Convective Dynamics Single cell cumulus. Review of Skew-T Diagram

Convective Dynamics

Single cell cumulus

Review of Skew-TDiagram

Desirable Properties of a thermodynamic diagram:

• Area on diagram proportional to energy • As many lines representing basic processes should be ~straight • Vertical coordinate proportional to height • Dry adiabatsshould be at near right angles to isotherms • Saturation adiabats should be at large angle to dry • adiabats in lower atmosphere

The Skew T-Log P diagramwas selected by the Air Weather Service as the most convenient thermodynamic diagram for general use.

•The most commonly used diagram in the United States.•Current soundings, model soundings, and archived soundings are available in Skew-T Log-P form at several websites and in analysis programs such as GARP.

Saturation Adiabats: the path that a saturated air parcel follows as it rises pseudo- moist-adiabatically through the atmosphere.

Pseudo-moist-adiabatically: All condensed moisture immediately precipitates from parcel. Moist adiabatically: All condensed moisture remains in parcel.

Winds are plotted in standard staff/barb format on the line to the right of the diagram

http://weather.uwyo.edu/upperair/sounding.html

Thunderstorms

• 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.

• We 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).

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.

The most basic classification includes: • Single-cell or air-mass storm Typically last 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.

Ordinary or "air-mass“ storm • Main Characteristics

– Consists of a single cell (updraft/downdraft pair)

– Forms in environment characterized by large conditional instability and weak vertical shear

– Vertically erect built-in self-destruction mechanism

– Can produce strong straight-line winds or microburst

– Life cycle is generally < 1 hour, usually 30-45 min

– These storms form in weakly-forced environments, and are driven primary by convective instability rather than the ambient winds

– They are some times called "air-mass" storm because they form within air-masses with more-or-less horizontal homogeneity

– 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.)

Photograph by: NSSL

• 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.

Life cycle of a non-severe single cell storm in weak wind shear

Radar history of the severe pulse storm – often with larger instability

Example of Single-cell Life Cycle

(a) – developing stage

(b) – maturing

(c) Mature stage

(d) – dissipating stage

Basic Dynamics (forces acting on an air parcel in the vertical)

Perturbation Vertical Momentum Equation (base-state hydrostatic equations has been

subtracted off on the RHS)

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

Three stages of single-cell storm development

Towering stage Mature Stage Dissipating Stage

LCL – Lifting condensation level

LFC – Level of freeconvection – level atwhich the parcel is warmer than theEnvironment

EL – Equilibrium Level – level at which the parcel’s T becomesthe same as the environment again

Development of 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 StormStep 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 ’ (remember earlier vertical momentum equation?). The saturated air parcel ascends following the moist adiabat, along which the equivalent potential temperature e 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)

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 the simple parcel theory which neglects the effect of mixing/friction, PGF and sometimes water loading.

• From CAPE, we can estimate the maximum vertical velocity that can be reached by a parcel

CAPEdt

Skew-T• The ‘negative’ area is equal

to CIN(Convective Inhibition)

• 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)

ARPS Simulation of a Single Cell Storm

• May 20, 1977 Del City Supercell Storm Sounding – used without environmental wind

Development of Single Cell Storm

Step 2 – continued …– When cloud forms, part of it is carried upward by the draft and the

other part falls off 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(’g L

10 x 3/300 ~ - 10 x 0.01 kg/kg

Therefore 10 g/kg of cloud or rain water will offset 3 K temperature surplus.

Development and decay of Single Cell Storm

• 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 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, 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 form a cold pool that propagates away from the cell above, further removing the lifting underneath the cell

The effect of pressure gradient force• In addition to buoyancy force and water loading, another force that is also

acting on the rising parcel is the vertical pressure gradient force (PGF)

• When an air parcel rises (due to buoyancy), it has to push off air above it, creating higher pressure (positive p’) above (imagine pushing yourself through a crowd)

• Below the rising parcel, a void is created (imagine a vacuum cleaner), leading to lower pressure at the cloud base

H

L

PGF

Effect 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, therefore acts to reduce the net upward forcing.

Effect of Pressure Gradient Force

• The degree of opposition to the buoyancy force depends on the aspect ratio of the cloud (L/H), or more accurately of the updraft

• The effect is larger for wider/large aspect-ratio cloud, and weaker for narrower/small aspect ratio cloud, because

– For 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 wider clouds, as a results, the net upward force (buoyancy – PGF) is significantly reduced, the cloud can only grow slowly. When B and PGF has 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.

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 updraft. Sill, it provides a useful upper-limit for the convection intensity


T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=0


T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=15min










Perturbation Pressure +qw+ref +wind t=0, 15min






Perturbation Pressure +qw+ref +wind t=45min

High pressure is seen undernearth the cold pool. Gust front circulation produces strong lifting.

Assignment #4• Use skew-T diagram to diagnose the convectionHow much surface heating (K) would it be to generate

the free convection from near the ground. How about given more moisture, such as?

Assuming adiabatic rising without water load, all CAPE would convert into the kinetic energy in the updraft, calculate the vertical velocity when the air parcel reach the LNB (level of neutral buoyancy)

Convective DynamicsMulticell Storms and Density Currents

Photographs © Todd Lindley

http://www.srh.noaa.gov/oun/gallery/

Where are single cells and multi cells?

Multicell Storm

• Multicell cluster storm - A group of cells moving as a single unit, often 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.

Multicell Storm Weather

• Multicell severe weather can be of any variety, and generally these storms are more potent than single cell storms, but considerably less so than supercells, because closely spaced updrafts compete for low-level moisture.

• Organized multicell storms have higher severe weather potential, although unorganized multicells can produce pulse storm-like bursts of severe events.

Multiple Cells as Seen by Radar

• Radar often reflects the multicell nature of these storms, as seen with the central echo mass and its three yellowish cores in the lower portion of this picture.

• Occasionally, a multicell storm will appear unicellular in a low-level radar scan, but will display several distinct tops when a tilt sequence is used to view the storm in its upper portion

4 cellsThis one might also contain multiple cells

n+1

n-2 n-1 n

Life Cycle of Multicell Storms

Life cycle - evolution of cells in a multicell cluster

• This illustration portrays a portion of the life cycle of a multicell storm.

As cell 1 dissipates at time = 0, cell 2 matures and becomes briefly dominant. Cell 2 drops its heaviest precipitation about 10 minutes later as cell 3 strengthens, and so on.

• A closer view at T = 20 minutes (from in the earlier slide) shows that cell 3 still has the highest top, but precipitation is undercutting the updraft in the lower levels. New echo development is occurring aloft in cells 4 and 5 in the flanking line, with only light rain falling from the dissipating cells 1 and 2 on the northeast side of the storm cluster.

• The inset shows what the low-level PPI (plane-position indicator) radar presentation might look like. This storm appears to be unicellular but the several distinct echo tops tell us otherwise.

A Real Example of Multicell storm

• Here is a real storm, with radar superimposed. Observe the physical similarities to the previous slide. This Texas Panhandle storm was non-severe. Looking north-northeast from about 20 miles. Note that

the updraft numbering is reversed.

The growth of a multicell storm

Time (0-21min)

Height (3-12km)

Cell Motion versus Storm Motion• Cells inside a storm (system)

do not necessarily move at the same speed and/or direction as the overall storm system

• The storm system can move as a result of the successive growth and decay of cells

• It can also move because the cell motion

• Environmental winds can have significant influence on the cell and/or storm movement, but the storms do not necessarily follow the wind.

Cell Regeneration in Multicell storms

• Before we discuss the cell regeneration in multicell storms, we will first look the gust front dynamics, which plays an important role in long-lasting convective systems

A Schematic Model of a Thunderstorm and Its Density Current Outflow

Downdraft Circulation- Density Current in a Broader Sense

(Simpson 1997)

Role of cold pool in a convective storms

At the surface, the cold pool propagates in the form of density or gravity current

Thunderstorm Outflow /Gust Front as reviewed by the blown dust

The Gust front propagates along the surface in the form of density or gravity current

A density current, or gravity current, is a region of dense fluid propagating into an environment of less dense fluid because of the horizontal pressure gradient across the frontal surface.

1/2( / )c k p

c u=0

In the Laboratory

Salt Water

Fresh Water

Schematic of a Thunderstorm Outflow(Goff 1976, based on tower measurements)

Rotor

Numerical simulation of density currents showing the pressure perturbations associated with density current

Pressure perturbations associated with rotors / rotating

(Kelvin-Halmholtz) eddies

Pressure perturbations in the

head region and associated (rotor)

circulation

Cell Regeneration in 2D multicell storms

• Representative modeling studies

– Fovell et al (1995 JAS), Fovell and Tan (1998, MWR)

– Lin et al (1998, 2001, JAS)

Fovell, R. G., and P. S. Dailey, 1995: The temporal behavior of numerically simulated multicell-type storms, Part I: Modes of behavior. J. Atmos. Sci., 52, 2073-2095.

Fovell, R. G., and P.-H. Tan, 1998: The Temporal Behavior of Numerically Simulated Multicell-Type Storms. Part II: The Convective Cell Life Cycle and Cell Regeneration. Mon. Wea. Rev., 126, 551-577.

Lin, Y.-L., R. L. Deal, and M. S. Kulie, 1998: Mechanisms of cell regeneration, development, and propagation within a two-dimensional multicell storm. J. Atmos. Sci., 55, 1867-1886.

Lin, Y.-L., and L. Joyce, 2001: A further study of mechanisms of cell regeneration, propagation and development within two-dimensional multicell storms. J. Atmos. Sci., 58, 2957–2988.

Summary of life cycle

Based on Lin et al 1998.

Formation and maintenance of the gust front updraft (GFU)

Rearward advection of the growing GFU

Cutting off of the growing cell (c1) from the GFU by the upstream compensating downdraft

Cell generation and coexistence of the growing (c2 and c3) and propagating (c1) cells

Conceptual Model of Lin et al (1998) for Cell Regeneration

In Lin et al (1998), the following processes are believed to repeat for cell regeneration (see previous illustration).

• (i) Near the edge of the gust front, the gust front updraft is formed by the low-level convergence ahead of the gust front near the surface.

• (ii) The upper portion of the gust front updraft grows by feeding on the midlevel inflow since the gust front propagates faster than the basic wind, creating mid-level as well as low-level convergence.

• (iii) The growing cell (C1) produces strong compensating downdrafts on both sides. The downdraft on the upstream (right) side cuts off this growing cell from the gust front updraft.

• (iv) The period of cell regeneration is inversely proportional to the midlevel, storm-relative wind speed.

(a) Skew-T plot of the temperature and dewpoint profiles used in the simulations. This is a smoothed version of the 1430 HNT 22 May 1976 sounding presented in Ogura and Liou (1980) . (b) Wind profiles used to initialize the simulations (from Lin et al 1998).

Life Cycle of simulation 2D-multicell storm

Vertical cross sections of vertical velocity (thin contours in intervals of 1 m s-1) for the U = 10 m s-1 case.

The cold pool / density current may be roughly represented by the 1 K potential temperature perturbation contour (bold dashed) near the surface.

The rainwater is shaded (>0.0005 g kg

1) and the cloud boundary is bold contoured (>0 g kg 1). The corresponding integration time is shown at the top of each panel (from Lin 1998).

Cell Regeneration in 2D multicell storms

• The three stages of a convective cell.

• Equivalent potential temperature (shaded) and vertical velocity (contoured) fields were taken from Fig. 3 . Note the reference, frame shown is not fixed in space, but rather tracks the cell’s principal updraft.

• For more details, read Fovell and Tan (1998). Link to the PDF format paper is found at the course web site.

Summary on Cell Regeneration Theories

• Looked harder, the two theories are more complementary than contradictory. Both examine the rearward movement of older cells and the separation of the cell from the new cells

• Lin et al focus on the environmental condition that affects the rearward cell movement then on the associated cell regeneration.

• Fovell’s work emphasizes cell and cold pool interaction and the associated gust-front forcing/lifting. The change in the gust-front lifting is considered to play an important role in modulating the intensity and generation of new cells at the gust front.

• Hence, Lin et al’s work looks to the external factor while Fovell et al’s work looks to the internal dynamics for an explanation of the multi-cellular behavior –in my opinion - each is looking at a different but complementary aspect of the problem.

Documents

Convective Dynamics Single cell cumulus. Review of Skew-T Diagram