Intense Squall Line Research Articles

Vertical divergence profiles as detected by two wind‐profiler mesonets over East China: Implications for nowcasting convective storms

AbstractLow‐level convergence is one of the most important dynamic variables in governing the convection initiation (CI) and development of storms. However, our ability to obtain high‐resolution horizontal divergence profiles remains limited due to the lack of high‐resolution vertical wind observations. In this study, horizontal divergence (and vertical motion) profiles are derived from horizontal winds measured by two radar wind‐profiler (RWP) mesonets in East China, and then used to examine their relationship with CI occurring within a mesonet and an intense squall line moving across another mesonet. Results show that the RWP mesonets together with automated surface observations could provide realistically the lower‐tropospheric profiles of horizontal divergence (and upward motion) associated with the CI in the presence of the urban heat island effects, and lake and sea breezes; and the evolution of the squall line respectively. High‐resolution surface observations resolve better divergence induced by localized CI than the RWP measurements, whereas the latter capture well significant convergence below 4‐km altitude that precedes the onset of squall precipitation. A statistical analysis of the RWP data during the summer months of 2018 indicates that the lower‐tropospheric convergence can be detected up to about 40 min in advance of the occurrence of heavy rainfall, and the correlation is higher when it is closer to the heavy rainfall time. Nearly 40% of heavy rainfall moments (at 6‐min intervals) are accompanied by strong convergence signals occurring mostly near the top of the boundary layer. This work indicates that the RWP and surface wind measurements should be integrated to aid in nowcasting the location and timing of CI and development of convective storms.

Is an NWP-Based Nowcasting System Suitable for Aviation Operations?

The growth of air transport demand expected over the next decades, along with the increasing frequency and intensity of extreme weather events, such as heavy rainfalls and severe storms due to climate change, will pose a tough challenge for air traffic management systems, with implications for flight safety, delays and passengers. In this context, the Satellite-borne and IN-situ Observations to Predict The Initiation of Convection for ATM (SINOPTICA) project has a dual aim, first to investigate if very short-range high-resolution weather forecast, including data assimilation, can improve the predictive capability of these events, and then to understand if such forecasts can be suitable for air traffic management purposes. The intense squall line that affected Malpensa, the major airport by passenger traffic in northern Italy, on 11 May 2019 is selected as a benchmark. Several numerical experiments are performed with a Weather Research and Forecasting (WRF) model using two assimilation techniques, 3D-Var in WRF Data Assimilation (WRFDA) system and a nudging scheme for lightning, in order to improve the forecast accuracy and to evaluate the impact of assimilated different datasets. To evaluate the numerical simulations performance, three different verification approaches, object-based, fuzzy and qualitative, are used. The results suggest that the assimilation of lightning data plays a key role in triggering the convective cells, improving both location and timing. Moreover, the numerical weather prediction (NWP)-based nowcasting system is able to produce reliable forecasts at high spatial and temporal resolution. The timing was found to be suitable for helping Air Traffic Management (ATM) operators to compute alternative landing trajectories.

High-resolution NU-WRF simulations of a deep convective-precipitation system during MC3E: Further improvements and comparisons between Goddard microphysics schemes and observations.

The Goddard microphysics was recently improved by adding a fourth ice class (frozen drops/hail). This new 4ICE scheme was developed and tested in the Goddard Cumulus Ensemble (GCE) model for an intense continental squall line and a moderate, less organized continental case. Simulated peak radar reflectivity profiles were improved in intensity and shape for both cases, as were the overall reflectivity probability distributions versus observations. In this study, the new Goddard 4ICE scheme is implemented into the regional-scale NASA Unified-Weather Research and Forecasting (NU-WRF) model, modified and evaluated for the same intense squall line, which occurred during the Midlatitude Continental Convective Clouds Experiment (MC3E). NU-WRF simulated radar reflectivities, total rainfall, propagation, and convective system structures using the 4ICE scheme modified herein agree as well as or significantly better with observations than the original 4ICE and two previous 3ICE (graupel or hail) versions of the Goddard microphysics. With the modified 4ICE, the bin microphysics-based rain evaporation correction improves propagation and in conjunction with eliminating the unrealistic dry collection of ice/snow by hail can replicate the erect, narrow, and intense convective cores. Revisions to the ice supersaturation, ice number concentration formula, and snow size mapping, including a new snow breakup effect, allow the modified 4ICE to produce a stronger, better organized system, more snow, and mimic the strong aggregation signature in the radar distributions. NU-WRF original 4ICE simulated radar reflectivity distributions are consistent with and generally superior to those using the GCE due to the less restrictive domain and lateral boundaries.

Impacts of Rainstorms during Austral Winter in Sao Paulo State, Brazil: A Case Study

September 2015 was an atypical austral winter rainy month. Rainstorms were observed in Sao Paulo State, Brazil (53oW-44oW; 20oS-25oS) with catastrophic consequences. In this paper the atmospheric conditions responsible for the rainstorms that occurred on 8th September 2015 in Sao Paulo State and their social and economic consequences were analyzed. The results showed that the rainstorms affected practically the entire Sao Paulo State causing floods and threw down trees, interrupting of the traffic on the streets and affecting the system of electric energy. Rainstorms accompanied by high winds and strong divergence at 200 hPa and ascending motion were observed due to intense squall line moving over Sao Paulo State. Satellite and radar images showed strong convective activity and the heavier rainfall (around 90 mm) in the center-east sector at 20:45 UTC. Hourly data from meteorological stations showed that the wind gust and the precipitation were higher than 65 km h-1 and 10 mm, respectively in almost the entire State. In Sao Paulo city, the rainfall was the highest for September in the last 20 years. The amount of precipitation was 3% higher than the amount expected for the entire September. Since each region of inundation in Sao Paulo city causes a daily damage higher than US$ 263,000,00 a great economic harm must be expected. A co-operative project between National Institute for Space Research (INPE) institution and Energisa Power Company is in progress aimed to mitigate the effects of adverse weather conditions as in the case of the rainstorm that occurred in 8th September 2015.

Interactions between cumulus convection and its environment as revealed by the MC3E sounding array

This study attempts to understand interactions between midlatitude convective systems and their environments through a heat and moisture budget analysis using the sounding data collected from the Midlatitude Continental Convective Clouds Experiment (MC3E) in central Oklahoma. Distinct large-scale structures and diabatic heating and drying profiles are presented for cases of weaker and elevated thunderstorms as well as intense squall line and supercell thunderstorm events during the campaign. The elevated cell events were nocturnal convective systems occurring in an environment having low convective available potential energy (CAPE) and a very dry boundary layer. In contrast, deeper convective events happened during the morning into early afternoon within an environment associated with large CAPE and a near-saturated boundary layer. As the systems reached maturity, the diagnosed diabatic heating in the latter deep convective cases was much stronger and of greater vertical extent than the former. Both groups showed considerable diabatic cooling in the lower troposphere, associated with the evaporation of precipitation and low-level clouds. The horizontal advection of moisture also played a dominant role in moistening the lower troposphere, particularly for the deeper convective events, wherein the near surface southeasterly flow allows persistent low-level moisture return from the Gulf of Mexico to support convection. The moisture convergence often was present before these systems develop, suggesting a strong correlation between the large-scale moisture convergence and convection. Sensitivity tests indicated that the uncertainty in the surface precipitation and the size of analysis domain mainly affected the magnitude of these analyzed fields rather than their vertical structures.

Anticipating a Rare Event Utilizing Forecast Anomalies and a Situational Awareness Display: The Western U.S. Storms of 18–23 January 2010

From 18 January to 23 January 2010, a series of winter storms impacted the western United States. During this period, a record-setting system produced severe convection, high winds, and heavy rain and snow on 21–22 January. The severe weather included tornadoes in California and gusts in excess of 40 m s−1 associated with an intense squall line affecting southeast California and Arizona. One of the primary impacts of the storms was a heavy precipitation event across Arizona. Rainfall amounts of 125–250 mm were recorded along the Mogollon Rim in central Arizona, while higher elevations in northern Arizona received 100–150 cm of snow, with one site setting the state's 24-h snowfall record. The heavy snow and high winds resulted in widespread power outages and paralyzed travel across portions of northern Arizona. All-time minimum pressure records were set across a large portion of the western United States from Oregon to Arizona. This was an extraordinary event that was well predicted. Standardized anomalies...

Mesoscale and radar analysis of the 30 June 1998 supercell

On 30 June 1998 an intense squall line swept the central-eastern Po valley: large hail, damaging winds and a supercell were observed with these storms. The forcing due to a baroclinic wave disturbance created the synoptic conditions favourable for storm development. A mesoscale analysis, conducted using the Local Analysis and Prediction System (LAPS), shows the characteristic circulation in the wind field close to the ground from the western and the eastern borders of the Po valley, advecting humidity and increasing the wind shear. Moreover, analysis of temperature and humidity permits to follow movement of the system its across the valley. Reflectivity and Doppler wind fields observed by the C-band radar located at S. Pietro Capofiume, near Bologna, document the position and the intensity of precipitation and the strong rotational pattern associated with supercell. Dual polarisation permits to infer characteristics on the type of the hydrometeors.

Bayesian estimation of precipitating cloud parameters from combined measurements of spaceborne microwave radiometer and radar

The objective of this paper is to evaluate the potential of a Bayesian inversion algorithm using microwave multisensor data for the retrieval of surface rainfall rate and cloud parameters. The retrieval scheme is based on the maximum a posteriori probability (MAP) method, extended for the use of both spaceborne passive and active microwave data. The MAP technique for precipitation profiling is also proposed to approach the problem of the radar-swath synthetic broadening; that is, the capability to exploit the combined information also where only radiometric data are available. In order to show an application to airborne data, two case studies are selected within the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). They refer to a stratiform storm region and an intense squall line of two mesoscale convective systems, which occurred over the ocean on February 20 and 22, 1993, respectively. The estimated rainfall rates and columnar hydrometeor contents derived from the proposed algorithms are compared to each other and to radar estimates based on reflectivity-rainrate (Z-R) relationships. Results in terms of reflectivity profiles and upwelling brightness temperatures, reconstructed from the estimated cloud structures, are also discussed. A database of combined measurements acquired at nadir during various TOGA-COARE flights, is used for applying the radar-swath synthetic broadening technique in the case of an along-track radar-failure countermeasure. A simulated test of the latter technique is performed using the case studies of February 20 and 22, 1993.

Shipborne Dual-Doppler Operations during TOGA COARE: Integrated Observations of Storm Kinematics and Electrification

Abstract Shipborne Doppler radar operations were conducted over the western Pacific warm pool during TOGA COARE using the Massachusetts Institute of Technology and NOAA TOGA C-band Doppler radars. Occasionally the ships carrying these radars were brought to within 50 km of each other to conduct coordinated dual-Doppler scanning. The dual-Doppler operations were considered a test of the logistical and engineering constraints associated with establishing a seagoing dual-Doppler configuration. A very successful dual-Doppler data collection period took place on 9 February 1993 when an oceanic squall line developed, intensified, and propagated through the shipborne dual-Doppler lobes. Later on the same day, NOAA P-3 aircraft sampled a more intense squall line located approximately 400 km to the southeast of the shipborne operations. This study provides an overview of the shipborne dual-Doppler operations, followed by a comparison of the kinematic and precipitation structures of the convective systems sampled b...

The Surface-Wind Response to Transient Mesoscale Pressure Fields Associated with Squall Lines

Abstract The two most prominent surface pressure features associated with squall lines are 1) a surface mesohigh, centered within the heavy-rain region, and 2) a wake low, located at the back edge of the trailing area of light rainfall. The surface flow near these features is often highly unbalanced due to propagation and transience of the pressure fields. Centers of surface divergence and convergence are typically displaced rearward of the mesohigh and wake-low axes, respectively. In an attempt to explain the basic mesoscale characteristics of the surface flow in the vicinity of squall lines, a simplified model of a propagating mesohigh-wake-low couplet is developed using a one-dimensional slab model of the boundary layer. The component of the flow normal to the squall line is predicted, with advective and frictional effects included but the Coriolis force neglected. Model results are compared to the observed airflow near the mesohigh and wake low associated with an intense squall line that moved through...

Heat and Moisture Budgets of an Intense Midlatitude Squall Line

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

Mesoscale Momentum Budget in a Midlatitude Squall Line: A Numerical Case Study

Abstract A mesoβ-scale momentum budget and its effect on larger-scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10–11 June 1985 PRE-STORM. It is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the mesoβ-scale internal structure and evolution of the squall line. The momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along-line flow. Both vertical and horizontal momentum advection have significant contributions, particularly to the vertical mixing of the along-line flow; and this component of the horizontal momentum is locally transported down-gradient. It is also found that for midlatitude convective systems, convectively generated downdrafts can play as prominent a role as updrafts i...

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

Abstract An intense rear-inflow jet, surface pressure perturbations, and stratiform precipitation associated with a squall line during 10–11 June 1985 are examined using a three-dimensional mesoscale nested-grid model. It is found that the large-scale baroclinity provides favorable and deep rear-to-front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow. However, latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow. The role of the rear inflow is generally to produce convergence ahead and divergence behind the system, and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions. In this case study, the rear-inflow jet appears to have caused the splitting of the surface wake low as well as the organized ...

Numerical Simulation of an Intense Squall Line during 10–11 June 1985 PRE-STORM. Part I: Model Verification

Abstract A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-diverg...

Thunderstorm Cloud Top Observations Using Satellite Stereoscopy

The present investigation has as objective to take a detailed look at the intense squall line over Oklahoma on May 2-3, 1979, using GOES stereoscopy combined with GOES infrared data. The synoptic situation and data sources are considered along with the stereoscopically observed cloud top ascent rates. Cloud top observations of intense thunderstorms are discussed, taking into account a contouring technique, the interpretation of infrared cloud top temperature patterns, and small-scale structure and its variability. It is found that GOES IR cloud top temperatures grossly underestimate the actual cloud top height observed stereoscopically, especially for immature storms. It is difficult to define growing storms below about 10 km in the GOES infrared data.

Lightning maps reveal makeup of storm

The clearest picture yet of the electrical activity that evolved from a tornado‐producing thunderstorm shows that cloud‐to‐ground lightning hugged the leading edge of the storm center and became active only after the tornado died off.This data has been gained from a National Oceanic and Atmospheric Administration study of last spring's major tornado in Lawton, Oklahoma. The storm was one of 11 produced when an intense squall line formed in northern Texas and central Oklahoma. The severe storm activity on April 10, 1979, killed 54 persons in Lawton, and in Wichita Falls and Vernon, Texas.

A Life History of an Intense Mesoscale Convective Storm in Oklahoma

Abstract A simple automated objective analysis scheme is developed to analyze upper air sounding data from the National Severe Storm Laboratory mesonetwork. This scheme uses a combination of Cressman' successive correction technique and cubic spline curve fitting. This scheme is applied to a squall line case that occurred on 8 June 1966 along the confluent line where and air from the desert southwest was brought into juxtaposition with moist air from the Gulf of Mexico. The first radar echoes were detected at about 1600 CST and a nearly continuous band was formed by 1830. At 2000 the intense squall line became disorganized and it dissipated by 2300. Serial sounding were started at 1400 and continued until 2300. The analysis results indicate that a well–defined narrow band of convergence (and consequently upward motion) with a width of about 100 km was present at low levels prior to the appearance of first radar echoes. The location and orientation of the line of maximum upward motion and of the subsequent...

VARIATIONAL SUBSYNOPTIC ANALYSIS WITH APPLICATIONS TO SEVERE LOCAL STORMS

Sasaki's variational analysis method is used to describe the subsynoptic surface conditions accompanying severe local storms. Observations are extracted from the network of surface stations that routinely report every hour. The variational analysis filters the observations by constraining the meteorological fields to satisfy a set of governing prognostic equations. The filtering is monotonic and is designed to admit space and time scales of the order of 500 km and 10 hr, respectively. The analysis is applied to a severe storm situation on June 10, 1968. The development of an intense squall line from the incipient to mature stage is depicted by an index coupling vertical motion and surface moisture. The results demonstrate that dynamically consistent time continuity can be achieved by using the variational method.