Knowledge of severe local storms has been increasing rapidly in recent years as a result of both observational studies and numerical modeling experiments. This paper reviews that knowledge as it relates to development of new applications for forecasting of severe local storms. Many of these new applications are based on physical understanding of processes taking place on the storm scale and thus allow forecasters to become less dependent on empirical relationships. Refinements in pattern recognition and severe weather climatology continue to be of value to the operational severe local storms forecasters, however. Current methodology for forecasting severe local storms at the National Severe Storms Forecast Center is described. Operational uses of new forecast applications, new “real-time” data sources (such as wind profilers and Doppler radars), and improved numerical model products are discussed.

Severe Local Storms Forecasting

The advent of kite and balloon-borne meteorograph soundings during the early 1900s and the subsequent deployment of regional rawinsonde networks provided the observational basis for the study of the spatial and temporal evolution of fronts, jet streams and the tropopause. During the mid-century years (1935–1965), researchers focused on the structural characteristics of fronts and their associated jet streams near the tropopause, and on the diagnosis of the frontogenetic processes and secondary circulations governing their life cycles. The pioneering observational study by J. Bjerknes and E. Palm n (1937) showed fronts to be transitional zones of finite width (~100 km) and depth (~1 km), rather than near zero-order discontinuities extending from the surface to the tropopause. Newton (1954) presented the most comprehensive diagnosis of all components of upper-level frontogenesis during this period, and Sawyer (1956) and Eliassen (1962) derived the diagnostic theory for geostrophically forced secondary circulations about fronts based on the semigeostrophic equations, which was later expanded to the temporal dimension by Hoskins (1971) and Hoskins and Bretherton (1972).

Fronts, Jet Streams and the Tropopause

Results are presented from a high-resolution three-dimensional simulation of shallow-to-deep convection transition based on idealization of observations made during the Large-Scale Biosphere–Atmosphere (LBA) experiment in Amazonia, Brazil, during the Tropical Rainfall Measuring Mission (TRMM)-LBA mission on 23 February. The doubly periodic grid has 1536 1536 256 grid cells with horizontal grid spacing of 100 m, thus covering an area of 154 154 km 2 . The vertical resolution varies from 50 m in the boundary layer to 100 m in the free troposphere and gradually coarsens to 250 m near the domain top at 25.4 km. The length of the simulation is 6 h, starting from an early morning sounding corresponding to 0730 local time. Convection is forced by prescribed surface latent and sensible heat fluxes and prescribed horizontally uniform radiative heating Despite a considerable amount of convective available potential energy (CAPE) in the range of 1600– 2400 J kg 1 , and despite virtually no convective inhibition (CIN) in the mean sounding throughout the simulation, the cumulus convection starts as shallow, gradually developing into congestus, and becomes deep only toward the end of simulation. Analysis shows that the reason is that the shallow clouds generated by the boundary layer turbulence are too small to penetrate deep into the troposphere, as they are quickly diluted by mixing with the environment. Precipitation and the associated cold pools are needed to generate thermals big enough to support the growth of deep clouds. This positive feedback involving precipitation is supported by a sensitivity experiment in which the cold pools are effectively eliminated by artificially switching off the evaporation of precipitation; in the experiment, the convection remains shallow throughout the entire simulation, with a few congestus but no deep clouds. The probability distribution function (PDF) of cloud size during the shallow, congestus, and deep phases is analyzed using a new method. During each of the three phases, the shallow clouds dominate the mode of the PDFs at about 1-km diameter. During the deep phase, the PDFs show cloud bases as wide as 4 km. Analysis of the joint PDFs of cloud size and in-cloud variables demonstrates that, as expected, the bigger clouds are far less diluted above their bases than their smaller counterparts. Also, thermodynamic properties at cloud bases are found to be nearly identical for all cloud sizes, with the moist static energy exceeding the mean value by as much as 4 kJ kg 1 . The width of the moist static energy distribution in the boundary layer is mostly due to variability of water vapor; therefore, clouds appear to grow from the air with the highest water vapor content available. No undiluted cloudy parcels are found near the level of neutral buoyancy. It appears that a simple entraining-plume model explains the entrainment rates rather well. The least diluted plumes in the simulation correspond to an entrainment parameter of about 0.1 km 1 .

High-Resolution Simulation of Shallow-to-Deep Convection Transition over Land

Intense, small-scale divergent outflows known as microbursts are held responsible for a number of aircraft accidents. This paper describes the morphology of microburst outflows observed in Colorado. Outflows are categorized into morphological types based on analysis of observation by Doppler radars and a surface meteorological network. Outflow life cycle is discussed, and the vertical and horizontal structure is described. Basic characteristics of microburst outflows are summarized from statistics compiled using both single and multiple Doppler analyses. The microburst outflows are classified into two types: individual microbursts and microburst lines. Examples of observations of each type are shown. Organization of microbursts into microburst lines results in much longer-lasting wind shear than exists with isolated microbursts. The greater lifetime of microburst lines, combined with the much larger area of divergence, can create a much greater potential for hazard to aircraft than is the case fo...

Structure and Life Cycle of Microburst Outflows Observed in Colorado

The International H2O Project (IHOP_2002) included four complementary research components: quantitative precipitation forecasting, convection initiation, atmospheric boundary layer processes, and instrumentation. This special issue introductory paper will review the current state of knowledge on surface-forced convection initiation and then describe some of the outstanding issues in convection initiation that partially motivated IHOP_2002. Subsequent papers in this special issue will illustrate the value of combining varied and complementary datasets to study convection initiation in order to address the outstanding issues discussed in this paper and new questions that arose from IHOP_2002 observations. The review will focus on convection initiation by boundaries that are prevalent in the U.S. southern Great Plains. Boundary layer circulations, which are sometimes precursors to deep convective development, are clearly observed by radar as reflectivity fine lines and/or convergence in Doppler velo...

A Review of Convection Initiation and Motivation for IHOP_2002

This paper presents the time-dependent analysis of the thunderstorm gust front with the use of Project NIMROD data. RHI cross sections of reflectivity and Doppler velocity are constructed to determine the entire vertical structure. The life cycle of the gust front is divided into four stages: (1) the formative stage; (2) the early mature stage; (3) the late mature stage; and (4) the dissipation stage. A new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground, while carrying some of the smaller precipitation ahead of the main echo core of the squall line. This feature is called a 'precipitation roll'. As determined from rawinsonde data, the cold air behind the gust front accounts for the observed surface pressure rise. Calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow. The equation governing the propagation speed of a density current accurately predicts the movement of the gust front.

The life cycle of thunderstorm gust fronts as viewed with Doppler radar and rawinsonde data

An attempt is made to estimate wind speed in a series of windstorms, which occurred in a 50-km wide zone from Chicago to Detroit on July 16, 1980, based on three types of airborne objects: a 180 kg chimney, a 1000 kg corn storage bin, and lumber from damaged roofs. The maximum wind speed obtained is 63 + or - 10 m/sec, or 140 + or - 25 mph. SMS/GOES pictures show that the parent cloud was oval-shaped, with a lifetime in excess of 12 hours. That the downbursts began when overshooting activities subsided is indicated by the rapid shrinking of overshooting areas enclosed by -66 C isotherms at the onset of the Chicago-area downbursts. Cloud-top features and wind effects on the ground are presented with no attempt to relate them, on the basis of current conceptual models.

Five Scales of Airflow Associated with a Series of Downbursts on 16 July 1980

A remarkable case of severe weather occurred near Springfield, Illinois on 6 August 1977. Aerial and ground surveys revealed that 17 cyclonic vortices, an anticyclonic vortex, 10 downbursts and 19 microbursts occurred in a limited (20 km x 40 km) area, associated with a bow-shaped radar echo. About half of the vortices appeared to have occurred along a gust front. Some of the others appear to have occurred within the circulation of a mesocyclone accompanying the bow echo, but these vortices seem to have developed specifically in response to localized boundary-layer vorticity generation associated with horizontal and vertical wind shears on the periphery of microbursts. Some of these vortices, and other destructive vortices in the literature, do not qualify as tornadoes as defined in the Glossary of Meteorology. A more pragmatic definition of a tornado is suggested.

A Concentrated Outbreak of Tornadoes, Downbursts and Microbursts, and Implications Regarding Vortex Classification

This paper presents an analysis of a thunderstorm system that spawned a downburst and an F4 anticyclonictornado in the West Bend, Wisconsin area in the early morning of 4 April 1981. The tornado caused threefatalities and was one of the strongest occurring in the United States during 1981. A weakly defined bowecho of level 3 intensity was observed by the radar located at Neenah, Wisconsin during the storm. Thethunderstorm tops were measured to only 7900 m (26 000 ft) and no apparent severe weather signature inthe infrared satellite imagery could be detected. The occurrence of this severe storm event is seen to haveimplications on present and future short-term operational forecasting technology.

The West Bend, Wisconsin Storm of 4 April 1981:A Problem in Operational Meteorology

Data from 27 PAM (Portable Automated Mesonet) stations, operational as a phase of project NIMROD (Northern Illinois Meteorological Research on Downburst), are presented. It was found that PAM-measured winds are influenced by the mesoscale obstruction of the Chicago metropolitan area, as well as by the misoscale obstruction of identified trees and buildings. The mesoscale obstruction was estimated within the range of near zero to 50%, increasing toward the city limits, while the misoscale obstruction was estimated as being as large as 58% near obstructing trees which were empirically calculated to cause a wind speed deficit 50-80 times their height. Despite a statistical analysis based on one-million PAM winds, wind speed and stability transmission factors could not be accurately calculated; thus, in order to calculate the airflow free from obstacle, PAM-measured winds must be corrected.

Roger M. Wakimoto

Papers

The life cycle of thunderstorm gust fronts as viewed with Doppler radar and rawinsonde data

Five Scales of Airflow Associated with a Series of Downbursts on 16 July 1980

A Concentrated Outbreak of Tornadoes, Downbursts and Microbursts, and Implications Regarding Vortex Classification

The West Bend, Wisconsin Storm of 4 April 1981:A Problem in Operational Meteorology

Effects of Miso- and Mesoscale Obstructions on PAM Winds Obtained during Project NIMROD