ATM 10 Severe and Unusual Weather Prof

ATM 10 Severe and Unusual Weather Prof. Richard Grotjahn L12 http://smartsite.ucdavis.edu Term Project • Term project: – emergency planning • Canvas, Go to: – Files (to find the assignment) – Assignments (to turn in online) • Part 1 – DRAFT (Due in 1 week!) • Parts 1 & 2 (FINAL of both due in 3 weeks) Online turn in: • http://canvas.ucda vis.edu/ • TERM PROJECT IN THE ASSIGNMENTS SECTION

• Click on Term Project. Then click submit

• Turn in term project. Notice that only certain file types will be accepted (so that we can provide feedback). Lecture topics: • Thunderstorm environment – Atmospheric conditions & energy source – climatology • Thunderstorm structure – Life cycle – Features and internal structure – Types • Airmass, • frontal, • supercell • Organization of thunderstorms – By frontal systems and squall lines – mesoscale convective clusters – the dryline

General Thunderstorm Properties

– Formation conditions / energy source – Climatology

What makes a cloud a thunderstorm?

• Cloud must have thunder (duh) and thunder is made by lightning • So, it must have lightning.

• Thunderstorm must be deep, as found in a cumulonimbus.

• To form such deep clouds, the air must become absolutely unstable

Thunderstorm Environment • dry air with lapse rate similar to dry adiabat (Γ∼Γd) above moist air with lapse rate near moist adiabat (Γ∼Γm) • Air parcel on chart: – start at 900mb. Above 700mb parcel warmer then environs, keeps rising. • Things that make parcels rise and air to become absolutely unstable: – daytime heating (e.g. sunshine on mountainsides, sunny morning, etc.) – forced lifting (front, convergence along line or area, gust front from another storm, etc.) • Q: Where is this common? • A: tropics, central US (warm season)

Fig. 17.15 Thunderstorm Climatology Thunderstorms occur most often in tropics ~1-2,000 at any moment. Over a year: 8 to 9 M thunderstorms occur!

Global low pressure zone around the equator is an area of convergence (ICZ) at the surface, rising air and cloud formation.

Individual thunderstorms often organized into groups

Thunderstorm Climatology (U.S.) • Fig. shows: days/year thunderstorm(s) observed • Over contiguous US: – Florida max – lesser max: • Rockies • KS & OK

Everglades, © R. Grotjahn Thunderstorm Structure

– Life cycle – Features & internal structures

Thunderstorm Life Cycle Stages • Typical AIR MASS thunderstorm lasts ~1 hour with these stages: 1. early growth (“towering cumulus”) stage: 2. mature phase: 3. dissipating: • If the motions are coordinated just right, the storm can last for much longer. Severe storms have such coordinated motions

© R Grotjahn Thunderstorm Life Cycle Stages 1. early growth stage: 1. Latent heat release provides growth energy: air warmer inside cloud than outside. (note error in figure: 0 oC line should bow up a bit in the cloud) 2. Inside cloud all air is rising (all updrafts inside). 3. Liquid water and active growth makes hard-edged cloud

© R. Grotjahn Thunderstorm Life Cycle Stages 1. early growth stage: 2. mature phase: • Downdraft starts, caused by: 1. downward motion of drops/hail drags air downward 2. drier air enters side of cloud, causes drops to evaporate, that cools air, makes it more dense (not drawn) • Downdraft reaches ground, spreads out making a gust front. Gust front can amplify updraft by forcing low level air to rise • Updraft can’t easily penetrate highly stable stratosphere so cloud spreads along tropopause making anvil shaped cloud (not drawn). precipitation begins falling, © R. Grotjahn Thunderstorm Life Cycle Stages 1. early growth stage: 2. mature phase: 3. dissipating: • cloud no longer expanding, edge mixes with drier air, cloud looses its hard edge • cloud mostly contains downdrafts and thus condensation is no longer occurring • with evaporation replacing condensation, storm looses energy © UCAR Thunderstorm Structures

NASA photo, thunderstorm over Africa Thunderstorm Structures

© R. Grotjahn Rain shaft / Hail shaft Downdraft Gust front Shelf cloud Anvil Overshooting top Other structures: mammatus, flanking line, vault, tornado Thunderstorm Structure • rain shaft • hail shaft (more streaky)

© R. Grotjahn Thunderstorm © R. Grotjahn Structure • other features – Rain shaft – Hail shaft – Downdraft • injection of dry air on “back side” • locations of heaviest rain relative to updraft & downdraft • Spreads out at ground to make gust front • gust front amplifies updraft of moist air

Thunderstorm Structure • gust front – downdraft spreads out when it reaches the ground – Often marked by a sudden increase in wind speed – Cool air behind

Gust Front near Lancaster CA © J Carroll

What does a radar ‘see’? Thunderstorm Structure • shelf cloud (above, smoother) • roll cloud (below, bumpy)

• anvil – Created when updraft is blocked by stratosphere – Notice the precipitation (arrow)

Thunderstorm Structure

• flanking line – Smaller clouds – Typically in rear of storm along gust front

© R. Grotjahn, Marshall CO Supercell Thunderstorm Structure • A few thunderstorms reach unusually strong intensity. – Called “Supercell” thunderstorms – Their Internal updrafts (yellow) and Downdrafts (blue) coordinated – Can develop internal rotation -- (in worst case: tornado)

Fig 17.22

Summary schematic is most like the lower left figure. Supercell Structure (visible)

• Two different events. • Supercell #1 (still photo) – View from SE looking NW – Rotation evident in ‘striations’ – Tornado seen vs larger cloud – Updraft inflow on lower half. • Supercell #2 (time lapse movie) – View is from S looking N – Backside (mid-left side) inflow feeding Rear Flank Downdraft – Inflow (right side) inflow feeding updraft.

Test your understanding

• Can you see the labelled parts? • Can you see the indicated flow?

Video of the day: Computer Simulation (Review) Organization of Thunderstorms

1. Cold fronts 2. Squall lines 3. Mesoscale convective clusters 4. The dryline

Recall: Weather Near COLD Fronts • Weather at a cold Front: (Fig. 12.14) • Cb-cumulonimbus, Ac-altocumulus, Cs-cirrostratus, Ci-cirrus • Narrow band (horiz. distance ~50km) of clouds & precipitation • Advancing cold air is forcing warm, possibly moist air upwards • Thunderstorms lined up along front if warm air conditionally unstable Cold front squall line: satellite image & weather map • Fig. 17.13a – taller (whiter) clouds above surface cold front boundary (text) • Narrow band (horiz. distance ~50km) of clouds & precipitation • Advancing cold air is forcing warm, possibly moist air upwards • Thunderstorms lined up along front if warm air conditionally unstable

Caution: Map is 2 hrs earlier than the satellite picture! Cold front squall line: radar • Fig. 17.11 • Narrow band (horiz. distance ~50km) of clouds & precipitation • Advancing cold air is forcing warm, possibly moist air upwards • Thunderstorms lined up along front if warm air conditionally unstable

Compare with Prior slide Recall: 12-14 March 1993 Superstorm • Unusually intense squall line was ahead of the Cold front

socir.mov (21 sec) = symbol indicating thunderstorm Mesoscale Convective Complex (MCC)

OKC tornadoes, 1999 Organized group of thunderstorms covering a large region is called a mesoscale convective complex (MCC). (bright area of deep cloud is a group of a dozen+ Thunderstorms)

When upper level winds weak, such MCC's can regenerate new storms and last for 12 hours and may bring hail, tornadoes, and flash floods.

They often form beneath a ridge of high pressure.

Figure 17.3 Mesoscale Convective Complex (MCC)

Downdrafts from individual thunderstorms reinforce the development of new thunderstorms downstream

One can recognize different parts of the complex when a radar loop is shown.

Figure 17.4 Figure 17.5 Mesoscale Convective Complex (MCC) •Cross section shows MCC stages •Panels in radar (fig. 17.4) & these schematics (fig. 17.5) correspond to same stages of MCC life cycle •Plotted: •Red arrows: updrafts •Blue arrows: downdrafts •White: snow & ice precipitation •Black dots: rain •Blue: evaporation-cooled air •Gray area: cloud Mesoscale Convective Broad area of precipitation (see horizontal Complex (MCC) radar) develops behind lead edge of MCC. Thunderstorm regeneration determines MCC Movement

1. Mid-troposphere winds often direct motion of individual thunderstorms.

2. But storm downdrafts spawn new Upper level storms: often on right, forward flank flow 3. So thunderstorm (composed of several cells) or MCC move to right relative to upper level winds • Figs: upper level winds move cells to the northeast, but downdrafts generate new cells to south that eventually cut View from behind View from above off moisture to old cell The Dryline

• So named since there is abrupt change of dew-point (Td), even though the air temperature (T) similar on both sides of dry line

• During spring and summer a dryline often forms in western TX, OK, and KS.

• Hot and dry cT air from the west rises up (so it cools) and over warm moist mT air from Gulf of Mexico. • Get surface convergence where air masses meet. • Creates potentially unstable profile Figure 15.16

Reference lapse rates Lecture Summary: • Thunderstorm environment – Thunderstorm local environment: • form in conditionally unstable air that has been made absolutely unstable by lifting or daytime heating below • ideal conditionally unstable profile has dry air above moist air with each layer close to respective adiabat – climatology • found throughout tropics, especially along ICZ • in US: most common in Florida, with 2nd max over Rockies and central plains – convection driven by latent heat release in updrafts • Thunderstorm structure – life cycle stages • early: all updraft in cloud, cloud has hard edge, no precipitation yet • mature: mix of up- & downdrafts, cloud develops anvil top where further rising blocked by stratosphere, precipitation (often heavy) from base and from anvil • decaying: downdrafts overwhelm updrafts & shut off source of energy (condensation) – structures: vault, overshooting top, anvil, rain shaft, gust front, up/downdrafts. – Some structures visible on radar images. Broad radar echo behind intense squall line or MCC due to precipitation from anvil • Organization of thunderstorms – by convergence lines that lift unstable air: such as along cold fronts and drylines – squall lines can form along or ahead of cold fronts, – downdrafts organize updrafts to maintain convection in squall lines & MCCs (next time, tornados!) End of lecture 12