Rear-inflow Jet Research Articles

Airflow and Precipitation Structure of Two Leading Stratiform Mesoscale Convective Systems Determined from Operational Datasets

An analysis of the airflow and precipitation structure of two leading stratiform (LS) mesoscale convective systems (MCSs) is presented. Leading stratiform systems are defined as linear MCSs that consist of a convective line with leading stratiform rain. Case studies of LS systems on 7 May 1997 and 30 April 2000 were conducted using the available operational datasets. Several of the features observed, though not all, appear as a mirror image of those seen in trailing stratiform (TS) mesoscale convective systems. Their horizontal reflectivity structure has similar aspects, with convective cells that are sometimes elongated and canted with respect to the convective line, a transition zone of lower reflectivity, and an area of enhanced stratiform rain. The 30 April case shows a leading mesolow that resembles a TS wake low, but its propagation characteristics (and presumably dynamics) differ. A descending leading inflow jet, the counterpart of a rear-inflow jet in a TS system, can be detected in both cases underneath a layer of strong ascending rear-to-front flow aloft. A few features of these LS systems are distinctive from TS systems. Cells in the convective line appear to be more discontinuous, and are elongated more than those of a TS system, although more work is needed to quantify these distinctions. Rear-feeding from an elevated equivalent potential temperature maximum behind the system is a distinguishing feature of these LS MCSs, since TS MCSs are typically fed from the boundary layer. Unlike the rear-inflow jet in TS systems, neither case shows a reversal in the leading inflow jet as it descends to low levels near the convective line. Both cases exhibit front-to-rear storm-relative surface flow throughout the LS systems. Finally, a conceptual model is presented that illustrates the structure observed in the two cases, based heavily on a single-Doppler radar analysis of 7 May 1997.

A Numerical Study of a Mesoscale Convective System over the Taiwan Strait

On 7 June 1998, a mesoscale convective system (MCS), associated with a mesoscale cyclone, was initiated on the south side of a mei-yu front near Hong Kong and developed over the Taiwan Strait. In this study, numerical simulations for this event are performed using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The model captures the evolution of the MCS, including the shapes of clouds and the rainfall rate. In the mature phase of the simulated MCS, the MCS is composed of several meso-β- and meso-γ-scale convective clusters possessing commalike shapes similar to that of a midlatitude occluded cyclone. The cluster at the head of the “comma” consists of convective clouds that are decaying, while the tail of the comma is made up of a leading active convective line. A mesoscale cyclone, associated with a mesolow, at the tailing region of the leading convective line is well developed below 500 hPa. At 850 hPa, a mesoscale low-level jet (mLLJ) is located on the south side of the mesolow, and is directed toward the comma-shaped convective clusters. At 300 hPa, a mesohigh develops over the leading cluster. A mesoscale upper-level jet (mULJ) is located on the east side of this mesohigh. Relative streamline and trajectory analyses show that the mLLJ, associated with low θe and sinking air motion, is a rear-inflow jet, while the mULJ is the outflow jet of the MCS. Monsoon air from the boundary layer in front of the MCS feeds deep convection within the MCS. Momentum budget calculations are performed in the regions of the mLLJ and mULJ, at the developing and mature stage of the MCS. The pressure gradient force and the horizontal advection are the main contributors to the development of mLLJ in the developing stage. Although the effects of the pressure gradient force are weakened considerably when the MCS reaches maturity, the horizontal advection continues to accelerate the mLLJ. Vertical advection tends to decelerate the mLLJ both in its developing and mature stages. The pressure gradient force and vertical advection are responsible for generating the mULJ in the early stages, and maintaining the mULJ in its mature stage. Strong convective upward motion, which carries the horizontal momentum upward, from the exit of the mLLJ to the entrance of the mULJ, is crucial in the vertical coupling of the mLLJ and mULJ.

Airborne Pseudo-Dual-Doppler Analysis of a Rear-Inflow Jet and Deep Convergence Zone within a Supercell

Abstract On 22 May 1995, numerous thunderstorms, several of which produced large hail and small tornadoes, formed along a dryline in the central and northern Texas Panhandle. The only long-lasting, daytime, severe storms developed later, south of the earlier storms. By late afternoon, new cells merged and evolved into two large supercells. The northern supercell produced three weak tornadoes and hail; the southern supercell did not produce a tornado, but did produce very large hail for an extended period of time. Verification of the Origins of Rotation in Tornadoes Experiment crews collected data in the southern supercell from its early stage through much of its mature stage. The National Center for Atmospheric Research Electra Doppler Radar collected pseudo-dual-Doppler radar data on this storm continuously for well over an hour. Detailed analyses of this high-resolution dataset revealed several unusual features, most notably an intense elevated rear-inflow jet at midlevels having maximum wind speeds app...

The oceanic mesoscale convective system and associated mesovortex observed 12 December 1992 during TOGA-COARE

This study documents the precipitation and kinematic structure of a mature/mature-to-decaying, westward propagating, near-equatorial oceanic mesoscale convective system (MCS) observed by airborne Doppler radars during TOGA-COARE, the Tropical Ocean/Global Atmosphere Coupled Ocean-Atmosphere Response Experiment. This system occurred on 12 December 1992 during the convectively active phase of an intraseasonal oscillation, and was followed, at its dissipating stage, by a redevelopment of convection that led to the MCS observed on 13 December 1992. These two successive MCSs were associated with a two-day atmospheric disturbance. Radar-deduced airflows at two time periods reveal many similarities with other tropical oceanic cloud clusters. In particular, a marked rear inflow jet associated with a midlevel cyclonic mesovortex was observed within the rear of the stratiform region. The mesovortex was better defined both in size and depth during the mature-to-decaying stage and was located more inside the system within a region of convergence, due to the westward propagation of the rear inflow. Countergradient transports of momentum normal to the mean orientation of the system at midlevels, and downgradient transports at lower and higher levels helped to intensify this rear inflow. These transports were mostly accomplished by eddy structures. Cyclonic vorticity was concentrated at the rear of the MCS and peaked at the centre of the observed closed wind circulation. As previously observed, stretching of the pre-existing vertical vorticity was the dominant dynamical mechanism which helped to amplify the mesovortex at mid-to-upper levels, while tilting of the horizontal vorticity into the vertical was a lesser mechanism and had an opposite effect. In most respects, advective processes were negatively correlated with the stretching and tilting mechanisms, but not sufficiently to be balanced. The resulting tendency was an increase of cyclonic vorticity at low-to-mid levels and a net decrease above.

The Influence of Convective Thermal Forcing on the Three-Dimensional Circulation around Squall Lines

Midlatitude squall lines are typically trailed by a large region of stratiform cloudiness and precipitation with significant mesoscale flow features, including an ascending front to rear flow; a descending rear inflow jet; line-end vortices; and, at later times, mesoscale convective vortices. The present study suggests that the mesoscale circulation in the trailing stratiform region is primarily determined by the time-mean pattern of heating and cooling in the leading convective line. Analysis of the line-normal circulation shows that it develops as thermally generated gravity waves spread away from the leading line. Midlevel line-end vortices are the result of diabatically driven tilting of horizontal vorticity generated by the time-mean thermal forcing. In the presence of the Coriolis force, a symmetric thermal forcing generates an asymmetric stratiform circulation and a pattern of vertical displacement that resembles the comma-shaped stratiform anvil observed in real systems; this suggests that asymmetries in the cloud and circulation behind midlatitude squall lines are not necessarily the result of asymmetries in the convective leading line.

Evolution and Structure of Tropical Squall Line Elements within a Moderate CAPE and Strong Low-Level Jet Environment

A three-dimensional cloud model is used to simulate the early development and propagation of an arc-shaped line element similar to that found in the GARP (Global Atmospheric Research Program) Tropical Atlantic Experiment (GATE) 4‐5 September 1974 squall line system. The simulated squall line element forms in a relatively unexplored environment with moderate convective available potential energy and a strong low-level jet (bulk Richardson number 5 37) associated with an easterly wave. The simulated line element develops in a large-scale convergence region from an initial cell that splits and elongates in a manner reminiscent of some midlatitude lines. Simulation features compare favorably with observed characteristics of some of the line elements including line orientation (approximately perpendicular to the average wind shear below the low-level jet), propagation speed (11 m s21 to the southwest), length (75 km), and maximum precipitation rate (187 mm h21). In addition, the simulated line merges with cells that form ahead as observed. The simulated arc-shaped line element consists of four regions several hours after its initiation. The northern region or right flank of the line contains a long-lived cell exhibiting three-dimensional characteristics similar to midlatitude supercells including long-lasting and steady updrafts with midlevel cyclonic rotation and movement to the right of the mean winds. Although the simulated supercell characteristics cannot be confirmed because of insufficient data from the limited GATE 4‐5 September observations, updrafts with strong vertical vorticity have been observed to occur in other GATE rainbands where waterspouts have been seen. The region just to the south of this cell is the site where a cell that formed out ahead of the line segment merges with the line and also develops supercell-like characteristics. Farther to the south, convection develops with quasi twodimensional characteristics below 3‐4 km but breaking into several multicells above. A rear-inflow jet does not accompany these features and the winds in the downdraft are oblique too and nonuniform along the associated gust front. Finally, on the southernmost or left flank of the line element, cells with both long and short lifetimes develop. Sensitivity tests indicate that once the early line structure has developed, its evolution and structure are not seriously altered by the removal of large-scale forcing. Further, the formation of the cold pool and gust front between the initial separating (splitting) cells is crucial to the filling in of the line. Changes in the thermodynamic profile consistent with nonsquall observations one-half day earlier result in only modest differences. Changes in the wind profile (based on the same observations) led to significant differences, such as the lack of convection between the initial separating cells and the merger taking place to the north of the right flank creating a convective complex in this region.

Cold frontal structure derived from radar wind profilers

AbstractColocated UHF and VHF wind profiler radars have been operated during the passage of a sharp cold front. The results demonstrate the utility of the profilers, in conjunction with a conceptual model, for identifying and quantifying many of the features commonly associated with such fronts: low‐level and upper‐level jets, and rearward‐sloping warm‐conveyor‐belt flow with underlying rear‐inflow. The UHF radar detected much of the transverse circulation and also the vertical distribution of precipitation, including a possible region of evaporation associated with the dry rear‐inflow jet. The VHF radar, in addition to extending the velocity measurements above the level of the precipitation, mapped the tropopause and, in particular, identified a tropopause fold, the position of which corresponded to the origin of the rear‐inflow jet detected by both profilers. Copyright © 1998 Royal Meteorological Society

“Bookend Vortex” Induced Tornadoes along the Natchez Trace

Abstract This study focuses on the evolution of the northern end of a bow echo that moved across parts of southwest Mississippi on 8 May 1995. A well-defined, cyclonically rotating “comma head echo” developed north of Natchez, Mississippi, and moved northeast for about 120 km (75 mi) before dissipating. The circulation associated with this comma head echo passed through several diameter changes during which the diameter varied between that of a classic mesocyclone and that more typical of a “bookend vortex.” The circulation and a strong rear inflow jet helped spawn small tornadoes (F0–F2) in Claiborne County, Mississippi, and wind damage in western Hinds County, Mississippi. The observed damage path from the tornadoes was more than 8 km (5 mi) long. For much of the track, the tornadoes paralleled the Natchez Trace, a scenic federal highway that extends from Natchez to Nashville, Tennessee.

Balanced and Unbalanced Circulations in a Primitive Equation Simulation of a Midlatitude MCC. Part I: The Numerical Simulation

Abstract A midlatitude mesoscale convective complex (MCC), which occurred over the central United States on 23–24 June 1985, was simulated using the Regional Atmospheric Modeling System (RAMS). The multiply nested-grid simulation agreed reasonably well with surface, upper-air, and satellite observations and ground-based radar plots. The simulated MCC had a typical structure consisting of a leading line of vigorous convection and a trailing region of less intense stratiform rainfall. Several other characteristic MCC circulations were also simulated: a divergent cold pool in the lower troposphere, midlevel convergence coupled with a relatively cool descending rear-inflow jet, and relatively warm updraft structure, and a cold divergent anticyclone in the tropopause region. Early in the MCC simulation, a mesoscale convectively induced vortex (MCV) formed on the eastern edge of the convective line. While frequently associated with MCCs and other mesoscale convective systems (MCSs), MCVs are more typically repo...

Recent progress in the operational forecasting of summer severe weather

Abstract Summer severe weather (SSW) can strike suddenly and unexpectedly with disastrous consequences for human activity. Considerable progress has been made in the past ten years in the operational forecasting of SSW. Traditionally, SSW was defined to consist of tornadoes, strong winds, hail, lightning and heavy rain. Hazardous types of strong winds have recently been expanded to include microbursts, macrobursts and surfacing rear inflow jet damage behind mesoscale convective systems. Doppler radar was used to relate surface damage to the appropriate atmospheric phenomena, first diagnostically and then prognostically. This improvement in classification has fedback to and improved the forecast process. Concurrent progress has been made in the use of synoptic observations. The concept of helical wind profiles and improved knowledge of the role of dry mid‐level air has improved the forecasting of tornadoes and strong gusty winds. Moisture flux convergence, derived from surface measurements, shows great promise in identifying areas of storm initiation. Satellite imagery has been used to identify dynamical atmospheric boundaries. Numerical modelling of the interaction of environmental wind profiles and individual thunderstorms has greatly contributed to the understanding of SSW. Studies of spatial and temporal patterns of lightning, both specific cases and climatology, contribute to the forecasting of severe storms. Polarization radar results have shown progress in separating the signals of hail from those of rain and in the improved measurement of heavy rainfalls. Radar observation of clear air boundaries and their interactions show potential for the forecasting of thunderstorm initiation. Though not traditionally considered part of SSW, hurricanes that evolve into extra‐tropical storms share many of the same hazardous features. The progress in computing, communications and display technologies has also made substantial contributions to operational forecasting and to the dissemination of weather warnings.

The Genesis of Severe, Long-Lived Bow Echoes

Abstract A series of idealized simulations using a nonhydrostatic cloud model is used to investigate the genesis of bow echoes (a bow-shaped system of convective cells that is especially noted for producing long swaths of damaging surface winds). It is hypothesized that severe, long-lived bow echoes represent a dynamically unique form of mesoconvective organization being produced for a restricted range of environmental conditions, including a convective available potential energy (CAPE) of at least 2000 m2 s2 and vertical wind shears of at least 20 m s−1 over the lowest 2.5–5 km AGL. The key structural features include a 40–100-km-long bow-shaped segment of convective cells, with a strong rear-inflow jet extending to the leading edge of the bow at 2–3 km AGL, and cyclonic and anticyclonic eddies (referred to as “bookend” vortices) on the northern and southern flanks of the bowed segment, respectively. This structure characteristically develops three to four hours into the lifetime of a convective system a...

The Momentum Budget of an Intense Midlatitude Squall Line

Abstract Rawinsonde data from OK PRE-STORM are used to calculate the momentum budgets of an intense midlatitude squall-line system with a trailing stratiform region. Budgets have been computed at three separate times during the mature through dissipating stages of the squall line using composited data over 3-hour intervals based on 90-minute releases. Budgets have been done in both ground-relative and system-relative reference frames, although system-relative terms are emphasized for comparison with other recent studies. Low-level radar data are used to partition the system into convective line and stratiform regions. The system contained a strong midlevel mesolow that contributed to rear-to-front pressure gradient acceleration in the vicinity of a rear-inflow jet. The mesolow, located within a larger region of low pressure sloping rearward with height, also led to strong front-to-rear acceleration elsewhere through a majority of the system. Convective-scale effects produced the primary accelerations oppo...

The Role of Convectively Generated Rear-Inflow Jets in the Evolution of Long-Lived Mesoconvective Systems

Abstract In this study, the structure of convectively generated rear-inflow jets and their role in the evolution of long-lived mesoconvective systems are investigated through an analysis of idealized three-dimensional simulations using a nonhydrostatic cloud model. Rear-inflow jets are generated within these systems in response to the upshear-tilting of the convective circulation, as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear. Within this framework, a wide range of rear-inflow strengths and structures are produced, depending on the magnitude of the ambient convective available potential energy (CAPE) and the vertical wind shear. In general, for environments characterized by weak-to-moderate vertical wind shear and weak-to moderate CAPE, the rear-inflow jet descends and spreads along the surface well behind the leading edge of the gust front, and the subsequent convective activity becomes weaker. However, for e...

The Wake Uw in a Midlatitude Mesoseale Convective System Having Complex Convective Organintion

Abstract An analysis has been carried out of the surface pressure field in a highly complex mesoscale convective system that occurred on 3-4 June 1985 during the Oklahoma-Kansas Preliminary Regional Expeximent for STORM-Central (OK PRE-STORM). During its mature stage the storm consisted of two primary intersecting convective bands approximately 200 km in length, one oriented NIE-SW (to the north) and the other N-S (to the south), with a stratiform precipitation region extending to the northwest of the bands. Stratifonn precipitation was weak to nonexistent in the southernmost portion of the storm. Although the organization of the storm was complex, the surface pressure field resembled those associated with simpler, quasi-linear squall systems containing trading stratifom regions: a mesohigh existed neat the convective line and a wake low was observed to the rear of the stratiform region. A strong system-relative, descending rear inflow jet was observed in the northern part of the storm near the wake low. ...

Heat and Moisture Budgets of an Intense Midlatitude Squall Line

Abstract Rawinsonde data from OK PRE-STORM are used to calculate the heat and moisture budgets during the late mature through dissipating stages of an intense squall line system with a trailing stratiform region. Budgets have been computed at three separate times using composited data from approximately 30 rawinsondes over three-hour intervals. Data spacing is marginally adequate to resolve features within the broad (∼200 km) stratiform region, but not the narrow (∼40 km) leading convective line. Low-level radar data and surface accumulated rainfall are used to partition the system into convective line and stratiform regions, and to cheek the accuracy of the heat and moisture budgets. The squall line had a pronounced descending rear inflow jet that extended toward the front of the system with time, and strong front-to-rear flow above and beneath this jet. The diagnosed vertical motion showed deep ascent in the leading convective line and upper-level ascent above lower-level descent in the trailing stratif...

Airflow patterns in two mesoscale convective systems

Single Doppler observations from an operational 5-cm radar of two mesoscale convective systems show mesoscale kinematic structures of the order of tens of kilometers. Two cases observed during routine operational scanning illustrate the presence of various low-level jets and their relationship with precipitation. One case illustrates dual rear jets associated with a mesoscale convective system (MCS) observed on 17 July 1986. The MCS developed on a wave along a warm front and was in steady state for much of the observation period. This wave was not resolvable in the synoptic data. Primary features were a leading narrow convective rain band 20 km in width, a low reflectivity transition zone 30 km in width, with a trailing region of rain which included a bright band at a height of 4 km. From the operational PPI radial velocity images, the patterns showed three-dimensional kinematic structure which included a mid-tropospheric rearward flow and two “dry” rear inflow jets. These jets were deduced to flow toward the rear of the convective band and then diverge with one descending anti-cyclonically and the other ascending cyclonically. These jets were below the bright band found in the trailing rain region. A “warm” cross-flow was also observed ahead of the convective line. A second case illustrates a MCS that evolved much more rapidly. The presence of the rear jet preceded the full development of the trailing rain region. These two MCS cases illustrate not only the complex three-dimensional structure of these systems which are very difficult to interpret for the operational nowcaster but also the mesoscale analogy to the conveyor-belt model of synoptic precipitation systems.

Numerical Simulation of an Intense Squall Line during 10–11 June 1985 PRE-STORM. Part II: Rear Inflow, Surface Pressure Perturbations and Stratiform Precipitation

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 ...

The Relationship of Surface Pressure Features to the Precipitation and Airflow Structure of an Intense Midlatitude Squall Line

Abstract Observations from the Oklahoma–Kansas Preliminary Regional Experiment for STORM-Central (OK PRE-STORM) have been used to document the surface pressure features accompanying an intense midlatitude squall line with trailing stratiform precipitation. Three well-known features are observed: a pre-squall mesolow, a squall mesohigh and a wake low. Particular attention is given to the wake low, its life cycle and association with the trailing stratiform portion of the squall line. During the formative stage, the pressure field to the rear of the squall line mesohigh is relatively flat with only weak stratiform precipitation present. As the squall line enters the developing-to-mature stages, a pronounced wake low appears at the back edge of the surface stratiform precipitation area. The squall line at this time is characterized by a strong rear-inflow jet, descending from the upper troposphere, as far as 500 km behind the leading convective line, to the lower troposphere just behind the line. The trailin...

A Three-Dimensional Simulation of a Tropical Squall Line: Convective Organization and Thermodynamic Vertical Transport

Abstract Three-dimensional convective-scale simulations of an African squall line, observed during the French COPT 81 experiment, are presented. Three simulations with different representations of large-scale forcing are performed on a domain of 50 km (along the line) by 80 km (across the line). They exhibit a similar circulation pattern characteristic of a squall line, but differ in intensity. The first simulation supposes an unperturbed environment and produces a slow-moving squall line (7 m s−1) with weaker total precipitation rate then observed (25%). The second one includes a representation of observed thermodynamic and dynamic environment modifications, and produces a fast-moving squall line (10 m s−1) still weaker than observations (50% or the rain rate). The third simulation takes into account the forcing induced by the rear inflow jet as depicted by Smull and Houze and observed on that day. It allows the system to reach an intensity in agreement with observations. The convective region (30 km wid...

The Oklahoma–Kansas Mesoscale Convective System of 10–11 June 1985: Precipitation Structure and Single-Doppler Radar Analysis

Abstract The 10–11 June mesoscale convective system observed in Kansas during PRE-STORM is studied using a variety of observations including conventional radar, satellite, and single-Doppler radar. This storm, at maturity, consisted of a strong line of convection trailed by a broad region of stratiform rain. The PRE-STORM Doppler radar observations show that the general airflow pattern is similar to that seen in previously analyzed cases; however, since the Doppler observations were quite extensive in time and space, they permit several details of the airflow to be revealed for the first time. A rear inflow jet, front-to-rear flow aloft, and a mesoscale updraft and downdraft were all present. The mesoscale downdraft commenced at the top of the slanted rear inflow jet. Sublimation and evaporation of hydrometeors in this flow apparently generated the necessary cooling to drive the mesoscale downdraft circulation. The intensity and slope of the rear inflow jet varied with location in the storm, which apparen...