Abstract

There have been some ambiguities in recent observational studies as to whether midlevel mesovortices are induced by latent heating or cooling, and develop in the descending or ascending portion of mesoscale convective systems (MCS's). In this study, a comprehensive examination of a cooling-induced mesovortex in the trailing stratiform region of a midlatitude squall line that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM) is presented using a 20-h high-resolution simulation of the squall system.

This cooling-induced midlevel vortex originates from the preexisting cyclonic vorticity associated with a traveling meso-α-scale short wave. The vortex is intensified in the descending rear-to-front (RTF) inflow as a result of continued sublimative melting and evaporative cooling in the stratiform region. It decouples from the front-to-rear (FTR) ascending and anticyclonic flow in the upper troposphere during the formative stage. The vortex tilts northward with height, resulting in a deep layer of cyclonic vorticity (up to 250 mb) near the northern end of the squall line. It has an across-line scale of 120–150 km and a longitudinal scale of more than 300 km, with its maximum intensity located above the melting level.

A three-dimensional vorticity budget shows that the cooling-induced vortex is initially maintained through the vertical stretching of its absolute vorticity associated with the short-wave trough. As the descending rear inflow develops within the system, the tilting of horizontal vorticity is about one order of magnitude larger than the stretching in determining the early intensification of the vortex. In most vortex layers, the stretching tends to destroy the vortex locally, owing to the existence of the divergent outflow in the lower troposphere. Only when the vortex propagates into the FTR-RTF flow interface does the stretching effect begin to control the final amplification of the vortex, and the tilting plays a negative role during the squall's decaying stage.

The model also reproduces well a narrow heating-induced (or warm-core) cyclonic vortex along the leading convective line and a deep anticyclonic-vorticity zone between the heating- and cooling-induced mesovortices. It is shown that the cyclonic vortex along the leading line develops through positive tilting and stretching, whereas the anticyclonic-vorticity zone is generated by tilting of horizontal vorticity by the FTR-ascending and RTF-descending flows, and later enhanced by negative stretching along the interface convergence zone. The warm-core vortex dissipates and eventually merges into the cooling-induced vortex circulation as the system advances into a convectively less favorable environment. The anticyclonic-vorticity zone rapidly diminishes as the cooling-induced vortex moves into the flow interface. At the end of the life cycle, the cooling-induced mesovortex becomes the only remaining element of the squall system that can be observed in a deep layer and at a larger scale in the low to midtroposphere. Different characteristics of heating-induced versus cooling-induced mesovortices and their relationships are discussed. The results suggest that mesovortices are ubiquitous in MCS's and that their pertinent mesoscale rotational flow may be the basic dynamic effect of MCS's on their larger-scale environments.

Abstract

A nocturnal torrential-rain-producing mesoscale convective system (MCS) occurring during the mei-yu season of July 2003 in east China is studied using conventional observations, surface mesoanalysis, satellite and radar data, and a 24-h multinested model simulation with the finest grid spacing of 444 m. Observational analyses reveal the presence of several larger-scale conditions that were favorable for the development of the MCS, including mei-yu frontal lifting, moderate cold advection aloft and a moist monsoonal flow below, and an elongated old cold dome left behind by a previously dissipated MCS.

Results show that the model could reproduce the evolution of the dissipating MCS and its associated cold outflows, the triggering of three separate convective storms over the remnant cold dome and the subsequent organization into a large MCS, and the convective generation of an intense surface meso-high and meso-β-scale radar reflectivity morphologies. In particular, the model reproduces the passage of several heavy-rain-producing convective bands at the leading convective line and the trailing stratiform region, leading to the torrential rainfall at nearly the right location. However, many of the above features are poorly simulated or missed when the finest model grid uses either 1.33- or 4-km grid spacing. Results indicate the important roles of isentropic lifting of moist monsoonal air over the cold dome in triggering deep convection, a low-level jet within an elevated moist layer in maintaining conditional instability, and the repeated formation and movement of convective cells along the same path in producing the torrential rainfall.

Abstract

This study uses a recently developed trajectory model to trace eyewall updrafts in a high-resolution Hurricane Wilma (2005) prediction to their roots in the maritime boundary layer (MBL) in order to better understand their thermodynamics and how they interact with the swirling winds. Out of 97 020 four-hour backward trajectories seeded from the upper troposphere, the 45% of them originating from the MBL are stratified into five subsamples binned by peak vertical velocity w _MAX. Of particular interest are the thermodynamic characteristics of parcels belonging to the w _MAX-Extreme subsample (i.e., those with w _MAX exceeding 20 m s⁻¹) that ascend through Wilma’s strongest convective burst (CB) cores. A vertical momentum budget computed along a selected w _MAX-Extreme trajectory confirms that the parcel possesses large positive buoyancy that more than compensates for negative hydrometeor loading to yield an upper-tropospheric w _MAX ~ 30 m s⁻¹. Comparing all 1170 w _MAX-Extreme trajectories with all 19 296 secondary circulation trajectories shows that the former tends to originate from the MBL where equivalent potential temperature θ _e and ocean surface heat and moisture fluxes are locally enhanced. The w _MAX-Extreme parcels become further differentiated from the background ascent in terms of their (i) greater updraft width and smaller θ _e reduction while ascending into the midtroposphere, implying lower environmental entrainment rates, and (ii) less hydrometeor loading in the z = 3–5-km layer. The Lagrangian analysis herein bridges two previous studies that focused separately on the importance of high SSTs and fusion latent heat release to the development of CBs, the latter of which may facilitate upper-level warm core development through their compensating subsidence.

Abstract

The effects of dissipative heating on hurricane intensity are examined using a 72-h explicit simulation of Hurricane Andrew (1992) with a state-of-the-art, three-dimensional, nonhydrostatic mesoscale (cloud resolving) model (i.e., MM5). It is found that the inclusion of dissipative heating increases the central pressure deficit of the storm by 5–7 hPa and its maximum surface wind by about 10% prior to landfall. It is shown that dissipative heating tends to warm the surface layer, causing a decrease (increase) in sensible heat flux at the sea surface (the top of the surface layer) that acts to cool the surface layer, although the net (sensible plus dissipative) heating rates are still 30%–40% greater than the sensible heating rates in the control simulation. Finally, the potential effects of energy transfer into the ocean, sea surface temperature changes within the inner core, and evaporation of sea spray, interacting with dissipative heating, on hurricane intensity are discussed.

Abstract

Recent observations have revealed that some mesoscale convective systems (MCSs) could undergo multiple cycles of convective development and dissipation, and, under certain environments, they appeared to be responsible for (barotropic) oceanic or tropical cyclogenesis. In this study, oceanic cyclogenesis, as induced by an MCS moving offshore and then driven by deep convection in a near-barotropic environment, is investigated by extending to 90 h the previously documented 18-h simulation of the MCSs that were responsible for the July 1977 Johnstown flash flood. It is demonstrated that the mesoscale model can reproduce very well much of the meso-β-scale structures and evolution of the long-lived MCS out to 90 h. These include the development and dissipation of the continental MCSs as well as the associated surface and tropospheric perturbations, the timing and location in the initiation of a new MCS after 36 h and in the genesis of a surface mesolow over the warm Gulf Stream water after 60-h integration, the track and the deepening of the surface cyclone into a “tropical storm,” the maintenance of a midlevel mesovortex/trough system, and the propagation of a large-scale cold front with respect to the surface cyclone.

It is found that the new MCS is triggered after the vortex/trough moved offshore and interacted with the land-ocean thermal contrasts during the afternoon hours. The oceanic cyclogenesis begins at 150–180 km to the south of the vortex, as the associated surface trough advances into the Gulf Stream and weakens. Then, the cyclone overpowers quickly the low-level portion of the vortex circulation and deepens 14 hPa in 24 h. A comparison with a dry sensitivity simulation shows that the cyclogenesis occurs entirely as a consequence of the convective forcing. Without it, an 84-h simulation produces only a surface trough with the minimum pressure being nearly the same as that left behind by the previous MCSs. It is shown that the vortex/trough provides persistent convergence at its southern periphery for the continued convective development, whereas the convectively enhanced low-level flow tends to (i) “pump” up sensible and latent heat fluxes from the warm ocean surface and (ii) transport the high-θ _e air in a slantwise fashion into the region having lower θ _e aloft, thereby causing further conditional instability, increased convection, and more rapid deepening. The interactions of the continental MCS/vortex and the oceanic cyclone/storm systems with their larger-scale environments are also discussed.

Abstract

This study examines the synoptic- and mesoscale processes leading to the generation of three extreme rainfall episodes with hourly rates of greater than 100 mm h⁻¹ over the southern, middle, and northern portions of the eastern foothills of Mt. Taihang in North China on 19–20 July 2016. The extreme rainfall episodes took place over the 200–600-m elevation zones in the southern and northern portions but also over the lower elevations in the middle portion of the target region, sequentially during late morning, early evening, and midnight hours. Echo training accounted for the development of a linear convective system in the southern region after the warm and moist air carried by a southeasterly low-level jet (LLJ) was lifted to condensation as moving across Mt. Yuntai. In contrast, two isolated circular-shaped convective clusters, with more robust convective cores in its leading segment, developed in the northern region through steep topographical lifting of moist northeasterly airflow, albeit conditionally less unstable. Extreme rainfall in the middle region developed from the convergence of a moist easterly LLJ with a northerly colder airflow associated with an extratropical cyclogenesis. Results reveal that the LLJs and associated moisture transport, the intensifying cyclone interacting with a southwest vortex and its subsequent northeastward movement, and the slope and orientation of local topography with respect to and the stability of the approaching airflows played different roles in determining the timing and location, the extreme rainfall rates, and convective organizations along the eastern foothills of Mt. Taihang.

Abstract

Hurricane Joaquin (2015) took a climatologically unusual track southwestward into the Bahamas before recurving sharply out to sea. Several operational forecast models, including the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS), struggled to maintain the southwest motion in their early cycles and instead forecast the storm to turn west and then northwest, striking the U.S. coast. Early cycle GFS track errors are diagnosed using a tropical cyclone (TC) motion error budget equation and found to result from the model 1) not maintaining a sufficiently strong mid- to upper-level ridge northwest of Joaquin, and 2) generating a shallow vortex that did not interact strongly with upper-level northeasterly steering winds. High-resolution model simulations are used to test the sensitivity of Joaquin’s track forecast to both error sources. A control (CTL) simulation, initialized with an analysis generated from cycled hybrid data assimilation, successfully reproduces Joaquin’s observed rapid intensification and southwestward-looping track. A comparison of CTL with sensitivity runs from perturbed analyses confirms that a sufficiently strong mid- to upper-level ridge northwest of Joaquin and a vortex deep enough to interact with northeasterly flows associated with this ridge are both necessary for steering Joaquin southwestward. Contraction and vertical alignment of the CTL vortex during the early forecast period may have also helped draw the low-level vortex center southward. The results indicate that for TCs developing in vertically sheared environments, improved inner-core sampling by means of cloudy radiances and aircraft reconnaissance missions may help reduce track forecast errors by improving the model estimate of vortex depth.

Abstract

When computing trajectories from model output, gridded winds are often temporally interpolated to a time step shorter than model output intervals to satisfy computational stability constraints. This study investigates whether trajectory accuracy may be improved for tropical cyclone (TC) applications by interpolating the model winds using advection correction (AC) instead of the traditional linear interpolation in time (LI) method. Originally developed for Doppler radar processing, AC algorithms interpolate data in a reference frame that moves with the pattern translation, or advective flow velocity. A previously developed trajectory AC implementation is modified here by extending it to three-dimensional (3D) flows, and the advective flows are defined in cylindrical rather than Cartesian coordinates. This AC algorithm is tested on two model-simulated TC cases, Hurricanes Joaquin (2015) and Wilma (2005). Several variations of the AC algorithm are compared to LI on a sample of 10 201 backward trajectories computed from the modeled 5-min output data, using reference trajectories computed from 1-min output to quantify position errors. Results show that AC of 3D wind vectors using advective flows defined as local gridpoint averages improves the accuracy of most trajectories, with more substantial improvements being found in the inner eyewall where the horizontal flows are dominated by rotating cyclonic wind perturbations. Furthermore, AC eliminates oscillations in vertical velocity along LI backward trajectories run through deep convective updrafts, leading to a ~2.5-km correction in parcel height after 20 min of integration.

Abstract

Considerable progress has been made in the past decades on understanding the life cycle of rapidly deepening winter cyclones. However, little attention has been paid to the role that mesoscale convective systems (MCSs) play during extratropical cyclogenesis within weak baroclinic environments. In this study, the impact of an MCS on the subsequent surface cyclogenesis is investigated by extending the previously documented 21-h simulation of the 10–11 June 1985 PRE-STORM squall line to 36 hours. The model reproduces the meteorological events from the initiation to the dissipation of the squall system and then to the formation of a surface cyclone and the amplification of midlevel baroclinic waves, as verified against all available observations.

It is found that the squall line is initiated ahead of a weak surface cold front with the aid of baroclinic forcing. Once initiated, however, the prefrontal squall system is primarily driven by the interaction of convectively generated circulations with a conditionally unstable environment. As it rapidly intensifies and accelerates east-ward, the squall system amplifies a midlevel short wave by warming the upper troposphere and cooling the lower troposphere, and then forces it to move with the system. On the other hand, the movement of a low to midlevel thermal wave is primarily determined by adiabatic processes. Thus, the convective system tends to enhance the larger-scale baroclinicity and increase the phase lag between the pressure and thermal waves such that the baroclinic environment becomes more favorable for the subsequent surface cyclogenesis.

The role of moist convection in the surface cyclogenesis is examined by comparing simulations with and without the convective system. It is found that, in the absence of moist convection, the model also produces a surface cyclone, but with much weaker intensity, much smaller horizontal extent, and much slower displacement. The relationships of convectively generated mesovortices and wake lows to the surface cyclogenesis are also examined.

Abstract

The interaction between internal gravity waves and a squall line that developed early in the evolution of the 1977 Johnston flood event is studied based on available surface observations and a three-dimensional model simulation of the flood-related mesoscale convective systems (MCSs). Several experimental simulators are carried out to investigate the mechanisms whereby gravity waves form and obtain energy. Both observations and model simulators of the wave/convection interaction fit certain theories of gravity wave propagation. Following the formation of the squall line, subsequent deep convection typically initiates behind a pressure trough associated with the lint and ahead of or along the axis of the trailing ridge. The zero contours of vertical motion correspond closely to the axis of the surface pressure trough. Positive potential temperature perturbations correspond with descending motion occurring ahead of the trough while negative perturbations occur with increasing ascending motion towards the approaching ridge axis. Model airflow trajectories show that the simulated gravity wave surface pressure perturbations (with amplitudes of about 1 mb) correspond to vertical parcel displacements of more than 30 mb.

The model simulations indicate that the gravity waves am initiated by a super-geostrophic low-level jet with strong horizontal wind shear over an area where an explosive convective development occurs, and then are enhanced by intense convection. The waves propagate at a speed significantly faster than a meso-α scale quasi-geostrophic wave that is partly responsible for the initial explosive development and that later plays a key role in controlling the evolution of a mesoscale convective complex (MCC). The fag moving gravity waves help the squall line accelerate eastward and separate from a trailing area of convection that later develops into the MCC. It appears that the waves and the squall line interact with each other constructively prior to the squall line's mature stage. Specifically, the line of deep convection seems to provide the waves with energy through enhancing mass convergence/divergence in a deep layer and acting as an “obstacle” to the sheared flow. The waves tend to help organize convective elements into a line structure and turn the line a little clockwise. After the squall line moves into a convectively less favorable environment, it slows down, whereas the accompanying gravity waves continue their eastward movement. Then the convection and gravity waves gradually become out of phase and interact with each other destructively. Because of the absence of low-level inversions and critical levels to duct the wave propagation, the gravity waves quickly diminish as they move away from the energy source region. Free-wave experimental simulations show many wave characteristics similar to the control simulation, indicating that the gravity waves determine the orientation, propagation and structure of the squall line. A sea breeze circulation and mountain waves associated with the Appalachians also occur in the model simulation, but do not seem to have a significant effect on the evolution of the daytime deep convection.

The results indicate that physical interaction between deep convection and internal gravity waves can be simulated by numerical models if a compatible grid resolution, proper model physics and good initial conditions are incorporated. In particular, the apparent relationship between the gravity waves and the squall ling suggests that preserving the components of layered internal gravity waves in the model initial conditions may be very important for successful model prediction of the timing and location of wave-related MCSs.