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

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.

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-θ _e 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.

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

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-θ _e , air from the boundary layer ahead of the line, and an overturning downdraft carrying low-θ _e , 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-θ _e , 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.

Abstract

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.

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.

Abstract

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., θ _e /z) and the absolute vorticity (v/ n −u/ 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.

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.