Single Cell Thunderstorms METR 4803 Hazardous Weather Detection and Prediction Spring, 2005 Semester Kelvin K. Droegemeier School of Meteorology University of Oklahoma

Thunderstorm n Definition: By definition, a thunderstorm is a local storm, invariably produced by a cumulonimbus cloud, that always is accompanied by lightning and thunder. It usually contains strong gusts of wind, heavy rain, and sometimes hail. Meteorologists often use the word "convection" to describe such storms in a general manner, though the term convection specifically refers to the motion of a fluid resulting in the transport and mixing of properties of the fluid. To be more precise, a convective cloud is one which owes its vertical development, and possibly its origin, to convection (upward air currents).

Thunderstorm Climatology n At any given time there are an estimated 2000 thunderstorms in progress, mostly in tropical and subtropical latitudes. About 45,000 thunderstorms take place each day. Annually, The U.S. experiences about 100,000 thunderstorms. About 16 million thunderstorms occur annually around the world!

Thunderstorm Climatology (storms per year)

Hail Climatology

Hail Days Climatology Courtesy H. Brooks, National Severe Storms Laboratory

Lightning Climatology

Wind Days Climatology Courtesy H. Brooks, National Severe Storms Laboratory

Modes of Convection / Storm Classification n Although a continuous spectrum of storms exists, meteorologists find it convenient to classify storms into specific categories according to their structure, intensity, environments in which they form, and weather produced. n Although a continuous spectrum of storms exists, meteorologists find it convenient to classify storms into specific categories according to their structure, intensity, environments in which they form, and weather produced.

Basic Classification n Single-cell or air-mass storm Typically lasts minutes. Pulse storms can produce severe weather elements such as downbursts, hail, some heavy rainfall and occasionally weak tornadoes. n Multicell cluster storm A group of cells moving as a single unit, with each cell in a different stage of the thunderstorm life cycle. Multicell storms can produce moderate size hail, flash floods and weak tornadoes. n Multicell Line (squall line) Storms - consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. Also known as squall lines, these storms can produce small to moderate size hail, occasional flash floods and weak tornadoes. n Supercells Defined as a thunderstorm with a rotating updraft, these storms can produce strong downbursts, large hail, occasional flash floods and weak to violent tornadoes.

Key Ingredients for Thunderstorms Static Instability Warm Air Cold Air Warm Moist Air Cold Dry Air

Convection and Buoyancy n Convection: transport of fluid properties by motions within that fluid n Buoyancy: vertically oriented force on a parcel of air due to density differences between between the parcel and surrounding air –Mathematically, the buoyancy force can be derived from the vertical equation of motion (we’ll do this later)

Vertical Wind Shear; Change in wind speed and/or direction with height; Severe storms need strong veering of wind with height and strong increase in speed Key Ingredients for Thunderstorms

n Mechanism to trigger the instability –Front –Terrain –Dryline –Daytime heating –Landmass inhomogeneities Key Ingredients for Thunderstorms

Types of Thunderstorms n Ordinary single storms –Most common –Last for less than an hour –Built-in self-destruct mechanism! –Occur all year long, mostly in summer –Can produce strong winds, hail, and lightning

Air Mass Thunderstorms n First studied just after World War II n Many commercial and military aircraft accidents n Newly developed radar was exploited for weather studies n The Thunderstorm Project n Resulted in first life cycle of a thunderstorm n Air mass thunderstorms are also referred to as “Garden Variety!”

Conditions of Formation of Air Mass Thunderstorms n Conditional instability (we’ll come to that later) n Warm, moist air near the ground n Localized source of lift (usually thermally driven) n Weak or no environmental vertical wind shear

Wind Shear n Definition: – –The change in the direction or speed of the wind over a distance. n Vertical Wind Shear – –The change with height in the direction or speed of the horizontal wind. n Low wind shear indicates little change in direction or speed of the wind over a distance.

Weak Wind Shear 7 kts 6 kts 7 kts 6 kts There is very little change in the speed or direction of the wind with height. Height

Three stages of single-cell storm development Developing stage Mature Stage Dissipating Stage

Example of Single-cell Life Cycle (a) – Developing (b) – Mature (c) Mature (d) – Dissipating

Cumulus Phase n Development of towering cumulus – –Region of low level convergence – –Warm moist air – –Updraft driven by latent heating n Nearby cumulus may merge to form a much larger cloud n Dominated by updraft n Mixing and entrainment occur in the updraft

Cumulus Phase c William Zender (2001) c David Shohami

Entrainment n Entrainment is the process by which saturated air from the growing cumulus cloud mixes with the surrounding cooler and drier (unsaturated) air. n Entrainment causes evaporation of the exterior of the cloud and tends to reduce the upward buoyancy there.

Mature Phase n Precipitation, formed by the Bergeron cold rain process, begins to reach the ground. n The precipitation drags some of the surrounding air down creating the downdraft.

Mature Phase

Downdraft n The downdraft is the descending column of air in a thunderstorm. n Created and maintained by three processes – –Evaporational cooling of entrained air – –Downward drag caused by falling precipitation – –Evaporational cooling of the air below the cloud base

Downdraft n When the downdraft reaches the ground, it spreads out in all directions. n The leading edge of this cold, often gusty wind is called the outflow boundary or gust front.

Reflectivity Radial Velocity 0.5 deg Elevation, 04:28 UTC LIT

Reflectivity Radial Velocity 0.5 deg Elevation, 04:34 UTC LIT

Reflectivity Radial Velocity 0.5 deg Elevation, 04:40 UTC LIT

Reflectivity Radial Velocity 0.5 deg Elevation, 04:34:12 UTC LIT Main body of storm (second part) Gust front; First part of storm

First part of the storm Second part of the storm LIT Radar Time = 04:34:12 UTC Radar LIT Heavy Precip Doppler Radial Velocity Reflectivity First part of the storm Second part of the storm

First part of the storm Second part of the storm LIT Radar Time = 04:40:02 UTC LIT Radar Heavy Precip Doppler Radial Velocity Reflectivity First part of the storm Second part of the storm

Gust Front Shelf Cloud National Severe Storms Laboratory

Downdraft n The outflow boundary behaves like a cold front: – –Strong wind shift (speed and direction) – –Much colder air behind the gust front – –Acts as a location for additional lift for future storm development. New Storm

Mature Phase n The mature phase represents the peak intensity of the storm. n Updrafts and downdrafts are about equal in strength. n Precipitation is typically heavy and may contain small hail n Gusty winds result from the downdraft spreading out on the ground. n The anvil, or cloud top, begins to turn to ice, or glaciate.

Mature Phase

Dissipating Phase n Eventually the downdraft overwhelms the updraft and convection collapses – because the cloud is vertically-oriented n Precipitation becomes lighter and diminishes. n Cloud begins to evaporate from the bottom up often leaving behind an “orphan anvil.” – –Cirrus Spissatus cumulonimbogenitus

Air Mass Thunderstorms n Usually weak (but can produce heavy rain in a short period of time). n Usually not severe n Usually move slowly (weak winds aloft) n Often develop and dissipate in less than one hour n Form in a weakly sheared environment and thus have a BUILT-IN SELF-DESTRUCT MECHANISM that guarantees a short lifetime

Life Cycle of Single Cell/Airmass Storm Visual Radar

Basic Concept of Buoyancy n Vertically oriented force on a parcel of air due to density differences between between the parcel and surrounding air n Now to the blackboard!!

Impact of Pressure Gradient Force n When an air parcel rises (due to buoyancy), it has to push through air above it, creating higher pressure (positive p’) above (imagine pushing yourself through a crowd – or drafting of race cars) n Below the rising parcel, a void is created, leading to lower pressure at the cloud base H L PGF

n The higher pressure above will push air to the side, making room for the rising parcel, while the lower pressure below “attracts” surrounding air to compensate for the displaced parcel n Such a positive-negative pattern of p perturbation creates a downward pressure gradient. The PGF force therefore opposes the buoyancy force, and therefore acts to reduce the net upward forcing. Impact of Pressure Gradient Force

n The degree of opposition to the buoyancy force depends upon the aspect ratio of the cloud (L/H), or more accurately of the updraft. This aspect ratio dependence ties directly into the degree of validity of the hydrostatic assumption (see Bluestein Vol. II ) n The effect is larger for a wider/large aspect-ratio cloud, and weaker for a narrower/small aspect ratio cloud, because –For a narrow cloud, a small amount of air has to be displaced/attracted by the rising parcel, therefore the p perturbation needed to achieve this is smaller, so that the opposing pressure gradient is smaller (often << buoyancy) so a narrow cloud can grow faster –PGF is stronger for a wide cloud: as a result, the net upward force (buoyancy – PGF) is significantly reduced, and the cloud can only grow slowly. When B and PGF have similar magnitude, the vertical motion becomes quasi-hydrostatic – this is typical of large scale broad ascent. n Dynamic stability analysis of inviscid flow shows that the infinitely narrow clouds grow the fastest, but in reality, the presence of turbulent mixing prevents the cloud from becoming too narrow, hence the typical aspect ratio of clouds is ~ 1. Impact of Pressure Gradient Force

Hazards of Air Mass Thunderstorms n Heavy Rain n Hail – –Usually not terribly large – –May be numerous n Downbursts or Microbursts – –Exceptionally strong downdrafts that, when they hit the earth, may have potentially destructive winds associated with them.

Hail Produced by an Ordinary Thunderstorm

Downbursts and Microbursts n Microburst – –An anomalously strong, concentrated downdraft that produces a pocket of dangerous wind shear near the ground over an area of 4 km or less in horizontal extent. – –Very short lived (last for 3-8 minutes) – –Very small and isolated (city block) n Associated with cumulonimbus clouds – –Can have heavy rain (Wet microbursts) – –Can have vanishing sprinkles (Dry microbursts)

Microburst

Microburst

Microburst

Dry and Wet Microbursts

Dry Microbursts n A microburst with little or no precipitation. n Very dry air is located beneath the cloud base. n Hydrometeors falling into the dry air will evaporate causing a pool of cold air just below cloud base. n This cold pool descends rapidly forming the dry microburst. n Often you can’t detect them until it is too late.

Dry Microburst

Wet Microbursts n Microbursts associated with moderate or heavy precipitation. n Some dry air above cloud top gets entrained in the top of the thunderstorm. n This dry air mixes with cloud air causing some evaporation of the cloud. n Evaporational cooling will form a pool of cold air near the top of the cloud. n This cold pool descends and adds to the downdraft to form a microburst. n Often there is a “rain gush” coincident with the microburst.

Wet Microburst

Microburst Damage

Visual Detection

Detection of Microbursts n Doppler Radar (Airport and Aircraft) – –Best when precipitation is present – –Terminal Doppler Weather Radar (TDWR)

Integrated Terminal Weather System (ITWS)

Detection of Microbursts n LLWAS – –Low level wind shear alert system – –A network of wind sensors positioned around the airport. – –Does not detect elevated microbursts or microbursts that are between sensors.

Microbursts and Aviation n Microbursts are extremely hazardous to low-flying aircraft because of –Low airpseed –Proximity to the ground –“Dirty” aerodynamic configuration (flaps out, gear down) –Difficulty of visual microburst detection –Rapid onset and short duration

Microburst Glide Slope Runway

Microburst Glide Slope Runway

Microburst Glide Slope Runway

Microburst Glide Slope Runway

Microburst Glide Slope Runway

Flight of Eastern 902

Flight of Eastern 66

Fatalities Associated with Aviation Wind Shear Accidents Wind Shear R&D Pilot Training TDWR