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Da-Lin Zhang

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.

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Man Zhang and Da-Lin Zhang

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.

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Da-Lin Zhang and Kun Gao

Abstract

An intense rear-inflow jet, surface pressure perturbations, and stratiform precipitation associated with a squall line during 10–11 June 1985 are examined using a three-dimensional mesoscale nested-grid model. It is found that the large-scale baroclinity provides favorable and deep rear-to-front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow. However, latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow. The role of the rear inflow is generally to produce convergence ahead and divergence behind the system, and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions. In this case study, the rear-inflow jet appears to have caused the splitting of the surface wake low as well as the organized rainfall.

As considerable mass within the rear inflow subsides, an intense surface wake low is formed at the back edge of the squall system. This result confirms previous observations that the surface wake low develops hydrostatically as a consequence of adiabatic warming and drying by the descending rear inflow. The wake low is shown to be an end product of complicated reactions involving condensate production, fallout cooling and induced subsiding motion. It does not have any significant effects on the evolution of atmospheric features ahead but contributes to vertical destabilization over the wake region.

The simulation shows that the squall line initially leans downshear and later upshear as the low-level cold pool progressively builds up and the system moves into a convectively stable environment. During the mature stage, there are three distinct airflows associated with the squall system: a leading overturning updraft and an ascending front-to-rear (FTR) current that both are driven by high-θe, air from the boundary layer ahead of the line, and an overturning downdraft carrying low-θe, air from the rear. These features resemble previously published results using nonhydrostatic cloud models. Due to continuous deposit of FTR momentum at the upper levels, the FTR updraft is responsible for the rearward transport of high-θe, air mass for the generation of the trailing stratiform precipitation.

Several sensitivity experiments are conducted. The generation of the descending rear inflow, and the surface and midlevel pressure perturbations are found to be most sensitive to the parameterized moist downdrafts, hydrostatic water loading, evaporative cooling and ice ice microphysics, in that order. Without any one of these model processes, neither the rear inflow reaches the surface nor the surface mesohigh and wake low become well developed. The results illustrate that the descending rear inflow is a product of the dynamic response to the latent-cooling-induced circulation. Different roles of the parameterized versus grid-resolved downdrafts in the development of the descending rear inflow are also discussed.

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Stéphane Bélair and Da-Lin Zhang

Abstract

Despite considerable progress in the understanding of two-dimensional structures of squall lines, little attention has been paid to the along-line variability of these convective systems. In this study, the roles of meso- and larger-scale circulations in the generation of along-line variability of squall lines are investigated, using an 18-h prediction of a frontal squall line that occurred on 26–27 June 1985 during PRE-STORM (Preliminary Regional Experiment for Stormscale Operational Research Meteorology). It is shown that the Canadian regional finite-element (RFE) model reproduces reasonably well a number of surface and vertical circulation structures of the squall system, as verified against available network observations. These include the initiation, propagation, and dissipation of the squall system, surface pressure perturbations, and cold outflow boundaries; a midlevel mesolow and an upper-level mesohigh; a front-to-rear (FTR) ascending flow overlying an intense rear-to-front (RTF) flow; and a leading convective line followed by stratiform precipitation regions.

It is found that across-line circulations at the northern segment of the squall line differ significantly from those at its southern segment, including the different types of precipitation, the absence of the RTF flow and midlevel mesolow, and the early dissipation of organized convection in the northern part. The along-line variability of the squall’s circulations results primarily from the interaction of convectively generated perturbations with a midlevel baroclinic trough. The large-scale trough provides an extensive RTF flow component in the southern portion of the squall system and an FTR flow component in the north, whereas the midlevel mesolow tends to enhance the RTF flow to the south and the FTR flow to the north of the mesolow during the mature to decaying stages. The along-line variability of the squall’s circulations appears to be partly responsible for the generation of different weather conditions along the line, such as the development of an upper-level stratiform region in the southern segment and a midlevel cloud region in the northern portion of the squall line.

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Da-Lin Zhang and Ning Bao

Abstract

The genesis of intense cyclonic vorticity in the boundary layer and the transformation of a low-level cold pool to a warm-core anomaly associated with the long-lived mesoscale convective systems (MCSs), which produced the July 1977 Johnstown flash flood and later developed into a tropical storm, are examined using a 90-h real-data simulation of the evolution from a continental MCS/vortex to an oceanic cyclone/storm system. It is shown that the midlevel vortex/trough at the end of the continental MCS's life cycle is characterized by a warm anomaly above and a cold anomaly below. The mesovortex, as it drifts toward the warm Gulf Stream water, plays an important role in initiating and organizing a new MCS and a cyclonic (shear) vorticity band at the southern periphery of the previously dissipated MCS. It is found from the vorticity budget that the vorticity band is amplified through stretching of absolute vorticity as it is wrapped around in a slantwise manner toward the cyclone center. Then, the associated shear vorticity is converted to curvature vorticity near the center, leading to the formation of a “comma-shaped” vortex and the rapid spinup of the surface cyclone to tropical storm intensity.

Thermodynamic budgets reveal that the vertical transfer of surface fluxes from the warm ocean and the convectively induced grid-scale transport are responsible for the development of a high-θe tongue, which is wrapped around in a fashion similar to the vorticity band, causing conditional instability and further organization of the convective storm. Because the genesis occurs at the southern periphery of the vortex/trough, the intensifying cyclonic circulation tends to advect the pertinent cold air in the north-to-northwesterly flow into the convective storm and the ambient warmer air into the cyclone center, thereby transforming the low-level cold anomaly to a warm-cored structure near the cyclone core. It is shown that the transformation and the evolution of the surface cyclone are mainly driven by the low-level vorticity concentrations.

It is found that many of the cyclogenesis scenarios in the present case are similar to those noted in previous tropical cyclogenesis studies and observed at the early stages of tropical cyclogenesis from MCSs during the Tropical Experiment in Mexico. Therefore, the results have significant implications with regard to tropical cyclogenesis from MCSs.

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Da-Lin Zhang and Ning Bao

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-θe air in a slantwise fashion into the region having lower θe 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.

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Da-Lin Zhang and Eric Altshuler

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.

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William Miller and Da-Lin Zhang

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.

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Rudi Xia and Da-Lin Zhang

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.

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William Miller and Da-Lin Zhang

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.

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