Single Cell Thunderstorms METR 4433 Mesoscale Meteorology Spring, 2008 Semester Adapted from Materials by Drs. Frank Gallagher III, Kelvin Droegemeier and Ming Xue School of Meteorology University of Oklahoma

Textbook materials for reading Bluestein Vol II pp. 444-462 Houze pp. 35-37 Cotton and Anthes p. 455-458

Thunderstorm 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 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 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 Single-cell or air-mass storm Typically lasts 20-30 minutes. Pulse storms can produce severe weather elements such as downbursts, hail, some heavy rainfall and occasionally weak tornadoes. 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. 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. 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.

                                                                                               

Convection and Buoyancy Convection: transport of fluid properties by motions within that fluid 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 Let’s derive it now!

Perturbation Vertical Momentum Equation (we just derived a very similar form) where L = liquid+ice water content. Note that the water vapor contributes to the buoyancy of air parcel and the combined effect can be expressed in terms of virtual potential temperature in the formulation we derived

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

Convective Available Potential Energy (CAPE) CAPE measures the amount of convective instability, or more accurately the potential energy of an environmental sounding – the energy that can be converted into kinetic energy when an air parcel rises from LFC to EL It is based on simple parcel theory which neglects the effect of mixing/friction, PGF and sometimes water loading. (The vertical momentum equation becomes simply Dw/Dt = B) From CAPE, we can estimate the maximum vertical velocity that can be reached by a parcel

CAPE

CAPE on a Skew-T diagram The positive area (where air parcel is warmer than environment) is equal to CAPE Lifted Index – temperature excess in 500mb environment over that of a parcel lifted from the low ‘moist’ layer (negative value indicates instability)

LCL – Lifting condensation level LFC – Level of free convection – level at which the parcel is warmer than the Environment EL – Equilibrium Level – level at which the parcel’s T becomes the same as the environment again

Skew-T analysis and Parcel Theory Skew-T analysis and Parcel Theory typically neglect the effect of PGF induced by vertical motion, essentially they assume that the environment is unchanged by the parcel motion. They also neglect the effect of mixing/friction Therefore, parcel theory tends to overestimate the intensity of the updraft. Still, it provides a useful upper limit for the convection intensity.

CIN Convective Inhibition The “negative area” on a thermodynamic diagram, or the area between the lifted surface parcel and ELR curves, in the layer where the parcel is colder than the environment. It is defined as the amount of energy beyond the normal work of expansion need to lift a parcel from the surface to the Level of Free Convection (LFC).

Example of CIN

How Can CAPE Increase?

How Can CAPE Increase? Hotter surface temperature More low-level moisture Cool the mid-levels

Td T

W(surface) = 11 g/kg

W(surface) = 14 g/kg

W(surface) = 16 g/kg

Key Ingredients for Thunderstorms 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

Environmental Shear CAPE and Vertical Environmental Wind Shear are the two most important factors in determining storm type Numerical models have been very effective tools to aid our understanding of such effects In general, single cell storms occur in environments with little vertical shear ( < 15 m/s over lowest 6 km) Multicell storms occur in environments with moderate vertical shear (15 – 25 m/s) Supercell storms occur in environments with strong vertical shear ( > 25 m/s)

Numerical Experiments of Weisman and Klemp (1982) Vertical wind profiles with unidirectional shear of different magnitudes Time series of max w for 5 experiments Supercell behavior is observed with us =25, 35 and 45m/s cases – quasi-steady updraft is found

Results from us = 15m/s and 35 m/s cases Multicell case (left) with u =15 m/s and supercell case (right) with u = 35m/s. Only the southern half of the computational domain is shown because the fields are symmetric about the central E-W axis The storm splitting is a result of rotational storm dynamics (more later). The member that moves to the right of vertical shear vector is called right mover, and the other the left mover. When the shear is not unidirectional, i.e., when shear changes direction with height, one of the members will be favored, again due to rotational storm dynamics

Maximum w as a function of CAPE and shear Multicell case The vertical axis is the low-level qv – higher value corresponds to higher CAPE First cell intensity increases with CAPE and decreases with shear Second cell occurs only with moderate shear. Supercell storm occurs in stronger vertical shear. Strong updrafts can survive in supercells because of the support of pressure perturbations associated with vertical rotation which initially comes from horizontal vorcitity in the environment via tilting. Supercell case

Predicting Thunderstorm Type: The Bulk Richardson Number The Bulk Richardson Number, BRN, is a measure of the relative importance of environmental instability effects to environmental shear effects Essentially the ratio of the KE of the updraft to the KE of the inflow Denominator is really the storm-relative inflow kinetic energy (sometimes called the BRN Shear) Large values associated with single-cell “pulse storms” Smaller values associated with supercells (~ 10 – 20) Must be used with caution. If CAPE and shear both low, can still get “supercell” values of R, but storms will be weak, if they form at all.

Results from Observations and Models

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

Types of Thunderstorms 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

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

Photograph by: NSSL This is a single cell storm, looking east from about 15 miles. The storm was moving east (into the photo). Some of the anvil cloud has been left behind the storm, but the greater portion of the anvil is blowing off in advance of the storm and is not observable from this perspective. (May storm in the Texas Panhandle near Amarillo.)

This late May storm in Oklahoma, looking northeast from about 20 miles, occurred with weak to moderate vertical wind shear. It did not produce any severe weather.

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

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

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

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

Cumulus Phase c David Shohami c William Zender (2001)

Development of a Single-Cell Storm Step 1 In the absence of frontal or other forcing, daytime heating of the PBL causes the convective temperature to be reached. Thus, there is no ‘negative area’ on the skew-T diagram for an air parcel rising from the surface – the lid is broken

Development of Single Cell Storm Step 2 Updraft forms – once the air reaches the LCL, latent heat is released due to condensation: - L dqv = Cp dT For every 1g/kg of water condensed, the atmosphere warms about 3 degrees. This feeds into the buoyancy term through an increase in q’ (remember earlier vertical momentum equation?). The saturated air parcel ascends following the moist adiabat, along which the equivalent potential temperature qe is conserved. Until the ‘Equilibrium level’ is reached, the air parcel is warmer than the environment, which keeps the buoyancy positive (without the effect of water loading – see later)

Development of Single Cell Storm Step 2 – continued … When a cloud forms, some of it is carried upward by the draft and some moves out of the updraft. The ‘weight’ of this liquid water makes the air parcel heavier, this ‘water loading’ effect acts to reduce the positive buoyancy. B = g(q’/q) - g L 10 x 3/300 ~ - 10 x 0.01 kg/kg Therefore 10 g/kg of cloud or rain water will offset a 3 K temperature surplus.

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

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

Mature Phase When the cloud grows to a stage that the updraft becomes too ‘heavy’ because of water loading, it will collapse, updraft then turns into downdraft. Another important process that contributes to the collapse is the evaporative cooling. When a cloud grows, cloud droplets turn into larger rain drops that fall out of the updraft, reaching the lower level where the air is sub-saturated. The rain drops will partially evaporate in this sub-saturated air, producing evaporative cooling. This cooling enhances the downdraft. In the absence of vertical wind shear, the cell is upright, and this downdraft would then disrupt the low-level updraft, causing the cell to dissipate. This is the built-in self-destruction mechanism mentioned earlier The cold downdraft sometimes forms a cold pool that propagates away from the cell above, further removing the lifting underneath the cell

Mature Phase

Mature Phase

Downdraft The downdraft is the descending column of air in a thunderstorm. 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 When the downdraft reaches the ground, it spreads out in all directions. 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 LIT

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

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

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

“thunderstorm just NW of the airport moving through the area now” Time = 04:34:12 UTC First part of the storm Doppler Radial Velocity Radar LIT Reflectivity Second part of the storm Heavy Precip Radar LIT Second part of the storm First part of the storm “thunderstorm just NW of the airport moving through the area now”

At 0442, ATCT advises “second part of the storm moving through” Time = 04:40:02 UTC First part of the storm Doppler Radial Velocity Radar LIT Reflectivity Heavy Precip Radar Second part of the storm LIT Second part of the storm First part of the storm At 0442, ATCT advises “second part of the storm moving through”

Gust Front Shelf Cloud National Severe Storms Laboratory

Downdraft The outflow boundary behaves like a cold front: New Storm 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 The mature phase represents the peak intensity of the storm. Updrafts and downdrafts are about equal in strength. Precipitation is typically heavy and may contain small hail Gusty winds result from the downdraft spreading out on the ground. The anvil, or cloud top, begins to turn to ice, or glaciate.

Mature Phase

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

Air Mass Thunderstorms Usually weak (but can produce heavy rain in a short period of time). Usually not severe Usually move slowly (weak winds aloft) Often develop and dissipate in less than one hour 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

Impact of Pressure Gradient Force 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) Below the rising parcel, a void is created, leading to lower pressure at the cloud base H PGF L

Impact of Pressure Gradient Force 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 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 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 433-434) 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. 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.

Hazards of Air Mass Thunderstorms Heavy Rain Hail Usually not terribly large May be numerous 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

ARPS Simulation of a Single Cell Storm May 20, 1977 Del City Supercell Storm Sounding – used without environmental wind

ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=0

ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=15min

ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=20min

ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=25min

ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=30min

ARPS Simulation of a Single Cell Storm T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=45min

ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=0, 15min

ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=20, 25min

ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=30, 45min

ARPS Simulation of a Single Cell Storm Perturbation Pressure +qw+ref +wind t=45min High pressure is seen undernearth the cold pool. Gust front circulation produces strong lifting.

ARPS Simulation of a Single Cell Storm Animations Buoyancy + Wind + qw + Reflectivity Eqivalent Potential Temperature + Wind + qw + Reflectivity Perturbation Pressure + Wind + qw + Reflectivity  

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

Dry Microburst

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

Wet Microburst

Wet Microburst

Wet Microburst

Microburst Damage

Visual Detection

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

Integrated Terminal Weather System (ITWS)

Integrated Terminal Weather System (ITWS)

Detection of Microbursts 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 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