Trailing Stratiform Regions Research Articles

Impact of Assimilating GOES-R Geostationary Lightning Mapper Flash Extent Density Data on Severe Convection Forecasts in a Warn-on-Forecast System

AbstractThe Geostationary Operational Environmental Satellite-R (GOES-R) Geostationary Lightning Mapper (GLM) instrument detects total lightning rate at high temporal and spatial resolution over the Americas and adjacent oceanic regions. The GLM observations provide detection and monitoring of deep electrified convection. This study explores the impact of assimilating the GLM-derived flash extent density (FED) on the analyses and short-term forecasts of two severe weather events into an experimental Warn-on-Forecast system (WoFS) using the ensemble Kalman filter data assimilation technique. Sensitivity experiments are conducted using two tornadic severe storm events: one with a line of individual supercells and the other one with both isolated cells and a severe convective line. The control experiment (CTRL) assimilates conventional surface observations and geostationary satellite cloud water path into WoFS. Additional experiments also assimilate either GLM FED or radar data (RAD), or a combination of both (RAD+GLM). It is found that assimilating GLM data in the absence of radar data into the WoFS improves the short-term forecast skill over CTRL in one case, while in the other case it degrades the forecast skill by generating weaker cold pools and overly suppressing convection, mainly owing to assimilating zero FED values in the trailing stratiform regions. Assimilating unexpectedly low FED values in some regions due to low GLM detection efficiency also accounts for the poorer forecasts. Although RAD provides superior forecasts over GLM, the combination RAD+GLM shows further gains in both cases. Additional observation operators should consider different storm types and GLM detection efficiency.

In Situ Observations of Microphysics, Electric Fields, and Lightning in the Trailing Stratiform Region of a Mesoscale Convective System

AbstractWe use in situ airborne observations to extend a previous analysis by Lang and Rutledge (2008; https://doi.org/10.1029/2006JD007709) of remotely sensed radar and lightning mapping array observations of the 11 June 2000 asymmetric mesoscale convective system (MCS) that moved through the primary observation region of the Severe Thunderstorm Electrification/Precipitation Study in northeastern Colorado and northwestern Kansas. We analyze in detail in situ aircraft observations, radar, and remotely mapped lightning discharges from a portion of the MCS that was starting to produce a bow echo during the time of the aircraft mission. The observations are interpreted to indicate the presence of a rearward and downward‐sloping positive charge layer detraining from a mature cell near the leading convective region. In the convective cell the positive charge region was at an altitude of ~10 km MSL. It then descended rearward, crossing the 6 km MSL altitude plane ~40 km to the rear of the leading convective region. During an 8‐min period, three major lightning discharges initiated in the convective region and propagated rearward into the trailing stratiform region in association with this positive charge layer. Airborne in situ microphysical and electrical observations suggest that the charge layer in the stratiform region of the storm during this period resulted from charge separation in the convective region and transport from there into the stratiform region. Microphysical and meteorological conditions in the stratiform region were not supportive of significant in situ charge separation in this portion of the stratiform region during this time period.

Particle Size Distribution Characteristics Within Different Regions of Mature Squall-Line Based on the Analysis of Global Precipitation Measurement Dual-Frequency Precipitation Radar Retrieval

Particle size distribution (PSD) characteristics of mature squall lines are investigated through global precipitation measurement (GPM) dual-frequency precipitation radar (DPR) measurements. These squall lines consist of a leading convective (LC) line, a weak-echo transition (WT) region and a trailing stratiform (TS) region. Their PSD characteristics are quite different from the existing conceptual models of mature squall line, given that many small raindrops/ice particles are found in the WT region while in the TS region raindrops/ice particles are sparse. Analysis shows that it is likely due to the short distance from LC to WT, where more particles may be dispersed from LC region and fall into WT region but barely have time to grow in size. In the TS region further behind LC, the particles have more time to get larger. Analysis also reveals that the mesoscale updraft generally occurs at mid-to-high levels in the TS region so that aggregations and collisions-coalescences could be promoted to increase the particle size but decrease the particle number. Through the GPM PSD data analysis, a refined conceptual model of a mesoscale convective system (MCS) with squall line is presented in this study.

Potential Vorticity Generation by West African Squall Lines

Abstract The West African summer monsoon features multiple, complex interactions between African easterly waves (AEWs), moist convection, variable land surface properties, dust aerosols, and the diurnal cycle. One aspect of these interactions, the coupling between convection and AEWs, is explored using observations obtained during the 2006 African Monsoon Multidisciplinary Analyses (AMMA) field campaign. During AMMA, a research weather radar operated at Niamey, Niger, where it surveilled 28 squall-line systems characterized by leading convective lines and trailing stratiform regions. Nieto Ferreira et al. found that the squall lines were linked with the passage of AEWs and classified them into two tracks, northerly and southerly, based on the position of the African easterly jet (AEJ). Using AMMA sounding data, we create a composite of northerly squall lines that tracked on the cyclonic shear side of the AEJ. Latent heating within the trailing stratiform regions produced a midtropospheric positive potential vorticity (PV) anomaly centered at the melting level, as commonly observed in such systems. However, a unique aspect of these PV anomalies is that they combined with a 400–500-hPa positive PV anomaly extending southward from the Sahara. The latter feature is a consequence of the deep convective boundary layer over the hot Saharan Desert. Results provide evidence of a coupling and merging of two PV sources—one associated with the Saharan heat low and another with latent heating—that ends up creating a prominent midtropospheric positive PV maximum to the rear of West African squall lines.

Measurements of Differential Reflectivity in Snowstorms and Warm Season Stratiform Systems

AbstractThe organized behavior of differential radar reflectivity (ZDR) is documented in the cold regions of a wide variety of stratiform precipitation types occurring in both winter and summer. The radar targets and attendant cloud microphysical conditions are interpreted within the context of measurements of ice crystal types in laboratory diffusion chambers in which humidity and temperature are both stringently controlled. The overriding operational interest here is in the identification of regions prone to icing hazards with long horizontal paths. Two predominant regimes are identified: category A, which is typified by moderate reflectivity (from 10 to 30 dBZ) and modest +ZDR values (from 0 to +3 dB) in which both supercooled water and dendritic ice crystals (and oriented aggregates of ice crystals) are present at a mean temperature of −13°C, and category B, which is typified by small reflectivity (from −10 to +10 dBZ) and the largest +ZDR values (from +3 to +7 dB), in which supercooled water is dilute or absent and both flat-plate and dendritic crystals are likely. The predominant positive values for ZDR in many case studies suggest that the role of an electric field on ice particle orientation is small in comparison with gravity. The absence of robust +ZDR signatures in the trailing stratiform regions of vigorous summer squall lines may be due both to the infusion of noncrystalline ice particles (i.e., graupel and rimed aggregates) from the leading deep convection and to the effects of the stronger electric fields expected in these situations. These polarimetric measurements and their interpretations underscore the need for the accurate calibration of ZDR.

Prediction of Convective Morphology in Near-Cloud-Permitting WRF Model Simulations

Abstract The Weather Research and Forecasting (WRF) model’s ability to forecast convective morphological evolution is examined for 37 convective systems. The simulations used Thompson microphysics with 3-km horizontal grid spacing. Ten convective mode classifications were used. An objective score was developed to determine the accuracy of the simulated morphologies considering a normalized duration of each mode simulated and its agreement with observations. Rapid Update Cycle analyses were used to compare larger-scale preinitiation conditions to simulated morphology accuracy, as well as to examine how the WRF model’s skill at predicting these larger-scale conditions influenced its prediction of morphology. Two case studies selected as representative of the most common simulated morphology deficiencies were examined in detail. The model simulated cellular systems relatively well but struggled more with linear systems, particularly bow echoes and squall lines having trailing stratiform rain regions. Morphological evolution was generally better simulated in environments with enhanced deep-layer shear and cooler potential temperatures at the level of maximum θe. Weaker deep-layer shear, cooler potential temperatures at the surface, and quickly warming potential temperatures with height increased the likelihood of timing errors. The first case study showed that a warmer cold pool, much larger line-normal shear, and excessive midlevel drying were present in the model run that failed to develop a trailing stratiform region. The second case study showed that weak shear and the absence of a well-developed cold pool may have played a role in the lack of bowing.

Microphysical and Thermodynamic Structure and Evolution of the Trailing Stratiform Regions of Mesoscale Convective Systems during BAMEX. Part I: Observations

Abstract 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.

Microphysical and Thermodynamic Structure and Evolution of the Trailing Stratiform Regions of Mesoscale Convective Systems during BAMEX. Part II: Column Model Simulations

Abstract 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.

MCS Rainfall Forecast Accuracy as a Function of Large-Scale Forcing

The large-scale forcing associated with 20 mesoscale convective system (MCS) events has been evaluated to determine how the magnitude of that forcing influences the rainfall forecasts made with a 10-km grid spacing version of the Eta Model. Different convective parameterizations and initialization modifications were used to simulate these Upper Midwest events. Cases were simulated using both the Betts–Miller–Janjić (BMJ) and the Kain–Fritsch (KF) convective parameterizations, and three different techniques were used to improve the initialization of mesoscale features important to later MCS evolution. These techniques included a cold pool initialization, vertical assimilation of surface mesoscale observations, and an adjustment to initialized relative humidity based on radar echo coverage. As an additional aspect in this work, a morphology analysis of the 20 MCSs was included. Results suggest that the model using both schemes performs better when net large-scale forcing is strong, which typically is the case when a cold front moves across the domain. When net forcing is weak, which is often the case in midsummer situations north of a warm or stationary front, both versions of the model perform poorly. Runs with the BMJ scheme seem to be more affected by the magnitude of surface frontogenesis than the KF runs. Runs with the KF scheme are more sensitive to the CAPE amount than the BMJ runs. A fairly well-defined split in morphology was observed, with squall lines having trailing stratiform regions likely in scenarios associated with higher equitable threat scores (ETSs) and nonlinear convective clusters strongly dominating the more poorly forecast weakly forced events.

Precipitation charge and size measurements inside a New Mexico mountain thunderstorm

We measured the charge and size of precipitation particles with an instrumented free balloon in a convective mountain thunderstorm over Langmuir Laboratory in central New Mexico. Using an instrument that measured precipitation charge from 2 to 220 pC and equivalent particle diameters from 0.6 to 3.8 mm, we deduced that (1) the charge of the main positive charge region is carried by cloud particles, the charge of the main negative charge region is carried by a mixture of negative charge bearing cloud particles and precipitation particles, and the charge of the lower positive charge region is carried almost entirely by precipitation; (2) there was a mixture of both polarities of precipitation charge at nearly all altitudes; (3) there was no relationship between precipitation charge and size; (4) our charge data appear to support the noninductive ice‐ice collisional charging mechanism; and (5) the sign of charge carried by the precipitation reverses with ambient temperature. In comparing these data to previous measurements that were collected in New Mexico mountain thunderstorms with different instrumentation, we found that in most ways our precipitation charge data are similar to the previous measurements. In comparing these data with data that we previously collected in the trailing stratiform regions of mesoscale convective systems, we found that there are substantial differences in the precipitation charge data between these different cloud types.

Precipitation charge and size measurements in the stratiform region of two mesoscale convective systems

We measured the charge and size of precipitation particles with instrumented free balloons in the trailing stratiform regions of two different mesoscale convective systems (MCSs) in Oklahoma. Each of these stratiform regions had a different type of vertical electric field structure, one of each of the two types commonly found in MCS stratiform regions. Using an instrument that measured precipitation charge from 4 to 400 pC and equivalent diameters from 0.8 to 8.0 mm, we found that (1) the particles measured were not the major charge carriers in these storms, implying that smaller particles were responsible for the measured field, (2) there was no obvious relation between precipitation charge and size or polarity and size, (3) the precipitation size distribution in and near the transition zone consisted of two independent populations, one centered at 2.6 mm and one at 4.9 mm, (4) the two lower charge layers in these storms were likely created by in situ charging processes, and (5) net precipitation charge density was small because nearly equal amounts of positive and negative precipitation charge existed at all altitudes measured.

The Dynamics of Circulations within the Trailing Stratiform Regions of Squall Lines. Part I: The 10–11 June PRE-STORM System

Abstract A dynamic version of the two-dimensional kinematic cloud model of Rutledge and Houze has been developed to investigate the effects of microphysics on circulations within the stratiform regions of mesoscale convective systems. The design of the model allows for specified inputs of hydrometeors, water vapor, and heat from the convective line. The stratiform region of the 10–11 June 1985 PRE-STORM squall line is simulated, with initialization based upon appropriate soundings, heat budgets, and ID cumulonimbus model results. The model accurately simulates the evolution of the stratiform rain area. Significant ascent occurs in the stratiform region where in situ condensate production contributes increasingly to the surface rainfall, reaching 65% of the total in the mature stage and averaging 44% over the entire simulation. The ratio of condensate produced within the mesoscale updraft to that advected from the convective line generally agrees with water budgets from other studies. Simulated horizontal ...

The Dynamics of Circulations within the Trailing Stratiform Regions of Squall Lines. Part II: Influence of the Convective Line and Ambient Environment

Abstract A dynamic version of the two-dimensional kinematic cloud model of Rutledge and Houze is used to study the individual roles of hydrometeor and heat advection from the convective line to the stratiform region of the 10–11 June 1985 PRE-STORM squall line. The design of the model allows for specified inputs of hydrometeors, heat, and water vapor from the convective line. Convective heating alone generates significant ascent and condensate in the anvil; however, surface rainfall is scarce without the advection of hydrometeors from the convective line. Hydrometeor advection alone does not produce strong ascent in the anvil cloud, implying the important additive effects of both heat and hydrometeor advection in generating broad regions of significant stratiform rainfall. The roles of individual microphysical processes within the stratiform region are examined, along with the sensitivity of stratiform region dynamics to postconvective region environmental conditions. Of the processes evaporation, melting...

Vertical Radar Reflectivity Structure and Cloud-to-Ground Lightning in the Stratiform Region of MCSs: Further Evidence for In Situ Charging in the Stratiform Region

Abstract This study presents further evidence in support of an in situ, noninductive charging mechanism as the process likely responsible for significant electrification of the trailing stratiform regions of mesoscale convective systems (MCSs). In contrast to previous studies of MCS electrification that have investigated observations of radar reflectivity and cloud-to-ground lightning in the horizontal (e.g., Orville et al.; Rutledge et al.), here the relationship between the location and occurrence of cloud-to-ground lightning in the stratiform regions of midlatitude and tropical MCSs and the vertical profile of radar reflectivity are examined. The vertical profile of radar reflectivity at elevations above the 0°C level is used as a proxy for the amount of mass present in the mixed-phase region of the stratiform clouds, which in turn is related to the generation of charge through a noninductive charging mechanism. To further explore the relationship between radar reflectivity, mixed-phase microphysics, a...

Amazon Coastal Squall Lines. Part I: Structure and Kinematics

Mesoscale to synoptic-scale squall lines that form along the northeastern coast of South America as sea-breeze-induced instability lines and propagate through the Amazon Basin are investigated using data collected during the April–May 1987 Amazon Boundary Layer Experiment (ABLE 2B). These systems, termed “Amazon coastal squall lines” (ACSL), have been noted by others, but details of the structure and evolution of the ACSL are limited. The present paper uses Geostationary Operational Environmental Satellite, radar, upper-air rawinsonde, and surface Portable Automated Mesonet data to describe the structure, dynamics, and life cycle of the ACSL. Twelve ACSL were sampled during ABLE 2B, and three cases are discussed in detail. The ACSL are discontinuous lines of organized mesoscale cloud clusters that propagate across the central Amazon Basin at speeds of 50–60 km h−1. The ACSL undergo six possible life cycle stages: coastal genesis, intensification, maturity, weakening, reintensification, and dissipation. Analysis also indicates that mesoscale clusters within the ACSL are composed of three distinct cloud components: a prestorm region that often contains towering cumulus, leading edge convection (LEC), and multiple, precipitating cloud layers in the trailing stratiform region (TSR). Divergence and vertical velocity calculations indicate deep vertical ascent in the LEC and a region of midlevel convergence (≈500 mb) in the TSR. The latter midlevel convergence is associated with a weak updraft above 500 mb and an unsaturated downdraft below. Vertical motions in the TSR are an order of magnitude smaller than in the LEC. Substantial shear in the low-level inflow occurs in all three case studies and, as suggested by model simulations, may play an important role in the longevity (24–48 h) of the ACSL. Profiles of equivalent potential temperature θe, taken from the prestorm, leading edge convection and trailing stratiform regions demonstrate that the ACSL stabilize the troposphere in their wake and remove a tropospheric minimum of θe. It is hypothesized that the removal of this minimum is accomplished both by direct mixing via vertical motions in the LEC ("hot towers") and also through detrainment in the multiple-layered TSR. Part I describes the structure and kinematics of the ACSL, while Part II deals with the heat and moisture transports of these systems.

A Numerical Investigation of Squall Lines. Part I: The Control Experiment

Abstract A two-dimensional, anelastic, cloud-resolving numerical model was used to simulate squall systems. Large domain and fine grid resolutions were utilized so that both the convective and mesoscale components of squall lines could be handled adequately. Detailed cloud microphysics including the ice phase and the Coriolis force have been included in the basic model. Both the life cycle and storm structure of observed squall systems have been simulated successfully. Some details in the observed precipitation and kinematic characteristics of squall lines, such as the locations of front-to-rear jet core, the base of the stratiform cloud, the formation of a transition zone, and the organized mesoscale updraft, have been simulated by the model. The storm-generated meso-γ-scale low pressure center located behind the convective updraft has been shown to be instrumental in the initiation and maintenance of the mesoscale circulation and the associated trailing stratiform region. Diagnostically, the horizontal ...

A Numerical Investigation of Squall Lines. Part II: The Mechanics of Evolution

The physical processes involved in the evolution of the model squall line presented in Part I of this study are examined. It is found that both the thermal and dynamic effects are important in the development of the midlevel meso-γ-scale low pressure zone located just to the rear of the convective core. Based on this observation, a positive feedback mechanism is proposed to explain the abrupt transformation of the more or less upright convection line into a quasi-steady meso-β-scale convective system possessing an extensive trailing stratiform region. In order to set the stage for this transformation to take place, the convective updraft is required to possess an initial upshear tilt. Qualitative arguments are given to show that this initial tilt might be the result of a moderate wind shear at low levels and none or reverse shear in the middle to high levels. Results from model sensitivity experiments are presented to support the theory. In addition to the dynamic effects of the low-level wind shear and the strength of the storm-induced cold pool, model results show that other storm-induced features and environmental factors such as the middle-level wind shear also play an important role in determining the evolution of squall systems.

The Formation of a Cooling-induced Mesovortex in the Trailing Stratiform Region of a Midlatitude Squall Line

Abstract There have been some ambiguities in recent observational studies as to whether midlevel mesovortices are induced by latent heating or cooling, and develop in the descending or ascending portion of mesoscale convective systems (MCS's). In this study, a comprehensive examination of a cooling-induced mesovortex in the trailing stratiform region of a midlatitude squall line that occurred on 10–11 June 1985 during the Preliminary Regional Experiment for STORM-Central (PRE-STORM) is presented using a 20-h high-resolution simulation of the squall system. This cooling-induced midlevel vortex originates from the preexisting cyclonic vorticity associated with a traveling meso-α-scale short wave. The vortex is intensified in the descending rear-to-front (RTF) inflow as a result of continued sublimative melting and evaporative cooling in the stratiform region. It decouples from the front-to-rear (FTR) ascending and anticyclonic flow in the upper troposphere during the formative stage. The vortex tilts northw...

Some characteristics of cloud‐to‐ground lightning in tropical northern Australia

Peak currents and polarities for the first return strokes of approximately 5000 cloud‐to‐ground lightning flashes have been studied. The flashes were associated with eight mesoscale convective systems (MCSs) that occurred in tropical northern Australia in 1989 during the second field phase of the Down Under Doppler and Electricity Experiment (Dundee). Statistical analysis of the lightning data produced an average peak current (independent of polarity) of 39 kA. When considered with the average cloud‐top heights of the tropical MCSs examined herein, the 39‐kA peak current average appears to be in general agreement with Orville's (1990) hypothesis of latitudinal variation in peak current. In six of the Dundee MCSs consisting of both convective and stratiform precipitation components, we found that positive peak current maxima tended to occur in the trailing stratiform region while positive peak current minima were generally situated in convective precipitation. We also observed this pattern in a middle‐latitude MCS that occurred on June 10–11, 1985. The magnitude of the positive peak current maxima in the tropical MCSs and the middle‐latitude MCS increased coincidentally in time with the growth of the stratiform regions, and reached peak values when the stratiform regions were most intense. Given the apparent dependence of positive peak current magnitude on the life‐cycle of the stratiform region and the spatial separation between positive peak current maxima and the convective line, we offer further support for a noninductive charging mechanism being responsible for the electrical charging and subsequent lightning observed in the trailing stratiform regions of MCSs.

Near-Tropopause Vertical Motion within the Trailing Stratiform Region of a Midlatitude Squall Line

Abstract Rawinsonde observations have been used to determine the flow structure in the vicinity of the tropopause atop the trailing stratiform precipitation region of an intense midlatitude squall line. Computations of vertical motion using the kinematic and thermodynamic methods show (i) upward motion in the mid- to upper troposphere within the stratiform cloud and (ii) downward motion in a thin layer 2–3 km deep centered near the tropopause at cloud top. The latter feature, which has received little attention and explanation, has recently been reported to exist above stratiform rain areas in the tropics, as measured directly by a wind profiler on the tropical western Pacific island Pohnpei. The thin layer of sinking at the top of the trailing stratiform region occurs along the sloping upper cloud boundary and is associated with downward-sloping isentropes in the lower stratosphere to the rear of the convective line. The deformation of the isentropes is associated with an upward bulging of the tropopause...