Synoptic Factors with Severe Convection

Review of AMS MonographSevere Conductive Storms

C.A. DoswellFor METR 515/815

Michele Blazek

List of Ingredients for DMC

• Deep Moisture Convection (DMC) requires three ingredients:– Moisture,– Instability– Lift

• With sufficient moisture & stability – still need lift to overcome Convective Inhibition (CIN)

Role of Lift in Convection

• Mass continuity – to lift parcels with “negative buoyancy,” there has to be a mass convergence under the ascending parcels.

• Boundaries show convergence – – e.g. fronts, drylines,

nonfrontal windshifts

Outline for Discussion

• Topographic Influences

• Solenoidal Circulations over Simple Terrains

• Monsoons

• Vertical Wind Shear

• Organized Convective Systems and Low Level Jet Streams

• Feedback to Synoptic Scale

Role of Synoptic Weather Systems in Convection?

• For a convective cloud – synoptic vertical motion is 2 O less than w = 10 m/s

• Besides to overcome CIN, the parcel must be lifted over that synoptic scale – – too much energy, too much time!– Subsynoptic processes are needed (fronts,

drylines, etc.)– But the “synoptic motions” weaken the

inhibitions – or prime conditions for convection

Topographic Influences

• Topography is MORE than orographic effects!

• Topography effects include– Orographic, – Thermal Contrasts– Water Bodies – Great

Lakes

No dice playing

• Synoptic scale cyclogenesis isn’t random – Most likely in lees of major mountainous areas

(Rockies) • Midwest (aided by low level moisture air masses

from the Gulf of Mexico)• Canadian Prairies (Moisture from GLs?)

– East Coast – due to the Gulf Stream– Be sure to mix in thermal contrasts!

And It’s Not Just North America!

• Non orographic– Great Lakes are

“allergic” to anticyclones in Winter but not Summer

– Mediterranean

• Air from the poles

• Low Level from Africa inhibits convection

• Easterlies ll increases convection

Mountains Provide Conditions for More Convection

• Complex terrrain increases possibility of lift and convective rain

• Subsynpotic scales but add enough moisture – BAM!! Thunderstorms

Influences of Air Masses

Superimpose Air Masses and you get wind shear!

Loaded Gun is Classic Example!

2. Solenoidal Circulations

• QG is not enough – can’t describe solenoids!– Fronts are well known to produce DMC– Other causes of solenoids

• Land sea breezes – local topography and terrain important ingredients – but you have to go with the synoptic flow!

• Outflow Boundaries – what goes down must go up!• Nonhomogenous topography – irrigated/non

irrigated; snow/no snow• And the mysterious dryline!

3. Monsoons

• What sea breezes are to diurnal periods – monsoons are to seasons!

• Monsoons don’t start convection – they start synoptic scale ascent!

4. Vertical Wind Shear

• Important ingredient for supercells – low level shear especially near updrafts (Rotunno and Klemp…)

• But its not just the shear it’s the helicity (Davies Jones, Lily)

And its in the Curves

• Ground level helicity– H(z) = - Σk (V * dv/dz) dz

• Storm Relative Helicity– H(z) = - Σk ((V-C)* dv/dz) dz

The Beauty of the Hodograph!

• For curved hodographs, then, it is likely that storms will encounter considerable storm-relative helicity.

• Fig. 20. (stippled) swept out by the ground-relative wind vectors along the hodograph from 0 to 3 km. Also shown is the area swept out by the storm-relative wind vectors (hatched

• Doswell, C.A., “ Review for Forecasters on Application of Hodographics to Forecast Severe Thunderstorms” 1991 REVIEW FOR

Need A lot of Energy to Overcome CIN (Sin?)

• Schematic comparing two soundings, each with a Level of Free Convection (LFC) 150 hPa above the surface. The trajectory followed by the surface parcel is suggested with the solid, dashed, and hatched lines that show the dry adiabatic ascent, the moist adiabatic ascent, and the mixing ratio of the surface parcel, respectively. The environmental temperature (T) and dewpoint temperature (Td) soundings are shown, and the associated CIN is indicated by the stippling.

Helicity and Thermal Wind

• Helicity is roughly proportional to the integrated advection of thermal wind

• k(Vx δv/ δz) = (g/fT)*V DelzT– Warm advection results veering of wind with

height – a great combo for DMC

5. Organized Systems and Low Level Jet Streams

• Mesoscale convective systems – are a big part of DMC

• MCCs occur in quiet conditions – In upper and mid troposphere with low level

thermal advection– Diurnal variations in the LLJS enhances

thermal advection (Maddox and Doswell, 1982)

– LLJS – regular cycle with cyclones and anticyclones

Feedbacks

• How does convection affect synoptic scale processes?– Mins of DMS in winter due to cool season and

reduced heat in low leevels– Warm seasons have less baroclinity – (less

shear!)– “Convection and ETC operate together to

maintain global heat balance”– Static stability is important on all scales

Quotable Quote

• “…the long term effect of convection must inevitably be an increase of static stability and a decrease in vertical wind shear, the short term effect on the environment can be to maintain conditions for DMC.”

Subtle Examples

• 28 Aug 1990 Plainfield IL – huge CAPE (>6000J/kg)

• 27 Mar 94 Palm Sunday Outbreak– No mid troposphere cyclone observed, but

CAPE, vertical wind shear, lljstream

Subtle Examples

Non tornadic Systems• 17 Aug 94 Lahoma OK

– Extreme CAPE, L/mid troposphere relative humidity, storm-relative helicity

– Intense Wind and Hail

• 14 June 90 Shadyside OH – flash flood– Shortwave trough in mid troposphere near large

anticyclone– High surface moisture/non frontal convergence

boundary (outflow)

Even More Subtle Examples

• 07 June 82 Kansas City derecho– Synoptic setting not alarming– Q-vector raingauge

• 09 June 72 Rapid city SD– Mid-troposphere anticyclone– 230 people died

Cold Season Severe Weather

• 21-22 January 1999 – severe thunderstorms and tornados (F3)

• A strong capping inversion eroded – severe convection and negatively tilted trough in mid-tropo– - like the superoutbreak in 1974

• Maybe the Sunday storm?

• Shreveport….