Wide Cold-frontal Rainband Research Articles

Synoptic-Scale Environments and Precipitation Morphologies of Tornado Outbreaks from Quasi-Linear Convective Systems in the United Kingdom

Abstract Nine tornado outbreaks (days with three or more tornadoes) have occurred in the United Kingdom from quasi-linear convective systems (QLCSs) in the 16 years between 2004 and 2019. Of the nine outbreaks, eight can be classified into two synoptic categories: type 1 and type 2. Synoptic categories are derived from the location of the parent extratropical cyclone and the orientation of the surface front associated with the QLCS. Environmental differences between the categories are assessed using ERA5 reanalysis data. Type 1 events are characterized by a confluent 500-hPa trough from the west, meridional cold front, strong cross-frontal wind veer (about 90°), cross-frontal temperature decrease of 2°–4°C, prefrontal 2-m dewpoint temperatures of 12°–14°C, a prefrontal low-level jet, and prefrontal 0–1- and 0–3-km bulk shears of 15 and 25 m s−1, respectively. In contrast, type 2 events are characterized by a diffluent 500-hPa trough from the northwest, zonal front, weaker cross-frontal wind veer (≤45°), much smaller cross-frontal temperature decrease, lower prefrontal 2-m dewpoint temperatures of 6°–10°C, and weaker prefrontal 0–1- and 0–3-km bulk shears of 10 and 15 m s−1, respectively. Analysis of the Met Office radar reflectivity mosaics revealed that narrow cold-frontal rainbands developed in all type 1 events and subsequently displayed precipitation core-and-gap structures. Conversely, type 2 events did not develop narrow cold-frontal rainbands, although precipitation cores developed sporadically within the wide cold-frontal rainband. Type 1 events produced tornadoes 2–4 h after core-and-gap development, whereas type 2 events produced tornadoes within 1 h of forming cores and gaps. All events produced tornadoes during a relatively short time period (1–3 h).

The Role of Vertically Propagating Gravity Waves Forced by Melting-Induced Cooling in the Formation and Evolution of Wide Cold-Frontal Rainbands

Abstract Realistic mesoscale model simulations using the Weather Research and Forecasting (WRF) Model and idealized dry simulations were used to study the mechanisms responsible for the formation and evolution of wide cold-frontal rainbands (WCFRs) associated with a wintertime cyclone that moved onto the Washington coast. The WRF simulation reproduced observed characteristics of three successively formed WCFRs, including their spacing and movement as well as the timing of the formation of two WCFRs behind the first. Sensitivity experiments showed that melting-induced cooling in the stratiform precipitation area behind the surface cold front was essential for the formation of the first and second WCFRs, whereas the third WCFR was formed by the release of potential instability within an ascent forced by upper-level frontogenesis. Enhanced frontal updrafts responsible for the first and second WCFRs were created by a superposition of a broad updraft caused by frontal dynamics and upward-propagating gravity waves generated by the melting-induced cooling. The dry simulations forced by specified cooling revealed specific mechanisms for the wave generation and the evolution of the first and second WCFRs. The gravity waves were generated at the intersection of the low-level frontal zone and the melting layer, where strong vertical shear of the cross-front wind and upshear-sloped cooling by melting cooperatively enhanced the wave generation. The formation of the second WCFR behind the first and subsequent dissipation of these WCFRs was attributed to the evolution of a wave pattern associated with the evolution of cross-front flow above the frontal zone.

Application of TRMM PR and TMI Measurements to Assess Cloud Microphysical Schemes in the MM5 for a Winter Storm

AbstractThis paper uses observations from Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and microwave imager (TMI) to evaluate the cloud microphysical schemes in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5; version 3.7.4) for a wintertime frontal precipitation system over the eastern Pacific Ocean. By incorporating a forward radiative transfer model, the radar reflectivity and brightness temperatures are simulated and compared with the observations at PR and TMI frequencies. The main purpose of this study is to identify key differences among the five schemes [including Simple ice, Reisner1, Reisner2, Schultz, and Goddard Space Flight Center (GSFC) microphysics scheme] in the MM5 that may lead to significant departures of simulated precipitation properties from both active (PR) and passive (TMI) microwave observations. Radiative properties, including radar reflectivity, attenuation, and scattering in precipitation liquid and ice layers are investigated. In the rain layer, most schemes are capable of reproducing the observed radiative properties to a reasonable degree; the Reisner2 simulation, however, produces weaker reflectivity and stronger attenuation than the observations, which is possibly attributable to the larger intercept parameter (N0r) applied in this run. In the precipitation ice layer, strong evidence regarding the differences in the microphysical and radiative properties between a narrow cold-frontal rainband (NCFR) and a wide cold-frontal rainband (WCFR) within this frontal precipitation system is found. The performances of these schemes vary significantly on simulating the microphysical and radiative properties of the frontal rainband. The GSFC scheme shows the least bias, while the Reisner1 scheme has the largest bias in the reflectivity comparison. It appears more challenging for the model to replicate the scattering signatures obtained by the passive sensor (TMI). Despite the common problem of excessive scattering in the WCFR (stratiform precipitation) region in every simulation, the magnitude of the scattering maximum seems better represented in the Reisner2 scheme. The different types of precipitation ice, snow, and graupel are found to behave differently in the relationship of scattering versus reflectivity. The determinative role of the precipitation ice particle size distribution (intercept parameters) is extensively discussed through sensitivity tests and a single-layer radiative transfer model.

Alongfront Variability of Precipitation Associated with a Midlatitude Frontal Zone: TRMM Observations and MM5 Simulation

Abstract On 19 February 2001, the Tropical Rainfall Measuring Mission (TRMM) satellite observed complex alongfront variability in the precipitation structure of an intense cold-frontal rainband. The TRMM Microwave Imager brightness temperatures suggested that, compared to the northern and southern ends of the rainband, a greater amount of precipitation ice was concentrated in the middle portion of the rainband where the front bowed out. A model simulation conducted using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is examined to explain the distribution of precipitation associated with the cold-frontal rainband. The simulation reveals that the enhanced precipitation ice production and the implied mean ascent along the central part of the front were associated with a synergistic interaction between a low-level front and an upper-level front associated with an intrusion of high-PV stratospheric air. The low-level front contributed to an intense bow-shaped narrow cold-frontal rainband (NCFR). The upper-level front was dynamically active only along the central to northern portion of the NCFR, where the upper-level PV advection and Q-vector convergence were most prominent. The enhanced mean ascent associated with the upper-level front contributed to a wide cold-frontal rainband (WCFR) that trailed or overlapped with the NCFR along its central to northern segments. Because of the combination of the forcing from both lower- and upper-level fronts, the ascent was deepest and most intense along the central portion of the front. Thus, a large concentration of precipitation ice, attributed to both the NCFR and WCFR, was produced.

The IMPROVE-1 Storm of 1–2 February 2001. Part IV: Precipitation Enhancement across the Melting Layer

Abstract Previous model simulations indicate that in stratiform precipitation, the precipitation rate can increase by 7% in the melting layer through direct condensation onto melting snow and the resultant cooled rain. In the present study, a model simulation of stratiform precipitation in a wide cold frontal rainband indicates that the precipitation rate can also increase by 5% in the melting layer through accretion, by melting snow and rain, of additional cloud water produced by the latent cooling of the ambient air associated with melting snow. The contribution of the combined processes, and therefore the additional precipitation gained through the latent cooling of melting snow within the melting layer, may contribute as much as 10% to the precipitation rate in stratiform precipitation.

The IMPROVE-1 Storm of 1–2 February 2001. Part III: Sensitivity of a Mesoscale Model Simulation to the Representation of Snow Particle Types and Testing of a Bulk Microphysical Scheme with Snow Habit Prediction

Abstract A mesoscale model simulation of a wide cold-frontal rainband observed in the Pacific Northwest during the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE-1) field study was used to test the sensitivity of the model-produced precipitation to varied representations of snow particles in a bulk microphysical scheme. Tests of sensitivity to snow habit type, by using empirical relationships for mass and velocity versus diameter, demonstrated the defectiveness of the conventional assumption of snow particles as constant density spheres. More realistic empirical mass–diameter relationships result in increased numbers of particles and shift the snow size distribution toward larger particles, leading to increased depositional growth of snow and decreased cloud water production. Use of realistic empirical mass–diameter relationships generally increased precipitation at the surface as the rainband interacted with the orography, with more limited increases occurring offshore. Changes in both the mass–diameter and velocity–diameter relationships significantly redistributed precipitation either windward or leeward when the rainband interacted with the mountain barrier. A method of predicting snow particle habit in a bulk microphysical scheme, and using predicted habit to dynamically determine snow properties in the scheme, was developed and tested. The scheme performed well at predicting the habits present (or not present) in aircraft observations of the rainband. Use of the scheme resulted in little change in the precipitation rate at the ground for the rainband offshore, but significantly increased precipitation when the rainband interacted with the windward slope of the Olympic Mountains. The study demonstrates the promise of the habit prediction approach to treating snow in bulk microphysical schemes.

The IMPROVE-1 Storm of 1–2 February 2001. Part II: Cloud Structures and the Growth of Precipitation

Abstract On 1–2 February 2001, a strong cyclonic storm system developed over the northeastern Pacific Ocean and moved onto the Washington coast. This storm was one of several that were documented during the first field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). In the 1–2 February case, soundings and wind profiler measurements showed that a wide cold-frontal rainband was coincident with the leading edge of an upper-level cold front in a classical warm occlusion. Ground-based radar observations revealed the presence of subbands within the wide cold-frontal rainband and two layers of precipitation generating cells within this rainband: one at 5–7 km MSL and the other at 9–10 km MSL. The lower layer of generating cells produced fallstreaks that were traced from the cells down to the radar bright band at 2 km MSL. Observations suggest a connection between the subbands and the lower layer of generating cells. A research aircraft, equipped for cloud microphysical measurements, passed through at least two generating cells in the 5–7-km region. These cells were in their formative stage, with elevated liquid water contents and low ice particle concentrations. The microphysical structure of the wide cold-frontal rainband was elucidated by particle imagery from a Cloud Particle Imaging (CPI) probe aboard the research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the rainband. The crystal habits are used to deduce the temperature and saturation conditions under which the crystals grew and, along with in situ measurements of particle size spectra, they are used to estimate particle terminal fall velocities, precipitation rates, radar reflectivities, and vertical air motions. The radar reflectivity derived in this way generally compared well with direct measurement. Both the derived and directly measured parameters are used to determine the primary particle growth processes in the wide cold-frontal rainband. Above the melting layer, vapor deposition was the dominant growth process in the rainband; growth of ice particles by riming was small. Significant aggregation of ice particles occurred in the region just above the melting layer. A doubling in the air-relative vertical precipitation mass flux occurred between the region where sheath ice crystals formed (−3° ≤ T ≤ −8°C) and the surface. Substantial amounts of liquid water were found within the melting layer where growth occurred by the accretion of cloud droplets and also by condensation. Growth by the collision and coalescence of raindrops was not significant below the melting layer.

Evolution of a Cold Front Encountering Steep Quasi-2D Terrain: Coordinated Aircraft Observations on 8–9 December 2001 during IMPROVE-2

Abstract Research aircraft observations from the 8–9 December 2001 case of the second phase of the Improvement of Microphysical Parameterization through Operational Verification Experiment (IMPROVE-2) describe the evolution of a wide cold-frontal rainband (WCFR) during its eastward advance from the Pacific coastline to a point 200 km inland over the Cascade Mountains of Oregon. This analysis has two objectives: first, to illustrate the rapid weakening of the circulation associated with a landfalling WCFR and the relationship of these changes to terrain-induced airflow modifications, and second, to quantify the degree to which this weakening impacted cloud microphysical properties such as liquid water content, ice particle concentrations, and precipitation rate. The kinematic structure of the WCFR is detailed using Doppler radar observations from a NOAA P-3 aircraft, while some concomitant cloud microphysical properties are documented using flight-level measurements from the University of Washington Convair-580 aircraft. An accompanying the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) control simulation (nested to a horizontal resolution of 4 km over the IMPROVE-2 domain) provides a mesosynoptic context and thermodynamic information to complement the aircraft observations. To the authors’ knowledge, this case study represents the most complete documentation obtained to date of the rapid modifications that may occur when a frontal rainband progresses from coastal waters into a region of prominent terrain.

The IMPROVE-1 Storm of 1–2 February 2001. Part I: Development of a Forward-Tilted Cold Front and a Warm Occlusion

Abstract Analysis of observations and the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) are used to study the development of a forward-tilted cold front off the coast of Washington State. The vertical velocity associated with the cold front produced a wide cold-frontal rainband. In the early stage of development the midtropospheric baroclinic zone (or upper cold front) moved forward with time over the warm sector to produce a structure similar to a split front. The movement of the upper cold front was due to horizontal transport and frontogenetical propagation. The frontogenetical propagation was produced by a combination of tilting and diabatic frontogenesis, which resulted in a negative/positive couplet of frontogenesis straddling the baroclinic zone. The lower-tropospheric cold front eventually caught up with the warm front to form a classical warm occlusion. In the initial occluding process the converging frontal zones tilted into a warm-type occlusion configuration due to the presence of a background vertical shear of the horizontal wind component perpendicular to the occluded front. Consequently, as the storm moved over the observing network, the occluded front had the structure of a warm occlusion (tilted forward) in the lower levels. Above the occlusion, the cold front was also tilted forward because it retained its split-front-like structure. Thus, the development of the split front and the warm occlusion were separate processes that occurred in sequence. Although the MM5 captured the basic forward tilt with time of the cold front, some key aspects of the midtropospheric frontal structure were not well simulated. Because diabatic heating was an important contributor to the maintenance and movement of the upper cold front, it is hypothesized that discrepancies in diabatic heating associated with deficiencies in the model’s explicit microphysical scheme may be responsible for deficiencies in reproducing the structure of the upper cold front.

Mesoscale analysis of the activation of a cold front during cyclogenesis

AbstractThe archetypal mesoscale structure of an ana‐cold front comprises a narrow cold‐frontal rainband (NCFR), associated with line convection, embedded within a wide cold‐frontal rainband (WCFR) due to the slantwise ascent associated with a strong transverse‐front mesoscale circulation. Whereas most mesoscale observational studies have concentrated on this mature stage, the present paper explores by means of a detailed case‐study how such a mature frontal system develops from pre‐existing features. the study is based upon the synthesis of data spanning many scales, in the form of satellite imagery, observations from conventional and multi‐parameter Doppler radar, and surface reports, together with diagnostics from an operational mesoscale forecast model. the cold‐frontal zone in this case is characterized by multiple cold fronts. the dominant second cold front is studied in detail from the point of view of its evolution and its relationship to neighbouring fronts, especially the leading front associated with the initial polar‐front cloud band. the dominant cold front started as a shallow ‘rope cloud’ in the cold sector, and at a fairly early stage it produced a NCFR due to line convection. Then, over a period of only a few hours, a dry intrusion and an associated strong direct transverse‐front circulation led to the formation of a canopy of mid‐level cloud and a WCFR. the WCFR was attributable not only to the rearward‐ascending outflow from the line convection but also to the re‐invigoration of ascent within the initial polar‐front cloud band that the line convection was undercutting. Although the transverse circulation was probably intensified by precipitation processes, it is inferred that it was mainly a manifestation of the larger‐scale environmental flow that existed independently of the major precipitation features.

A wide cold-frontal rainband and its relationship to frontal topography

A wide cold-frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high-resolution Doppler radar. It was found that the movement of the rainband was closely related to the velocity of the frontal surface, and that the substructural features of the rainband corresponded closely with the topographical features of that surface. The evolution of the topography of the frontal surface is described, and some processes that might affect frontal topography, and therefore the structure and movement of rainbands, are examined.

A wide cold‐frontal rainband and its relationship to frontal topography

AbstractA wide cold‐frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high‐resolution Doppler radar. It was found that the movement of the rainband was closely related to the velocity of the frontal surface, and that the substructural features of the rainband corresponded closely with the topographical features of that surface. The evolution of the topography of the frontal surface is described, and some processes that might affect frontal topography, and therefore the structure and movement of rainbands, are examined.

Numerical modeling of cloud and precipitation chemistry associated with two rainbands and some comparisons with observations

A diagnostic modeling study is described of the chemistry of a pair of rainbands, consisting of a narrow and a wide cold‐frontal rainband, which were associated with a cold front on the mid‐Atlantic coast of the United States. Precipitation growth in the narrow cold‐frontal rainband (NCFR) was dominated by collection processes. The wide cold‐frontal rainband (WCFR), which followed the NCFR, had most of its precipitation associated with frontal lifting. Surface sulfate and nitrate deposition rates predicted by the model for the WCFR were of the same order as those obtained from measurements on the ground. When the two rainbands were allowed to interact in the model, allowing the NCFR to process the air that entered the WCFR, sulfate deposition from the WCFR decreased, but nitrate deposition remained unchanged. The total sulfate deposition from the two bands, with and without interaction, differed by only ∼2%. The effects on the sulfate and nitrate deposition of the initial concentrations of their gaseous precursors in the clear air ahead of each rainband were examined. For most cases modeled, the sulfate and nitrate depositions responded approximately linearly to variations in the concentrations of the precursors. Examination of the pathways for oxidizing aqueous sulfur dioxide shows that iron‐catalyzed aerobic oxidation may be an important mechanism for converting sulfur dioxide to sulfate.

Organization and Structure of Clouds and Precipitation on the Mid-Atlantic Coast of the United States. Part II: The Mesoscale and Microscale Structures of Some Frontal Rainbands

Abstract The mesoscale and microscale structures of the clouds and precipitation associated with a frontal system on the mid-Atlantic Coast of the United States are investigated using radar reflectivity and Doppler velocity data, surface mesonet, conventional surface, upper-air and microphysical data. The frontal structure showed similarities to a warm occlusion, with code-air advection aloft preceding cold-air advection at lower levels. Six rainbands were observed in association with the frontal system. All of these rainbands developed within the region of coverage of the NWS WSR-57 radar at Cape Hatteras, North Carolina. Two were upper-level features, associated with a prefrontal surge of cold air and the main push of cold dry air aloft. These rainbands were similar in structure to prefrontal surge and wide cold-frontal rainbands, respectively, observed on the Pacific Northwest Coast. The microphysical and small mesoscale structures of the wide cold-frontal rainband are examined. Three of the rainbands ...

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. Part XV: A Numerical Modeling Study of Frontogenesis and Cold-Frontal Rainbands

Abstract A two-dimensional, hydrostatic, primitive-equation model is used to investigate the dynamics of frontogenesis in a moist atmosphere. The development of a cold front is simulated through shear-deformation associated with the non-linear evolution of an Eady wave. Simulations are performed with 5, 10, 40 and 80 km horizontal resolutions and 14 levels in the vertical (four in the boundary layer). Compared to the dry case, the inclusion of moisture in the model produces a stronger low-level jet ahead of the front and a stronger upper-level jet. Moisture also produces a stronger ageostrophic circulation across the front and a more concentrated updraft just ahead of the surface front. The updraft develops a banded structure above and behind the surface front, with a wavelength of about 70 km. Bands form near the back edge of the cloud shield and move toward the surface front with a relative velocity of ∼1 m s−1. These characteristics agree with observations of wide cold-frontal rainbands. The banded str...

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. XI: Comparisons between Observational and Theoretical Aspects of Rainbands

Abstract Comparisons are made between the characteristics of several types of rainbands observed in an extratropical cyclone and dynamical mechanisms relevant on the mesoscale. The warm-sector flow ahead of the cold front and above the cold-frontal zone aloft was unstable to conditional symmetric instability, and theoretical predictions for this mechanism am consistent with several aspects of the warm-sector and wide cold-frontal rainbands. In the case of the warm-sector rainbands, other mechanisms (e.g., wave-CISK and mixed dynamic/convective instabilities) may have also played a role. The core structure of the narrow cold-frontal rainbands appeared to be affected by an instability that derived its energy from the horizontal shear across the surface front. Also, many aspects of the narrow cold-frontal rainband were similar to a density current. Shear-induced gravity waves appeared to be responsible for the wavelike rainbands observed in the vicinity of the cold-frontal zone aloft. The orientation of the ...

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. IX: Some Effects of Orography on Rainbands

Abstract The effects of orography on the mesoscale structures and precipitation processes in warm-frontal, warm-sector, wide cold-frontal, narrow cold-frontal and post-frontal rainbands in four Pacific cyclones are described. The rainbands were tracked with a Doppler radar as they approached the Washington coast and then for ∼150 km inland as they passed over topographic features ranging from modest hills to mountain ranges. The rainbands were affected in a variety of ways by orography, ranging from dissipation to formation, and precipitation from the bands was enhanced and reduced in different situations. These effects are discussed with respect to the large-scale flow, mesoscale air motions and precipitation growth mechanisms.

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. VII: Formation, Development, Interaction and Dissipation of Rainbands

Mesoscale measurements on five Pacific cyclones are used to investigate the formation, movement, development, interaction and dissipation of warm-sector, prefrontal cold-surge, narrow cold-frontal, wide cold-frontal, wavelike and postfrontal rainbands. Warm-sector rainbands formed near the leading edge of the cold front and often moved away from the front. Prefrontal cold-surge and wide cold-frontal rainbands formed aloft and behind the surface cold front and they also advanced relative to the front. The clearest interactions between rainbands occurred when wide cold-frontal rainbands overtook narrow cold-frontal rainbands; in this case the narrow cold-frontal rainband may be either temporarily or permanently dissipated, or the wide cold-frontal rainband may be dissipated, depending on the relative strengths of the rainbands. Wavelike rainbands, with very uniform properties, were initiated primarily in the vicinity of the cold front aloft; despite their small scale, these rainbands were relatively long-lived. Postfrontal rainbands, some of which contained oriented precipitation cores, moved with the winds within the postfrontal airmass, and exhibited a variety of lifecycles.

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. VI: Wavelike Rainbands Associated With a Cold-Frontal Zone

Abstract The cloud and precipitation structure and the airflow associated with wavelike rainbands in a cold-frontal zone have been investigated with Doppler radar, instrumented aircraft, rawinsondes and a network of ground stations. The rainbands were oriented perpendicular to the cold front and embedded within wide cold-frontal rainbands. The wavelike rainbands were 20–40 km long, 3–6 km wide, spaced 9–13 km apart and their tops ranged from 3-5 km in height. The radar reflectivities, convergence/divergence and airflow show regular patterns associated with the rainbands. There is evidence that wavelike rainbands were associated with generating cells aloft. These rainbands may have been initiated by shear instability in the frontal zone, since the resonant mode for such an instability had a similar orientation, movement and spacing to those observed for the rainbands.

The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. I: A Case Study of a Cold Front

Abstract Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations. The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between ∼3—6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was p...