severe convective storms, theory Pieter Groenemeijer FMI Helsinki, 2 May 2005
“one-slide introduction” of myself I am Pieter Groenemeijer • M.Sc. in Physics and Astronomy at Utrecht University • Oklahoma University (spring semester 2002) • ESWD (European Severe Weather Database) • “Sounding-derived parameters associated with large hail and tornadoes in the Netherlands“ • Co-initiator of ESTOFEX (with Johannes Dahl and Christoph Gatzen), Oct, 2002.
my contribution this morning 1. Ingredients-based forecasting - instability - lift 2. Storm structure - wind shear: multicells and supercells - other factors: linear convective systems _________________________________________ (short break) • Convection parameters • Severe weather hazards - a study in Holland • A case Questions, discussion
what will we discuss? severe convective storms: storms that produce hazardous weather like: • lightning • heavy rain (leading to flash floods) • strong winds (straight-line winds) • large hail • tornadoes
ingredients-based forecasting (Doswell, 2004) • What is“ingredients-based forecasting”? an “ingredient” is something necessary for some event to occur I will cover the theory by exploring those ingredients
ingredients for convective storms • latent instability • lift (rising motion)
instability • lapse rate definition: dT/dz > 1.0 C/km in dry air or: dT/dz > moist adiabatic lapse rate in saturated air these are the definitions of absolute and conditional instability
instability • layer definition: when lifting a layer, saturation occurs and dT/dz becomes > moist adiabatic lapse rate Or equivalently: theta-e (and theta-w) decrease with height potential instability
instability a convective bubble is more like a parcel than a layer... • parcel definition: parcel becomes warmer than environment after lift latent instability (Normand, 1937) several “convective parameters” are based on the concept of latent instability: • CAPE (in all its forms) • LI (Lifted Index) • Showalter Index
limitations of parcel theory Realize that parcel theory is a simplification of reality: • what in reality is a parcel? is it undiluted? • and its environment? is it not influenced by convection? objection: We neglect pressure perturbation forces! (come back to that later)
lift latent instability ≠ storms • a “cap”, CIN may be present, or • entrainment may inhibit the development of convective storms lift • can weaken the “cap”, or • is associated with convergence at the surface: - helps to sustain initiating convective bubbles
we have identified... two ingredients for convective storms... • latent instability • (sufficient) lift okay... but when should we become worried about extreme events? are other ingredients required?
storm structures / convective modes • some severe events are associated with particular storm structures (or convective modes) EXAMPLES from my home country multicell clusters isolated supercell multicell line
storm structures / convective modes • some severe events are associated with particular storm structures (or convective modes), others are not, e.g.: - strong tornadoes are known to occur mostly with supercell storms - extreme rainfall and lightning can occur with any storm structure, but generally... anticipating storm structure is very important to predict the quantity and quality of the severe weather that may occur
factors influencing storm structures • vertical wind shear • other factors
vertical wind shear • vertical wind shear has a strong influence on convective organisation it affects • storm propagation • vertical speeds in up- and downdrafts • storm longevity
storm in weak vertical shear weak shear: single-cell storms • updraft grows • precipitation forms • cold pool forms and spreads out • updraft ceases • storm ceases
reality a gust front made visible by blowing dust and sand
storm in moderate vertical shear moderate shear: multicell storms • updraft grows • precipitation forms • cold pool forms and spreads out >>>>> • updraft ceases • storm ceases • new updraft grows • precipitation forms • cold pool forms and spreads out >>>>> • updraft ceases • storm ceases 1. new updraft grows 2. precipitation forms 3. cold pool grows and spreads out 4. updraft ceases 5. storm ceases time new cells form at the edge of the cold pool....
RKW-theory from Rotunno, Klemp and Wilhelmson, 1988 when horizontal vorticity produced by the cold pool and that of the environments are roughly equal the strongest lift will occur new cells form at the edge of the cold pool....
RKW-theory no vertical wind shear from Xue et al., 1997
RKW-theory low-level vertical wind shear from Xue et al., 1997
RKW-theory RKW-theory is not undisputed... it seems to work better in the laboratory than in reality
storm in moderate vertical shear multicell cluster the cells may not be distinguished by a radar scanning at a low elevation....
storm in moderate vertical shear multicell line: squall line watch the cells forming at the front of the system that move backward w.r.t. the system
storm in strong vertical shear strong shear: supercell storms
supercell definition: a supercell is a storm with a persistent, deep rotating updraft (that is, a mesocyclone) • a few characteristics: • very strong updrafts • often: very strong downdrafts • ...resulting in a high potential for severe weather • don’t move with the mean wind
storm-relative helicity vertical shear implies horizontal vorticity
storm-relative helicity storm-relative helicity (e.g. Davies, 1985; Droegemeier et al., 1993)