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Da-Lin Zhang and Richard Harvey

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.

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Guangxing Zhang, Da-Lin Zhang, and Shufang Sun

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A high-latitude low-level easterly jet (LLEJ) and downslope winds, causing severe dust storms over the Tacheng basin of northwestern China in March 2006 when the dust source regions were previously covered by snow with frozen soil, are studied in order to understand the associated meteorological conditions and the impact of complex topography on the generation of the LLEJ. Observational analyses show the development of a large-scale, geostrophically balanced, easterly flow associated with a northeastern high pressure and a southeastern low pressure system, accompanied by a westward-moving cold front with an intense inversion layer near the altitudes of mountain ridges. A high-resolution model simulation shows the generation of an LLEJ of near-typhoon strength, which peaked at about 500 m above the ground, as well as downslope windstorms with marked wave breakings and subsidence warming in the leeside surface layer, as the large-scale cold easterly flow moves through a constricting saddle pass and across a higher mountain ridge followed by a lower parallel ridge, respectively. The two different airstreams are merged to form an intense LLEJ of cold air, driven mostly by zonal pressure gradient force, and then the LLEJ moves along a zonally oriented mountain range to the north. Results indicate the importance of the lower ridge in enhancing the downslope winds associated with the higher ridge and the importance of the saddle pass in generating the LLEJ. We conclude that the intense downslope winds account for melting snow, warming and drying soils, and raising dust into the air that is then transported by the LLEJ, generated mostly through the saddle pass, into the far west of the basin.

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Da-Lin Zhang, Lin Zhu, Xuejin Zhang, and Vijay Tallapragada

Abstract

A series of 5-day numerical simulations of idealized hurricane vortices under the influence of different background flows is performed by varying vertical grid resolution (VGR) in different portions of the atmosphere with the operational version of the Hurricane Weather Research and Forecasting Model in order to study the sensitivity of hurricane intensity forecasts to different distributions of VGR. Increasing VGR from 21 to 43 levels produces stronger hurricanes, whereas increasing it further to 64 levels does not intensify the storms further, but intensity fluctuations are much reduced. Moreover, increasing the lower-level VGRs generates stronger storms, but the opposite is true for increased upper-level VGRs. On average, adding mean flow increases intensity fluctuations and variability (between the strongest and weakest hurricanes), whereas adding vertical wind shear (VWS) delays hurricane intensification and then causes more rapid growth in intensity variability. The stronger the VWS, the larger intensity variability and bifurcation rate occur at later stages. These intensity differences are found to be closely related to inner-core structural changes, and they are attributable to how much latent heat could be released in higher-VGR layers, followed by how much moisture content in nearby layers is converged. Hurricane intensity with higher VGRs is shown to be much less sensitive to varying background flows, and stronger hurricane vortices at the model initial time are less sensitive to the vertical distribution of VGR; the opposite is true for relatively uniform VGRs or weaker hurricane vortices. Results reveal that higher VGRs with a near-parabolic or Ω shape tend to produce smoother intensity variations and more typical inner-core structures.

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Da-Lin Zhang, Ekaterina Radeva, and John Gyakum

Abstract

In this study, a series of sensitivity experiments is performed to study the relative influence of latent heating, surface friction, and surface heat fluxes on the development of a family of frontal cyclones that occurred over the western Atlantic Ocean, using the simulation presented in Part I as a control run. It is shown that dry dynamics determines the initiation and track of all the frontal cyclones, and it accounts for about 59% of the deepening of a major frontal cyclone. Vorticity budget calculations reveal that in the absence of latent heating, preexisting upper-level cyclonic vorticity associated with a ring of potential vorticity provides the necessary forcing for the initiation and movement of the frontal cyclones, whereas the low-level thermal advection is responsible for a large portion of their amplifications as well as for their shallow circulations.

The impact of surface sensible and latent heat fluxes on the frontal cyclogenesis depends on the cyclones’ location with respect to the warm water surface. In the absence of latent heating, the surface fluxes have very weak impact, through modifying the low-level baroclinicity, on the evolution and final intensity of the frontal cyclones. When latent heating is allowed, however, the surface fluxes could result in more rapid cyclogenesis as a result of reduced static stability and increased moisture content in the maritime boundary layer; the impact is as pronounced as the latent heating. It is found that (dry) frontal cyclogenesis could still occur over a vast continental surface, although it is the slowest moving and deepening system among all the sensitivity tests being conducted.

The results reveal that (i) the frontal cyclones in the present case are baroclinically driven in nature, although they are markedly modulated by diabatic heating and surface fluxes; and (ii) the rapid frontal cyclogenesis phenomena tend to occur more frequently over a warm ocean surface due to its associated weak surface friction and its generated weak static stability in the maritime boundary layer.

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Da-Lin Zhang and J. Michael Fritsch

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A 36-h nested-grid numerical simulation of the life cycle of a convectively generated, inertially stable, warm-core mesovortex is presented. The vortex evolved from a mesoscale convective complex that developed from a squall line over Oklahoma during 7–8 July 1982. A modified version of the Pennsylvania State University /National Center for Atmospheric Research mesoscale hydrostatic model with a fine-mesh grid resolution of 25 km is utilized for this study. The model simultaneously incorporates parameterized convection and a grid-resolved convective scheme containing the effects of hydrostatic water loading, condensation (evaporation), freezing (melting) and sublimation.

Genesis, intensification and maintenance of a low- to midtropospheric closed meso-β scale cyclone as well as the associated surface pressure perturbations, the evolution of moist convection, and the distribution and magnitude of total rainfall are simulated by the model. Similarly, the observed amplification of a 700-mb meso-α scale short-wave trough, the development of a midlevel warm-core structure and an upper-level mesoanticyclone during the mature stage, the quasi-stationary nature of the vortex circulation, and the vertical distribution of horizontal wind and relative vorticity in the vicinity of the rotating mesoscale convective system (MCS) are all reasonably well simulated up to 36 h. During the mature stage of the rotating MCS, both the observed and simulated vertical structure are characterized by a low-level mesohigh in association with a cool pool and sinking motion, a midtropospheric warm-core structure, and an upper-level cold dome with an associated anticyclonic circulation. The horizontal momentum and equivalent potential temperature are uniformly distributed in the vortex layer with the vorticity maximum located between 600 and 700 mb.

The model simulation shows that the upward motion and cyclonic vorticity associated with the front and vortex system are out of phase. The phase difference appears to be a propagation mechanism of the rotating MCS and the low-level front. Another important finding is that most of the vortex properties tilt downstream with height during the decay period. Such a vertical distribution helps explain why a well-defined and long-lived hydrostatic surface mesolow did not form in either the observations or the simulation. It also helps explain why other midlatitude rotating MCSs often exhibit weak surface pressure predictions.

It is found that a propagating mesoscale vorticity disturbance, preexisting low-level frontal forcing and a convectively favorable environment ahead of the front help generate an organized area of upward motion wherein the vortex develops. However, it is the resolvable-scale latent heat release that appears to be directly responsible for producing the rotating MCS. The quasi-stationary nature of the rotating MCS is related to the fact that the vortex develops within a slow-moving, low-level horizontal deformation field. The vortex is well maintained because of the weak horizontal and vertical shear in the deformation zone and the generated 1arge inertial stability of the vortex. The results indicate that in some situations, numerical forecasts of the genesis, evolution and rainfall of rotating MCSs are possible up to 36 h using the currently available observations if a high-grid resolution model can be utilized.

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Chanh Q. Kieu and Da-Lin Zhang

Abstract

This comment presents some concerns with the study of Stern et al. and their misinterpretation of the contraction of the radius of the maximum wind (RMW) in tropical cyclones. It is shown that their geometrical RMW contraction model provides little dynamical understanding of the RMW contraction during tropical cyclone intensification, and it differs fundamentally from the RMW contraction model of Willoughby et al. that was derived from the directional derivative concept. Moreover, it is demonstrated that Stern et al. were mistaken in commenting on the derivation of the governing equation for the RMW contraction in Kieu.

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Da-Lin Zhang and J. Michael Fritsch

Abstract

A 12 h nested-grid numerical simulation of a warm-season mesoscale convective weather system (Zhang and Fritsch, 1986) is utilized as a control run in order to 1) test the sensitivity of the numerical simulation to different types of initial conditions; 2) examine the need for an observing system that would resolve mesoscale features; and 3) determine which meteorological variables need to be most carefully considered in observing system design and preprocessing analysis.

It is found that improved observational capabilities are likely to have an important impact on the successful prediction of the timing and location of summertime mesoscale convective weather systems if mesoscale features can be resolved. In particular, the resolution of the moisture field significantly affects the prediction of the evolution of the convective weather systems. Correspondingly, the mesoscale distribution of precipitation is substantially affected, especially the location of the areas of heavy rain. It is also found that procedures to account for the effects of convective systems that are in progress at the time of initialization can make significant contributions to the prediction of the evolution of the meteorological events and to the improvement of the quantitative precipitation forecasts. In particular, in weak-gradient summertime situations, mesoscale convective systems can severely alter their near environment within a short time period by producing strong mesoscale circulations, thermal boundaries, moist adiabatic stratification etc.

For summertime situations where the large-scale gradients are weak, detailed temperature and moisture fields appear to be more important than the detailed wind fields in determining the development and evolution of deep convection. However, poor resolution of the wind field such that wind speed magnitudes and gradients are underestimated tends to reduce the degree of mesoscale organization. It also alters the magnitude and distribution of low-level convergence, and this affects the evolution of the thermodynamic fields and the deep convection.

Incorporation of dense surface observations into the initial conditions can be very important in improving forecasts of meso-β-scale structures such as moist (dry) tongues, thermal boundaries, and, in particular, pressure distribution. Most significantly, the large (meso-α)-scale environment appears to contain some type of signal such that the general evolution of events is similar, even when the initial mesoscale structure and the simulated meso-β-scale evolution of events are significantly different. On the other hand, poor resolution of meso-α-scale gradients can substantially alter the predicted evolution of meso-β-scale features and the location of heavy rain.

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Jung Hoon Shin and Da-Lin Zhang

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This study examines the relative roles of moist frontogenesis and tropopause undulation in determining the intensity, size, and structural changes of Hurricane Sandy using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy’s four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea level pressure (SLP) falls, and (iv) reintensification. Results show typical correlations between intensity changes, sea surface temperature, and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy’s northward movement into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, three spiral frontogenetic zones and associated rainbands develop internally in the outer northeastern quadrant during the last three stages when Sandy’s southeasterly warm current converges with an easterly cold current associated with an eastern Canadian high. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical storm–force wind and structural changes, while deep convection in the eyewall and merging of the final two surviving frontogenetic zones generate a spiraling jet in Sandy’s northwestern quadrant, leading to its reintensification prior to landfall. The authors conclude that a series of moist frontogenesis plays a more important role than the lower-stratospheric warmth in determining Sandy’s size, intensity, and structural changes.

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Da-Lin Zhang and Han-Ru Cho

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This paper presents evidence on the development of negative moist potential vorticity (MPV) or moist symmetric instability (MSI) in the stratiform region of a midlatitude squall line, based on a three-dimensional (3D) numerical simulation of a case that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM). The results show that the stratiform region, though convectively stable to pure vertical displacement, is considerably unstable to slantwise displacement along the system's broad front-to-rear (FTR) saturated ascending flow. It is found that this instability evolves from boundary-layer convective instability that has previously been removed by upright convection over the leading portion of the squall system. The negative MPV in the stratiform region is mainly the result of upward and rearward transport of the low-level convectively unstable air along the sloping FTR ascending flow and of processes that reverse the signs of both the convective stability parameter (i.e., θe/z) and the absolute vorticity (v/ nu/ s+f). The resulting symmetric instability appears to considerably enhance the vertical motion and precipitation rate in the stratiform clouds. In the stratiform region of the squall system, the negative MPV leads to a region of negative absolute vorticity or inertial instability at the upper levels, and it may be responsible for the strong anticyclonic divergent outflow in that region. Thus, the effect of the squall system is to process the low-level negative MPV in such a way as to symmetrically stabilize the lower troposphere and inertially destabilize the upper troposphere. The roles of convective, symmetric, and inertial instabilities in the development of the squall system and their implications with respect to intense oceanic storms are discussed.

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Zonghui Huo, Da-Lin Zhang, and John Gyakum

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In this study, a methodology is proposed to improve the model initial conditions, based on available surface temperature observations from ships, buoys, and drifters. It is tested with the numerical prediction of the 12–14 March 1993 superstorm that is initialized at its incipient stage over the Gulf of Mexico. In this methodology, the authors make use of the piecewise potential vorticity (PV) inversion technique and treat the temperature errors at the lowest level as a surrogate PV anomaly. After inverting the wind and mass perturbations from the surface thermal anomaly and its pertinent interior PV anomaly, a three-dimensional, dynamically consistent set of “errors” are obtained and added to the model initial conditions to improve the representation of the lower troposphere over the data-sparse ocean.

It is found that the numerical model prediction, initialized with the modified initial conditions, exhibits significant improvements in the early rapid deepening and the track of the superstorm over ocean, the development of a prefrontal squall line, and the central sea level pressure traces during the life cycle of the cyclone, as verified against observations. These results show that the methodology proposed is promising in improving the representation of lower-tropospheric meteorological variables in the model initial conditions, based on available surface observations over data-sparse regions.

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