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Vertical profiles of temperature, relative humidity, cloud particle concentration, median mass dimension, and mass content were derived using instruments on the NOAA P-3 aircraft for 37 spiral ascents/descents flown within five mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) project, and 16 spiral descents of the NOAA P-3 within 10 MCSs during the 2003 Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The statistical distribution of thermodynamic and microphysical properties within these spirals is presented in context of three primary MCS regions—the transition zone (TZ), enhanced stratiform rain region (ESR), and the anvil region (AR)—allowing deductions concerning the relative importance and nature of microphysical processes in each region. Aggregation was ubiquitous across all MCS zones at subfreezing temperatures, where the degree of ambient subsaturation, if present, moderated the effectiveness of this process via sublimation. The predominately ice-supersaturated ESR experienced the least impact of sublimation on microphysical characteristics relative to the TZ and AR. Aggregation was most limited by sublimation in the ice-subsaturated AR, where total particle number and mass concentrations decreased most rapidly with increasing temperature. Sublimation cooling at the surface of ice particles in the TZ, the driest of the three regions, allowed ice to survive to temperatures as high as +6.8°C. Two spirals executed behind a frontal squall line exhibited a high incidence of pristine ice crystals, and notably different characteristics from most other spirals. Gradual meso- to synoptic-scale ascent in this region likely contributed to the observed differences.

This study examines microphysical and thermodynamic characteristics of the 20 June 2015 mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) experiment, specifically within the transition zone (TZ), enhanced stratiform rain region (ESR), anvil region, melting layer (ML), and the rear inflow jet (RIJ). Analyses are developed from airborne optical array probe data and multiple-Doppler wind and reflectivity syntheses using data from the airborne NOAA Tail Doppler Radar (TDR) and ground-based Weather Surveillance Radar-1988 Doppler (WSR-88D) radars. Seven spiral ascents/descents of the NOAA P-3 aircraft were executed within various regions of the 20 June MCS. Aggregation modified by sublimation was observed in each MCS region, regardless of whether the sampling was within the RIJ. Sustained sublimation and evaporation of precipitation in subsaturated layers led to a trend of downward moistening across the ESR spirals, with greater degrees of subsaturation maintained when in the vicinity of the descending RIJ. In all cases where melting was observed, the ML acted as a prominent thermodynamic boundary, with differing rates of change in temperature and relative humidity above and below the ML. Two spiral profiles coincident with the rear inflow notch provided unique observations within the TZ and were interpreted in the context of similar observations from the 29 June 2003 Bow Echo and Mesoscale Convective Vortex Experiment MCS. There, sublimation cooling and enhanced descent within the RIJ allowed ice particles to survive to temperatures as warm as +6.8°C before completely sublimating/evaporating.

The development of elevated potential instability within the comma head of a continental winter cyclone over the north-central United States is examined using a 63-h Weather Research and Forecasting (WRF) Model simulation. The simulation is first compared to the observed cyclone. The distribution of most unstable convective available potential energy (MUCAPE) within the comma head is then analyzed. The region with positive MUCAPE was based from 2- to 4-km altitude with MUCAPE values up to 93 J kg−1. Backward trajectories from five sublayers within the region of elevated convection in the comma head were calculated to investigate how elevated potential instability developed. Air in the lowest sublayer, the source air for convective cells, originated 63 h earlier near Baja California at elevations between 2.25- and 2.75-km altitude. Air atop the layer where convection occurred originated at altitudes between 9.25 and 9.75 km in the Arctic, 5000 km away from the origin of air in the lowest sublayer. All air in the layer in which convection occurred originated over the Pacific coast of Mexico, the Pacific Ocean, or arctic regions of Canada. Diabatic processes strongly influenced air properties during transit to the comma head. Air underwent radiative cooling, was affected by mixing during passage over mountains, and underwent interactions with clouds and precipitation. Notably, no trajectory followed an isentropic surface during the transit. The changes in thermodynamic properties along the trajectories led to an arrangement of air masses in the comma head that promoted the development of potential instability and elevated convection.

Cloud-top generating cells (GCs) are a common feature atop stratiform clouds within the comma head of winter cyclones. The dynamics of cloud-top GCs are investigated using very high-resolution idealized WRF Model simulations to examine the role of shear in modulating the structure and intensity of GCs. Simulations were run for the same combinations of radiative forcing and instability as in Part II of this series, but with six different shear profiles ranging from 0 to 10 m s−1 km−1 within the layer encompassing the GCs. The primary role of shear was to modulate the organization of GCs, which organized as closed convective cells in simulations with radiative forcing and no shear. In simulations with shear and radiative forcing, GCs organized in linear streets parallel to the wind. No GCs developed in the initially stable simulations with no radiative forcing. In the initially unstable and neutral simulations with no radiative forcing or shear, GCs were exceptionally weak, with no clear organization. In moderate-shear (Δu/Δz = 2, 4 m s−1 km−1) simulations with no radiative forcing, linear organization of the weak cells was apparent, but this organization was less coherent in simulations with high shear (Δu/Δz = 6, 8, 10 m s−1 km−1). The intensity of the updrafts was primarily related to the mode of radiative forcing but was modulated by shear. The more intense GCs in nighttime simulations were either associated with no shear (closed convective cells) or strong shear (linear streets). Updrafts within GCs under conditions with radiative forcing were typically ~1–2 m s−1 with maximum values < 4 m s−1.

This paper examines the impact of the Laurentian Great Lakes (GL) on atmospheric structure, stability, and precipitation within the Chicago–Milwaukee urban corridor during the passage of the 1–2 February 2011 extratropical cyclone. This storm produced the third largest snowfall [53.8 cm (21.2 in.)] recorded in a 130-yr period in the city of Chicago. Two simulations of the storm using the Weather Research and Forecasting (WRF) Model are described: the first with the GL present, and the second with the lakes replaced with land having characteristics of adjacent shores. The GL were found to alter the surface temperature and moisture fields in their lee during cyclone passage. The changes were limited to the layer below the frontal inversion, but were significant enough to reduce the mean sea level pressure in some locations by 2.0–2.5 hPa, and raise the surface temperature and dewpoint temperature by 2°–4°C across several states downwind. In the Chicago–Milwaukee metropolitan corridor where the heavy snow occurred, the surface temperature and dewpoint temperature increased from +3° to +6°C as a result of heating and moistening of the lower atmosphere by the GL. Enhanced convergence also occurred along the downwind shoreline. Despite these changes, the areal impact on precipitation was surprisingly small, with liquid equivalent precipitation increases exceeding 5 mm limited to a small area over metropolitan Chicago late in the storm. The reason for the limited impact appeared to be the shallow nature of the cold air mass below the frontal inversion. Nevertheless, over metropolitan Chicago, as much as 20% of the snowfall could be attributed to the presence of the GL.

The newly developed High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) Cloud Radar (HCR) is an airborne, W-band, dual-polarization, Doppler research radar that fits within an underwing pod on the National Center for Atmospheric Research Gulfstream-V HIAPER aircraft. On 2 February 2015, the HCR was flown on its maiden research voyage over a cyclone along the Northeast coast of the United States. Six straight flight legs were flown over 6 h between the northern tip of Delaware Bay and Bangor, Maine, crossing the rain–snow line, and passing directly over Boston, Massachusetts, which received over 16 in. of snow during the event. The HCR, which recorded reflectivity, radial velocity, spectral width, and linear depolarization ratio with a 0.7° beam, was pointed at nadir from a flight altitude of 12,800 m (42,000 ft). The along-track resolution ranged between 20 and 200 m, depending on range, at aircraft speeds varying between 200 and 275 m s−1. The range resolution was 19.2 m. Remarkably detailed finescale structures were found throughout the storm system, including cloud-top generating cells, upright elevated convection, layers of turbulence, vertical velocity perturbations across the melting level, gravity waves, boundary layer circulations, and other complex features. Vertical velocities in these features ranged from 1 to 5 m s−1, and many features were on scales of 5 km or less. The purpose of this paper is to introduce the HCR and highlight the remarkable finescale structures revealed within this Northeast U.S. cyclone by the HCR.

An analysis of the microphysical structure of elevated convection within the comma head region of two winter cyclones over the midwestern United States is presented using data from the Wyoming Cloud Radar (WCR) and microphysical probes on the NSF/NCAR C-130 aircraft during the Profiling of Winter Storms campaign. The aircraft penetrated 36 elevated convective cells at various temperatures T and distances below cloud top zd. The statistical properties of ice water content (IWC), liquid water content (LWC), ice particle concentration with diameter > 500 μm N>500, and median mass diameter Dmm, as well as particle habits within these cells were determined as functions of zd and T for active updrafts and residual stratiform regions originating from convective towers that ascended through unsaturated air. Insufficient data were available for analysis within downdrafts. For updrafts stratified by zd, distributions of IWC, N>500, and Dmm for all zd between 1000 and 4000 m proved to be statistically indistinct. These results imply that turbulence and mixing within the updrafts effectively distributed particles throughout their depths. A decrease in IWC and N>500 in the layer closest to cloud top was likely related to cloud-top entrainment. Within residual stratiform regions, decreases in IWC and N>500 and increases in Dmm were observed with depth below cloud top. These trends are consistent with particles falling and aggregating while entrainment and subsequent sublimation was occurring.

Since the advent of dual-polarization radar, methods of classifying hydrometeors by type from measured polarization variables have been developed. The deterministic approach of existing hydrometeor classification algorithms of assigning only one dominant habit to each radar sample volume does not properly consider the distribution of habits present in that volume, however. During the Profiling of Winter Storms field campaign, the “NSF/NCAR C-130” aircraft, equipped with in situ microphysical probes, made multiple passes through the comma heads of two cyclones as the Mobile Alabama X-band dual-polarization radar performed range–height indicator scans in the same plane as the C-130 flight track. On 14–15 February and 21–22 February 2010, 579 and 202 coincident data points, respectively, were identified when the plane was within 10 s (~1 km) of a radar gate. For all particles that occurred for times within different binned intervals of radar reflectivity ZHH and of differential reflectivity ZDR, the reflectivity-weighted contribution of each habit and the frequency distributions of axis ratio and sphericity were determined. This permitted the determination of habits that dominate particular ZHH and ZDR intervals; only 40% of the ZHH–ZDR bins were found to have a habit that contributes over 50% to the reflectivity in that bin. Of these bins, only 12% had a habit that contributes over 75% to the reflectivity. These findings show the general lack of dominance of a given habit for a particular ZHH and ZDR and suggest that determining the probability of specific habits in radar volumes may be more suitable than the deterministic methods currently used.

On 8 June 2003, an expansive squall line along a surface cold frontal boundary was sampled during the Bow Echo and Mesoscale Convective Vortex Experiment. The Naval Research Laboratory P-3 aircraft and the National Oceanic and Atmospheric Administration P-3 aircraft simultaneously sampled the leading and trailing edge of this squall line, respectively, with X-band Doppler radars. Data from these two airborne radar systems have been synthesized to produce a pseudo-quad-Doppler analysis of the squall line, yielding a detailed three-dimensional kinematic analysis of its structure. A simulation of the squall line was carried out using the Weather Research and Forecasting Model to complement the pseudo-quad-Doppler analysis. The simulation employed a 3-km, convection-allowing, nested domain centered over the pseudo-quad-Doppler domain, along with a 9-km parent domain to capture the larger synoptic-scale cyclone.The pseudo-quad-Doppler analysis reveals that the convective line was embedded within the upper-tropospheric jet stream, causing local decelerations and deviations in the jet-level flow. The vertical transport of low momentum air from the boundary layer via convective updrafts is shown to significantly decelerate jet-level flow. Pressure perturbations associated with the intrusion of low momentum air into the jet stream–level flow led to deviation of the jet stream flow around the squall line that resulted in counter-rotating ribbons of vertical vorticity parallel to the squall line. Model results indicate that disturbances in the jet stream structure persisted downwind of the squall line for several hours.

Recent field observations suggest that cloud-top precipitation generating cells (GCs) are ubiquitous in the warm-frontal and comma-head regions of midlatitude winter cyclones. The presence of fallstreaks emanating from the GCs and their persistence either to the surface or until merging into precipitation bands suggests that GCs are a critical component of the precipitation process in these cyclones. This paper is the second part of a three-part series that investigates the dynamics of GCs through very-high-resolution idealized Weather Research and Forecasting (WRF) Model simulations. This paper assesses the role of cloud-top instability paired with nighttime, daytime, or no radiative forcing on the development and maintenance (or lack) of GCs. Under initially unstable conditions at cloud top, GCs develop regardless of radiative forcing but only persist clearly with radiative forcing. Cloud-top destabilization due to longwave cooling leads to development of GCs even under initially neutral and stable conditions, providing a physical explanation for the observed ubiquity of GCs atop winter cyclones. GCs do not develop in initially stable simulations with no radiation. Decreased range in vertical velocity spectra under daytime radiative forcing is consistent with offset of the destabilizing influence of longwave cooling by shortwave heating.

This paper assesses the influence of radiative forcing and latent heating on the development and maintenance of cloud-top generating cells (GCs) in high-resolution idealized Weather Research and Forecasting Model simulations with initial conditions representative of the vertical structure of a cyclone observed during the Profiling of Winter Storms campaign. Simulated GC kinematics, structure, and ice mass are shown to compare well quantitatively with Wyoming Cloud Radar, cloud probe, and other observations. Sensitivity to radiative forcing was assessed in simulations with longwave-only (nighttime), longwave-and-shortwave (daytime), and no-radiation parameterizations. The domain-averaged longwave cooling rate exceeded 0.50 K h−1 near cloud top, with maxima greater than 2.00 K h−1 atop GCs. Shortwave warming was weaker by comparison, with domain-averaged values of 0.10–0.20 K h−1 and maxima of 0.50 K h−1 atop GCs. The stabilizing influence of cloud-top shortwave warming was evident in the daytime simulation’s vertical velocity spectrum, with 1% of the updrafts in the 6.0–8.0-km layer exceeding 1.20 m s−1, compared to 1.80 m s−1 for the nighttime simulation. GCs regenerate in simulations with radiative forcing after the initial instability is released but do not persist when radiation is not parameterized, demonstrating that radiative forcing is critical to GC maintenance under the thermodynamic and vertical wind shear conditions in this cyclone. GCs are characterized by high ice supersaturation (RHice > 150%) and latent heating rates frequently in excess of 2.00 K h−1 collocated with vertical velocity maxima. Ice precipitation mixing ratio maxima of greater than 0.15 g kg−1 were common within GCs in the daytime and nighttime simulations.

Data from airborne W-band radar are used in conjunction with thermodynamic fields from the Weather Research and Forecasting Model and air-parcel back trajectories from the HYSPLIT model to investigate the finescale reflectivity, vertical motion, and airmass structure of the comma head of a winter cyclone in the vicinity of the Great Lakes. Cloud-top generating cells formed along an upper-level frontal boundary vertically separating dry air, which 48 h earlier was located in the upper troposphere over south-central Canada, from moist air, which was located in the lower troposphere over the southeast United States. The stronger updrafts within the generating cells had vertical velocities ranging from 1 to 3 m s−1. The generating cells were important to precipitation production within the comma head. Precipitation trails formed within the generating cells could sometimes be followed to the boundary layer before merging. Boundary layer air beneath the cyclone’s comma head exhibited convective circulations and was turbulent. Gravity waves were sometimes observed at the base of the stable layer atop the convective boundary layer. Trajectory analyses showed that boundary layer air sampled by radar beneath the aircraft path had a history of crossing the Great Lakes. The magnitude of updrafts and downdrafts in the boundary layer were 1–2 m s−1, while wave circulations exhibited maximum updrafts and downdrafts of ~3 m s−1. The tops of some boundary layer convective circulations and gravity waves exhibited enhancements in radar reflectivity. The data presented illustrate the impact of the Great Lakes on cyclone mesostructure during the passage of a cyclone through the region.

This paper presents analyses of the microphysical structure of comma head stratiform precipitation in 14 continental cyclones, focusing on fall streaks of hydrometeors produced by cloud-top convective generating cells. Data were obtained at temperatures between −4° and −45°C using in situ instrumentation and the W-band University of Wyoming Cloud Radar, all operated aboard the National Science Foundation/National Center for Atmospheric Research C-130. Analyses are presented first for a case study of one cyclone, followed by statistical analyses of the full dataset. Using radar-based objective classifications, the statistical percentile number concentrations averaged 1.9 times larger within the fall streaks compared to the regions between them, and the corresponding ice water content and median mass diameter values averaged 2.2 and 1.1 times larger. Ice-phase conditions were predominant within the stratiform precipitation, with deposition and aggregation the primary ice growth mechanisms. No distinct vertical velocity signatures were associated with the fall streaks, and similar ice growth mechanisms were common within and between them. Combined with observations of cloud-top generating cells in many of the same cyclones, these analyses provide a more complete description of the comma head microphysical structure and the physical processes producing precipitation. Whereas the generating cells are critical to nucleation and initial ice growth, the majority of ice growth (exceeding 90% of the median ice water contents in the case study) typically occurred below the generating-cell level, where enhanced moisture associated with synoptic-scale ascent was present.

Gamma distributions represent particle size distributions (SDs) in mesoscale and cloud-resolving models that predict one, two, or three moments of hydrometeor species. They are characterized by intercept (N0), slope (λ), and shape (μ) parameters prognosed by such schemes or diagnosed based on fits to SDs measured in situ in clouds. Here, ice crystal SDs acquired in arctic cirrus during the Indirect and Semi-Direct Aerosol Campaign (ISDAC) and in hurricanes during the National Aeronautic and Space Administration (NASA) African Monsoon Multidisciplinary Analyses (NAMMA) are fit to gamma distributions using multiple algorithms. It is shown that N0, λ, and μ are not independent parameters but rather exhibit mutual dependence. Although N0, λ, and μ are not highly dependent on choice of fitting routine, they are sensitive to the tolerance permitted by fitting algorithms, meaning a three-dimensional volume in N0–λ–μ phase space is required to represent a single SD. Depending on the uncertainty in the measured SD and on how well a gamma distribution matches the SD, parameters within this volume of equally realizable solutions can vary substantially, with N0, in particular, spanning several orders of magnitude. A method to characterize a family of SDs as an ellipsoid in N0–λ–μ phase space is described, with the associated scatter in N0–λ–μ for such families comparable to scatter in N0, λ, and μ observed in prior field campaigns conducted in different conditions. Ramifications for the development of cloud parameterization schemes and associated calculations of microphysical process rates are discussed.

This paper presents analyses of the microphysical structure of cloud-top convective generating cells at temperatures between −10° and −55°C across the comma head of 11 continental cyclones, using data collected by the W-band Wyoming Cloud Radar and in situ instrumentation aboard the National Science Foundation (NSF)/NCAR C-130. A case study of one cyclone is presented, followed by statistical analyses of the entire dataset. Ice particle number concentrations averaged 1.9 times larger inside generating cells compared to outside, and derived ice water contents and median mass diameters averaged 2.2 and 1.1 times larger in cells, respectively. Supercooled water was directly measured at temperatures between −31.4° and −11.1°C, with the median and 95th-percentile liquid water content increasing from ~0.09 to 0.12 g m−3 and 0.14 to 0.28 g m−3 over this temperature range, respectively. Liquid water was present in 26% of observations within cells and 18% of observations between cells over the same temperature range, and it was nearly ubiquitous at temperatures above −16°C. The larger ice particle concentrations in cells are consistent with greater ice production in convective updrafts. The increased mass and diameter of the ice particles demonstrate that generating cells provide environments favorable for enhanced particle growth. The impact of water saturation and supercooled water in the cells was evident, with rapid particle growth by diffusion and sometimes riming apparent, in addition to aggregation. Turbulent mixing lessened the observed differences between cells and surrounding regions, with supercooled water observed within and between cells, similar habits within and between cells, and rimed particles evident even in ice-phase conditions.

Data from airborne W-band radar, thermodynamic fields from the Weather Research and Forecasting (WRF) Model, and air parcel back trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model are used to investigate the finescale reflectivity, vertical motion, and airmass structure of the comma head of a winter cyclone that produced 15–25 cm of snow across the U.S. Midwest on 29–30 January 2010. The comma head consisted of three vertically stacked air masses: from bottom to top, an arctic air mass of Canadian origin, a moist cloud-bearing air mass of Gulf of Mexico origin, and a drier air mass originating mostly at low altitudes over Baja California and the Mexican Plateau. The drier air mass capped the entire comma head and significantly influenced precipitation distribution and type across the storm, limiting cloud depth on the warm side, and creating instability with respect to ice-saturated ascent, cloud-top generating cells, and a seeder–feeder process on the cold side. Convective generating cells with depths of 1.5–3.0 km and vertical air velocities of 1–3 m s−1 were ubiquitous atop the cold side of the comma head. The airmass boundaries within the comma head lacked the thermal contrast commonly observed along fronts in other sectors of extratropical cyclones. The boundary between the Gulf and Canadian air masses, although quite distinct in terms of precipitation distribution, wind, and moisture, was marked by almost no horizontal thermal contrast at the time of observation. The higher-altitude airmass boundary between the Gulf of Mexico and Baja air masses also lacked thermal contrast, with the less-stable Baja air mass overriding the stable Gulf of Mexico air.

The vertical motion and physical structure of elevated convection and generating cells within the comma heads of three continental winter cyclones are investigated using the Wyoming W-band cloud radar mounted on the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) C-130, supplemented by analyses from the Rapid Update Cycle model and Weather Surveillance Radar-1988 Doppler (WSR-88D) data. The cyclones followed three distinct archetypical tracks and were typical of those producing winter weather in the midwestern United States. In two of the cyclones, dry air in the middle and upper troposphere behind the Pacific cold front intruded over moist Gulf of Mexico air at lower altitudes within the comma head, separating the comma head into two zones. Elevated convection in the southern zone extended from the cold-frontal surface to the tropopause. The stronger convective updrafts ranged from 2 to 7 m s−1 and downdrafts ranged from −2 to −6 m s−1. The horizontal scale of the convective cells was approximately 5 km. The poleward zone of the comma head was characterized by deep stratiform clouds topped by cloud-top generating cells that reached the tropopause. Updrafts and downdrafts within the generating cells ranged from 1 to 2 m s−1, with the horizontal scale of the cells from about 1 to 2 km. Precipitation on the poleward side of the comma head conformed to a seeder–feeder process—the generating cells seeding the stratiform cloud—which was forced by synoptic-scale ascent. In one case, shallow clouds behind the cyclone’s cold front were also topped by cloud-top generating cells, with vertical motions ranging from 1 to 2 m s−1.

This paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16-storm climatology analyzing the distribution of lightning within the comma head. A case study of a deep cyclone is presented illustrating how upper-tropospheric dry air associated with the dry slot can intrude over moist Gulf air, creating two zones of precipitation within the comma head: a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection. Lightning, when it occurred, originated from the elevated convection. A second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection. Updrafts within convective cells in both storms approached 6–8 m s−1, and convective available potential energy in the cell environment reached approximately 50–250 J kg−1. Radar measurements obtained in convective updraft regions showed enhanced spectral width within the temperature range from −10° to −20°C, while microphysical measurements showed the simultaneous presence of graupel, ice particles, and supercooled water at the same temperatures, together supporting noninductive charging as an important charging mechanism in these storms. A climatology of lightning flashes across the comma head of 16 winter cyclones shows that lightning flashes commonly occur on the southern side of the comma head where dry-slot air is more likely to overrun lower-level moist air. Over 90% of the cloud-to-ground flashes had negative polarity, suggesting the cells were not strongly sheared aloft. About 55% of the flashes were associated with cloud-to-ground flashes while 45% were in-cloud flashes.

A 1-km Weather Research and Forecasting model simulation of Hurricane Dennis was used to identify precursors in vertical velocity and latent heating distributions to rapid intensification (RI). Although the observed structure qualitatively replicated data obtained during P-3 and Earth Resources-2 (ER-2) flights, the simulated reflectivity was overestimated. During the 6 h preceding RI, defined as 0000 UTC 8 July 2005 close to the time of simulated maximum central pressure deepening, the asymmetric convection transformed into an eyewall with the maximum 10-m wind speed increasing by 16 m s−1. Contour by frequency altitude diagrams showed unique changes in the breadth of simulated vertical velocity (w) distributions before and after RI. Outliers of w distributions at 14 km preceded RI onset, whereas the increase in w outliers at 6 km lagged it. Prior to RI there was an increase in the upward flux of hydrometeors between 10 and 15 km, with increased contributions from w > 6 m s−1. Increases in lower-level updraft airmass fluxes did not lead RI, but the 14-km positive w outliers were better indicators of RI onset than positive w averages. The area of convective bursts did not strongly increase before RI, but it continually increased after RI. Latent heating was dominated by contributions from w < 2 m s−1, meaning increases in positive w outliers before RI did not cause the increase in latent heating seen during RI. The location of convective bursts and outliers of positive and negative w distributions contracted toward the eye as the simulated Dennis intensified.

This study examines the development, structure, and forcing of the rear inflow jet (RIJ) through the life cycle of a small, short-lived squall line over north-central Kansas on 29 June 2003. The analyses were developed from airborne quad-Doppler tail radar data from the NOAA and NRL P-3 aircraft, obtained over a 2-h period encompassing the formation, development, and decay of the squall line during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). The strengthening of the system-relative rear inflow to 17 m s−1 was concurrent with the formation of a bow echo, an increased dynamic pressure gradient beneath the rearward-tilted updraft, and two counterrotating vortices at either end of the bow. The later weakening of the RIJ to 8 m s−1 was concurrent with the weakening of the bow, a decreased dynamic pressure gradient at midlevels behind the bow, and the weakening and spreading of the vortices. In a modeling study, Weisman quantified the forcing mechanisms responsible for the development of an RIJ. This present study is the first to quantitatively analyze these mechanisms using observational data. The forcing for the horizontal rear inflow was analyzed at different stages of system evolution by evaluating the contributions of four forcing mechanisms: 1) the horizontal pressure gradient resulting from the vertical buoyancy distribution (δPB), 2) the dynamic pressure gradient induced by the circulation between the vortices (δPV), 3) the dynamic irrotational pressure gradient (δPI), and 4) the background synoptic-scale dynamic pressure gradient (δPS). During the formative stage of the bow, δPI was the strongest forcing mechanism, contributing 50% to the rear inflow. However, during the mature and weakening stages, δPI switched signs and opposed the rear inflow while the combination of δPB and δPV accounted for at least 70% of the rear inflow. The δPS forced 4%–25% of the rear inflow throughout the system evolution.

This study employed a nondynamic microphysical column model to evaluate the degree to which the microphysical processes of sublimation, melting, and evaporation alone can explain the evolution of the relative humidity (RH) and latent cooling profiles within the trailing stratiform region of mesoscale convective systems observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). Simulations revealed that observations of a sharp change in the profile of RH, from saturated air with respect to ice above the melting layer to subsaturated air with respect to water below, developed in response to the rapid increase in hydrometeor fall speeds from 1–2 m s−1 for ice to 2–11 m s−1 for rain. However, at certain times and locations, such as the first spiral descent on 29 June 2003 within the notch of lower reflectivity, the air may remain subsaturated for temperatures (T) < 0°C. Sufficiently strong downdrafts above the melting level, such as the 1–3 m s−1 downdrafts observed in the notch of lower reflectivity on 29 June, could enable this state of sustained subsaturation. Sensitivity tests, where the hydrometeor size distributions and upstream RH profiles were varied within the range of BAMEX observations, revealed that the sharp contrast in the RH field across the melting layer always developed. The simulations also revealed that latent cooling from sublimation and melting resulted in the strongest cooling at altitudes within and above the melting layer for locations where hydrometeors did not reach the ground, such as within the rear anvil region, and where sustained subsaturated air is present for T < 0°C, such as is observed within downdrafts. Within the enhanced stratiform rain region, the air is typically at or near saturation for T < 0°C, whereas it is typically subsaturated for T > 0°C; thus, evaporation and melting result in the primary cooling in this region. The implications of these results for the descent of the rear inflow jet across the trailing stratiform region are discussed.

This study used airborne and ground-based radar and optical array probe data from the spiral descent flight patterns and horizontal flight legs of the NOAA P-3 aircraft in the trailing stratiform regions (TSRs) of mesoscale convective systems (MCSs) observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) to characterize microphysical and thermodynamic variations within the TSRs in the context of the following features: the transition zone, the notch region, the enhanced stratiform rain region, the rear anvil region, the front-to-rear flow, the rear-to-front flow, and the rear inflow jet axis. One spiral from the notch region, nine from the enhanced stratiform rain region, and two from the rear anvil region were analyzed along with numerous horizontal flight legs that traversed these zones. The spiral performed in the notch region on 29 June occurred early in the MCS life cycle and exhibited subsaturated conditions throughout its depth. The nine spirals performed within the enhanced stratiform rain region exhibited saturated conditions with respect to ice above and within the melting layer and subsaturated conditions below the melting layer. Spirals performed in the rear anvil region showed saturation until the base of the anvil, near −1°C, and subsaturation below. These data, together with analyses of total number concentration and the slope to gamma fits to size distributions, revealed that sublimation above the melting layer occurs early in the MCS life cycle but then reduces in importance as the environment behind the convective line is moistened from the top down. Evaporation below the melting layer was insufficient to attain saturation below the melting layer at any time or location within the MCS TSRs. Relative humidity was found to have a strong correlation to the component of wind parallel to the storm motion, especially within air flowing from front to rear.

During the Bow Echo and Mesoscale Convective Vortex Experiment, the NOAA P-3 research aircraft executed 17 spiral descents to the rear of convective lines to document the vertical variability of hydrometeors above, within, and below the stratiform melting layer. Ten spirals were behind lines that exhibited bowing at some stage in their evolution. Although quick descents on some spirals forced sampling of different particle zones, clear trends with respect to temperature were seen. For 16 spirals, the ambient relative humidity with respect to ice was in the range of 100% ± 4% at temperatures between −10°C and the melting layer, but exhibited steady decreases below the melting layer to an average relative humidity with respect to water of 77% ± 15% at 9°C. In contrast, one spiral conducted on 29 June 2003 directly behind a developing bow echo had a relative humidity with respect to ice averaging 85% at heights above the 0°C level and relative humidity with respect to ice further decreased below the 0°C level to a minimum relative humidity with respect to water of 48% at 9°C. Vertical profiles of particle shapes, size distributions (SDs), total mass contents (TMC), number concentrations, and parameters of gamma distributions fit to SDs were computed using optical array probe data in conjunction with measurements of radar reflectivity from the P-3 X-band tail radar. For spirals with humidity at or near saturation above the melting layer, melting particles occurred through about 300 m of cloud depth between 0° and 2° or 3°C. Above the melting layer, number concentrations, dominated by smaller crystals, decreased at 19% ± 10% °C−1, faster than the 10% ± 7% °C−1 decrease of TMC dominated by larger particles. Increases in the numbers of crystals with a maximum dimension

The University of Alabama in Huntsville Mobile Integrated Profiling System 915-MHz profiler was deployed in January and February of 2004 to measure vertical air velocities in finescale precipitation bands in winter cyclones. The profiler was placed to sample the “wraparound” quadrant of three winter cyclones in the central and southern United States, and it obtained high-resolution measurements of the vertical structure of a series of bands in each storm. The data revealed bands that were up to 6 km deep, 10–50 km wide, and spaced about 5–20 km apart. Measurements of vertical air motion w within these bands were retrieved from the Doppler spectra using the lower-bound method, adapted to account for the effects of spectral broadening caused by the horizontal wind, wind shear, and turbulence. Derived vertical air motions ranged from −4.3 to 6.7 m s−1, with an uncertainty of about ±0.6 m s−1. Approximately 29% of the 1515 total derived values were negative, 35% exceeded 1 m s−1, and 9% exceeded 2.0 m s−1. These values are consistent with studies in the Pacific Northwest, except that more extreme values were observed in one band than have been previously reported. There was a high correlation between values of signal-to-noise ratio (SNR) and w within each band (0.60 ≤ r ≤ 0.85), in the composite of bands from each cyclone (0.59 ≤ r ≤ 0.79), and in the overall analysis (r = 0.68). The strongest updrafts were typically between 2.0 and 4.0 m s−1 and were located near the center of each band in regions of high SNR. Regions of downdrafts within the bands had maximum values between −1.0 and −4.3 m s−1 and were typically located along the edges of the bands in regions of low SNR. These results are consistent with snow growth and sublimation processes. The magnitudes of the vertical velocities in the core of the bands were comparable to theoretical predictions for moist symmetric instability (MSI) under inviscid conditions but would appear to be somewhat larger than expected for MSI when turbulent mixing is considered, suggesting that other instabilities, such as potential instability, may have contributed to the band development in these storms.

The dynamic and thermodynamic structure and associated frontal circulations within the trowal and warm-frontal regions of two extratropical winter cyclones are examined using numerical simulations. In each cyclone, the warm, moist airstream originating in the warm sector was found to bifurcate upon reaching the warm front. One branch of the flow turned anticyclonically eastward, corresponding to the warm conveyor belt, while the second branch turned cyclonically westward becoming the trowal airstream. The dynamic forcing of vertical motion within the two airstreams was investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5), both from an analysis of the Sawyer–Eliassen (SE) equation in two dimensions and from complete model solutions. Shearing deformation, associated with the jet stream and the low-level cyclonic flow, dominated confluent deformation near the trowal in both cases. The shearing deformation was accompanied by cold advection associated with an intrusion of cold, dry air aloft. The configuration of isentropes and the wind field led to frontogenesis on the equatorward side of the trowal and frontolysis farther south on the poleward side of the jet stream. The SE solution showed a circulation centered on the frontogenesis–frontolysis couplet, with air rising in the trowal and sinking within the dry air mass on the trowal’s equatorward side. The rising branch of the circulation was responsible for the wide swath of snowfall coincident with the trowal. From the vicinity of the bifurcation axis eastward along the warm-frontal zone, confluent deformation dominated within the troposphere. Frontogenesis in this region produced a direct circulation whose rising branch accounted for the production of precipitation over the warm-frontal zone. Diabatic processes associated with latent heating and cooling produced frontogenesis–frontolysis couplets and significantly modified the transverse frontal circulations. The ascending motion was amplified by a factor of 2 or greater compared with the ascending motion solely due to horizontal deformation. The width of the ascending branch was also narrowed compared with that solely from deformation. Vertical tilting, a result of the secondary circulation generated by horizontal deformation, produced frontogenesis–frontolysis couplets that acted to oppose and reduce the magnitude of the secondary circulation. A conceptual model of the effect of these processes on the production and organization of snowfall in the two cyclones is presented.

This paper compares the structure of the trough of warm air aloft (trowal)–warm-frontal region of two continental wintertime cyclones. The cyclones were observed over the central Great Lakes region during the Lake-Induced Convection Experiment/Snowband Dynamics Project field campaign. The cyclones had different origins, with the first forming east of the Colorado Rockies and the second forming over the Gulf of Mexico. They were associated with different upper-level flow regimes, one located just north of a nearly zonal jet and the other located just west of a nearly meridional jet. Both storms produced heavy swaths of snow across the states of Illinois, Wisconsin, and Michigan. High-resolution observations of frontal structure were made during flights of the National Center for Atmospheric Research Electra aircraft using dropsondes and the Electra Doppler Radar tail radar system. The high-resolution observations suggest a different arrangement of air masses in the trowal region compared with the classical occlusion model, where the trowal axis forms at the intersection of a warm front and a cold front that has overtaken and subsequently ascended the warm front. In both cyclones dry air intruded over the warm front, isolating the warm, moist airflow within the trowal airstream. Very sharp moisture gradients were present at the leading edge of the dry air in both cyclones. In each case, relative humidity differences of over 50% were observed over distances of 10–20 km. The thermal gradient near the leading edge of the dry air in one cyclone was diffuse, so that the moist–dry boundary could best be characterized as an upper-level humidity front. In the other cyclone, the thermal gradient was sharper and aligned with the moisture boundary and was best characterized as a cold front aloft. The analyses suggest that the classical conceptual model of the trowal, at least in some cyclones such as the two illustrated here, needs to be revised to include the possibility that the warm moist airstream aloft may sometimes be bounded on its south side by an upper-level front rather than a surface-based cold front. Since the two cyclones discussed here had different origins, tracks, and flow regimes, the similarity of their structure suggests that these features may be common.

This study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines. The forcing chosen was a cold front, simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front. Use of a realistic, balanced, and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing. Three-dimensional squall lines were modeled with and without the front present, in low and high bulk Richardson number environments. The forced convection evolved in significantly different ways than their isolated, unforced counterparts. In low-shear conditions, the line of isolated convective cells split, with the adjacent split cells interfering destructively with neighboring cells in the line. With forcing present, differences in anticyclonic cell intensity and propagation prevented this interaction from occurring, leading to longer-lived cyclonic convection despite a near-normal orientation between cloud-bearing shear and the convective line. The split-cell interaction also failed to occur under higher-shear conditions due to anticyclonic cell decay given the greater cyclonic hodograph curvature. In both low- and higher-shear environments, a strong bias toward cyclonic storms was noted with forcing present, due to shallower anticyclonic cells with the front present and to preexisting vorticity in the environment; updraft–vorticity correlations were skewed accordingly. Forcing also reduced the sensitivity of the evolving convection to detailed aspects of the initialization.

In the 19 April 1996 Illinois tornado outbreak, cell mergers played a very important role in the convective evolution. With a large number of cells forming within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells as described in Part I of this study. Twenty-six mergers were documented and analyzed. Storm-rotation-induced differential cell propagation accounted for 58% of these 26 cell mergers while differing cell speeds prompted 27% of the mergers. Cell merger characterizations were utilized to describe the cell reflectivity coalescence morphology including aspects of new cell development, development along the periphery of an existing cell, or an upward pulse in the cell intensity of a dominant cell. In cases where the merging cells were of similar intensity, a rapidly developing cellular pulse “bridging” the two echoes was often observed. When the relationship between short-term cell intensity changes and cell mergers was examined, it was found that the maximum reflectivity tendency showed a bias toward higher reflectivity for the product storm. Depending upon the radar elevation angle utilized, 27%–44% of mergers were associated with an increase in peak reflectivity while 40%–58% of the storms realized little or no increase. With respect to short-term cell rotation changes, the merger signal was marked. Depending upon the length of the evaluation window, in 44%–60% of the mergers, there was evidence of a merger-associated increase in cell rotation. When considering the association between tornado occurrence and cell mergers, a striking 54% of the tornadoes occurred within 15 min before or after a cell merger. This high percentage is strongly suggestive of a physical relationship between storm mergers and tornadogenesis. A discussion is presented of potential merger scenarios and favorable ambient environmental conditions that may have been conducive to tornadogenesis in this event. Suggestions are presented to raise the awareness level of forecasters to key aspects of cell evolution and interaction in nowcasting severe convection.

In this study of the 19 April 1996 Illinois tornado outbreak, 109 cells were tracked using radar data to understand the transition of the cell configuration from a considerable number of initial cells to a small subset of supercells after several hours of evolution. Of these 109 cells, 85 developed along three synoptic boundaries (dryline, warm front, and dryline–warm front occlusion) between 1940 and 2230 UTC. A large majority of these 85 cells formed in a 1-h period between 2040 and 2140 UTC. With a considerable number of cells initiating within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells. Cell-type initiation analysis revealed that storms that would become supercells were initiated, on average, 17 min before nonsupercell storms. Cyclonic supercells, with mean storm life spans of 214 min, had much longer lives than nonsupercell storms. Anticyclonic supercells resulting from storm splits were the second longest lived at 166 min. In comparison, the largest nonsupercell category, those cells that dissipated in relative isolation, only had 35-min life spans. Supercell isolation resulted from storm mergers due to differential cell propagation and the frequent attrition of cells that formed along a common boundary. The varying rotational properties of individual cells enhanced the probability for numerous mergers while fostering a scenario where, after a few hours, the supercells became increasingly isolated. Suggestions are presented to raise the awareness level of forecasters to key aspects of cell evolution and interaction in nowcasting severe convection. In Part II of this study, storm interactions are examined in the context of merger morphology, merged cell intensity changes, and the association between storm mergers and tornadogenesis.

The Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX) is a research investigation using highly mobile platforms to examine the life cycles of mesoscale convective systems. It represents a combination of two related investigations to study (a) bow echoes, principally those that produce damaging surface winds and last at least 4 h, and (b) larger convective systems that produce long-lived mesoscale convective vortices (MCVs). The field phase of BAMEX utilized three instrumented research aircraft and an array of mobile ground-based instruments. Two long-range turboprop aircraft were equipped with pseudo-dual-Doppler radar capability, the third aircraft was a jet equipped with dropsondes. The aircraft documented the environmental structure of mesoscale convective systems (MCSs), observed the kinematic and thermodynamic structure of the convective line and stratiform regions (where rear-inflow jets and MCVs reside), and captured the structure of mature MCVs. The ground-based instruments augmented sou...

On 14 February 1992, a long-lived moderate-amplitude mesoscale gravity wave formed in Kansas during the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test (STORM-FEST). Wave formation was evident in correlated surface pressure and wind fields. The wave of depression, accompanied by a weak rainband, tracked across the state. A wealth of data was collected on the mature wave as it passed over the STORM-FEST dual-Doppler domain. However, the mechanism of genesis remained difficult to ascertain, since wave formation occurred in a region of less comprehensive observations. The genesis of the STORM-FEST gravity wave is successfully simulated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (Penn State–NCAR) Mesoscale Model (MM5), which was run at 6-km grid spacing in the innermost domain. The lee cyclone movement, dry airmass development, and gravity wave formation over Kansas were successfully captured by the model. Results ...

No Abstract available.

On 14–15 February 1992 a long-lived mesoscale gravity wave was observed over the Storm-scale Operational and Research Meteorology-Fronts Experiment Systems Test observational network. A precipitation band formed and intensified as the wave moved across Kansas and Missouri. The disturbance was tracked for 14 h. Surface, dual-Doppler radar, and wind profiler analyses, as well as isentropic analyses derived from a special rawinsonde network, are used to examine the origin and evolution of this wave. The wave originated at the leading edge of a dry air mass associated with downslope flow in the lee of the Rockies. The earliest surface pressure signatures of wave motion began as a dry air mass, associated with the downslope flow, ascended a warm front east of a lee cyclone. A weak rainband developed simultaneously with the wave at the leading edge of the dry air mass. The mesoscale gravity wave, and convection, remained tied to the leading edge of the advancing dry air mass during the first 8–10 h of ...

A severe 5—day lake—effect storm resulted in eight deaths, hundreds of injuries, and over $3 million in damage to a small area of northeastern Ohio and northwestern Pennsylvania in November 1996. In 1999, a blizzard associated with an intense cyclone disabled Chicago and much of the U.S. Midwest with 30—90 cm of snow. Such winter weather conditions have many impacts on the lives and property of people throughout much of North America. Each of these events is the culmination of a complex interaction between synoptic—scale, mesoscale, and microscale processes. An understanding of how the multiple size scales and timescales interact is critical to improving forecasting of these severe winter weather events. The Lake—Induced Convection Experiment (Lake—ICE) and the Snowband Dynamics Project (SNOWBAND) collected comprehensive datasets on processes involved in lake—effect snowstorms and snowbands associated with cyclones during the winter of 1997/98. This paper outlines the goals and operations of thes...

Over the past several years, the Department of Atmospheric Sciences at the University of Illinois has developed a computerized weather laboratory that permits interactive access to real-time data from observing sites around the United States and to output from numerical weather prediction models at the operational centers. Such a setup, with timely access to observations and numerical model forecasts from any networked terminal, personal computer, or workstation, is a valuable tool for education and research in meteorology. The University of Illinois system acts as a real-time, on-line, in-class instructional meteorology laboratory for students. The data-display software is based on the X-windows protocol, which is network transparent and system independent. In addition to software packages distributed by the University Data Project (UNIDATA), software tools developed by the National Center for Supercomputing Applications and the University of Illinois are used to display, animate, and manipulate...

A 3-h intermittent data assimilation system (Mesoscale Analysis and Prediction System—MAPS) configured in isentropic coordinates was developed and implemented in real-time operation. The major components of the system are data ingest, objective quality control of the observation, objective analysis, and a primitive equation forecast model, all using isentropic coordinates to take advantage of the improved resolution near frontal zones and greater spatial coherence of data that this coordinate system provides. Each 3-h forecast becomes the background for the subsequent analysis; in this manner, a four-dimensional set of observations can be assimilated. The primary asynoptic data source used in current real-time operation of this system is air-craft data, most of it automated. Data from wind profilers, surface observations, and radiosondes are also included in MAPS. Statistics were collected over the last half of 1989 and into 1990 to study the performance of MAPS and compare it with that of the Re...

The results of numerical simulations of severe thunderstorms with rotating updrafts and supercell characteristics are analyzed to determine their sources, sinks, and transports of momentum and kinetic energy. Two simulations are used, one initiated in an idealized environment with unidirectional shear, and the other simulating a real tornadic storm in an environment with a curved hodograph. For the unidirectional shear storm, we carry out the analysis at 10-minute intervals throughout the 2.5 hour duration of the simulation, during which the storm develops a fairly steady amplitude after the first hour but continues to grow in areal extent and in disturbance kinetic energy. We analyze just one time level for the tornadic storm. For both storms the vertical flux of horizontal momentum is strongly down the velocity gradient, and the corresponding rate of transfer of disturbance kinetic energy from the mean flow is comparable to that of buoyant energy release. The mean-flow kinetic energy is in both...