very highly (negatively) correlated to MPI, with little or no relationship to convective instability, Coriolis parameter, mid‐level humidity, or the empirical genesis parameter.In some cases, tropical cyclones are found to form spontaneously from random convection. This formation is due to a cooperative interaction between large‐scale moisture, long‐wave radiation, and locally enhanced sea‐surface fluxes, similar to the ‘aggregation’ of convection found in previous studies. Copyright © 2007 Royal Meteorological Society

Probabilistic Evaluation of the Dynamics and Predictability of the Mesoscale Convective Vortex of 10–13 June 2003

Abstract This study examines the dynamics and predictability of the mesoscale convective vortex (MCV) of 10–13 June 2003 through ensemble forecasting. The MCV of interest developed from a preexisting upper-level disturbance over the southwest United States on 10 June and matured as it traveled northeastward. This event is of particular interest given the anomalously strong and long-lived nature of the circulation. An ensemble of 20 forecasts using a 2-way nested mesoscale model with horizontal grid increments of 30 and 10 km are employed to probabilistically evaluate the dynamics and predictability of the MCV. Ensemble mean and spread as well as correlations between different forecast variables at different forecast times are examined. It is shown that small-amplitude large-scale balanced initial perturbations may result in very large ensemble spread, with individual solutions ranging from a very strong MCV to no MCV at all. Despite similar synoptic-scale conditions, the ensemble MCV forecasts vary greatly depending on intensity and coverage of simulated convection, illustrating the critical role of convection in the development and evolution of this MCV. Correlation analyses reveal the importance of a preexisting disturbance to the eventual development of the MCV. It is also found that convection near the center of the MCV the day after its formation may be an important factor in determining the eventual growth of a surface vortex and that a stronger midlevel vortex is more conducive to convection, especially on the downshear side, consistent with the findings of previous MCV studies.